Influence of Mixing Sequence on Cement-Admixture Interaction

Abstract

Dynamic rheology was used to investigate cement pastes prepared using various mixing sequences and containing a high-range water-reducing admixture. Delayed admixture addition, delayed water addition, double mixing, and vigorous mixing techniques were used to prepare samples. Mixing sequence effects varied with cement composition, admixture dosage, and addition time. Of these mixing sequences, the most effective at dispersion enhancement was delayed admixture addition. A 2.5 minute delay time was sufficient to disperse all cement-admixture combinations. An optimum amount of delayed water exists with delayed water addition. Pastes with longer between mixing time intervals were dispersed more with double mixing. In general, pastes with vigorous mixing were dispersed by the admixture. At low admixture dosages, however, the dispersing effect was not apparent. Study results demonstrate that mixing procedures and batching sequences must be taken into account when comparing different concrete batches, even through they have the same raw materials and mixture proportions. Study results suggest, moreover, a direction for optimizing the batching sequence and mixing procedure for both paste and concrete.