Workability of superplasticized microsilica-portland cement concretes

The effect of the addition time of superplasticizers on the workability of concretes containing 0 or 10% microsilica was studied at different polymer dosages, water/cement ratios and mixing times. Cement pastes were also prepared and examined for workability, zeta potential and polymer adsorption. The water demand and the slump loss of the two types of concretes, as well as the dosage of superplasticizer, are significantly reduced when the addition of the polymer is delayed. Regardless of the addition time of the admixture, the difference in water demand between the two types of concretes is greatly reduced when the polymer dosage is above 2% by weight of cement. The data on cement pastes suggest that the delayed addition of superplasticizer might be assimilated to an immediate addition to a cement with a lower fireness.