Hardened property of concrete or mortar is mainly influenced by property of cement. Normally once the concrete or mortar get hardened, they do not go under expansion or contraction. i.e. the volume of concrete or mortar does not change once they are set.
Soundness of cement is very important property. Soundness refers to the ability of cement paste to retain its volume after it has get hardened.
Also Read: Various Uses of Cement
After the drying of cement mortar or concrete, it should not undergo any appreciable change in volume. If it does, it will cause expansion/contraction on drying.Therefore, there are chances of development of cracks. Unnecessary expansion or contraction occurring as a result of disintegration is called unsound cement. Unsound cement can cause serious troubles to the durability of structures when such cement is used. A good unsound cement never goes under any volume changes once it sets and there is no expansion or contraction after drying.
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Normally, due to the presence of free lime (CaO) or magnesia (MgO) in cement, the paste expands after it has set which also impairs its strength. Therefore, Portland cement specifications limit the presence of free lime & magnesia in cement content.
During the manufacturing process of cement, raw materials are fed into the kiln. Various raw materials like lime (Cao) and some acidic oxide mixed inside the kiln. If burning and cooling processes are not satisfactory, the excess lime will remain in a free condition. This hard-burnt lime hydrates very slowly and slaked lime occupies the larger volume than the original free calcium oxide. Slow hydration process therefore affects the hardened property of mortar or concrete. The difference of hydration rate between free lime and slaked lime leads to change in volume of mortar or concrete. The cement which exhibits this type of expansion is described as unsound.
A cement can also be unsound due to the presence of magnesium oxide (MgO), which reacts with water in a manner similar to lime. The third compound liable to cause expansion is calcium sulfate (CaSo4). In this case calcium sulfoaluminate is formed inside the kiln. It may be recalled that a hydrate of calcium sulfate – gypsum is added to cement clinker in order to prevent flash set. But if gypsum is present in an excess amount it will react with C3A during setting and unsoundness in the form of a slow expansion will result. For this reason, the standards limit very strictly the amount of gypsum that can be added to clinker. The limits are well on the safe side as far as the dangers of unsoundness are concerned.