Influence of silica fume on rheology of cement paste: Fid-Bau Portal

Influence of silica fume on rheology of cement paste

Vikan, H. / Justnes, H.

The influence of silica fume on the rheology of cementitious paste has been studied utilising the capacity of a parallel plate Physica MCR 300 Rheometer. Rheological parameters extracted are plastic viscosity, yield point and gel strength. The plasticizers used were sodium naphthalene sulphonate - formaldehyde condensate (SNF) dispersing by electrostatic dispersion and polyacrylate with grafted polyether chains (SSP) designed to disperse by steric hindrance. The effect of delayed addition versus addition with the mix water was investigated as well. The totalparticle volume was kept constant (0.442) corresponding to w/c = 0.40 in the paste while the amount of silica fume (SF) was replacing cement in volume increments of 0.1 from 0.0 to 0.6 (corresponding to mass SF of total powder from 0.0 % to 9.9 %). The paste was 20 minutes old when performing the first measurement and measuring cycles were repeated 4 times with 20 minutes difference in order to see time dependencies. The polyacrylate with grafted polyether chains (SSP) seems to be much better suited for self-compacting concrete (SCC) production than naphthalene sulphonate-formaldehyde condensate (SNF) since shear stress is kept low over a wide range of shear rates (200 s^-1 to 2 s^-1). Delayed addition of superplasticizer lowered the shear stress substantially for cementitious pastes with SNF but only marginally for pastes with SSP, which removes the necessity for delayed addition of SSP in SCC production. When analysing the flow curves with the linear Bingham approach for a medium and low shear rate range, the obtained yield stresses (ordinate intercept) increases with increasing SF replacement in all cases. The plastic viscosity has a decreasing tendency with increasing SF replacement when SSP is used, in particular at low shear rates. The plastic viscosity increases, on the other hand, when SNF is used as a plasticizer. SNF leads to a shear thinning behaviour of the paste (thinking from low towards high shear rates). SSP leads only to a marginal shear thinning without SF and for lowest SF dosage that transfers info a substantial shear thickening behaviour at the highest SF replacement (10 vol%). Such SSP behaviour may be beneficial when making SCC matrix stabilised with SF. The change in rheological properties as a function of time (measured every 20 minutes for 80 minutes) was much smaller for pastes plasticized with SSP compared with SNF.