Expansive cement, when mixed with
water, forms a paste that, after setting, tends to increase in volume to a
significantly greater degree than Portland cement paste. This expansion may be
used to compensate for the volume decrease due to shrinkage, or to induce
tensile stress in reinforcement.
Types of Expansive cements:
An expansive cement containing
anhydrous tetra calcium alumino-sulphate, which is burnt simultaneously with a
Portland cement composition, or burnt separately, when it is to be interground
with Portland cement clinker or blended with Portland cement, calcium sulphate
and free lime.
It is a mixture of Portland cement, calcium-aluminate cement
and calcium sulphate
It is a Portland cement, containing a large
compound of tricalcium-aluminate content and modified by an excess of calcium
sulphate, above the usual optimum content.
In all cases, the specific
surfaces or fineness of expansive cement has a major influence on its expansion
The increase in specific surface
accelerates very early formation of ettingite in the plastic mix, and as a
result, with the increase in the specific surface for a given sulphate content,
the amount of expansion decreases with increasing surface area.