CEMENT

CEMENT It can be defined as material having adhesive and cohesive properties which make it capable of bonding material fragments in to a compact mass. Cement is obtained by burning together, in a definite proportion, a mixture of naturally occurring calcareous (containing calcium carbonate or lime) and argillaceous (containing alumina) material to be partial fusion at high temperature about 1450◦c. The product obtained on burning called clinker, is cooled and ground to the required fineness. During grinding of clinker gypsum (CaSO4) is added to it to adjust the setting time and to improve the soundness of cement.

Classification of cements 1. Natural cement - burning limestone containing 20-40 % clay and crushing it to powder. - brown in colour and sets very quickly when mixed with water. 2. Artificial cement (i) portland cement - ordinary portland cement - rapid hardening cement - low heat cement (ii) special cements – quick setting cement - high alumina cement - blast furnace cement - calcium chloride cement - white cement - coloured cement - expanding cement - super sulphate cement - sulphate reisting cement

Early History of Cement In ancient times, Romans, Egyptians and Indians used some kind of cementing materials in their construction Egyptians used burnt gypsum (CaSo4) as cementing materials - the analysis of mortar used in Great pyramid showed that it contained 81.5% of calcium sulphate and about 9.5% of calcium carbonate Not much is known about the cementing materials used by Indians in the construction of cities of Harappa and Mohenjo-Daro. Early Romans & Greeks used cementing materials obtained by calcinations (burning) of lime stone. Latter Romans & Greeks learnt that cementing materials may be obtained by mixing volcanic ash and tuff with lime stone, sand and water. - the tuff used in the mix was found in the village Pozzuoli near the mount pozzolona. Afterwards any material natural of artificial having the same properties as those of tuff or ash was called pozzolona.

Early History of Cement…… In the absence of natural volcanic ash, Romans used powdered pottery or tiles as pozzolona, In India Surkhi (brick powder) has been used as pozzolona in mortar Romans added milk, blood & lard in the mix to improve the workability The blood Hemoglobin is a powerful air entraining agent and plasticizer. After 1756, John Smeaton carried out extensive experiments to find out the best material to withstand severe action of sea water. In 1796 hydraulic cement was produced by burning the nodules of argillaceous lime stones - in 1800 it was given the name Roman cement - it was in use till 1850 then OPC was introduced

History of OPC Invented in 1824 by Joseph Aspdin, a brick layer of Leeds (Yorkshire) England - Prepared by heating a mixture of finely divided clay and hard lime stone in furnace until CO2 had been driven off. 1845 – modern cement was made by Issac Johnson In India,OPC was first manufactured in 1904 near madras Bet’n 1912 & 1913, the India cement Co.was established at Porbander produced 1000 tons(1914). By 1918 three more cement factories were established in India and produced about 8500 tons every year During 1951 -56 production rose from 2.69 million tons to 4.6 million tons. By 1969 the production of cement in India was 13.2 million tons (9th place in the world USSR being the first with production of 89.4 million tons) In 2002 the production in India crossed 100 million tons (2nd position after China)

Raw materials Oxide % Function Lime, CaO 60-65 Bulk to cement, Lesser - sets quickly and reduction in strength More - unsound, expansion & disintegration Silica, SiO2 17-25 Imparts strength Alumina, Al2O3 3-8 Setting, imparts quick setting, excess make weak Iron oxide, Fe2O3 0.5-6 Grey colour, strength to cement & hardness Magnesia, MgO 0.5-4 soundness Sulphur trioxide, SO3 1-2 Reduce the setting time, combined with lime gives calcium sulphate Soda(Na2O) and Potash (K2O) (alkalis) 0.5-1 react with agg. Cause disintegration of concrete, affect the rate of gain of strength

manufacture of Cement

Major compounds of cement Tricalcium silicate (3CaO.SiO2) C3S 54.1% Dicalcium silicate (2CaO.SiO2) C2S 16.6% Tricalcium aluminate (3CaO.Al2O3) C3A 10.8% Tetra calcium alumino ferrite (4CaO. Al2O3.Fe2O3) C4AF 9.1% The calculation of main compounds in cement C3S = 4.07(CaO) - 7.6(SiO2) - 6.72(Al2O3) - 1.43(Fe2O3) - 2.85(SO3) C2S = 2.87(SiO2) - 0.754(C3S) C3A = 2.65(Al2O3) - 1.69(Fe2O3) C4AF = 3.04(Fe2O3)

