The history of cementing material is as old as the history of engineering construction. Some kind of cementing material were used by Egyptians romans and Indians in their ancient construction. It is believed that the early Egyptians mostly used cementing materials, obtained by burning gypsum. Not much light has been thrown on cementing material, used in the construction of the cites of Harappa and Mohenjo-Daro.
An analysis of mortar from the great Pyramid showed that it contained 81.5 per cent calcium sulphate and only 9.5 per cent carbonate. The early Greeks and Romans used carbonate. The remarkable obtained by burning limestone. The remarkable hardness of the mortar used in the early roman brickwork. Some of which still exit, Is presenting sufficient evidence of the perfection which the art of cementing material had attained in ancient times, the superiority of roman mortar has been attributed to thoroughness of mixing and long continued ramming.
The Greeks and Romans later became aware the face the certain volcanic ash and tuff, when mixed with lime and sand yielded mortar possessing superior strength and better durability in fresh or salt water. Roman builders used volcanic tuff found near Pozzuoli village near mount Vesuvius in Italy. This volcanic tuff or ash mostly siliceous in nature thus acquired the name pozzolana. Later on, the name pozzolona was applied to any other material natural or artificial, having nearly the same composition as that of volcanic tuff or ash found at Pozzuoli, the Romans, in the absence of natural volcanic ash, used powered tiles or pottery as pozzolana. In India. Powered brick named surkhi has been used in mortar. The Indian practice of through mixing and long continued ramming of lime mortar with or without the addition of surkhi yielded strong and impervious mortar which confirmed the secret of superiority of Roman mortar.
It is learnt that the Romans added blood, milk and lard to their mortar and concrete to achieve better workability. Hemoglobin in a powerful air-entraining agent and plasticizer which perhaps is yet another reason for the durability of Roman structure. Probably they did not know about the durability aspect but used them a workability agents. The cementing material made by Romans using lime and natural or artificial pozzolana retained its position as the chief building material for all work particularly. For hydraulic construction belabor a principal authority in hydraulic construction, recommended an intimate mixture of tiles, stone chips and scales from black smith’s forge, carefully ground wadhed free coal and drit dried and sifted and them mixed with fresh slaked lime for making good concrete .
When we come to more recent times, the most important advance in the knowledge of cements, the forerunner to the discoveries and manufacture of all modern cements is undoubtedly the investigations carried out by John Smeaton. When he was called upon to rebuild the Eddystone Light-house in 1756, he made extensive enquiries into the state of art existing in those days and also conducted experiments with a view to find out the best material to withstand the severe action of sea water. Finally, he concluded that lime-stones which contained considerable proportion of clayey matter yielded better lime possessing superior hydraulic properties. In spite of the success of Smeaton’s experiments, the use of hydraulic lime made little progress, and the old practice of mixture of lime and pozzolana remained popular for a long period. In 1976 hydraulic cement was made by calcining nodules of argillaceous lime-stones. In about 1800 the product thus obtained was called Roman cement. This type of cement was in use till about 1850 after which this was outdated by portland cement.
Early History of Modern Cement:
The investigations of L.J. Vicat led him to prepare an artificial hydraulic lime by calcining an intimate mixture of limestone and clay. This process may be regarded as the leading knowledge to the manufacture of Portland cement. James Frost also patented a cement of this kind Joseph Aspdin’s first cement works, around 1823, at Kirkgate in in 1811 and established The story of the invention of Portland cement is, however, attributed to Joseph Aspdin, a Leeds builder and bricklayer, even though similar procedures had been adopted by other inventors. Joseph Aspdin took the patent of portland cement on 21st October 1824. The fancy name of portland was given owing to the resemblance of this hardened cement to the natural stone occurring at Portland in England. In his process Aspdin mixed and ground hard limestones and finely divided clay into the form of slurry and calcined it in a furnace similar to a lime kiln till the CO2 was expelled. The mixture so calcined was then ground to a fine powder. Perhaps, a temperature lower than the clinkering temperature was used by Aspdin. Later in 1845 Isaac Charles Johnson burnt a mixture of clay and chalk till the clinkering stage to make better cement and established factories in 1851. In the early period, cement was used for making mortar only. Later the use of cement was extended for making concrete. As the use of Portland cement was increased for making concrete, engineers called for consistently higher standard material for use in major works. Association of Engineers, Consumers and Cement Courtesy : Ambuja Technical Literature
Manufacturers have been established to specify standards for cement. The German standard specification for Portland cement was drawn in 1877. The British standard specification was first drawn up in 1904. The first ASTM specification was issued in 1904.
In India, Portland cement was first manufactured in 1904 near Madras, by the South India Industrial Ltd. But this venture failed. Between 1912 and 1913, the Indian Cement Co. Ltd., was established at Porbander (Gujarat) and by 1914 this Company was able to deliver about 1000 tons of Portland cement. By 1918 three factories were established. Together they were able to produce about 85000 tons of cement per year. During the First Five-Year Plan (1951-1956) cement production in India rose from 2.69 million tons to 4.60 million tons. By 1969
the total production of cement in India was 13.2 million tons and India was then occupying the 9th place in the world, with the USSR producing 89.4 million tonnes and the USA producing 70.5 million tonnes1.1. Table 1.1 shows the Growth of Cement Industry through Plans.
Prior to the manufacture of Portland cement in India, it was imported from UK and only a few reinforced concrete structures were built with imported cement. A three storeyed structure built at Byculla, Bombay is one of the oldest RCC structures using Portland cement in India. A concrete masonry building on Mount Road at Madras (1903), the har-ki-pahari bridge at Haridwar (1908) and the Cotton Depot Bombay, then one of the largest of its kind in the world (1922) are some of the oldest concrete structures in India
The perusal of table 1.2 shows that per capita cement consumption in India is much less than world average. Considerable infrastructural development is needed to build modern India. Production of more cement, knowledge and economical utilisation of cement is the need of the day.
The early scientific study of cements did not reveal much about the chemical reactions that take place at the time burning. A deeper study of the fact that the clayey constituents of limestone are responsible for the hydraulic properties in lime (as established by John Smeaton) was not taken for further research. It may be mentioned that among the earlier cement technologists, Vicat, Le Chatelier and Michaelis were the pioneers in the theoretical and practical field.
Systematic work on the composition and chemical reaction of Portland cement was first begun in the United States. The study on setting was undertaken by the Bureau of Standards and since 1926 much work on the study of Portland cement was also conducted by the Portland Cement Association, U.K. By this time, the manufacture and use of Portland cement had spread to many countries. Scientific work on cements and fundamental contributions to the chemistry of Portland cements were carried out in Germany, Italy, France, Sweden, Canada and USSR, in addition to Britain and USA. In Great Britain with the establishment of Building Research Station in 1921 a systematic research programme was undertaken and many major contributions have been made. Early literatures on the development and use of Portland cements may be found in the Building Science Abstracts published by Building Research Station U.K. since 1928, “Documentation Bibliographique” issued quarterly since 1948 in France and “Handbuch der Zement Literature” in Germany.
Manufacture of Portland Cement
The raw materials required for manufacture of Portland cement are calcareous materials, such as limestone or chalk, and argillaceous material such as shale or clay. Cement factories are established where these raw materials are available in plenty. Cement factories have come up in many regions in India, eliminating the inconvenience of long distance transportation of raw and finished materials.
The process of manufacture of cement consists of grinding the raw materials, mixing them intimately in certain proportions depending upon their purity and composition and burning them in a kiln at a temperature of about 1300 to 1500°C, at which temperature, the material sinters and partially fuses to form nodular shaped clinker. The clinker is cooled and ground to fine powder with addition of about 3 to 5% of gypsum. The product formed by using this procedure is Portland cement.
There are two processes known as “wet” and “dry” processes depending upon whether the mixing and grinding of raw materials is done in wet or dry conditions. With a little change in the above process we have the semi-dry process also where the raw materials are ground dry and then mixed with about 10-14 per cent of water and further burnt to clinkering temperature.
For many years the wet process remained popular because of the possibility of more accurate control in the mixing of raw materials. The techniques of intimate mixing of raw materials in powder form was not available then. Later, the dry process gained momentum with the modern development of the technique of dry mixing of powdered materials using compressed air. The dry process requires much less fuel as the materials are already in a dry state, whereas in the wet process the slurry contains about 35 to 50 per cent water. To dry 6 " Concrete Technology the slurry we thus require more fuel. In India most of the cement factories used the wet process. Recently a number of factories have been commissioned to employ the dry process method. Within next few years most of the cement factories will adopt dry process system.
In the wet process, the limestone brought from the quarries is first crushed to smaller fragments. Then it is taken to a ball or tube mill where it is mixed with clay or shale as the case may be and ground to a fine consistency of slurry with the addition of water. The slurry is a liquid of creamy consistency with water content of about 35 to 50 per cent, wherein particles, crushed to the fineness of Indian Standard Sieve number 9, are held in suspension. The slurry is pumped to slurry tanks or basins where it is kept in an agitated condition by means of rotating arms with chains or blowing compressed air from the bottom to prevent settling of limestone and clay particles. The composition of the slurry is tested to give the required chemical composition and corrected periodically in the tube mill and also in the slurry tank by blending slurry from different storage tanks. Finally, the corrected slurry is stored in the final storage tanks and kept in a homogeneous condition by the agitation of slurry.
The corrected slurry is sprayed on to the upper end of a rotary kiln against hot heavy hanging chains. The rotary kiln is an important component of a cement factory. It is a thick steel cylinder of diameter anything from 3 metres to 8 metres, lined with refractory materials, mounted on roller bearings and capable of rotating about its own axis at a specified speed.
The length of the rotary kiln may vary anything from 30 metres to 200 metres. The slurry on being sprayed against a hot surface of flexible chain loses moisture and becomes flakes. These flakes peel off and fall on the floor. The rotation of the rotary kiln causes the flakes to move from the upper end towards the lower end of the kiln subjecting itself to higher and higher temperature. The kiln is fired from the lower end. The fuel is either powered coal, oil or natural gass. By the time the material rolls down to the lower end of the rotary kiln, the dry material
Concrete Technology A view of Limestone quarry, raw material preparation : The prime raw material limestone after blasting in mines is broken into big boulders. Then it is transported by dumpers, tippers to limestone crusher where it is crushed to 15 to 20 mm size
STACKER FOR CRUSHED LIMESTONE RECLAIMER FOR CRUSHED LIMESTONE
After crushing, the crushed limestone is piled longitudinally by an equipment called stacker. The stacker deposits limestone longitudinally in the form of a pile. The pile is normally 250 to 300 m long and 8-10 m height. The declaimer cuts the pile vertically, simultaneously from top to bottom to ensure homogenization of limestone. Reclaimer for homogenization of crushed limestone.
undergoes a series of chemical reactions until finally, in the hottest part of the kiln, where the temperature is in the order of 1500°C, about 20 to 30 per cent of the materials get fused.
