Abstract

C(3)A is the most highly reactive phase in clinker and the one with the greatest affinity for superplasticiser admixtures. The amount of C(3)A in cement, the sulfate content in the medium and the type and amount of admixture largely determine paste, mortar and concrete rheology. Many unknowns remain, however, around the effect of SP structure on admixture adsorption onto (cubic or orthorhombic) C(3)A polymorphs. Isotherms were found for polycarboxylate ether and naphthalene-based admixture adsorption onto synthetic cubic and orthorhombic C(3)A to determine that effect, given their different structure and nature. The impact of sulfates on adsorption was also explored. The conclusion drawn was that admixture structure and sulfate content in the media were the factors with the greatest impact on adsorption onto cubic C(3)A. Orthorhombic C(3)A was observed to react more intensely to the presence of sulfate and consequently to have less affinity for the admixtures. In the presence of soluble sulfates the addition of superplasticisers was shown to retard the appearance of the main cubic-C(3)A calorimetric signal more effectively when admixture-sulfate competition was more intense. The presence of SP admixtures has no impact on the peak heat flow time in orthorhombic-C(3)A hydration. The affinity of this polymorph for sulfates is so high that admixture adsorption is much smaller than observed in cubic-C(3)A. Therefore, the SPs have a scant effect on orthorhombic-C(3)A hydration. (C) 2014 Elsevier Ltd. All rights reserved.