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Properties of cement compounds


 These compounds contribute to the properties of cement in different ways
 
  1.  Tricalcium aluminate, C3A:-It liberates a lot of heat during the early stages of hydration, but has little strength contribution. Gypsum slows down the hydration rate of C3A. Cement low in C3A is sulfate resistant. 
  2. Tricalcium silicate, C3S:-This compound hydrates and hardens rapidly. It is largely responsible for portland cement’s initial set and early strength gain. 
  3.  Dicalcium silicate, C2S: C2S hydrates and hardens slowly. It is largely responsible for strength gain after one week.
  4.  Ferrite, C4AF:This is a fluxing agent which reduces the melting temperature of the raw materials in the kiln (from 3,000o F to 2,600o F). It hydrates rapidly, but does not contribute much to strength of the cement paste.

By mixing these compounds appropriately, manufacturers can produce different types of cement to suit several construction environments.

Setting and Hardening of Cement:-  The chemical reaction between cement and water is called Hydration of Cement or Simply Cement hydration .The phenomenon by virtue of which the plastic cement change into a solid mass is known as setting of cement. The phenomenon by virtue of which the cement paste sets and develops strength is known as hardening of cement.
          The rate of setting and hardening of cement , the rate of evolution of heat and resistance to sulphate attack are affected by the proportions of different cement compounds. The sum of percentage of tricalcium silicate and dicalcium silicate for Portland cement varies from 70% to 80%. The tricalcium silicate hydrates more rapidly then dicalcium silicate and develops strength in cement for the first 7 days. The tricalcuim aluminate  and tetracalcium alumino ferrite compounds  are responsible for the initial setting of cement

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