2 C3S + 6 H --------> C3S2H3 + 3 Ca (OH)2 Hydration of cement Upon hydration, both C3S and C2S give the same product called calcium silicate hydrate (C3S2H3) and calcium hydroxide (Ca (OH)2) 2 C3S + 6 H --------> C3S2H3 + 3 Ca (OH)2 (100) (24) (75) (49) 2 C2S + 4 H ------ > C3S2H3 + Ca (OH)2 (100) (21) (99) (22) Note:C3S2H3 – mainly responsible for strength for bond and strength Ca (OH)2 is responsible for sulphate attack , to compensate this add poz.mat Tricalcium silicate(C3S ) giving a faster rate of reaction accompanied by greater heat evolution develops early strength. On the other hand, dicalcium silicate (C2S) hydrates and hardens slowly and provides much of the ultimate strength. C3S and C2S need approximately 24% and 21% water by weight respectively for chemical reaction but C3S liberates 2 times as much calcium hydroxide on hydration as C2S. however C2S provides more resistance to chemical attack. The amount of Ca(OH)2 is not a desirable product in the concrete mass as it is soluble in water and may get leached out making the concrete porous, weak and not durable.

Ca(OH)2 also reacts with sulphates present in water or soil to form calcium sulphates which further reacts with C3A and cause deterioration of concrete The only advantage of calcium hydroxide is that it being alkaline in nature, maintains the Ph value of water around 13 in the concrete which reduces the corrosion of reinforcement. The reaction of C3A with water is very rapid and produces enormous heat. The hydrated product obtained is calcium alumina hydrate (C3AH6) which will bind the materials. The approximate reaction being C3A + 6 H -----> C3AH6 (100) (40) (140) The reaction of C4AF with water gives a product called calcium ferrite hydrate (CFH) in addition to calcium alumina hydrate. It does not contribute any strength to concrete. The calcium ferrite hydrate shows a comparatively higher resistance to attack of sulphate than the hydrates of calcium aluminates. The reaction being C4AF + 7 H -----> C3AH6 + CFH

Applications Hydraulic structures Mass concreting works Marine structures Masonry mortars and plastering Under aggressive conditions. All other application where OPC is used.

Sulphate Resisting Portland Cement (SRC) C3A is restricted to less than 5 % 2 C3A + C4AF lower than 25% use of SRC is particularly beneficial in such conditions where the concrete is exposed to the risk of deterioration due to sulphate attack. Ex: in contact with soils and ground waters containing excessive amounts of sulphates as well as for concrete in seawater or exposed directly to seacoast.

Sulphate attack The Ca(OH)2 react with sulphate gives calcium sulphate resulting in the formation of gypsum (CaSO4.2H2O) with an increase in volume of solid phase by 124% This calcium sulphate reacts with hydrated product of C3A and C4AF forms calcium sulpho aluminate which causes expansion of cement ( increase the solid phase by 227%), hence the disintegration of concretee This increase in volume by any type of reaction is called g* attack.

Applications Foundations, piles Basements and underground structures Sewage and Water treatment plants Chemical, Fertilizers and Sugar factories Food processing industries and Petrochemical projects Coastal works. Also for normal construction works where opc is used .

Soundness Le Chate Auto -lier Clave Max Max Setting Time (Minutes) Physical Characteristics of OPC (BIS Requirement) grade Of cement Fineness (sq.m/kg) Min Soundness Le Chate Auto -lier Clave Max Max Setting Time (Minutes) Initial Final Min Max Compressive Strength (MPa) 3day 7day 28day 53 225 10 mm 0.8 % 30 600 27 37 53 43 23 33 43 33 16 22 33

Portland Pozzolana Cement (PPC) OPC clinker along with gypsum and pozzolanic materials in certain proportions either inter-grinding or grinding separately and thoroughly blending Pozzolana is a natural or artificial material containing silica in a reactive form, no cementitious properties, chemically react with calcium hydroxide to form stable calcium silicates which have cementitious properties. PPC produces less heat of hydration and offers greater resistance to the attack of aggressive waters