Lime, silica and alumina get recombined. The fused mass turns into nodular form of size 3 mm to 20 mm known as clinker. The clinker drops into a rotary cooler where it is cooled under controlled conditions The clinker is stored in silos or bins. The clinker weighs about 1100 to 1300 gms per litre. The litre weight of clinker indicates the quality of clinker.
The cooled clinker is then ground in a ball mill with the addition of 3 to 5 per cent of gypsum in order to prevent flash-setting of the cement. A ball mill consists of several compartments charged with progressively smaller hardened steel balls. The particles crushed to the required fineness are separated by currents of air and taken to storage silos from where the cement is bagged or filled into barrels for bulk supply to dams or other large work sites.
In the modern process of grinding, the particle size distribution of cement particles are maintained in such a way as to give desirable grading pattern. Just as the good grading of aggregates is essential for making good concrete, it is now recognised that good grading pattern of the cement particles is also important.
In the dry and semi-dry process the raw materials are crushed dry and fed in correct proportions into a grinding mill where they are dried and reduced to a very fine powder. The dry powder called the raw meal is then further blended and corrected for its right composition and mixed by means of compressed air. The aerated powder tends to behave almost like liquid and in about one hour of aeration a uniform mixture is obtained.
The blended meal is further sieved and fed into a rotating disc called granulator. A quantity of water about 12 per cent by wright is added to make the blended meal into pellets.
This is done to permit air flow for exchange of heat for further chemical reactions and conversion of the same into clinker further in the rotary kiln.
The equipments used in the dry process kiln is comparatively smaller. The process is quite economical. The total consumption of coal in this method is only about 100 kg when compared to the requirement of about 350 kg for producing a ton of cement in the wet process. During March 1998, in India, there were 173 large plants operating, out of which 49 plants used wet process, 115 plants used dry process and 9 plants used semi-dry process.
Since the time of partial liberalisation of cement industry in India (1982), there has been an upgradation in the quality of cement. Many cement companies upgraded their plants both in respect of capacity and quality. Many of the recent plants employed the best equipments, such as cross belt analyser manufactured by Gamma-Metrics of USA to find the composition of limestone at the conveyor belts, high pressure twin roller press, six stage preheater, precalciner and vertical roller mill. The latest process includes stacker and reclaimer, on-line X-ray analyser, Fuzzy Logic kiln control system and other modern process control. In one of the recently built cement plant at Reddypalayam near Trichy, by Grasim Indistries, employed Robot for automatic collection of hourly samples from 5 different places on the process line and help analyse the ame, throughout 24 hours, untouched by men, to avoid human errors in quality control. With all the above sophisticated equipments and controls, consistent quality of clinker is produced.
The methods are commonly employed for direct control of quality of clinker. The first method involves reflected light optical microscopy of polished and etched section of clinker
" Concrete Technology Close circuit grinding technology is most modern grinding system for raw mix as well as for clinker grinding. The systems are in compound mode and are equipped with high efficiency Roller press and separators. The above mentioned system enables to maintain low power consumption for grinding as well as Electronic packers: it has continuous narrow particle size distribution. With this circuit, it is weighing system and it ensures that the possible to manufacture higher surface area of product bags separating from the nozzles have as per customers, requirement. accurate weight of cement. The weight of filled bag is also displayed on the packer. Multi-compartment silo. Cross section of multi-compartment silo. Jumbo bag transportation. followed by point count of areas occupied by various compounds. The second method, which is also applicable to powdered cement, involves X-ray diffraction of powder specimen. Calibration curves based on known mixtures of pure compounds, help to estimate the compound composition. As a rough and ready method, litre weight (bulk density) of clinker is made use of to ascertain the quality. A litre weight of about 1200 gms. is found to be satisfactory. Jumbo bag packing. It is important to note that the strength properties of cement are considerably influenced by the cooling rate of clinker.
Concrete Technology It can be seen from the table that a moderate rate of cooling of clinker in the rotary cooler will result in higher strength. By moderate cooling it is implied that from about 1200°C, the clinker is brought to about 500°C in about 15 minutes and from the 500°C the temperature is brought down to normal atmospheric temperature in about 10 minutes.
The rate of cooling influences the degree of crystallisation, the size of the crystal and the amount of amorphous materials present in the clinker. The properties of this amorphous material for similar chemical composition will be different from the one which is crystallined.
The raw materials used for the manufacture of cement consist mainly of lime, silica, alumina and iron oxide. These oxides interact with one another in the kiln at high temperature to form more complex compounds. The relative proportions of these oxide compositions are responsible for influencing the various properties of cement; in addition to rate of cooling and fineness of grinding. Table 1.4 shows the approximate oxide composition limits of ordinary Portland cement.
In addition to the four major compounds, there are many minor compounds formed in the kiln. The influence of these minor compounds on the properties of cement or hydrated compounds is not significant. Two of the minor oxides namely K2O and Na2O referred to as alkalis in cement are of some importance. This aspect will be dealt with later when discussing alkali-aggregate reaction. The oxide composition of typical Portland cement and the corresponding calculated compound composition
Schematic presentation of various compounds in clinker Courtesy : All the photographs on manufacture of cement are by Grasim Industries Cement Division Tricalcium silicate and dicalcium silicate are the most important compounds responsible for strength. Together they constitute 70 to 80 per cent of cement. The average C3S content in modern cement is about 45 per cent and that of C2S is about 25 per cent. The sum of the contents of C3A and C4AF has decreased slightly in modern cements. The calculated quantity of the compounds in cement varies greatly even for a relatively small change in the oxide composition of the raw materials. To manufacture a cement of stipulated compound composition, it becomes absolutely necessary to closely control the oxide composition of the raw materials. An increase in lime content beyond a certain value makes it difficult to combine with other compounds and free lime will exist in the clinker which causes unsoundness in cement. An increase in silica content at the expense of the content of alumina and ferric oxide will make the cement difficult to fuse and form clinker. Cements with a high total alumina and high ferric oxide content is favourable to the production of high early strengths in cement. This is perhaps due to the influence of these oxides for the complete combining of the entire quantity of lime present to form tricalcium silicate.
The advancement made in the various spheres of science and technology has helped us to recognise and understand the micro structure of the cement compounds before hydration and after hydration. The X-ray powder diffraction method, X-ray fluorescence method and use of powerful electron microscope capable of magnifying 50,000 times or even more has helped to reveal the crystalline or amorphous structure of the unhydrated or hydrated cement.
Both Le Chatelier and Tornebohm observed four different kinds of crystals in thin sections of cement clinkers. Tornebohm called these four kinds of crystals as Alite, Belite, Celite and Felite. Tornebohm’s description of the minerals in cement was found to be similar to Bogue’s description of the compounds. Therefore, Bogue’s compounds C3S, C2S, C3A and C4AF are sometimes called in literature as Alite, Belite, Celite and Felite respectively.
Hydration of Cement:
Anhydrous cement does not bind fine and coarse aggregate. It acquires adhesive property only when mixed with water. The chemical reactions that take place between cement and water is referred as hydration of cement.
The chemistry of concrete is essentially the chemistry of the reaction between cement and water.On account of hydration certain products are formed. These products are important because they have cementing or adhesive value. The quality, quantity, continuity, stability and the rate of formation of the hydration products are important. Anhydrous cement compounds when mixed with water, react with each other to form hydrated compounds of very low solubility. The hydration of cement can be visualized in two ways. The first is “through solution” mechanism. In this the cement compounds dissolve to produce a supersaturated solution from which different hydrated products get precipitated. The second possibility is
" Concrete Technology that water attacks cement compounds in the solid state converting the compounds into hydrated products starting from the surface and proceeding to the interior of the compounds with time. It is probable that both “through solution” and “solid state” types of mechanism may occur during the course of reactions between cement and water. The former mechanism may predominate in the early stages of hydration in view of large quantities of water being available, and the latter mechanism may operate during the later stages of hydration.
Heat of Hydration:
The reaction of cement with water is exothermic. The reaction liberates a considerable quantity of heat. This liberation of heat is called heat of hydration. This is clearly seen if freshly mixed cement is put in a vaccum flask and the temperature of the mass is read at intervals.
The study and control of the heat of hydration becomes important in the construction of concrete dams and other mass concrete constructions. It has been observed that the temperature in the interior of large mass concrete is 50°C above the original temperature of the concrete mass at the time of placing and this high temperature is found to persist for a prolonged period. Fig 1.2 shows the pattern of liberation of heat from setting cement1.4 and during early hardening period.
On mixing cement with water, a rapid heat evolution, lasting a few minutes, occurs. This heat evolution is probably due to the reaction of solution of aluminates and sulphates (ascending peak A). This initial heat evolution ceases quickly when the solubility of aluminate is depressed by gypsum. (decending peak A). Next heat evolution is on account of formation of ettringite and also may be due to the reaction of C3S (ascending peak B). Refer Fig. 1.2.
Different compounds hydrate at different rates and liberate different quantities of heat.
Fig. 1.3 shows the rate of hydration of pure compounds. Since retarders are added to control the flash setting properties of C3A, actually the early heat of hydration is mainly contributed from the hydration of C3S. Fineness of cement also influences the rate of development of heat but not the total heat. The total quantity of heat generated in the complete hydration will depend upon the relative quantities of the major compounds present in a cement.
Analysis of heat of hydration data of large number of cements, Verbec and Foster1.5
computed heat evolution of four major compounds of cement. Table 1.7. shows the heats of hydration of four compounds.
Since the heat of hydration of cement is an additive property, it can be predicted from an expression of the type
H = aA + bB + cC + dD
Where H represents the heat of hydration, A, B, C, and D are the percentage contents of C3S, C2S, C3A and C4AF. and a, b, c and d are coefficients representing the contribution of 1 per cent of the corresponding compound to the heat of hydration.
Normal cement generally produces 89-90 cal/g in 7 days and 90 to 100 cal/g in 28 days.
The hydration process is not an instantaneous one. The reaction is faster in the early period and continues idenfinitely at a decreasing rate. Complete hydration cannot be obtained under a period of one year or more unless the cement is very finely ground and reground with excess of water to expose fresh surfaces at intervals. Otherwise, the product obtained shows unattacked cores of tricalcium silicate surrounded by a layer of hydrated silicate, which being relatively impervious to water, renders further attack slow. It has been observed that after 28
days of curing, cement grains have been found to have hydrated to a depth of only 4µ. It has also been observed that complete hydration under normal condition is possible only for cement particles smaller than 50µ.
A grain of cement may contain many crystals of C3S or others. The largest crystals of C3S
or C2S are about 40µ. An average size would be 15-20µ. It is probable that the C2S crystals present in the surface of a cement grain may get hydrated and a more reactive compound like C3S lying in the interior of a cement grain may not get hydrated.
