低温环境下液体速凝剂的适应性及强度增强机理研究

Adaptation and Strength Enhancement Mechanisms of Liquid Accelerators in Low Temperature

速凝剂广泛应用于喷射混凝土中,可实现混凝土的快速凝结硬化。本文研究了碱性和无碱液体速凝剂在20和5℃环境下的促凝机理和强度增强规律,并结合水化热、XRD、TG和SEM等测试手段,分析了不同类型速凝剂对水泥水化进程和促凝机理的影响。结果表明:当无碱速凝剂掺量为8%(质量分数,下同)和有碱速凝剂掺量为5%时即可满足《喷射混凝土用速凝剂》(GB/T 35159—2017)中初凝时间不大于5 min,终凝时间不大于12 min的要求;低温(5℃)环境会延缓净浆终凝时间,降低掺速凝剂砂浆的早期强度;无碱速凝剂在20和5℃环境下的28 d强度仍在增加,而有碱速凝剂出现强度倒缩现象。高浓硫酸铝、常规硫酸铝、含氢氟酸类速凝剂均是通过水化生成钙矾石(AFt)和水化硅酸钙(C-S-H)凝胶等水化产物来提供早期强度和缩短凝结时间,而偏铝酸钠类速凝剂则是通过消耗石膏和生成水化铝酸钙(C-A-H)凝胶和氢氧化钙(CH)等水化产物来提供早期强度和缩短凝结时间;低温环境会使高浓硫酸铝、常规硫酸铝、含氢氟酸类速凝剂作用的水泥中AFt和C-S-H等水化产物含量减少,使强度下降。而掺偏铝酸钠类速凝剂的水泥水化产物受低温环境的影响,导致CH的生成量减少,减弱了CH对后期强度的不利影响。

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.