The Ramco Cements Limited answers your FAQs on cement & concrete

What is cement?
Cement is an adhesive material that is capable of bonding together fragments or particles of solid matter into a compact whole. Cement , from the construction point of view, is restricted to materials containing compounds of lime as their principal constituent .
Can you list the different types of cements ?
The various types of cements as per Bureau of Indian Standards :
Ordinary Portland Cement – 33 grade – IS 269
Ordinary Portland Cement - 43 grade – IS 8112
Ordinary Portland Cement – 53 grade – IS 12269
Rapid Hardening Portland Cement – IS 8041
Portland Slag Cement – IS 455
Portland Pozzolona Cement ( Fly ash based) – IS 1489 – Part 1
Portland Pozzolona Cement ( Calcined clay based) – IS 1489 – Part 2
Hydrophobic Cement – IS 8043
Low Heat Portland Cement – IS 12600
Sulphate resisting Portland Cement – IS 12330
Masonry Cement – IS 3466
What are the types of cement commonly available in our Country ?
Portland Pozzolona Cement ( PPC ) is widely available . Other varieties of cement that are available are OPC – 43 grade , OPC – 53 grade and Portland Slag Cement
What is Portland Cement?
Portland cement is a cementing material that is obtained by intimately mixing together other lime-bearing material with, if required, silica, alumina or iron-oxide bearing materials, burning the mixture at a clinkering temperature, grinding the resultant clinker with a few percentage of gypsum to regulate the setting behaviour of cement. This final product is akin to a product found near Portland, UK in physical characteristics and that is how it attained the name Portland cement.
What are the different grades of cement available in our country ?
Ordinary Portland Cement ( OPC ) is categorized into three grades based on the 28 - day mortar strength – 33 grade , 43 grade and 53 grade .
On what basis are the different grades of OPC categorized?
The cement that gives a 28 day compressive strength of 33 MPa is known as 33 grade OPC , a cement with 28 day compressive strength of 43 MPa is 43 grade OPC , a cement with a 28 day compressive strength of 53 MPa is termed as 53 grade OPC.
What are blended cements?
Cements that are derived out of blending complementary materials such as fly ash, slag, etc, with clinker and gypsum are termed as blended cements . Two types of blended cements are popularly manufactured in India – Portland Pozzolona Cement ( PPC) and Portland Slag Cement ( PSC) .
What is PPC ( Portland Pozzolona Cement ) ?
PPC is manufactured either by intergrinding Portland Cement Clinker , flyash and gypsum to the required fineness or by blending ground OPC with fine flyash. Fly ash is a byproduct in coal based thermal plants .
What is PSC ( Portland Slag Cement ) ?
Portland slag cement is manufactured using OPC and granulated blast furnace slag, or in short slag. Slag is a byproduct obtained from steel plants. Slag is generated in molten form, which then is granulated through the process of quenching.
Is there any Indian Standard for blending materials like flyash & slag ?
The physical and chemical requirements for flyash and slag are governed by Bureau of Indian Standards .
The BIS codes for flyash and slag are IS 3812 ( Flyash for use as Pozzolona and admixture) and IS 12089 ( Granulated slag
The supporting codal specifications are as follows:
Methods of physical tests for hydraulic cement (Part I to XV) 4031 - 1986/89
Method of chemical analysis for Hydraulic cement 4032 - 1968
Methods of test for pozzolonic material 1727 - 1968
Fly ash for use as pozzolan and admixture 3812 - 1981
Granulated slag for the manufacture Of Portland slag cement 12089 - 1987
Cement is always used in composite form such as mortar and concrete. Why not in neat form?
Cement is not used in neat form because of both technical and economical reasons.
Cement releases considerable quantities of heat during hydration that creates shrinkage cracks during cooling. This phenomenon causes tensile stresses resulting in development of cracks in the matrix. Without using aggregate the matrix form of cement is not effective hence dimensional stability is at stake. Aggregates such as sand and crushed stone are less expensive than cement. Mortar is prepared by mixing cement and sand or any other fine aggregate together with water. Concrete is prepared by mixing cement, sand, coarse aggregate and together with water.
How could concrete become a product for versatile applications?
