Sulfate resistance of portland-pozzolanic cements in relation to strength

Sulfate resistance of Portland-pozzolanic cements, prepared by gradual replacement of Portland cement by pozzolana, were studied in relation to their transverse strengths. Common clay and China clay, after calcination, were used as two different pozzolana. Strengths of samples, after curing separately in water and in 5% MgSO₄ solution, were measured. On sulfate-curing the strength increased remarkably upto a maximum value beyond which it decreased. This was thought to be dependent on the mode of formation of ettringite in the set cement. Strength on water-curing was not much improved. This was due to the non-availability of lime by the reacting pozzolana because of slower rate of diffusion of lime from the hydrating cement.