New insights into the retardation mechanism of PCE superplasticisers on cement hydration

In this study, the retardation mechanism of polycarboxylate (PCE) superplasticisers on cement hydration from the viewpoint of the dissolution–precipitation process was investigated. Effects of PCE superplasticisers on hydration of Portland cement (OPC), C3A (tricalcium aluminate, Ca3Al2O6)−gypsum and pure C3S (tricalcium silicate, Ca3SiO5) and on precipitation of synthetic calcium silicate hydrates (C−S−H) were studied by means of calorimetry and solution analysis. Results show that PCE with a higher charge density and shorter side chain exhibits a stronger retardation effect on cement hydration. The PCE additions inhibit C3S hydration in a similar manner as in the cement pastes, while promoting the depletion of gypsum and secondary hydration of C3A in the C3A−gypsum paste. Furthermore, in OPC systems, the PCE delays C−S−H precipitation without inhibiting dissolution of C3A and C3S, which is responsible for the retardation. Based on the experimental results, a complete view on the retardation mechanism of PCE is proposed: the extension of the dormant period originates from the heightened saturation index required for C−S−H nucleation and growth because of the enhanced stabilisation of C−S−H nuclei by the adsorbed PCE.