The hydrated product of the cement compound in a grain of cement adheres firmly to the unhydrated core in the grains of cement. That is to say unhydrated cement left in a grain of cement will not reduce the strength of cement mortar or concrete, as long as the products of hydration are well compacted. Abrams obtained strength of the order of 280 MPa using mixes with a water/cement ratio as low as 0.08. Essentially he has applied tremendous pressure to obtain proper compaction of such a mixture. Owing to such a low water/cement ratio, hydration must have been possible only at the surface of cement grains, and a considerable portion of cement grains must have remained in an unhydrated condition.
The present day High Performance concrete is made with water cement ratio in the region of 0.25 in which case it is possible that a considerable portion of cement grain remains unhydrated in the core. Only surface hydration takes place. The unhydrated core of cement grain can be deemed to work as very fine aggregates in the whole system.
Calcium Silicate Hydrates:
During the course of reaction of C3S and C2S with water, calcium silicate hydrate, abbreviated C-S-H and calcium hydroxide, Ca(OH)2 are formed. Calcium silicate hydrates are the most important products. It is the essence that determines the good properties of concrete.
" Concrete Technology It makes up 50-60 per cent of the volume of solids in a completely hydrated cement paste.
The fact that term C-S-H is hyphenated signifies that C-S-H is not a well defined compound. The morphology of C-S-H shows a poorly crystalline fibrous mass.
It was considered doubtful that the product of hydration of both C3S and C2S results in the formation of the same hydrated compound. But later on it was seen that ultimately the hydrates of C3S and C2S will turn out to be the same. The following are the approximate equations showing the reactions of C3S and C2S with water.
However, the simple equations given above do not bring out the complexities of the actual reactions.
It can be seen that C3S produces a comparatively lesser quantity of calcium silicate hydrates and more quantity of Ca(OH)2 than that formed in the hydration of C2S. Ca(OH)2 is not a desirable product in the concrete mass, it is soluble in water and gets leached out making the concrete porous, particularly in hydraulic structures. Under such conditions it is useful to use cement with higher percentage of C2S content.
C3S readily reacts with water and produces more heat of hydration. It is responsible for early strength of concrete. A cement with more C3S content is better for cold weather concreting. The quality and density of calcium silicate hydrate formed out of C3S is slightly inferior to that formed by C2S. The early strength of concrete is due to C3S.
C2S hydrates rather slowly. It is responsible for the later strength of concrete. It produces less heat of hydration. The calcium silicate hydrate formed is rather dense and its specific surface is higher. In general, the quality of the proudct of hydration of C2S is better than that produced in the hydration of C3S. Fig 1.4 shows the development of strength of pure compounds.
The other products of hydration of C3S and C2S is calcium hydroxide. In contrast to the C-S-H, the calcium hydroxide is a compound with a distinctive hexagonal prism morphology.
It constitutes 20 to 25 per cent of the volume of solids in the hydrated paste. The lack of durability of concrete, is on account of the presence of calcium hydroxide. The calcium hydroxide also reacts with sulphates present in soils or water to form calcium sulphate which further reacts with C3A and cause deterioration of concrete. This is known as sulphate attack.
To reduce the quantity of Ca(OH)2 in concrete and to overcome its bad effects by converting it into cementitious product is an advancement in concrete technology. The use of blending Cement " 21
materials such as fly ash, silica fume and such other pozzolanic materials are the steps to overcome bad effect of Ca(OH)2 in concrete. This aspect will be dealt in greater detail later.
The only advantage is that Ca(OH)2, being alkaline in nature maintain pH value around 13 in the concrete which resists the corrosion of reinforcements.
Calcium Aluminate Hydrates:
The hydration of aluminates has been the subject of numerous investigations, but there is still some uncertainty about some of the reported products. Due to the hydration of C3A , a calcium aluminate system CaO – Al2O3 – H2O is formed. The cubic compound C3 AH6 is probably the only stable compound formed which remains stable upto about 225°C.
The reaction of pure C3 A with water is very fast and this may lead to flash set. To prevent this flash set, gypsum is added at the time of grinding the cement clinker. The quantity of gypsum added has a bearing on the quantity of C3 A present. The hydrated aluminates do not contribute anything to the strength of concrete. On the other hand, their presence is harmful to the durability of concrete particularly where the concrete is likely to be attacked by sulphates. As it hydrates very fast it may contribute a little to the early strength. On hydration, C4AF is believed to form a system of the form CaO – Fe2O3 – H2O. A hydrated calcium ferrite of the form C3FH6 is comparatively more stable. This hydrated product also does not contribute anything to the strength. The hydrates of C4AF show a comparatively higher resistance to the attack of sulphates than the hydrates of calcium aluminate.
From the standpoint of hydration, it is convenient to discuss C3A and C4AF together, because the products formed in the presence of gypsum are similar. Gypsum and alkalies go into solution quickly and the solubility of C3A is depressed. Depending upon the concentration of aluminate and sulphate ions in solution, the pricipitating crystalline product is either the calcium aluminate trisulphate hydrate (C6A S 3H32) or calcium aluminate monosulhphate hydrate (C4A S H18). The calcium aluminate trisulphate hydrate is known as ettringite.
Ettringite is usually the first to hydrate and crystallise as short prismatic needle on account of the high sulphate/aluminate ratio in solution phase during the first hour of hydration. When sulphate in solution gets depleted, the aluminate concentration goes up due to renewed hydration of C3A and C4AF. At this stage ettringite becomes unstable and is gradually converted into mono-sulphate, which is the final product of hydration of portland cements containing more than 5 percent C3A.
The amount of gypsum added has significant bearing on the quantity of aluminate in the cement. The maintenance of aluminate-to-sulphate ratio balance the normal setting 22 " Concrete Technology behaviour of cement paste. The various setting phenomena affected by an imbalance in the A/ S ratio is of practical significance in concrete technology.
Many theories have been put forward to explain what actually is formed in the hydration of cement compounds with water. It has been said earliiar that product consisting of (CaO.SiO2.H2O) and Ca(OH)2 are formed in the hydration of calcium silicates. Ca(OH)2 is an unimportant product, and the really significant product is (CaO.SiO2.H2O). For simplicity’s sake this product of hydration is sometime called tobermorite gel because of its structural similarity to a naturally occurring mineral tobermorite. But very commonly the product of hydration is referred to as C – S – H gel.
It may not be exactly correct to call the product of hydrations as gel. Le chatelier identified the products as crystalline in nature and put forward his crystalline theory. He explained that the precipitates resemble crystals interlocked with each other. Later on Michaelis put forward his colloidal theory wherein he considered the precipitates as colloidal mass, gelatinous in nature. It is agreed that an element of truth exists in both these theories. It is accepted now that the product of hydration is more like gel, consisting of poorly formed, thin, fibrous crystals that are infinitely small. A variety of transitional forms are also believed to exist and the whole is seen as bundle of fibres, a fluffy mass with a refractive index of 1.5 to 1.55, increasing with age.
Since the gel consists of crystals, it is porous in nature. It is estimated that the porosity of gel is to the extent of 28%. The gel pores are filled with water. The pores are so small that the specific surface of cement gel is of the order of 2 million sq. cm. per gm. of cement. The porosity of gel can be found out by the capillary condensation method or by the mercury porosimetry method.
Structure of Hydrated Cement:
To understand the behaviour of concrete, it is necessary to acquaint ourselves with the structure of hydrated hardened cement paste. If the concrete is considered as two phase material, namely, the paste phase and the aggregate phase, the understanding of the paste phase becomes more important as it influences the behaviour of concrete to a much greater extent. It will be discussed later that the strength, the permeability, the durability, the drying shrinkage, the elastic properties, the creep and volume change properties of concrete is greatly influenced by the paste structure. The aggregate phase though important, has lesser influence on the properties of concrete than the paste phase. Therefore, in our study to understand concrete, it is important that we have a deep understanding of the structure of the hydrated hardened cement paste at a phenomenological level.
Concrete is generally considered as two phase material i.e., paste phase and aggregates phase. At macro level it is seen that aggregate particles are dispersed in a matrix of cement paste. At the microscopic level, the complexities of the concrete begin to show up, particularly in the vicinity of large aggregate particles. This area can be considered as a third phase, the transition zone, which represents the interfacial region between the particles of coarse aggregate and hardened cement paste. Transition zone is generally a plane of weakness and, therefore, has far greater influence on the mechanical behaviour of concrete.
Although transition zone is composed of same bulk cement paste, the quality of paste in the transition zone is of poorer quality. Firstly due to internal bleeding, water accumulate below elongated, flaky and large pieces of aggregates. This reduces the bond between paste Cement " and aggregate in general. If we go into little greater detail, the size and concentration of crystalline compounds such as calcium hydroxide and ettringite are also larger in the transition zone. Such a situation account for the lower strength of transition zone than bulk cement paste in concrete.
Due to drying shrinkage or temperature variation, the transition zone develops microcracks even before a structures is loaded. When structure is loaded and at high stress levels, these microcracks propagate and bigger chracks are formed resulting in failure of bond.
Therefore, transition zone, generally the weakest link of the chain, is considered strength limiting phase in concrete. It is because of the presence of transition zone that concrete fails at considerably lower stress level than the strength of bulk paste or aggregate.
Sometimes it may be necessary for us to look into the structure of hardening concrete also. The rate and extent of hydration of cement have been investigated in the past using a variety of techniques. The techniques used to study the structure of cement paste include measurements of setting time, compressive strength, the quantity of heat of hydration evolved, the optical and electron microscope studies coupled with chemical analysis and thermal analysis of hydration products. Continuous monitoring of reactions by X-ray diffractions and conduction calorimetry has also been used for the study.
Measurements of heat evolved during the exothermic reactions also gives valuable insight into the nature of hydration reactions. Since approximately 50% of a total heat
The mechanical properties of the hardened concrete depend more on the physical structure of the products of hydration than on the chemical composition of the cement. Mortar and concrete, shrinks and cracks, offers varying chemical resistance to different situations, creeps in different magnitude, and in short, exhibits complex behaviour under different conditions. Eventhough it is difficult to explain the behaviour of concrete fully and exactly, it is possible to explain the behaviour of concrete on better understanding of the structure of the hardened cement paste. Just as it is necessary for doctors to understand in great detail the anatomy of the human body to be able to diagnose disease and treat the patient with medicine or surgery, it is necessary for concrete technologists to fully understand the structure of hardened cement paste in great detail to be able to appreciate and rectify the ills and defects of the concrete.
hardening paste consists of hydrates of various compounds, unhydrated cement particles and water. With further lapse of time the quantity of unhydrated cement left in the paste decreases and the hydrates of the various compounds increase. Some of the mixing water is used up for chemical reaction, and some water occupies the gel-pores and the remaining water remains in the paste. After a sufficiently long time (say a month) the hydrated paste can be considered to be consisting of about 85 to 90% of hydrates of the various compounds and 10 to 15 per cent of unhydrated cement. The mixing water is partly used up in the chemical reactions. Part of it occupies the gel-pores and the remaining water unwanted for hydration or for filling in the gel-pores causes capillary cavities. These capillary cavities may have been fully filled with water or partly with water or may be fully empty depending upon the age and the ambient temperature and humidity conditions. Figure 1.6 (a) and (b) schematically depict the structure of hydrated cement paste. The dark portion represents gel. The small gap within the dark portion represents gel-pores and big space such as marked “c” represents capillary cavities.1.6 Fig. 1.7 represents the microscopic schematic model of structure of hardened cement paste.
