Experimental evidence for a possible dispersion mechanism of polycarboxylate-type superplasticisers

Abstract

In this paper, a brief overview of the current literature on the dispersion mechanism of polycarboxylate-type superplasticisers (PCEs) is first presented. Then, the available free water amount and fluidity of cement paste with various water-to-cement ratios (w/c) and PCE dosages are measured. The phase and particle size distribution of early cement hydrates are analysed by X-ray diffraction and laser particle size analysis, respectively. As a result, a surprising tendency is observed: the increase rate of fluidity increases with decreasing w/c ratio when the dosage of PCE gradually increases. This phenomenon cannot be explained by the steric effect. The authors also conclude that not less than 2·0% of solids volume of ‘AFt–PCE’ structures (early hydrates of ettringite (AFt) coated with PCE, < 200 nm) are dispersed well in the interstitial pore space along with non-adsorbed PCE. Based on these results, the authors present a possible dispersion mechanism in the PCE–cement system studied here based on the repulsion originating from the exclusion of free polymers (non-adsorbed PCE, ‘AFt–PCE’ structure and some other fine matters coated with PCE) from the gap between cement particles.