Impacts of sulphates on rheological property and hydration performance of cement paste in the function of polycarboxylate superplasticizer

Abstract

Effects of sulphates on the dispersibility as well as adsorption behavior of polycarboxylate superplasticizer (PCE) were investigated, and the hydration process of cement paste in their synergistic function was studied systematically. Results show that with increased dosage of sulphate added into cement paste, the adsorption-dispersing behavior of PCE decreases. Alkali sulphates (Na2SO4 and K2SO4) most remarkably impact the adsorption behavior as well as the dispersing capability of PCE. Low amount of sulphates added into cement paste can result in the retardation of cement hydration, while high amount of sulphates can advance cement hydration process. The increased content of MgSO4 continuously and most remarkably delays the maximum hydration peak in acceleration period and boosts the transformation from AFt to AFm. The addition of sulphates decreases the production of Ca(OH)2 (CH) due to the reaction between CH and sulphates to produce gypsum. Among the four types of sulphates, MgSO4 most significantly influences the microstructure of cement matrix. Due to the reaction between MgSO4 and calcium silicates hydrates (C-S-H gel), the Ca/Si ratio of hydration products is greatly reduced.