The mechanism of alkali-free liquid accelerator on the hydration of cement pastes

Aluminum sulfate type alkali-free liquid accelerator is widely used in shotcrete. For obtaining a better understanding of the accelerating mechanism, the effect of the accelerator on the hydration of C3A + gypsum, C3S and C3S + C3A + gypsum was investigated. The hydration products were analysed using X-ray diffraction, themogravimetric analysis and scanning electron microscopy. The results show that, after the accelerator's addition, it reacted with Ca(OH)2, C3A, and gypsum in the cement to accelerate the formation of ettringite, which typically formed on the surface of C3S particles that were not fully hydrated, thus slightly delaying C3S hydration at a later stage.