Water Requirements for Hydration:
It has been brought out earlier that C3S requires 24% of water by weight of cement and C2S requires 21%. It has also been estimated that on an average 23% of water by weight of cement is required for chemical reaction with Portland cement compounds. This 23% of water chemically combines with cement and, therefore, it is called bound water. A certain quantity of water is imbibed within the gel-pores. This water is known as gel-water. It can be said that bound water and gel-water are complimentary to each other. If the quantity of water is inadequate to fill up the gel-pores, the formations of gel itself will stop and if the formation of gel stops there is no question of gel-pores being present. It has been further estimated that about 15 per cent by weight of cement is required to fill up the gel-pores. Therefore, a total 38 per cent of water by weight of cement is required for the complete chemical reactions and to occupy the space within gel-pores. If water equal to 38 per cent by weight of cement is Diagrammatic representation of the Hydration process and formation of cement gel.
" Concrete Technology only used it can be noticed that the resultant paste will undergo full hydration and no extra water will be available for the formation of undesirable capillary cavities. On the other hand, if more than 38 per cent of water is used, then the excess water will cause undesirable capillary cavities. Therefore greater the water above the minimum required is used (38 per cent), the more will be the undesirable capillary cavities. In all this it is assumed that hydration is taking place in a sealed container, where moisture to and from the paste does not take place.
It can be seen that the capillary cavities become larger with increased water/cement ratio.
With lower w/c ratio the cement particles are closer together. With the progress of hydration, when the volume of anhydrous cement increases, the product of hydration also increases. The increase in the volume of gel due to complete hydration could fill up the space earlier occupied by water upto a w/c ratio of 0.6 or so. If the w/c ratio is more than 0.7, the increase in volume of the hydrated product would never be sufficient to fill up the voids created by water. Such concrete would ever remain as porous mass. This is to say that gel occupies more and more space, that once occupied by mixing water. It has been estimated that the volume of gel would be about twice the volume of unhydrated cement.
The diagrammatic representation of progress of hydration is sown in Fig. 1.8. Fig. 1.8
(a) represents the state of cement particles immediately when dispersed in an aqueous solution. During the first few minutes, the reaction rate is rapid and the calcium silicate hydrate forms a coating around the cement grains See Fig. 1.8 (b). As hydration proceeds, hydration products, including calcium hydroxide are precipitated from the saturated solution and bridge the gap between the cement grains, and the paste stiffens into its final shape, see Fig. 1.8
(c). Further hyudration involving some complex form of diffusion process results in further deposition of the cement gel at the expense of the unhydrated cement and capillary pore-water Fig. 1.8 (d).
What has been described briefly is the approximate structure of hardened cement paste on account of the hydration of some of the major compounds. Very little cognisance is taken of the product of hydration of the other major and minor compounds in cement. The morphology of product of hydration and the study of structure of hardened cement paste in its entirety is a subject of continued research.
The development of high voltage electron microscopy, combined with developments of skill in making very thin sections is making possible high resolution photography and diffractometry while at the same time reducing damage to the specimen while under observation. The scanning electron provides stereographic images and a detailed picture of structure of cement paste. These facilitate further to understand aggregate cement bond, micro fracture and porosity of cement gel.
TYPE OF CEMENT AND TESTING OF CEMENT
I n the previous chapter we have discussed various properties of Portland cement in general. We have seen that cements exhibit different properties and characteristics depending upon their chemical compositions. By changing the fineness of grinding or the oxide composition, cement can be made to exhibit different properties. In the past continuous efforts were made to produce different kinds of cement, suitable for different situations by changing oxide composition and fineness of grinding. With the extensive use of cement, for widely varying conditions, the types of cement that could be made only by varying the relative proportions of the oxide compositions, were not found to be sufficient. Recourses have been taken to add one or two more new materials, known as additives, to the clinker at the time of grinding, or to the use of entirely different basic raw materials in the manufacture of cement. The use of additives, changing chemical composition, and use of different raw materials have resulted in the availability of many types of cements
" Concrete Technology to cater to the need of the construction industries for specific purposes. In this chapter we shall deal with the properties and use of various kinds of cement. These cements are classified as Portland cements and non-Portland cements. The distinction is mainly based on the methods of manufacture. The Portland and Non-Portland cements generally used are listed below: Indian standard specification number is also given against these elements.
TYPE OF CEMENT:
(a) Ordinary Portland Cement
(i ) Ordinary Portland Cement 33 Grade– IS 269: 1989
(ii) Ordinary Portland Cement 43 Grade– IS 8112: 1989
(iii) Ordinary Portland Cement 53 Grade– IS 12269: 1987
(b) Rapid Hardening Cement – IS 8041: 1990
(c) Extra Rapid Hardening Cement
(d) Sulphate Resisting Cement– IS 12330: 1988
(e) Portland Slag Cement– IS 455: 1989
(f) Quick Setting Cement
(g) Super Sulphated Cement– IS 6909: 1990
(h) Low Heat Cement – IS 12600: 1989
(j) Portland Pozzolana Cement
– IS 1489 (Part I) 1991 (fly ash based)
– IS 1489 (Part II) 1991 (calcined clay based)
(k) Air Entraining Cement
(l) Coloured Cement: White Cement – IS 8042: 1989
(m) Hydrophobic Cement – IS 8043: 1991
(n) Masonry Cement – IS 3466: 1988
(o) Expansive Cement
(p) Oil Well Cement – IS 8229: 1986
(q) Rediset Cement
(r) Concrete Sleeper grade Cement– IRS-T 40: 1985
(s) High Alumina Cement– IS 6452: 1989
(t) Very High Strength Cement
Before we discuss the above cements, for general information, it is necessary to see how Portland cement are classified under the ASTM (American Society for Testing Materials) standards. As per ASTM, cement is designated as Type I, Type II, Type III, Type IV, Type V and other minor types like Type IS, Type IP and Type IA IIA and IIIA.
For use in general concrete construction where the special properties specified for Types II, III, IV and V are not required (Ordinary Portland Cement).
For use in general concrete construction exposed to moderate sulphate action, or where moderate heat of hydration is required.
For use when high early strength is required (Rapid Hardening Cement).
For use when low heat of hydration is required (Low Heat Cement).
For use when high sulphate resistance is required (Sulphate Resisting Cement). ASTM standard also have cement of the type IS. This consist of an intimate and uniform blend of Portland Cement of type I and fine granulated slag. The slag content is between 25 and 70 per cent of the weight of Portland Blast-Furnace Slag Cement.
This consist of an intimate and uniform Cross Section of Multi-compartment Silo for blend of Portland Cement (or Portland Blast storing different types of cement. Furnace Slag Cement) and fine pozzolana in Courtesy : Grasim Industries Cement Division which the pozzolana content is between 15 and 40 per cent of the weight of the total cement.
Type IA, IIA and IIIA
These are type I, II or III cement in which air-entraining agent is interground where air-entrainment in concrete is desired
Ordinary Portland Cement
Ordinary Portland cement (OPC) is by far the most important type of cement. All the discussions that we have done in the previous chapter and most of the discussions that are going to be done in the coming chapters relate to OPC. Prior to 1987, there was only one grade of OPC which was governed by IS 269-1976. After 1987 higher grade cements were introduced in India. The OPC was classified into three grades, namely 33 grade, 43 grade and 53 grade depending upon the strength of the cement at 28 days when tested as per IS 4031-1988. If the 28 days strength is not less than 33N/mm2, it is called 33 grade cement, if the strength is not less than 43N/mm2, it is called 43 grade cement, and if the strength is not less then 53 N/mm2, it is called 53 grade cement. But the actual strength obtained by these cements at the factory are much higher than the BIS specifications.
It has been possible to upgrade the qualities of cement by using high quality limestone, modern equipments, closer on line control of constituents, maintaining better particle size distribution, finer grinding and better packing. Generally use of high grade cements offer many advantages for making stronger concrete. Although they are little costlier than low grade cement, they offer 10-20% savings in cement consumption and also they offer many other hidden benefits. One of the most important benefits is the faster rate of development 30 " Concrete Technology of strength. In the modern construction activities, higher grade cements have become so popular that 33 grade cement is almost out of the market. Table 2.9 shows the grades of cement manufactured in various countries of the world.
The manufacture of OPC is decreasing all over the world in view of the popularity of blended cement on account of lower energy consumption, environmental pollution, economic and other technical reasons. In advanced western countries the use of OPC has come down to about 40 per cent of the total cement production. In India for the year 1998-99 out of the total cement production i.e., 79 million tons, the production of OPC in 57.00
million tons i.e., 70%. The production of PPC is 16 million tone i.e., 19% and slag cement is 8 million tons i.e., 10%. In the years to come the use of OPC may still come down, but all the same the OPC will remain as an important type for general construction.
The detail testing methods of OPC is separately discribed at the end of this chapter.
Rapid Hardening Cement (IS 8041–1990)
This cement is similar to ordinary Portland cement. As the name indicates it develops strength rapidly and as such it may be more appropriate to call it as high early strength cement. It is pointed out that rapid hardening cement which develops higher rate of development of strength should not be confused with quick-setting cement which only sets quickly. Rapid hardening cement develops at the age of three days, the same strength as that is expected of ordinary Portland cement at seven days.
The rapid rate of development of strength is attributed to the higher fineness of grinding (specific surface not less than 3250 sq. cm per gram) and higher C3S and lower C2S content.
A higher fineness of cement particles expose greater surface area for action of water and also higher proportion of C3S results in quicker hydration. Consequently, capid hardening cement gives out much greater heat of hydration during the early period. Therefore, rapid hardening cement should not be used in mass concrete construction.
The use of rapid heading cement is recommended in the following situations: (a) In pre-fabricated concrete construction.
(b ) Where formwork is required to be removed early for re-use elsewhere, (c ) Road repair works,
(d ) In cold weather concrete where the rapid rate of development of strength reduces the vulnerability of concrete to the frost damage.
The physical and chemical requirements of rapid hardening cement are shown in Tables 2.5 and 2.6 respectively.
Extra Rapid Hardening Cement:
Extra rapid hardening cement is obtained by intergrinding calcium chloride with rapid hardening Portland cement. The normal addition of calcium chloride should not exceed 2 per cent by weight of the rapid hardening cement. It is necessary that the concrete made by using extra rapid hardening cement should be transported, placed and compacted and finished within about 20 minutes. It is also necessary that this cement should not be stored for more than a month.
