Abstract
The effects of different inorganic salt accelerators (CaBr2, NaBr, Ca(NO3)2, NaNO3) and an alkali-free liquid accelerator were researched at a low temperature of 10 °C. The results showed the effects of 1.5% NaBr and 1.5% NaNO3 inorganic accelerator were pronounced. The 1-d compressive strengths of the mortar with these two inorganic salts were increased by 185.8% and 184.2%, respectively, and the final setting times were shortened from 7.74 to 6.08 min and 6.12 min, respectively. The hydration temperatures at 10 °C were measured, and the promotion effects of the inorganic accelerators were calculated: the relationship between the hydration degree was αAS + NN > αAS + NB > αAS + CB > αAS + CN > αAS. In addition, the reaction of C3A with NaBr and NaNO3 was used to analyze the products in an ettringite phase, i.e., Ca4Al2O6Br210·H2O, 3CaOAl2O3Ca(NO3)2X·H2O. The formation of these phases was detected in the hydration products of the cement paste hydration for 12 h, 24 h, and 28 d. Combined with the mass loss of the ettringite phase at 90–120 °C, determined using TG/DTG, the synergetic acceleration mechanism of the inorganic accelerators was comprehensively inferred.
Highlights
Shotcretes have been widely used in tunnel and mine construction
Shotcrete has the characteristics of rapid setting and early strength, its early compressive strength development depends on ambient temperature, just like ordinary concrete
For ordinary concrete, when the average temperature is higher than 20 ◦C, the compressive strength of the concrete develops well; when the temperature is lower than 10 ◦C, the strength of the concrete decreases and the curing cycle is prolonged [6]
Summary
Shotcretes have been widely used in tunnel and mine construction. It has the advantages of a fast setting time and a high early strength, speeding up the construction process [1,2]. Shotcrete has the characteristics of rapid setting and early strength, its early compressive strength development depends on ambient temperature, just like ordinary concrete. Many researchers used chemical admixtures to promote low-temperature hydration to improve early strength development. For shotcrete, an alkali-free accelerator could shorten the setting time and improve the early compressive strength, and environmental temperatures have a significant effect on it. There is little research on the effects of low-temperature environments on the performance of shotcrete. The influence of the environmental temperature on the cement hydration is directly reflected in the performance of the accelerator, such as initial and final setting times and early strength. We propose to use the synergetic effect of inorganic salt accelerators to solve the problems of shotcrete in a low-temperature environment to reduce the occurrence of events that affect the shotcrete construction due to low ambient temperature.
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