Low temperature environment affects the development of early performance of tunnel shotcrete.This study investigated the effects of silica fume on the fluidity,compressive strength,and impermeability of alkali-free accelerator-modified cement mortars at low temperatures.XRD,TG-DTG and MIP were used to analyze the changes in hydration products and pore structure.The results show that using silica fume significantly improves the 1 d compressive strength of mortar at low temperatures with an increase of over 300.0％.At 28 d,incorporating 5％(mass fraction)silica fume also increased the compressive strength by 4.7％.Moreover,the impermeability of mortar increases with the increase of the silica fume content.Especially,the impermeability is remarkably improved when the content of silica fume exceeded 10％.Silica fume modification can exert the pozzolanic effect and micro-filling effect at low temperatures,which transformes more-harmful pores towards less-harmful ones,refining the pore structure and making the paste structure denser,thereby improving the compressive strength and impermeability of mortars.
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