John J. Earley, prominent sculpture of 1920s explained the versatile characteristics of concrete in an aesthetic way in following lines:
“Concrete as an artistic medium becomes doubly interesting when we realize that in addition to its economy it possesses those properties which are the most desirable to both metal and stone. Metal is cast, it is an exact mechanical reproduction of the artist’s work, as is concrete - -- - Stone (Sculpture) is an interpretation of an original work and more often than is not carried out by another artist. But stone has the advantage of color and texture which enable it to fit easily into varied surroundings, a capability lacking in metal. Concrete, treated as in the Foundation of Time, presents a surface almost entirely of stone with all its visual advantages while at the same time offering the precision of casting that would otherwise only be attained in metal.” Time goes, you say? Ah no, Alas time stays as well as concrete; we go”
What are the basic raw materials in Portland cement production?
The raw materials consist of calcareous and argillaceous base containing lime (CaO), Silica (SiO2), Alumina (Al2O3) and Iron oxide (Fe2O3). These can be used either from naturally occurring minerals or from industrial byproducts. Out of calcareous base, limestone and chalk are the most common forms of calcium carbonate, CaCO3. Of argillaceous base clay is the important material. Clays are basically aluminium silicate hydrates.
What is the chemical composition of Portland Cement clinker – the principal component of cement ?
Portland cement clinker principally constitutes of four mineralogical phases as follows:
Tricalcium silicate: 3 CaO. SiO2 C3S
Dicalcium silicate: 2 CaO. SiO2 C2S
Tricalcium aluminate 3 CaO. Al2O3 C3A
Tetracalcium aluminoferrite 4 CaO. Al2O3. Fe2O3 C4AF
The above phases aggregate to about 90% in clinker, the remaining 10% being free lime, MgO, alkalis, sulfates and LOI.
What are the parameters that are normally checked during cement manufacturing ?
Lime saturation factor (LSF): It is evident that all the major mineralogical phases of Portland cement clinker are lime-based hence it is important that a sufficient lime should be provided in the raw meal to achieve the optimum phase formation. This is generally decided by a factor known as Lime saturation factor (LSF), obtained through a comprehensive formula as follows. LSF is computed based on the lime demand of each component, SiO2, Al2O3 and Fe2O3 and composing the total. LSF = CaO/ 2.8 SiO2+1.2 Al2O3+0.65 Fe2O3 LSF is one of the important quality control parameter of the clinker with a target of 0.9 to 0.96. The proportioning of raw meal is so adjusted that LSF of clinker is always within the limits. In case of OPC the formula differs as follows keeping in view the gypsum addition: LSF = CaO - 0.7 SO3/ 2.8 SiO2+1.2 Al2O3+0.65 Fe2O3
Silica Modulus: This is another quality control parameter that is computed as follows: Silica Modulus (SM) = SiO2/ Al2O3+ Fe2O3 This indicates a measure of burnability of clinker because higher the SM value it becomes difficult to attain optimum clinkerisation. This also leads to the formation of coating in the kiln. This is generally controlled at a value between 1.9 to 3.2in clinker.
Alumina Modulus: Alumina modulus indicates the liquid phase formation in the clinker that creates an environment in the kiln for the formation of calcium silicate phases. This is computed as follows: Alumina Modulus (AM) = Al2O3/ Fe2O3 This is generally targeted in the zone of 1.5 to 2.5 for normal Portland cement clinkers. If it is below this zone, the cement becomes slow setting and above this zone the cement becomes fast setting, demanding addition of higher gypsum. In order to limit the slow setting, BIS specifies this modulus with minimum 0.66. Good quality clinker of desired physico-chemical and mineralogical composition can be manufactured only when the above parameters are maintained well in the limits and then by maintaining the optimum kiln operation by countering various other operating impediments.
How is the quality of raw meal controlled?
Raw meal was studied for its chemical analysis and the above moduli. Depending on the values corrections in the raw meal are taken care of. Conventionally chemical analysis was done through earlier techniques such as gravimetric and volumetric methods. However with the need for high capacity cement plants there are many advancements in technology right from raw material analysis. One of the recent such advancements is the Cross Belt Analyser (CBA). The mechanism is elemental analysis of the material using gamma ray as the detecting agency. The system works on software and hence all the calculations are done automatically. When the material passes though CBA minute-wise analyses are recorded and at the end of every hour the average of 60 readings are produced together with LSF and other moduli. Depending on these values corrective mechanism in the raw meal is done. This is more efficient system because every hour about 250 tons of material analysed, facilitating the uniformity in the total raw materials.
