Effect of the cement type on compatibility with carboxylate superplasticisers
Keywords:rheology, superplasticizer, Portland cement, calcium sulfoaluminate, calcium silicate hydrate
An empirical study was conducted to gain a fuller understanding of the interactions taking place in cementsuperplasticiser systems. To this end, two clinkers of known chemical and phase composition were prepared in this study to gain insight into such interactions. One contained no tricalcium aluminate (C1), while the other had a 9% C3A content (C2). These clinkers were ground to approximately 340 m2/kg and blended with gypsum only or gypsum and Klein compound (3CaO·3Al2O3·CaSO4) (1, 2). Sufficient compound was added to C1 to ensure the formation of about the same amount of ettringite after 0.5 and 1 h of hydration as found in cement C2 + gypsum. The admixture used was a carboxylate superplasticiser. Rheology measurements showed that while paste yield stress was correlated to ettringite formation, no such simple relationship was observed for plastic viscosity. Plastic viscosity depended on the total hydrates formed, i.e., not only as ettringite but also as C-S-H gel. The findings revealed that in clinkers with very low sulfate and potassium contents, the rheology of carboxylate-containing cement paste is primarily controlled by ettringite formation.
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