Impact of molecular weight of block polycarboxylate superplasticisers on the dispersion of cement paste

Abstract

Polycarboxylate ether superplasticiser (PCE) with different molecular weights but the same length and density of side chains with a block distribution was successfully synthesised through reversible addition-fragmentation chain transfer polymerisation (RAFT). The influence of molecular weight on the performance in cement paste was systematically investigated by measuring the flowability of fresh cement paste, rheology properties, conformation in solution and adsorption characteristics. The results illustrate that when the molecular weight is relatively small, the number of carboxylic groups from the backbone accessible for the cement particle surfaces is too small to have good dispersion. However, PCE with a relatively large molecular weight may be co-precipitated with early cement hydration products owing to rapid adsorption kinetics. Moreover, PCE of much larger molecular weight leads to larger adsorption amounts from the view of both kinetics and thermodynamics. In addition, the flowability and the yield stress are closely related to adsorption amount. An important finding is that the dispersion ability of PCE can be improved with an increase in molecular weight within a certain interval.