What types of fuels are used in cement manufacturing?
Any type of fuel, solid, liquid or gas can be used in cement manufacturing. However, fuel has to be decided in advance and accordingly the kilns are to be designed. As India is having major source of coal the kilns are normally designed to handle solid fuel.
What are the check mechanisms for clinker quality?
Like raw meal and kiln feed clinker is also studied for its chemical analysis in regular intervals. In order to meet the higher rate of production the chemical analysis is shifted to X-Ray techniques. This is known as x-Ray Florescence spectrometer (XRF). It takes very short time of few minutes to complete the total analysis. Hence corrective measures, if any, can be taken immediately thereby maintaining the variations in the quality of clinker at narrow zone.
How does color of OPC influence the technical parameter?
The cement attains color basically because of iron and in some cases because of manganese present in the clinker. The color of the clinker can be changed over a shade; greenish -black to yellowish- brown due to iron and, a shade of brown because of manganese. As a general rule there is no impact of color on technical virtues of cement. Color consciousness is more of a mind set and conviction, rather than technical performance. As long as the operating conditions of kiln and the rate of cooling are maintained within the stipulated limits, cements with any color behave similarly.
What is the need to add gypsum to cement and what is its role?
Gypsum is used as to regulate the setting time of cement . This is achieved by retarding the hydration of aluminate phase.
How does the fineness of cement influence the reactions?
The fineness of the cement is generally studied in terms of Blaine specific surface area, that gives a value in m2/kg or cm2/gm. However ever since high strength cements have become order of the day, concentration is also put on particle size distribution (PSD). As the previous theories established that the particles in the range of 3 to 30 micron contribute for high early strength, the cement industry has set a specific target in terms of PSD as a measure for cement. However keeping in view the cement rheology & related issues and also the economic feasibility of grinding, the fine fractions have to be restricted to some extent. Accordingly cement is generally ground to achieve a residue of 30 to 35% on 25 micron. With regard to relation between specific surface area and PSD, in terms of Lea, “A given specific surface in a cement could be produced with a variety of  PSD”. Thus it is established that there is no linear relation to PSD and Blaine surface area.
What is normal consistency of cement?
Normal consistency (NC) is a measure for water content needed to bring the paste to a standard condition of wetness. The study aims to wet each and every cement grain and bring into a state of gel formation. Then the mix becomes viscous and plastic. The type and the surface area of the cement influence normal consistency. In case of OPC there is a threshold value of surface area within which NC is maintained around 28%. Beyond this threshold value as the surface area increases NC increases. However the NC of cement and workability of concrete are not interrelated.
Popovics explained the same as follows:
“A comparison of experimental results reveals that cement with a higher water requirement for NC does not necessarily need a higher water content to produce a specified slump. In other words, the result of the NC test cannot be used for the prediction of water need of the same cement in a concrete. The reason for this paradox is that the two tests measure two properties of the cement paste. The Vicat test measures primarily the viscosity, whereas the result of the slump test is influenced mostly by the lubricating capability of the paste. These two paste properties are not correlated, or at least the relationship is not a simple linear one.” This observation proves true in certain pozzolan cements where the NC increases over that of the control cement but in the slump test the workability increases over OPC concrete. The NC is determined by Vicat apparatus and the test method is briefly described in the annexure.
What is setting time of cement and what is the implication?
