Effect of sulfate carrier addition on the strength of calcined clay blended cements

The use of calcined clays as Supplementary Cementitious Material (SCM) comes with the need for sulfate carrier in the binder. Otherwise, the early hydration reaction of the binder is altered in such a way that the early age compressive strength of mortar or concrete is significantly lowered. In this article, the effect of sulfate carrier addition on the properties of different calcined clay blended cements is analyzed. Portland and Portland limestone cement are blended with 1:1 dominated, common and 2:1 dominated clays at various sulfate carrier contents so that 122 different binder compositions are studied. The compressive strengths of their mortars are evaluated as Activity Indexes and compared with calorimeter measurements on pastes. The Activity Index of 2:1 dominated clay blends increases by up to 10 % with sulfate carrier addition and oversulfation can already be achieved with more than 1 wt% sulfate carrier addition. In some 1:1 dominated clay blends, this is not even reached with 9 wt% addition and Activity Indexes of these mortars increase up to 60 %. The influence of sulfate carrier addition on the strength was greater at 2 days than at 28 days. The effect of sulfate carrier addition on calcined clay blended cements is different for the two reference cements, but the correlation between hydration heat and Activity Index of the blended cements at 2 days is cement-independent and basically very high (R 2 ≈ 0.90). When using 2:1 dominated clays, the strength prediction by this correlation is only possible to a limited extent. • 1:1 dominated clays demand more sulfate addition than 2:1 dominated clays. • Strength gain due to sulfate addition is higher with 1:1 than with 2:1 dominated clay. • Blends with illitic and smectitic clays behave the same with sulfate addition. • Strength change of blends vary due to sulfate addition depending on the cement type. • Calorimetric measurements can be used to predict strength of sulfate added blends.