Adsorption of PCE and PNS superplasticisers on cubic and orthorhombic C3A. Effect of sulfate

• Sulfate-SPs compatibility on cubic and orthorhombic C3A polymorphs is studied.
• Sulfates exhibit greater affinity for orthorhombic than for cubic C3A.
• SPs structure on admixture-sulfate competition is lower in orthorhombic C3A.
• Application for working with concretes with high orthorhombic C3A content.

C3A is the most highly reactive phase in clinker and the one with the greatest affinity for superplasticiser admixtures. The amount of C3A 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) C3A polymorphs. Isotherms were found for polycarboxylate ether and naphthalene-based admixture adsorption onto synthetic cubic and orthorhombic C3A 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 C3A. Orthorhombic C3A 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-C3A 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-C3A hydration. The affinity of this polymorph for sulfates is so high that admixture adsorption is much smaller than observed in cubic-C3A. Therefore, the SPs have a scant effect on orthorhombic-C3A hydration.