Extra rapid hardening cement accelerates the setting and hardening process. A large quantity of heat is evolved in a very short time after placing. The acceleration of setting, hardening and evolution of this large quantity of heat in the early period of hydration makes the cement very suitable for concreting in cold weather, The strength of extra rapid hardening Types of Cement "
cement is about 25 per cent higher than that of rapid hardening cement at one or two days and 10–20 per cent higher at 7 days. The gain of strength will disappear with age and at 90
days the strength of extra rapid hardening cement or the ordinary portland cement may be nearly the same.
There is some evidence that there is small amount of initial corrosion of reinforcement when extra rapid hardening cement is used, but in general, this effect does not appear to be progressive and as such there is no harm in using extra rapid hardening cement in reinforced concrete work. However, its use in prestress concrete construction is prohibited.
In Russia, the attempt has been made to obtain the extra rapid hardening property by grinding the cement to a very fine degree to the extent of having a specific surface between 5000 to 6000 sq. cm/gm. The size of most of the particles are generally less than 3 microns2.1.
It is found that this very finely ground cement is difficult to store as it is liable to air-set. It is not a common cement and hence it is not covered by Indian standard.
Sulphate Resisting Cement (IS 12330–1988):
Ordinary Portland cement is susceptible to the attack of sulphates, in particular to the action of magnesium sulphate. Sulphates react both with the free calcium hydroxide in set-cement to form calcium sulphate and with hydrate of calcium aluminate to form calcium sulphoaluminate, the volume of which is approximately 227% of the volume of the original aluminates. Their expansion within the frame work of hadened cement paste results in cracks and subsequent disruption. Solid sulphate do not attack the cement compound. Sulphates in solution permeate into hardened concrete and attack calcium hydroxide, hydrated calcium aluminate and even hydrated silicates.
The above is known as sulphate attack. Sulphate attack is greatly accelerated if accompanied by alternate wetting and drying which normally takes place in marine structures in the zone of tidal variations.
To remedy the sulphate attack, the use of cement with low C3A content is found to be effective. Such cement with low C3 A and comparatively low C4AF content is known as Sulphate Resisting Cement. In other words, this cement has a high silicate content. The specification generally limits the C3A content to 5 per cent.
Tetracalcium Alumino Ferrite (C3AF) varies in Normal Portland Cement between to 6 to 12%. Since it is often not feasible to reduce the Al2O3 content of the raw material, Fe2O3 may be added to the mix so that the C4AF content increases at the expense of C3A. IS code limits the total content of C4AF and C3A, as follows.
2C3A + C4AF should not exceed 25%.
In many of its physical properties, sulphate resisting cement is similar to ordinary Portland cement. The use of sulphate resisting cement is recommended under the following conditions: (a ) Concrete to be used in marine condition;
(b ) Concrete to be used in foundation and basement, where soil is infested with sulphates;
(c ) Concrete used for fabrication of pipes which are likely to be buried in marshy region or sulphate bearing soils;
(d ) Concrete to be used in the construction of sewage treatment works.
Portland Slag Cement (PSC) (IS 455–1989)
Portland slag cement is obtained by mixing Portland cement clinker, gypsum and Concrete Technology granulated blast furnace slag in suitable proportions and grinding the mixture to get a thorough and intimate mixture between the constituents. It may also be manufactured by separately grinding Portland cement clinker, gypsum and ground granulated blast furnace slag and later mixing them intimately. The resultant product is a cement which has physical properties similar to those of ordinary Portland cement. In addition, it has low heat of hydration and is relatively better resistant to chlorides, soils and water containing excessive amount of sulphates or alkali metals, alumina and iron, as well as, to acidic waters, and therefore, this can be used for marine works with advantage. The manufacture of blast furnace slag
cement has been developed primarily to utilize blast furnace slag, a waste product from blast furnaces. The development of this type of cement has considerably increased the total output of cement production in India and has, in addition, provided a scope for profitable use for an otherwise waste product. During 98-99 India produced 10% slag cement out of 79 million tons. The quantity of granulated slag mixed with portland clinker will range from 25-65 per cent. In different countries this cement is known in different names. The quantity of slag mixed also will vary from country to country Schematic representation of production of the maximum being upto 85 per cent. Early blast furnace slag. strength is mainly due to the cement clinker
fraction and later strength is that due to the slag fraction. Separate grinding is used as an easy means of verying the slag clinker proportion in the finished cement to meet the market demand. Recently, under Bombay Sewage disposal project at Bandra, they have used 70%
ground granulated blast furnace slag (GGBS) and 30% cement for making grout to fill up the trench around precast sewer 3.5 m dia embedded 40 m below MSL.
Portland blast furnace cement is similar to ordinary Portland cement with respect to fineness, setting time, soundness and strength. It is generally recognised that the rate of hardening of Portland blast furnace slag cement in mortar or concrete is somewhat slower than that of ordinary Portland cement during the first 28 days, but thereafter increases, so that at 12 months the strength becomes close to or even exceeds those of Portland cement. The heat of hydration of Portland blast furnace cement is lower than that of ordinary Portland cement. So this cement can be used in mass concrete structures with advantage. However, in cold weather the low heat of hydration of Portland blast furnace cement coupled with moderately low rate of strength development, can lead to frost damage.
Extensive research shows that the presence of GGBS leads to the enhancement of the intrinsic properties of the concrete both in fresh and hardened states. The major advantages currently recognised are:
(a ) Reduced heat of hydration;
(b ) Refinement of pore structure;
(c ) Reduced permeability;
(d ) Increased resistance to chemical attack.
It is seen that in India when the Portland blast furnace slag cement was first introduced it met with considerable suspicion and resistance by the users. This is just because some manufacturers did not use the right quality of slag. It has been pointed out that only glassy granulated slag could be used for the manufacture of slag cement. Air-cooled crystallined slag cannot be used for providing cementitious property. The slag which is used in the manufacture of various slag cement is chilled very rapidly either by pouring it into a large body of water or by subjecting the slag stream to jets of water, or of air and water. The purpose is to cool the slag quickly so that crystallisation is prevented and it solidifies as glass. The product is called granulated slag. Only in this form the slag should be used for slag cement. It the slag prepared in any other form is used, the required quality of the cement will not be obtained.
Portland slag cement exhibits very low diffusivity to chloride ions and such slag cement gives better resistance to corrosion of steel reinforcement.
Application of GGBS Concrete
In recent years the use of GGBS concrete is well recognised. Combining GGBS and OPC
at mixer is treated as equivalent to factory made PSC. Concrete with different properties can be made by varying the proportions of GGBS.
While placing large pours of concrete it is vital to minimise the risk of early age thermal cracking by controlling the rate of temperature rise. One of the accepted methods is through the use of GGBS concrete containing 50% to 90% GGBS. Generally, a combination of 70%
GGBS and 30% OPC is recommended. Resistance to chemical attack may be enhanced by using GGBS in concrete. Resistance to acid attack may be improved through the use of 70% GGBS. To counter the problem of sulphate and chloride attack 40% to 70% GGBS may be used. There is a general consensus among concrete technologists that the risk of ASR can be minimised by using at least 50% GGBS. GGBS concrete is also recommended for use in water retaining structures. Aggressive water can affect concrete foundations. In such conditions GGBS concrete can perform better.
Quick Setting Cement
This cement as the name indicates sets very early. The early setting property is brought out by reducing the gypsum content at the time of clinker grinding. This cement is required to be mixed, placed and compacted very early. It is used mostly in under water construction where pumping is involved. Use of quick setting cement in such conditions reduces the pumping time and makes it economical. Quick setting cement may also find its use in some typical grouting operations.
Super sulphated cement is manufactured by grinding together a mixture of 80-85 per cent granulated slag, 10-15 per cent hard burnt gypsum, and about 5 per cent Portland cement clinker. The product is ground finer than that of Portland cement. Specific surface must not be less than 4000 cm2 per gm. The super-sulphated cement is extensively used in Belgium, where it is known as “ciment metallurgique sursulfate.” In France, it is known as “ciment sursulfate”.
This cement is rather more sensitive to deterioration during storage than Portland cement.
Super-sulphated cement has a low heat of hydration of about 40-45 calories/gm at 7 days and 45-50 at 28 days. This cement has high sulphate resistance. Because of this property this cement is particularly recommended for use in foundation, where chemically aggressive conditions exist. As super-sulphated cement has more resistance than Portland blast furnace slag cement to attack by sea water, it is also used in the marine works. Other areas where super-sulphated cement is recommended include the fabrication of reinforced concrete pipes which are likely to be buried in sulphate bearing soils. The substitution of granulated slag is responsible for better resistance to sulphate attack.
Super-sulphated cement, like high alumina cement, combines with more water on hydration than Portland cements. Wet curing for not less than 3 days after casting is essential as the premature drying out results in an undesirable or powdery surface layer. When we use super sulphated cement the water/cement ratio should not be less than 0.5. A mix leaner than about 1:6 is also not recommended.
Low Heat Cement (IS 12600-1989)
It is well known that hydration of cement is an exothermic action which produces large quantity of heat during hydration. This aspect has been discussed in detail in Chapter 1. Formation of cracks in large body of concrete due to heat of hydration has focussed the attention of the concrete technologists to produce a kind of cement which produces less heat or the same amount of heat, at a low rate during the hydration process. Cement having this property was developed in U.S.A. during 1930 for use in mass concrete
construction, such as dams, where temperature rise by the heat of hydration can become excessively large. A Law heat cement is made use of in
low-heat evolution is achieved by reducing the contents construction of massive dams.
of C3S and C3A which are the compounds evolving the maximum heat of hydration and increasing C2S. A reduction of temperature will retard the chemical action of hardening and so further restrict the rate of evolution of heat. The rate of evolution of heat will, therefore, be less and evolution of heat will extend over a longer period.
Therefore, the feature of low-heat cement is a slow rate of gain of strength. But the ultimate strength of low-heat cement is the same as that of ordinary Portland cement. As per the Indian Standard Specification the heat of hydration of low-heat Portland cement shall be as follows: 7 days — not more than 65 calories per gm.
28 days — not more than 75 calories per gm.
The specific surface of low heat cement as found out by air-permeability method is not less than 3200 sq. cm/gm. The 7 days strength of low heat cement is not less than 16 MPa in contrast to 22 MPa in the case of ordinary Portland cement. Other properties, such as setting time and soundness are same as that of ordinary Portland cement.
Portland Pozzolana Cement (IS 1489–1991)
The history of pozzolanic material goes back to Roman’s time. The descriptions and details of pozzolanic material will be dealt separately under the chapter ‘Admixtures’. However a brief description is given below.
Portland Pozzolana cement (PPC) is manufactured by the intergrinding of OPC clinker with 10 to 25 per cent of pozzolanic material (as per the latest amendment, it is 15 to 35%).