Setting time of cement is of practical implication. Basically the initial setting determines the length of time in which the cement paste remains plastic and workable. As per the description of setting time “The term setting implies solidification of the plastic cement paste. The beginning of solidification, called the initial set marks the point in time when the paste has become unworkable. Accordingly, placement, compaction, and finishing of concrete beyond this stage will be very difficult. The paste does not solidify suddenly; it requires considerable time to become fully rigid. The time taken to solidify completely marks the final set, which should not be too long”. To put the setting phenomenon in a technical way the following text helps: In a correctly retarded cement, there is an initial rapid reaction over the first few minutes followed by an induction period of slow reaction usually lasting from a half to two hours. During this induction period coatings of hydration products form over the cement grains and there is a slow build-up of hydration products of colloidal dimensions in the paste. The more rapid reaction that follows the induction period is ascribed to the break-up of the coatings and setting to the coagulation of the paste. …….The dormant period is followed by a more rapid chemical reaction leading to initial and final set, which are arbitrarily defined degrees of firmness. This is the reason why the difference between initial and final set generally is around 60 to 100 minutes. The setting time of the cement is studied with Vicat apparatus using different needles. The brief description of the method is given in annexure.
How is the soundness of cement evaluated?
To check the soundness of cement there are two test methods; one is Le Chatelier expansion and the other is autoclave expansion. Le Chatelier method generally indicates the role of free lime. Autoclave is a study under induced conditions of a pressure of about 22 Bar, which is expected to simulate the late age impact. Unless otherwise the cement fails in these two tests there are little chances for the failure of the same in the field. The meaning of failure is that the bar in the autoclave should totally bend or crumbled. As long as the values of autoclave and Le Chatelier tests are within the levels it may be considered that there is no threat on soundness because of the above three phases.
What is concrete?
Concrete is a composite material that consists essentially of a binding medium within which are embedded particles or fragments of aggregates when the whole mass is added with water and hardened subsequently. The coarse aggregate occupies the major volume and acts as filler. Next comes the fine aggregate that fills up the voids in between coarse aggregate. Finally comes the cement, which in association with water works like a binding medium. If independently seen, coarse aggregate is very strong, but in the absence of proper binding media the strength of the coarse aggregate is of no value as an ingredient to concrete. Hence concrete is a homogenous product that consists of heterogeneous materials.
What are aggregates?
Aggregate is the granular material, such as sand, gravel, crushed stone. The term coarse aggregate refers to aggregate particles larger than 4.75 mm and the term fine aggregate refers to aggregate particles smaller than 4.75 mm. Gravel is one of the coarse aggregates resulting from natural disintegration and abrasion of rock and processing of weakly bound conglomerate. However due to its low strengths this is not generally used in good concrete. The crushed stone is the product resulting from industrial crushing of rocks, boulders or large cobblestones. In some areas the coarse aggregate is available in already crushed form. Sand comes in the class of fine aggregate. The sand forms due to natural disintegration and abrasion of rock or processing of friable sandstone. Of late manufacture of sand from quarry stones is also becoming popular. The quality and gradation of aggregates are very important to manufacture a good concrete. IS 383: specification for coarse and fine aggregates from natural sources for concrete, discusses the appropriate gradation for both the fine and coarse aggregates. Properly graded aggregates help to manufacture concrete with well-formed matrix. This is possible when the interstitial space in concrete matrix is filled with different sizes of aggregate thereby mitigating the chance for porosity.
What is mix design?
Adam Neville illustrates the mix design as the process of selecting suitable ingredients of concrete and determining the relative quantities with the object of producing as economically as possible concrete of certain minimum properties, namely consistency, strength and durability. Mix design is a sound engineering principle to rationalize various parameters in the application of concrete. Mix design facilitates to know right input of cement, aggregate and ratio of water to cement to attain the desired workability and pronounced strength; all culminating in techno-economic optimization associated with feasibility.
What is Grade of concrete?
Concrete grade indicates the characteristic strength on 28-day that is defined as the strength of the material below which not more than 5% of the test results are expected to fall. It is generally mentioned as M-20, M-30 and so on. It means that M-20 concrete should attain a characteristic strength of 20 MPa ( Mega Pascal) by 28-day. IS 456:2000 defines that the target mean strength should be equal to the characteristic strength plus 1.65 times the standard deviation. IS:10262-1982 gives suggested value for standard deviation that is 2 to 5 for concretes from M 10 to M 30 and up to 6.8 for concretes of M 60.
What are the various steps to be taken care in mix design calculations?
Before going for mix design calculations the characteristics of the materials those go into the concrete should be studied. For this the following are generally considered.
