Influence of Silica Fume on the Rheology of Cementitious Paste
H. Man and H. Justnes
Appears on pages(s):
gel strength; rheology; silica fume; viscosity; yield stress
The influence of silica fume (SF) on the plastic viscosity, yield point and gel strength of cementitious paste has been studied. A super-plasticizer based on polyacrylate with grafted polyether chains (SSP) was used. The effect of delayed addition of super-plasticizer versus addition with the mix water was investigated as well as the effect of densified versus untreated silica fume. Inert limestone slurries with SF replacement were used as comparison. The results showed that there was not much to gain in terms of lower shear stress in flow curves by delayed addition of this particular SSP. The plastic viscosity had a de-creasing tendency with increasing SF replacement, while yield stress was rather constant. Plastic viscosity increased with increasing time. There was a substantial increase in 10 min gel strength with increasing SF replacement. Gel strength was lower for mixes with densified SF versus untreated SF. Delayed SSP addition reduced gel strength for cementitious pastes with 10% SF. Neutral limestone pastes with increased SF replacement had increasing gel strength from 6 vol% replacement. Densified silica fume gave 10 times less gel strength than untreated SF. Increasing pH from 8 to 13.2 in limestone slurries with 10 vol% SF increased the shear stresses substantially.