Effect of polycarboxylate ether (PCE) superplasticizer on thixotropic structural build-up of fresh cement pastes over time

Abstract

Water reducing agents including polycarboxylate ether superplasticizer (PCE) have been found to have retardation effect on hydration heat and setting of cementitious materials, which is a long term effect after 8 h. However, the short term effect within 3 h of PCE on workability and workability loss, which could result in failure during SCC pumping or continuous concrete 3D printing, is not clear. This study explores the effects of PCE on flowability and structural build-up of cement pastes in fresh states using rheometer. Strength and modulus, such as the dynamic and static yield stress, and storage modulus are measured over time up to 2.5 h. They are measured by step down protocol, stress development protocol (i.e. low constant shear rate test), and small amplitude oscillatory sweep, respectively. Plastic viscosity fitted in Bingham model and critical strain using oscillatory test are also recorded. The effect of dosage of PCE addition and time are studied. It is found that in both cases where w/c are 0.3 and 0.4, low dosage of PCE addition exhibits much higher increasing rates of structural build-up than plain cement paste or at high dosage of PCE addition. It highlights attention needed on structural build-up at fresh state with low dosage of PCE addition. Possible mechanisms, especially effect of PCE polymers on colloidal flocculation and early hydrates formation, are also discussed and reasoned.