Sieve analysis of fine and coarse aggregate Dry-roded unit weight of coarse aggregate Bulk specific gravity of the materials Absorption capacity of the materials After arriving to these values the other studies to be conducted are: Variation in the approximate mixing water requirement for the specified slump, air content grading of the available aggregate Relationship between strength and w/c for available combinations of cement and aggregate. Then all the above are to be assimilated to comply with the requirement of job specifications if any, viz., maximum w/c, minimum air content, minimum slump, maximum size of aggregate, and strength at 28-days.
How is water addition decided in concrete?
Water cement ratio (w/c) plays a crucial role in producing the good quality concrete. Depending on the grade of concrete (target strength for ultimate concrete) w/c is considered from the reference charts vide IS:10262-1982, Recommended guidelines for concrete mix design. The w/c reflects on workability of concrete. With the advent of chemical admixtures in concrete, w/c has become more of practical implication because high slumps can be achieved even with low water cement ratio.
What is workability of concrete?
Workability of concrete is defined in ASTM C 125 as the property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity. This parameter is studied in terms of slump using a slump cone apparatus or in terms of consistency using Vee-Bee apparatus. However slump is the most popular study because of the easiness and convenience. Workability of even field concrete can also be studied using slump cone approach. Workability in concrete technology has a lot of significance. It is one of the key properties that must be satisfied. Regardless of the sophistication of the mix design procedure and other considerations, a concrete mixture that cannot be placed easily or compacted fully is not likely to yield the expected strength and durability characteristics. Workability is influenced by internal factors such as water content, cement content, aggregate characteristics with special reference to shape, and chemical admixtures. There is one more factor to influence the workability; that is the weather condition. Though code suggests placement condition Vs degree of required workability, in practice it cannot be followed. The reason for this malady is the influence of weather conditions viz., temperature, humidity and rate of airflow on the workability of the concrete. To illustrate further, for medium degree of workability the slump range suggested is 50 to 100 mm. But for a given mix design and w/c, the slump varies between summer and winter, without any exception during noon and evening in the summer. In a recent study, it is observed that the slump has reached collapse for a particular concrete when prepared during evening time, though the same concrete with same w/c has given a slump of around 80 mm when prepared around noon. Based on this phenomenon consider a site where concreting activity continues from morning to evening. What control on water addition can be exercised if slump is the ultimate criterion? Hence it is to be understood that the concrete preparation is influenced by external factors.
What are the factors to be taken care while executing concrete work in the field?
Formwork should be done in a rigid a way so as to withstand the live loads of movements as well as dead load of concrete placement. This has more implication when the concrete from RMC plants is pumped on the formwork. The spacers beneath the rebars should be perfectly placed. This helps in maintaining the correct and uniform cover to concrete, in turn, to protect the rebars from getting exposed to the environment. Concrete preparation should be taken care with special reference to adding water. In general, the mixer operator adds water to such a workability so as to fill the container with concrete in “cone shaped top” that facilitates maximum mobilization of concrete per each head-load. This generally renders a slump of 50-60 mm that also offers better placement of concrete. However once the concrete reaches the placement area, if the mason gets stuck in pushing the concrete into beams, particularly at those sites where no vibrator is deployed and/or if the rebars are closely oriented, the concrete is diluted with addition of water, taking the slump to as high as 160-200 mm without using any admixtures. This is where the problems such as exposed aggregate at the corners of the beams occur. To avoid such problems vibrator has to be used while casting work is going on. This is much more important for column and beams because of their orientation. Using vibrator contributes for better compaction or consolidation of the concrete. However too much of vibration is not advisable because it leads to segregation and bleeding. The superfines of cement reach the surface along with water resulting in shrinkage cracks.
What are the tips for curing after execution is completed?
