Interaction of polycarboxylate-based superplasticiser with clay in Portland cement systems

It is widely accepted that polycarboxylate-based superplasticisers (PCEs) are sensitive to the presence of clay in cement and concrete. The impact of clay content on the adsorption behaviour of PCE on cement grain surfaces was investigated, with the adsorption of PCE on clay and cement grains determined by means of total organic carbon analysis. In addition, the effect of the mixing sequence of clay and PCE in cement paste was also investigated to explore the mechanism of the low clay-tolerance of PCE. The results indicated that the addition of clay reduced the adsorption of PCE on cement grain surfaces. Accordingly, the workability of the cement paste mixed with PCE was also changed with the addition of clay. In the presence of clay, although the total consumption of PCE was increased, the fluidity of the paste decreased. The results from different mixing sequences revealed that, in the presence of clay, the adsorption amount of PCE on cement grain surfaces was highly reduced, leading to poor dispersion of the cement particles.