Soundness of Cement
To keep cement sound we need to be careful in some aspects. It is essential that a cement paste, once it has set, does not undergo a large change in volume. Such changes in volume may take place due to the delay in hardening cement, namely free lime, magnesia and calcium sulphate.
If the raw materials fed into the kiln contain more lime (CaO) than that can combine with the acidic oxides, the excess will remain in a free condition. The free lime will hydrate very slowly in a subsequent stage and the mortar and concrete prepared with such cement is therefore liable to expand and crack after a few months or a year. Cements which exhibit has expansion are known as unsound cements.
Cement can be unsound due to the presence of magnesia (MgO), which reacts with water in a manner similar to quick lime (CaO). However, only crystalline variety of magnesia (Periclasue MgO) is deleterious, but magnesia present in glass form is of course harmless.
Calcium sulphate is the third compound liable to cause expansion; in this case calcium sulpho-aluminate is formed. A hydrate of calcium sulphate-gypsum is added to cement clinker in order to prevent flash setting. But if gypsum is resent in excess of the amount that can react with C2A during setting, unsoundness in the form of a low expansion will result. For this reason, it is essential to limit very strictly the amount of gypsum that can be added to clinker, but the limits are well on the safe side as far as the danger of unsoundness is concerned.