Along the S‐Curve – How Superplasticizers Affect the Yield Stress of Cement Paste

Abstract

AbstractIn cementitious systems, superplasticizers have two core purposes. On the one hand, they are used to lower the yield stress of a cement paste without changing the water to cement (w/c) ratio. On the other hand, they can reduce the w/c ratio without changing the yield stress. The relationship between slump flow and dosage of a superplasticizer containing cement paste can be described as an S‐curve. Below a critical dosage, the superplasticizer does not increase the flow. Above this dosage, the flow increases linearly until the saturation dosage is reached. Further addition of polymer will therefore not increase the fluidity of the cement paste. The minimum dosage which corresponds to an onset of increased flow compared to a dispersant free paste is referred to as the critical dosage.In this study, we analysed the dosage curve for three different polymers: a mela‐mine based (MFS), a polycarboxylate ether (PCE) and a polyphosphate ether (PPE) superplasticizer. The dosage curves of each superplasticizer were measured with a slump flow test at two different w/c ratios. To observe even small changes in the ultra‐low dosages, MFS was measured using a rheometer. We have specifically examined the areas defined as critical dosage and saturation dosage. We have found that these definitions need to be revised, not with regards to their values, but in terms of a proper understanding.