Curing of concrete, within the stipulated time, is like nurturing the newly born baby. If the baby is not taken care in the early months the whole anatomy of the body gets affected and the baby becomes undernourished. Once this occurs any amount of nourishment becomes waste and the baby becomes vulnerable against several diseases. When curing is not effective in the early days of casting the chemistry will not be progressive which hampers the microstructure development. Under such circumstances instead of dense microstructure the concrete develops porous microstructure, which becomes vulnerable against chemical attacks eventually resulting in distress of concrete. With this basic principle in the forefront, the operation of curing is associated with two objects; one to prevent the loss of moisture and the other to control the temperature of concrete for a period sufficient to achieve a desired strength level. To achieve the objects curing in the initial days has to be very effective. For this purpose, ponding incase of flat slabs or wrapping with wet gunny bags in case of vertical elements like column, is more convenient and efficient. The other method is to use curing compounds. Suitable method may be selected depending on the convenience. Sometimes there is controversy as to commencement of curing. As a general rule as long as the inherent moisture appears on the concrete surface it is an indication of progress in hydration, dispensing away the need of curing. In other words, no curing should start unless the product attains final set and hardened. If water is added before the final set, there is possibility that the hydration gets disturbed and in the process more water gets entrapped in the surface mortar. Thereby surface mortar becomes more permeable paving the way for ingress of pollutants and moisture in the long run. Hence care should be taken in deciding the curing time for the concrete. There is a school of thought that water should be sprayed on green concrete to avoid moisture evaporation and quench heat of hydration. This can be served by covering with plastic sheet, but spraying water during green state is not advisable. In case of blended cement concretes the curing has another dimension. The progress and efficiency of pozzolanic reactions depend on the moisture availability. Otherwise the concrete matrix becomes porous thereby vetoing very purpose of using blended cements. Hence curing in the initial days should be more effective to protect the value of the blended cements. Hence curing is one of the important unit operations to have a concrete with long-term durability.
What are the basic issues that influence the durability of concrete?
Durability of concrete is its ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration; this means durable concrete retains its original form, quality, and serviceability when exposed to various environments. As durability is not an intrinsic property of the material, as well as concrete in the present case, conducive environment has to be created in the system so as to uphold the characteristics in the best way. In cement/concrete there are two factors viz.,surplus Ca(OH)2 and heat of hydration that become the root cause for distress of concrete as discussed below:
Lime in cement:The natural embodiment:
Lime has a specified role for the performance of cement in a secured way. However, as much as the ‘nature’ is punctured environmentally to satiate the needs of mankind, cement system is also abused to meet the need i.e., the ‘high early strength’. In the process, the mineralogy and fineness are maneuvered and the ‘protective action of lime’ is ignored. Though remedial steps are identified in blended cement route, many a times certain antagonists comment about exhaustion of lime and threat to passivity film through reduction of pH on account of formation of secondary mineralogical hydrates. If the cement chemistry is not realized in holistic perspective one may tend to come to such conclusions. In this context it is quite interesting to learn the intricate role of lime in cement at the pre and post hydration stages: Lime exists as CaO, combined in anhydrous mineralogy of cement, but does not get liberated unless the cement is subjected for hydration. When cement hydration takes place progressively, lime is released gradually out of which major portion gets into hydrated mineralogy and some portion remains as hydrated free lime [Ca(OH)2] to maintain pH. Because of progressive release over ages, the hydrated cement system is assured of hydrated lime, required to maintain pH in concrete system that has two roles to play viz., maintain the hydrated mineralogy under equilibrium and protect the passivity film of reinforcement in RCC. By ‘fine grinding’ the cement, in the anxiety of attaining early strength, rapid hydrations are induced, affecting the nature’s locus for progressive hydration and sustained availability of lime till late ages. This is one of the many reasons for the shorter life of concretes made of high-grade cements in contrast to the long service life of structures made of low-grade cements in yester-decades.
Thermal shrinkage due to heat of hydration:
The heat generated from the cement hydration increases the temperature within the concrete, which further increases due to the poor dissipation characteristics of the concrete system. This phenomenon manifests in the expansion, which under restraint causes compressive stresses. However, the low elastic modulus at early ages helps in high stress relaxation mitigating the influence of compressive stresses. Subsequently the concrete cools down to ambient temperature during which it is subjected to the tensile strains. With low tensile strength materials such as concrete, it is the shrinkage strain from cooling that is more important than the expansion from heat generated by cement hydration. This impact is more in concretes made of high early strength cements in which the larger potential of stress relaxation is availed much before the tensile stresses come into force. Under these circumstances, depending on the factors viz., the elastic modulus, the degree of restraint, and stress relaxation due to creep, the resulting tensile stresses can be large enough to cause cracking.
How to counter the above issues to enhance the concrete durability?
