Effect of non-adsorbed superplasticizer molecules on fluidity of cement paste at low water-powder ratio

Non-adsorbed superplasticizer molecules increase the fluidity of cement paste with low water-powder ratio, but their function is unclear. The authors used superplasticizers having different molecular structures to investigate the influence of non-adsorbed superplasticizer on the fluidity of cement paste. The paste consisted of low-heat Portland cement, ultra-fine silica particle, and polycarboxylate-based superplasticizer solution. First, a superplasticizer having moderate molecular length was added to the paste, and the adsorption saturated. Subsequently, another superplasticizer was added to the paste. The subsequent additions of shorter super plasticizers decreased the adsorption of the first superplasticizer but the fluidity did not change. The subsequent additions of moderately long superplasticizers did not change the adsorption but increased the fluidity. In contrast, the subsequent additions of longer superplasticizers decreased the fluidity. The authors consider that the fluidity is related to the size of the subsequent superplasticizer, i.e. small superplasticizer molecules intercalate between the particles and aid in dispersion.