Soundness:

The phenomenon through virtue of that cement does not undergo large change in volume when treated with water is called as soundness.

If the quantity of free lime and magnesia is present in excess during the manufacture of cement, that is liable to remain uncombined and be over-burnt in the kiln. The cement concrete or mortar of such cement is liable to expand after the setting action is completed. It is a single of the causes of cracking of cement. Soundness test is performed to accelerate the slaking action of free lime and magnesia, if they exist by the application of heat, thereby detecting the defect in a short time.

The unsoundness may be reduced by

(a) Fine grinding,

(b) Thorough mixing,

(c) Limiting the MgO content to less than 0.5 %, and

(d) Permitting the cement to aerate for several days.

Unsoundness is commonly expressed by the expansion of cement paste by Le-Chatelier method.

Le-Chatelier apparatus consist of a small split cylinder of spring brass of thickness 0.5 mm, forming a mould 30 mm high. Two indicators with pointed ends are attached on both sides of the split (which should not be more than 0.5 mm). The distance from these ends to the center of the cylinder is 165 mm.