One of the primary requirements to enhance the durability of concrete is to engage surplus Ca(OH)2 with complimentary cementing material (CCM) that results in the following reaction : fast OPC + H Hydrated primary mineralogy + CH slow CCM + CH + H Hydrated secondary mineralogy As per the established data, the hydration of OPC yields approximately 75% strength rendering mineralogical phases. The balance 25% is Ca(OH)2 that does not contribute for strength; on the other hand, renders deleterious effect to concrete as discussed later. However, the same Ca(OH)2 attains significance in the chemistry with CCM towards formation of secondary mineralogical phases, majorly contributing for additional strength. As a typical case study, the enrichment of hydrated mineralogical phases through secondary reactions of a given fly ash with OPC have been computed and given vide enclosed table. The above reactions attain technical significance from three main features: The reaction is slow and rate of heat This reduces micro-cracking liberation is accordingly slow : and improves soundness. The reaction is lime consuming : This leaves little chance for chemical attacks that results in deleterious reactions and, in turn, deterioration of concrete. The reaction results in the formation This contributes for the mechanism of hydrated secondary mineralogical of pore refinement and grain refinement, phases : resulting in enhanced strength, impermeability and strong transition zone. Strength and durability are two important features for concrete performance, which can be addressed by CCM through the above mechanism if incorporated in cements and concretes. With this background blended cements have made an entry into cement scenario.
Which type of CCM( Complimentary Cementing Materials) can be used in blended cement?
Any CCM can be used as long as it offers better performance in cement /concrete. Pozzolans have been in use since millennia which were of natural origin and volcanic base. ASTM defines pozzolan as: "a siliceous or siliceous and aluminous material which in itself possesses little or no cementitious value but which will, in finely divided form and in the presence of moisture, chemically reacts with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties". Of various pozzolanic materials, more popularly used CCM are fly ash and granulated blast furnace slag.
What is the mechanism in blended cements that improves the durability?
Permeability is the prime cause for the problems of concrete associated with sulphate attack, alkali-aggregate reaction, carbonation and corrosion of steel. These aspects get aggravated with high contents of surplus Ca(OH)2 remain in concrete, where high early strength cements are used.
Surplus Ca(OH)2 plays key role in effecting deleteriously the concrete through interacting with reactive chemicals like SO2, CO2, O2, Cl- etc., that ingress into permeable concretes.
Transition zone (interface between cement paste and aggregate) is another area that influences the micro-cracking and durability of concrete. Incidentally, for a variety of reasons, transition zone is the weakest link of concrete in general and it is more so in permeable concretes and those made of high grade cements. Due to accumulation of part of bleed water at the aggregate surface in the concrete, the zone becomes high in w/c providing space for the formation of porous products. As a result higher Ca(OH)2 crystals of 10-20  form on the aggregate The thickness of the transition zone produced on aggregate is proportional to the quantity of surplus Ca(OH)2 produced at early age of hydration. Hence, where cements with high content of C3S are used, obviously transition zone further increases. When such concrete is subjected to stress of whatsoever nature micro-cracks form readily through this product, due to the porosity and weak constitution. Subsequently, when subjected to various weathering and loading effects concrete loses water tightness; thereby pores and micro-cracks become interconnected, slowly leading to deterioration of concrete. Mehta vividly explained the total mechanism in his holistic model. This is where the role of complementary cementing materials becomes significant. When these products are added to cement/concrete, the formation of microporous secondary mineralogical hydrates around these particles tends to fill the large capillary pores. This mechanism, during which large pores slowly get transformed into microporous products containing numerous fine pores, is known as "pore-size refinement". Transition zone offers conducive environment for pozzolanic reactions because cement paste concentrated with large and oriented crystals of Ca(OH)2 and the particles of CCM exist here in a relatively high water to cement ratio. Under these circumstances Ca (OH)2 slowly gets consumed by the particles of CCM to form numerous, small and less oriented crystals and poorly crystalline reaction products. The mechanism where large grains transform into a product of small grains is known as "grain-size refinement". While pore-size refinement contributes for the impermeability of concrete, grain-size refinement influences the transition-zone towards densification thereby minimising the chances for micro cracking. Such improved microstructure of cement paste contributes for the durability of concrete.

 

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