Adaptation and Strength Enhancement Mechanisms of Liquid Accelerators in Low Temperature
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.
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维普
