New insights into the role of superplasticizer addition time on yield stress of cementitious materials

The timing of superplasticizer addition is known to impact both the yield stress and hydration kinetics of cementitious systems. Recent results demonstrate that these changes can be directly related to changes in the specific surface area of the cement paste. As this results from a change in hydrate morphology, in particular ettringite, two main reasons may explain yield stress changes in relation to addition time. On the one hand, these may result from changes in adsorption. On the other hand, they may be due to changes in the maximum packing of the cement paste. To examine both scenarios, we have applied the YODEL, a Yield Stress mODEL that has been recognized as an effective model for predicting the yield stress of cement-based materials. By doing this, we could evaluate the relative roles of changes in adsorption and of maximum packing in cement-limestone pastes. Results indicate that, in the range of yield stresses and volume fractions measurable in our experiments, changes in adsorption, rather than variations in maximum packing, are the primary difference between direct and delayed addition.