A pozzolanic material is essentially a silicious or aluminous material which while in itself possessing no cementitious properties, which will, in finely divided form and in the presence of water, react with calcium hydroxide, liberated in the hydration process, at ordinary temperature, to form compounds possessing cementitious properties. The pozzolanic materials generally used for manufacture of PPC are calcined clay (IS 1489 part 2 of 1991) or fly ash (IS
1489 part I of 1991). Fly ash is a waste material, generated in the thermal power station, when powdered coal is used as a fuel. These are collected in the electrostatic precipitator. (It is called pulverised fuel ash in UK). More information on fly ash as a mineral admixture is given in chapter 5.
It may be recalled that calcium silicates produce considerable quantities of calcium hydroxide, which is by and large a useless material from the point of view of strength or durability. If such useless mass could be converted into a useful cementitious product, it considerably improves quality of concrete. The use of fly ash performs such a role. The pozzolanic action is shown below:
Calcium hydroxide + Pozzolana + water → C – S – H (gel) Portland pozzolana cement produces less heat of hydration and offers greater resistance to the attack of aggressive waters than ordinary Portland cement. Moreover, it reduces the leaching of calcium hydroxide when used in hydraulic structures. It is particularly useful in marine and hydraulic construction and other mass concrete constructions. Portland pozzolana cement can generally be used where ordinary Portland cement is usable. However, it is important to appreciate that the addition of pozzolana does not contribute to the strength at early ages. Strengths similar to those of ordinary Portland cement can be expected in general only at later ages provided the concrete is cured under moist conditions for a sufficient period.
Status of PPC in India:
Over 60 million tones of fly ash is generated from over 75 thermal power stations. But the qualities of such fly ash are generally not satisfactory to be used in PPC. In western countries fly ash generated in thermal power plants are further processed to render it fit for using in PPC. Because of the poor quality of fly ash, lack of awareness and fear psychics on the part of users, PPC is not popular. In India only 19% of total cement production is PPC.
(1998-1999) and about 10% is slag cement. Government of India has set up an organisation called Fly Ash mission to promote the use of fly ash as mineral admixture or in manufacturing PPC. It has been realised by all experts in the world that more and more blended cement has to be used for sustainable development of any country.
Due to the shortage of electrical power, many cement factories have their own dedicated thermal power plant. They use their own fly ash for manufacturing PPC. As they know the importance of the qualities of fly ash, they take particular care to produce fly ash of good qualities to be used in PPC. The PPC produced by such cement plant is of superior quality. The chemical and physical qualities of properties of such PPC show much superior values than what is prescribed in BIS standard. The physical and chemical properties of PPC as given in IS: 1489 (part-I) 1991 is given in table 2.5
Birla Plus, Suraksha, Silicate Cement, Birla Bonus are some of the brand names of PPC in India.
Grading of PPC:
In many countries, PPC is graded like OPC depending upon their compressive strength at 28 days. In India, so far PPC is considered equivalent to 33 grade OPC, strengthwise, although some brand of PPC is as good as even 53 grade OPC. Many cement manufacturers have requested BIS for grading of PPC just like grading of OPC. They have also requested for upper limits of fly ash content from 25% to 35%. Recently BIS has increased the fly ash content in PPC from 10–25% to 15–35%.
Portland pozzolana cement can be used in all situations where OPC is used except where high early strength is of special requirement. As PPC needs enough moisture for sustained pozzolanic activity, a little longer curing is desirable. Use of PPC would be particularly suitable for the following situations:
(a) For hydraulic structures;
(b ) For mass concrete structures like dam, bridge piers and thick foundation; (c ) For marine structures;
(d ) For sewers and sewage disposal works etc.
Air-entraining cement is not covered by Indian Standard so far. This cement is made by mixing a small amount of an air-entraining agent with ordinary Portland cement clinker at the time of grinding. The following types of air-entraining agents could be used: (a) Alkali salts of wood resins.
(b ) Synthetic detergents of the alkyl-aryl sulphonate type.
(c ) Calcium lignosulphate derived from the sulphite process in paper making.
(d) Calcium salts of glues and other proteins obtained in the treatment of animal hides.
" Concrete Technology These agents in powder, or in liquid forms are added to the extent of 0.025–0.1 per cent by weight of cement clinker. There are other additives including animal and vegetable fats, oil and their acids could be used. Wetting agents, aluminium powder, hydrogen peroxide could also be used. Air-entraining cement will produce at the time of mixing, tough, tiny, discrete non-coalesceing air bubbles in the body of the concrete which will modify the properties of plastic concrete with respect to workability, segregation and bleeding. It will modify the properties of hardened concrete with respect to its resistance to frost action. Air-entraining agent can also be added at the time of mixing ordinary Portland cement with rest of the ingredients. More about this will be dealt under the chapter “Admixtures.”
Coloured Cement (White Cement IS 8042–1989) For manufacturing various coloured cements either white cement or grey Portland cement is used as a base. The use of white cement as a base is costly. With the use of grey cement only red or brown cement can be produced.
Coloured cement consists of Portland cement with 5-10 per cent of pigment. The pigment cannot be satisfactorily distributed throughout the cement by mixing, and hence, it is usual to grind the cement and pigment together. The properties required of a pigment to be used for coloured cement are the durability of colour under exposure to light and weather, a fine state of division, a chemical composition such that the pigment is neither effected by the cement nor detrimental to it, and the absence of soluble salts.
The process of manufacture of white Portland cement is nearly same as OPC. As the raw materials, particularity the kind of limestone required for manufacturing white cement is only available around Jodhpur in Rajasthan, two famous brands of white cement namely Birla White and J.K. White Cements are manufactured near Jodhpur. The raw materials used are high purity limestone (96% CaCo3 and less than 0.07% iron oxide). The other raw materials are china clay with iron content of about 0.72 to 0.8%, silica sand, flourspar as flux and selenite as retarder. The fuels used are refined furnace oil (RFO) or gas. Sea shells and coral can also be used as raw materials for production of white cement.
The properties of white cement is nearly same as OPC. Generally white cement is ground finer than grey cement. Whiteness of white cement as measured by ISI scale shall not be less than 70%. Whiteness can also be measured by Hunters Scale. The value as measured by Hunters scale is generally 90%. The strength of white cement is much higher than what is stated in IS code 8042 of 1989.
Hydrophobic cement (IS 8043-1991)
Hydrophobic cement is obtained by grinding ordinary Portland cement clinker with water repellant film-forming substance such as oleic acid, and stearic acid. The water-repellant film formed around each grain of cement, reduces the rate of deterioration of the cement during long storage, transport, or under unfavourable conditions. The film is broken out when the cement and aggregate are mixed together at the mixer exposing the cement particles for normal hydration. The film forming water-repellant material will entrain certain amount of air in the body of the concrete which incidentally will improve the workability of concrete. In India certain places such as Assam, Shillong etc., get plenty of rainfall in the rainy season had have high humidity in other seasons. The transportation and storage of cement in such places cause deterioration in the quality of cement. In such far off places with poor communication system, cement perforce requires to be stored for long time. Ordinary cement gets deteriorated and loses some if its strength, whereas the hydrophobic cement which does not lose strength is an answer for such situations.
The properties of hydrophobic cement is nearly the same as that ordinary Portland cement except that it entrains a small quantity of air bubbles. The hydrophobic cement is made actually from ordinary Portland cement clinker. After grinding, the cement particle is sprayed in one direction and film forming materials such as oleic acid, or stearic acid, or pentachlorophenol, or calcium oleate are sprayed from another direction such that every particle of cement is coated with a very fine film of this water repellant material which protects them from the bad effect of moisture during storage and transportation. The cost of this cement is nominally higher than ordinary Portland cement.
Masonry Cement (IS 3466 : 1988)
Ordinary cement mortar, though good when compared to lime mortar with respect to " Concrete Technology strength and setting properties, is inferior to lime mortar with respect to workability, water-retentivity, shrinkage property and extensibility.
Masonry cement is a type of cement which is particularly made with such combination of materials, which when used for making mortar, incorporates all the good properties of lime mortar and discards all the not so ideal properties of cement mortar. This kind of cement is mostly used, as the name indicates, for masonry construction. It contains certain amount of air-entraining agent and mineral admixtures to improve the plasticity and water retentivity.
Concrete made with ordinary Portland cement shrinks while setting due to loss of free water. Concrete also shrinks continuously for long time. This is known as drying shrinkage.
Cement used for grouting anchor bolts or grouting machine foundations or the cement used in grouting the prestress concrete ducts, if shrinks, the purpose for which the grout is used will be to some extent defeated. There has been a search for such type of cement which will not shrink while hardening and thereafter. As a matter of fact, a slight expansion with time will prove to be advantageous for grouting purpose. This type of cement which suffers no overall change in volume on drying is known as expansive cement. Cement of this type has been developed by using an expanding agent and a stabilizer very carefully. Proper material and controlled proportioning are necessary in order to obtain the desired expansion.
Generally, about 8-20 parts of the sulphoaluminate clinker are mixed with 100 parts of the Portland cement and 15 parts of the stabilizer. Since expansion takes place only so long as concrete is moist, curing must be carefully controlled. The use of expanding cement requires skill and experience.
One type of expansive cement is known as shrinkage compensating cement. This cement when used in concrete, with restrained expansion, induces compressive stresses which approximately offset the tensile stress induced by shrinkage. Another similar type of cement is known as Self Stressing cement. This cement when used in concrete induces significant compressive stresses after the drying shrinkage has occurred. The induced compressive stresses not only compensate the shrinkage but also give some sort of prestressing effects in the tensile zone of a flexural member.
IRS-T 40 Special Grade Cement
IRS-T-40 special grade cement is manufactured as per specification laid down by ministry of Railways under IRS- T40: 1985. It is a very finely ground cement with high C3S content designed to develop high early strength required for manufacture of concrete sleeper for Indian Railways. This cement can also be used with advantage for other applications where high early strength concrete is required. This cement can be used for prestressed concrete elements, high rise buildings, high strength IRS-T 40 special grade cement was originally made for concrete. manufacturing concrete sleeper for railway line.
Oil-Well Cement (IS 8229-1986)
Oil-wells are drilled through stratified sedimentary rocks through a great depth in search of oil. It is likely that if oil is struck, oil or gas may escape through the space between the steel casing and rock formation. Cement slurry is used to seal off the annular space between steel casing and rock strata and also to seal off any other fissures or cavities in the sedimentary rock layer. The cement slurry has to be pumped into position, at considerable depth where the prevailing temperature may be upto 175°C. The pressure required may go upto 1300 kg/cm2.
