Effect of silica fume on cement rheology properties in presence of superplasticisers

Abstract

The effect of silica fume (SF) at different mass concentrations (0, 2, 4, 6, 8 and 10%) on the ordinary Portland cement (C) paste rheology, in the presence of superplasticisers, has been studied. The shear stress versus shear rate was obtained by using the rheometer AR 2000. Two types of superplasticisers (PCEs) have been conducted which are acrylic copolymer (PA) and modified polycarboxylate (PC), and the saturation point of each one was 2% on the amount of cement. The water-cement ratio is equal to 0.35 (W/C = 0.35). The rheological properties of which have been defined applying a rotational viscometer with co-axial cylinders were yield stress, consistence, behavior index and thixotropy. The thixotropy was evaluated by hysteresis loop test which was performed on paste specimen with 6% of SF at 10, 40 and 70 min. The rheological behavior of cement and C-SF solutions was shear-thinning with yield stress. The flow curves were modelled by Hurshel-Bulkly (H-B) law (R2 > 0.99). Hence, it has been noted that the fluidizing effect of the organic additives on cement pastes is conditioned by the presence of silica fume addition. The yield stress and apparent viscosity of the mixtures were increased significantly by raising SF amount. In fact, the yield stress and consistency values were higher for samples added to with SF than for samples without SF. In addition, the results indicated that the PC-admixture effect on the C-SF paste fluidity is more than the PA-admixture one. In other hand, the C-6% SF mixture presented a slight thixotropy value in presence of PC-admixture in comparison with PA-admixture thixotropy especially at 70 min.