Role of silica fume in alkali-activated slag/glass powder paste

The influences of silica fume (SF) on the properties of sodium silicate-based alkali-activated slag/glass powder (AASG) pastes were systematically investigated. Evaluation of fresh properties (including setting time, flow spread, and rheological behavior) and compressive strength of AASG pastes were carried out. Dispersion properties of the precursors in the alkali solution were observed to explain the rheological behavior. The heat evolutions and microstructural properties of AASG pastes were evaluated to understand the mechanism of setting time developments and compressive strength, respectively. The test results showed that the use of SF prolonged the setting time of AASG pastes and different sources of SF, of which the pH values were different, had different influences. Incorporating SF can significantly increase the yield stress of the fresh AASG paste and its plastic viscosity at a low shear rate (<10 s⁻¹), while decreasing its plastic viscosity at a high shear rate (>20 s⁻¹). The compressive strength of AASG paste increased first and then decreased with the increase of SF content, and the optimal SF content was about 10%. The increase in compressive strength was due to the filling and nucleation effect of SF and the formation of additional C-(N)-A–S–H gel with a lower Ca/Si ratio, which resulted in a reduced amount of large capillary pores (100 nm to 1000 nm) and increased amount of small capillary pores (10 nm to 100 nm). When too much SF (>15%) was incorporated in AASG paste, it increased the amount of large capillary pores, leading to reduced strength.