Sulphate durability of low carbon cements with high contents of calcined clay. Reasons for the resistant phenomenon

Abstract

In this research, the resistance against to sulphate ion of several cements which could be included in the current European cement standard EN 197–1:2011 (CEM type IV/A-SR and IV/B-SR). To confirm the initial hypothesis of this research three Portland cements with different crystalline compositions were used, (from 0% to 14% of C3A phase, approximately) and a thermally activated clay with a reactive alumina content known. In all the blended cements prepared, the amount of calcined clay present was greater than 40% and less than 80% by weight. Three series of blended cements were carried out: in the case of the first one, the setting regulator content was adjusted to 7% SO3 in the cement, the second, without additional contribution of gypsum to change the hydration reactions, and the last one, prepared with extra calcium hydroxide content. Each blended cement and the three Portland cements (PC1, PC2 and PC3) were submitted to the Le Chatelier–Anstett test.In the first place, the results obtained confirm the decrease in sulphate resistance of all of the Portland cements when they are replaced by calcined clay. This performance found is a consequence of the chemical effect from the pozzolanic activity of the reactive alumina of the calcined clay selected in this research. An opposite behaviour was detected when the sustituion levels of calcined clay in blended cements was higher than 50%, due to an important decrease in dissolved portlandite levels of paste liquid phase oh these cement pastes that produce the sulphate resistance phenomenon. This lower amount of portlandite prevented the formation of all the rapid formation ettringite amounts of calcined clay. With the external addition of portlandite, the calcined clay was able to show its true chemical character and behaviour against the attack of sulphates.