Effect of some superplasticizers on the rheological properties of blended cement pastes containing condensed silica fume

In silica fume blended cements, the water of consistency increases with the silica fume content due to its high surface area. Therefore, the porosity is increased and consequently the compressive strength value decreased. To get full benefits of the partial substitution by silica fume it was decided to demonstrate the effect of superplasticizer, which reduces the water demand and/or improves the workability of cement paste. In this program, the effect of Conplast SP4302 and SP600 (which are based on naphthalene formaldehyde sulfonate and polycarboxylate/polyether compounds) on the shear stress-shear rate relationship was investigated. The cement pastes were prepared by replacing of cement by different silica fume ratios ranging from 0 to 20\% while superplasticizers dosage were ranging from 0 to 1.5 mass\%. The shear stress values were determined at different shear rate values ranging 0.3 to 146 s(-1). Other tools, mainly chemically combined water, and Ca++ ions concentration confirmed the obtained results. It was found that replacing cement by silica fume decreases the workability of pastes while addition of superplasticizer is enhancing its workability.