Abstract: Accelerators are generally applied in shotcrete to achieve rapid setting and hardening of the concrete. Alkaline and alkali-free liquid accelerators were investigated for their setting-promoting mechanism and strength enhancement at 20 and 5 ℃, and the effect of different types of accelerators on the hydration process and setting-promoting mechanism of cement were analyzed by using heat of hydration analysis, XRD, TG and SEM. The results show that when alkali-free liquid accelerators content of 8% (mass fraction, the same below), alkaline liquid accelerators content of 5% the requirements of the initial setting time not more than 5 min, the final setting time not more than 12 min were met. The environment 5 ℃ will delay final setting time of the slurry and reduce early strength of the mortar mixed with accelerator. Alkali-free accelerator still increases in strength at 28 d in 20 and 5 ℃ environments, while alkaline accelerator shows strength collapse. Highly-concentration aluminum sulfate, conventional aluminum sulfate and hydrofluoric acid are hydrated to generate ettringite (AFt) and calcium silicate hydrate (C-S-H) gel to provide early strength and shorten the setting time, while sodium aluminate accelerator is through the consumption of gypsum and the generation of C-S-H and calcium hydroxide (CH) to provide early strength and shorten the setting time. The low-temperature environment will reduce hydration products content such as AFt and C-S-H in cement acted by high-concentration aluminum sulfate, conventional aluminum sulfate, and hydrofluoric acid-based accelerator, resulting in a decrease in strength. The hydration product of cement mixed with sodium meta-aluminate type accelerator is reduced by the generation of CH under the influence of low-temperature environment, which is able to attenuate the adverse effect of CH on the late strength.

Key words: accelerator, temperature, hydration, compressive strength, microstructure, setting-promoting mechanism