The slurry should remain sufficiently mobile to be able to flow under these conditions for periods upto several hours and then hardened fairly rapidly. It may also have to resist corrosive conditions from sulphur gases or waters containing dissolved salts. The type of cement suitable for the above conditions is known as Oil-well cement. The desired properties of Oil-well cement can be obtained in two ways: by adjusting the compound composition of cement or by adding retarders to ordinary Portland cement. Many admixtures have been patented as retarders. The commonest agents are starches or cellulose products or acids. These retarding agents prevent quick setting and retains the slurry in mobile condition to facilitate penetration to all fissures and cavities. Sometimes workability agents are also added to this cement to increase the mobility.
Acclerating the setting and hardening of concrete by the use of admixtures is a common knowledge. Calcium chloride, lignosulfonates, and cellulose products form the base of some of admixtures. The limitations on the use of admixtures and the factors influencing the end properties are also fairly well known.
High alumina cement, though good for early strengths, shows retrogression of strength when exposed to hot and humid conditions. A new product was needed for use in the precast concrete industry, for rapid repairs of concrete roads and pavements, and slip-forming.
In brief, for all jobs where the time and strength relationship was important. In the PCA laboratories of USA, investigations were conducted for developing a cement which could yield high strengths in a matter of hours, without showing any retrogression. Regset cement was the result of investigation. Associated Cement Company of India have developed an equivalent cement by name “REDISET” Cement.
High Alumina Cement (IS 6452 : 1989)
High alumina cement is obtained by fusing or sintering a mixture, in suitable proportions, of alumina and calcareous materials and grinding the resultant product to a fine powder. The raw materials used for the manufacture of high alumina cement are limestone and bauxite.
These raw materials with the required proportion of coke were charged into the furnace. The furnace is fired with pulverised coal or oil with a hot air blast. The fusion takes place at a temperature of about 1550-1600°C. The cement is maintained in a liquid state in the furnace.
Afterwards the molten cement is run into moulds and cooled. These castings are known as pigs. After cooling the cement mass resembles a dark, fine gey compact rock resembling the structure and hardeness of basalt rock.
The pigs of fused cement, after cooling are crushed and then ground in tube mills to a finess of about 3000 sq. cm/gm.
Hydration of High Alumina Cement
The important reaction during the setting of the high alumina cement (HAC) is the formation of monocalcium aluminate decahydrate (CAH10), dicalcium aluminate octahydrate (C2 AH8) and alumina gel (AH n). These aluminates give high strength to HAC concrete but they are metastable and at normal temperature convert gradually to tricalcium alumina hexahydrate (C3AH6) and gibbsite which are more stable. The change in composition is accompanised by a loss of strength and by a change in crystal form from hexagonal to cubical form with the release of water which results in increased porosity of concrete. The precise manner in which these changes take place depends on the temperature, water/cement ratio and chemical environment.
The change in composition accompanied by loss of strength and change in crystal form from hexagonal to cubic shape is known as conversion.
Experimental evidence suggests that in the important reaction of the conversion from CAH10 to C3AH6 and alumina hydrate, temperature effects the decomposition. The higher the temperature, the faster the rate of conversion. Experimental studies have also shown that the Types of Cement "
It should be noted that this reaction liberates all the water needed for the conversion process to continue. The conversion reaction will result in a reduction in volume of the solids and an increase in the porosity, since the overall dimensions of specimens of cement paste or concrete remain sensibly constant.
High Alumina Cement Concrete
The use of high alumina cement concrete commenced in the U.K. in 1925 following its introduction in France where it had been developed earlier to make concrete resistant to chemical attack, particularly in marine conditions. The capability of this concrete to develop a high early strength offers advantages in structural use. However, its high cost prevented extensive use of high alumina cement for structural purposes. All the same during 1930’s many structures were built in European countries using high alumina cement. Following the collapse of two roof beams in a school at Stepney in U.K. in February 1974, the Building Research Establishment (BRE) of U.K. started field studies and laboratory tests to establish the degree of risk likely in buildings with precast prestressed concrete beams made with high alumina cement. The results of the BRE investigations are summarised below: 1.
Measurements of the degree of conversion of the concrete used in the buildings indicated that high alumina cement concrete reaches a high level of conversion within a few years. The concrete specimens cut from beams indicated that some concrete suffered substantial loss of strength when compared to one day strength on which the design was earlier based
Very High Strength Cement
(a) Macro-defect-free cements (MDF)2.4. The engineering of a new class of high strength cement called Macro-defect-free (MDF) cements is an innovation. MDF refers to the absence of relatively large voids or defects which are usually present in conventional mixed cement pastes because of entrapped air and inadequate dispersion. Such voids and defects limit the strength. In the MDF process 4-7% of one of several water-soluble polymers (such as hydroxypropylmethyle cellulose, polyacrylamide of hydrolysed polyvinylacetate) is added as rheological aid to permit cement to be mixed with very small amount of water. Control of particle size distribution was also considered important for generating the strength. At final processing stage entrapped air is removed by applying a modest pressure of 5 MPa.
With this process a strength of 300 MPa for calcium aluminate system and 150 MPa for Portland cement system can be achieved.
(b) Densely Packed System (DSP). New materials termed DSP (Densified system containing homegeneously arranged ultre-fine particles) is yet another innovation in the field of high strength cement. Normal Portland cement and ultra-fine silica fume are mixed. The size of cement particles may very from 0.5 to 100µ and that of silica fume varies from 0.005 to 0.5µ.
Silica fume is generally added from 5 to 25 %. A compressive strength of 270 MPa have been achieved with silica fume substituted paste.
The formation of typical DSP is schematically represented in Fig. 2.4.
(c) Pressure Densification and Warm Pressing. For decades uncertainties existed regarding the theoretical strength of hydrated cement paste. Before 1970, the potential strength of cement paste at theoretical density (What T.C. Powers called “intrinsic strength”) had never been achieved because of considerable porosity (20 to 30% or more) always remain ofter completing hydration of cement. A new approach has ben developed for achieving very high strength by a method called “Warm Pressing” (applying heat and pressure simultaneously) to cement paste. Some modest increase in strength was achieved by application of pressure alone.
Compressive strength as much as 650 MPa and tensile strength up to 68 MPa have been obtained by warm pressing Portland and calcium aluminate cements. Enormous increases in strength resulted from the removal of most of the porosity and generation of very homogeneous, fine micro-structures with the porosities as low as 1.7%.
(d) High Early Strength Cement. Development of high early strength becomes an important factor, sometimes, for repair and emergency work. Research has been carried out in the recent past to develop rapid setting and hardening cement to give materials of very high early strength.
Lithium salts have been effectively used as accelerators in high alumina cement. This has resulted in very high early strength in cement and a marginal reduction in later strength.
Strength as high as 4 MPa has been obtained within 1 hour and 27 MPa has been obtained within 3 hours time and 49 MPa in one day.
(e) Pyrament Cement. Some cement industries in USA have developed a super high early strength and durable cement called by trade name “Pyrament Cement”. This product is a blended hydraulic cement. In this cement no chlorides are added during the manufacturing process. Pyrament cement produces a high and very early strength of concrete and mortar which can be used for repair of Air Field Run-ways. In India Associated Cement Company in collaboration with R & D Engineers, Dighi, Pune have also produced high early strength cement for rapid repair of airfields.
The Pyrament cement showed the following strength. Refer Table 2.4.
(f) Magnesium Phosphate Cement (MPC). Magnesium Phosphate Cement, an advanced cementing material, giving very high early strength mortar and concrete has been developed by Central Road Research Institute, New Delhi. This cement can be used for rapid repair of damaged concrete roads and airfield pavements. This is an important development for emergency repair of airfields, launching pads, hard standing and road pavements suffering damage due to enemy bombing and missile attack.
TESTING OF CEMENT
Testing of cement can be brought under two categories: (a) Field testing
(b) Laboratory testing.
It is sufficient to subject the cement to field tests when it is used for minor works. The following are the field tests:
(a) Open the bag and take a good look at the cement. There should not be any visible lumps. The colour of the cement should normally be greenish grey.
(b) Thrust your hand into the cement bag. It must give you a cool feeling. There should not be any lump inside.
(c) Take a pinch of cement and feel-between the fingers. It should give a smooth and not a gritty feeling.
(d) Take a handful of cement and throw it on a bucket full of water, the particles should float for some time before they sink.
" Concrete Technology (e) Take about 100 grams of cement and a small quantity of water and make a stiff paste.
From the stiff paste, pat a cake with sharp edges. Put it on a glass plate and slowly take it under water in a bucket. See that the shape of the cake is not disturbed while taking it down to the bottom of the bucket. After 24 hours the cake should retain its original shape and at the same time it should also set and attain some strength.
If a sample of cement satisfies the above field tests it may be concluded that the cement is not bad. The above tests do not really indicate that the cement is really good for important works. For using cement in important and major works it is incumbent on the part of the user to test the cement in the laboratory to confirm the requirements of the Indian Standard specifications with respect to its physical and chemical properties. No doubt, such confirmations will have been done at the factory laboratory before the production comes out from the factory. But the cement may go bad during transportation and storage prior to its use in works. The following tests are usually conducted in the laboratory.
(a) Fineness test.
(b) Setting time test.
(c) Strength test.
(d ) Soundness test.
(e) Heat of hydration test.
(f ) Chemical composition test.
The fineness of cement has an important bearing on the rate of hydration and hence on the rate of gain of strength and also on the rate of evolution of heat. Finer cement offers a greater surface area for hydration and hence faster the development of strength, The fineness of grinding has increased over the years. But now it has got nearly stabilised. Different cements are ground to different fineness. The disadvantages of fine grinding is that it is susceptible to air-set and early deterioration. Maximum number of particles in a sample of cement should have a size less than about 100 microns. The smallest particle may have a size of about 1.5 microns. By and large an average size of the cement particles may be taken as about 10 micron. The particle size fraction below 3 microns has been found to have the predominant effect on the strength at one day while 3-25 micron fraction has a major influence on the 28 days strength. Increase in fineness of cement is also found to increase the drying shrinkage of concrete. In commercial cement it is suggested that there should be about 25-30 per cent of particles of less than 7 micron in size.
Fineness of cement is tested in two ways :
(a) By seiving.
(b) By determination of specific surface (total surface area of all the particles in one gram of cement) by air-premeability appartus. Expressed as cm2/gm or m2/kg. Generally Blaine Airpermeability appartus is used.
Weigh correctly 100 grams of cement and take it on a standard IS Sieve No. 9 (90
microns). Break down the air-set lumps in the sample with fingers. Continuously sieve the sample giving circular and vertical motion for a period of 15 minutes. Mechanical sieving devices may also be used. Weigh the residue left on the sieve. This weight shall not exceed 10% for ordinary cement. Sieve test is rarely used.
