Influence of silica fume on the rheological properties of the matrices of high-performance concretes

The influence of two types of silica fume (SF) on the rheological properties of cement pastes with water-cement ratios typical of very high performance concrete was investigated. The sensitivity of cement to the sulphonated naphthalene formaldehyde type of superplasticizer (SP) and its optimum content was determined from apparent viscosity as afunction of three parameters. It was found that the values characterizing apparent viscosity of the pastes depend non-linearly on the amount of C³A, and that the relation is linked to the content of free CaO in cement. Values of pH for the cement suspensions are also highly correlated with the content of free CaO, and may be regarded as an indicator for selection of cements that have good workability with SP. The quality of SF had no significant influence on the rheological behaviour of the cement pastes with SP that had the lowest content of C³A, free CaO and alkalis. For all the cements with higher contents of these components, a decrease in viscosity was observed only after the addition of the lower reactivity SF. It has been proved that SF can improve the efficiency of the SP and may reduce the water content provided that its reactivity is limited. The chemical processes at the beginning of hydration are restrained by the same phenomenon.