Air Permeability Method
This method of test covers the procedure for determining the fineness of cement as represented by specific surface expressed as total surface area in sq. cm/gm. of cement. It is also expressed in m2/kg. Lea and Nurse Air Permeability Appartus is shown in Fig. 2.6. This appartus can be used for measuring the specific surface of cement. The principle is based on the relation between the flow of air through the cement bed and the surface area of the particles comprising the cement bed. From this the surface area per unit weight of the body material can be related to the permeability of a bed of a given porosity. The cement bed in the permeability cell is 1 cm. high and 2.5 cm. in diameter. Knowing the density of cement the weight required to make a cement bed of porosity of 0.475 can be calculated. This quantity of cement is placed in the permeability cell in a standard manner. Slowly pass on air 50 " Concrete Technology through the cement bed at a constant velocity. Adjust the rate of air flow until the flowmeter shows a difference in level of 30-50 cm. Read the difference in level (h1) of the manometer and the difference in level (h2) of the flowmeter. Repeat these observations to ensure that steady conditions have been obtained as shown by a constant value of h1/h2. Specific surface Sw is calculated from the following formula:
Standard Consistency Test
For finding out initial setting time, final setting time and soundness of cement, and strength a parameter known as standard consistency has to be used. It is pertinent at this stage to describe the procedure of conducting standard consistency test. The standard consistency of a cement paste is defined as that consistency which will permit a Vicat plunger having 10 mm diameter and 50 mm length to penetrate to a depth of 33-35 mm from the top of the mould shown in Fig. 2.8. The appartus is called Vicat Appartus. This appartus is used to find out the percentage of water required to produce a cement paste of standard consistency.
The standard consistency of the cement paste is some time called normal consistency (CPNC).
The following procedures is adopted to find out standard consistency. Take about 500
gms of cement and prepare a paste with a weighed quantity of water (say 24 per cent by weight of cement) for the first trial. The paste must be prepared in a standard manner and filled into the Vicat mould within 3-5 minutes. After completely filling the mould, shake the mould to expel air. A standard plunger, 10 mm diameter, 50 mm long is attached and brought down to touch the surface of the paste in the test block and quickly released allowing it to sink into the paste by its own weight. Take the reading by noting the depth of penetration of the plunger. Conduct a 2nd trial (say with 25 per cent of water) and find out the depth of penetration of plunger. Similarly, conduct trials with higher and higher water/cement ratios till such time the plunger penetrates for a depth of 33-35 mm from the top. That particular percentage of water which allows the plunger to penetrate only to a depth of 33-35 mm from the top is known as the percentage of water required to produce a cement paste of standard consistency. This percentage is usually denoted as ‘P’. The test is required to be conducted in a constant temperature (27° + 2°C) and constant humidity (90%).
Setting Time Test
An arbitraty division has been made for the setting time of cement as initial setting time and final setting time. It is difficult to draw a rigid line between these two arbitrary divisions. For convenience, initial setting time is regarded as the time elapsed between the moment that the water is added to the cement, to the time that the paste starts losing its plasticity. The final setting time is the time elapsed between the moment the water is added to the cement, and the time when the paste has completely lost its plasticity and has attained sufficient firmness to resist certain definite pressure.
In actual construction dealing with cement paste, mortar or concrete certain time is required for mixing, transporting, placing, compacting and finishing. During this time cement paste, mortar, or concrete should be in plastic condition. The time interval for which the cement products remain in plastic condition is known as the initial setting time. Normally a minimum of 30 minutes is given for mixing and handling operations. The constituents and fineness of cement is maintained in such a way that the concrete remains in plastic condition for certain minimum time. Once the concrete is placed in the final position, compacted and finished, it should lose its plasticity in the earliest possible time so that it is least vulnerable to damages from external destructive agencies. This time should not be more than 10 hours Concrete Technology which is often referred to as final setting time. Table 2.5 shows the setting time for different cements.
The Vicat Appartus shown in Fig. 2.8 is used for setting time test also. The following procedure is adopted. Take 500 gm. of cement sample and guage it with 0.85 times the water required to produce cement paste of standard consistency (0.85 P). The paste shall be guaged and filled into the Vicat mould in specified manner within 3-5 minutes. Start the stop watch the moment water is added to the cement.
Initial Setting Time
Lower the needle (C) gently and bring it in contact with the surface of the test block and quickly release. Allow it to penetrate into the test block. In the beginning, the needle will completely pierce through the test block. But after some time when the paste starts Vicat Apparatus and Automatic Vicat Apparatus. losing its plasticity, the Accessories. needly may penetrate only to a depth of 33-35 mm from the top. The period elapsing between the time when water is added to the cement and the time at which the needle penetrates the test block to a depth equal to 33-35 mm from the top is taken as initial setting time.
Final Setting Time
Replace the needle (C) of the Vicat appartus by a circular attachment (F) shown in the Fig 2.8. The cement shall be considered as finally set when, upon, lowering the attachment gently cover the surface of the test block, the centre needle makes an impression, while the circular cutting edge of the attachment fails to do so. In other words the paste has attained such hardness that the centre needle does not pierce through the paste more than 0.5 mm.
The compressive strength of hardened cement is the most important of all the properties.
Therefore, it is not surprising that the cement is always tested for its strength at the laboratory before the cement is used in important works. Strength tests are not made on neat cement paste because of difficulties of excessive shrinkage and subsequent cracking of neat cement.
Strength of cement is indirectly found on cement sand mortar in specific proportions. The standard sand is used for finding the strength of cement. It shall conform to IS 650-1991. Take 555 gms of standard sand (Ennore sand), 185 gms of cement (i.e., ratio of cement to sand is 1:3) in a non-porous enamel tray and mix them with a trowel for one minute, then add water of quantity P + 3.0 per cent of combined 4 weight of cement and sand and mix the three ingredients thoroughly until the mixture is of uniform colour. The time of mixing should not be less than 3 minutes nor more than 4 minutes. Immediately after mixing, the mortar is filled into a cube mould of size 7.06 cm. The area of the face of the cube will be equal to 50 sq cm. Compact the mortar either by hand compaction in a standard specified manner or on the vibrating equipment (12000 RPM) for 2 minutes.. Moulding of 70.7 mm Mortar Cube Vibrating Machine. Keep the compacted cube in the mould at a temperature of 27°C ± 2°C and at least 90 per cent relative humidity for 24 hours. Where the facility of standard temperature and humidity room is not available, the cube may be kept under wet gunny bag to simulate 90 per cent relative humidity. After 24
hours the cubes are removed from the mould and immersed in clean fresh water until taken out for testing.
Three cubes are tested for compressive strength at the periods mentioned in Table 2.5.
The periods being reckoned from the completion of vibration. The compressive strength shall be the average of the strengths of the three cubes for each period respectively.
It is very important that the cement after setting shall not undergo any appreciable change of volume. Certain cements have been found to undergo a large expansion after setting causing disruption of the set and hardened mass. This will cause serious difficulties for the durability of structures when such cement is used. The testing of soundness of cement, to ensure that the cement does not show any appreciable subsequent expansion is of prime importance.
The unsoundness in cement is due to the presence of excess of lime than that could be combined with acidic oxide at the kiln. This is also due to inadequate burning or insufficiency in fineness of grinding or thorough mixing of raw materials. It is also likely that too high a proportion of magnesium content or calcium sulphate content may cause unsoundness in cement. For this reason the magnesia content allowed in cement is limited to 6 per cent. It can be recalled that, to prevent flash set, calcium sulphate is added to the clinker while grinding. The quantity of gypsum added will vary from 3
to 5 per cent depending upon C3A content. If the addition of gypsum is more than that could be combined with C3A, excess of gypsum will remain in the cement in free state. This excess of gypsum leads to an expansion and consequent disruption of the set cement paste.
Unsoundness in cement is due to excess of lime, excess of magnesia or excessive proportion of sulphates.
Unsoundness in cement does not come to surface for a considarable period of time. Therefore, accelerated tests are required to detect it. There are number of such tests in common use. The appartus is shown in Fig. 2.9. It consists of a small split cylinder of spring brass or other suitable metal. It is 30 mm in diameter and 30 mm high.
On either side of the split are attached two indicator arms 165 mm long with pointed ends. Cement is gauged with 0.78 times the water required for standard consistency (0.78 P), in a standard manner and filled into the mould kept on a glass plate. The mould is covered on the top Autoclave.
Heat of Hydration
The reaction of cement with water is exothermic.
The reaction liberates a considerable quantity of heat.
This can be easily observed if a cement is gauged with water and placed in a thermos flask. Much attention has been paid to the heat evolved during the hydration of cement in the interior of mass concrete dams. It is estimated that about 120 calories of heat is generated in the hydration of 1 gm. of cement. From this it can be assessed the total quantum of heat produced in a conservative system such as the interior of a mass concrete dam. A temperature rise of about 50°C has been observed. This unduly high temperature
developed at the interior of a concrete dam causes serious expansion of the body of the dam and with the subsequent cooling considerable shrinkage takes place Heat of hydration Apparatus. resulting in serious cracking of concrete.
" Concrete Technology The use of lean mix, use of pozzolanic cement, artificial cooling of constituent materials and incorporation of pipe system in the body of the dam as the concrete work progresses for circulating cold brine solution through the pipe system to absorb the heat, are some of the methods adopted to offset the heat generation in the body of dams due to heat of hydration of cement.
Test for heat of hydration is essentially required to be carried out for low heat cement only. This test is carried out over a few days by vaccum flask methods, or over a longer period in an adiabatic calorimeter. When tested in a standard manner the heat of hydration of low heat Portland cement shall not be more than 65 cal/gm. at 7 days and 75 cal/g, at 28 days.
Chemical Composition Test
A fairly detailed discussion has been given earlier regarding the chemical composition of cement. Both oxide composition and compound composition of cement have been discussed.
At this stage it is sufficient to give the limits of chemical requirements. The Table 2.6 shows the various chemical compositions of all types of cements.
Every cement company is continuously testing the cement manufactured in their factory.
They keep a good record of both physical and chemical properties of the cement manufactured applying a batch number. Batch number indicates date, month and year.
They also issue test certificate. Every purchaser is eligible to demand test certificate.
A typical test certificate of Birla super 53 grade cement for the week number 35 is given in Table 2.7 for general information.
Some cement companies also work out the standard deviation and coefficient of variation for 3 months or 6 months or for one year period subjecting the various parameters obtained from their test results. Table 2.8 shows the typical standard deviation for 3 days, 7 days and 28 days strength in respect of 53 grade cement Birla super. Standard deviation has been worked out for the whole year from Jan. 99 to Dec. 99.
The properties of cements, particularly the strength property shown in Table No. 2.5 is tested as per the procedures given by BIS. In different countries cement is tested as per their own country’s code of practice. There are lot of variations in the methods of testing of cement with respect to w/c ratio, size and shape of specimen, material proportion, compacting methods and temperature. Strength of cement as indicated by one country may not mean the same in another country. This will present a small problem when export or import of cement from one country to another country is concerned. Table No. 2.9. Shows the cements testing procedure and various grades of cement manufactured in some countries. There is suggestion that all the countries should follow one method recommended by International standards organisation for testing of cement. If that system is adopted properties indicated by any one country will mean the same to any other country.