Study On Improving Technology Of Low Temperature Hardening Performance Of Alkali-activated Slag Binder

The strength development of cement concrete is closely related to the hydration process.The rate of hydration will slow down due to the decrease in temperature.Increasing the low-temperature hydration and hardening rate of cementitious materials is essential to ensure construction and application under low-temperature conditions.There is an urgent need to find a cementitious material that can harden and strengthen quickly in a low temperature environment.At room temperature,alkali-activated slag binder has mang excellent properties such as set quickly,fast hardening,high early strength,and has the potential of low-temperature hydration and hardening due to the presence of lye.At present,domestic and foreign literature reports on the low-temperature hydration behavior of alkali-activated slag cement are still lacking,and it is difficult to guide engineering applications.This paper studied the influence of water-binder ratio,alkali equivalent,and water glass modulus on the early strength of alkali-activated slag mortar under low temperature curing（0℃）by setting orthogonal experiments,and according to the early mechanical properties,the optimal mix ratio was determined.Basis on this,the influence of the mold entering temperature,sodium hydroxide incorporation method,and slag fineness on the setting time,low-temperature mechanical properties,and internal temperature of the alkali-activated slag cement was explored,the effect of curing temperature on its hydration and hardening behavior was analyzed by XRD,TG,SEM and other test techniques.The effects of early strength minerals on the strength development of AAS mortar at low temperature have been tested and studied.The basic laws revealed are as follows:（1）Under the curing condition of 0℃,it could be shown that the effect of the mixing ratio parameters on the 4 h compressive strength of alkali-activated slag mortar decreased in order of alkali equivalent,water-binder ratio,water glass modulus,and on the 1 d compressive strength of alkali-activated slag mortar decreased in order of water-binder ratio,alkali equivalent,water glass modulus.（2）The mold entering temperature,sodium hydroxide incorporation method,and slag fineness affect the early hardening performance of AAS.Cured at 0℃,alkali equivalent=8%,W/C=0.35,Ms=1.2,when the mold-in temperature was within the range of 12～19℃,the decrease in temperature would prolong the setting time of AAS paste and reduce the mortar.Early compressive strength the decrease of the mold entering temperature would prolong the setting time of the AAS paste and reduce the early compressive strength of the mortar;When Na OH was directly incorporated into the AAS system in the form of flakes or powder,it could shorten the setting time of the AAS paste and improve the 2 h compressive strength of the mortar due to the increase in the mold entering temperature;The slag fineness was increased from 450 m²/kg to 790 m²/kg,the hydration reaction rate at low temperature became faster,the setting time of the paste was shortened.The improvement effect of the compressive strength of the mortar in the first6 h was obvious.The 4 h and 6 h compressive strength increased by more than 50%.（3）Under negative temperature,the initial strength development of alkali-slag cementation material decreased significantly with the decrease in temperature.Under the conditions of this study,when curing at-10℃,the slag fineness was 450 m^2/kg,the 1 d compressive strength of the AAS mortar was less than 3 MPa,which was only 8.3%of the same age curing at 20℃;When curing at 10℃,the slag fineness was 790 m²/kg,the1 d compressive strength of alkali slag mortar was less than 8 MPa,which was only 13.5%of the same age curing at 20℃.（4）CaO/Ca（OH）₂,cement clinker,AAS powder,high-strength sulphoaluminate cement could all improve the low-temperature hardening performance of AAS binder.By comparison,CaO worked best,when the solid phase temperature was 0℃,the setting time of AAS paste could be controlled within 15 min～39 min;When curing at 0℃,the best early strength could be obtained.The 2 h,4 h,6 h,and 1 d compressive strength of AAS mortar could reach 9.5 MPa,13.4 MPa,15.3 MPa,and 24.6 MPa,respectively.（5）The three composite minerals of CaO-clinker,CH-clinker and CSA-clinker improved the low-temperature hardening performance of AAS better than the single-admixture group.By comparison,CaO-clinker worked best,when the solid phase temperature was 0℃,the initial setting time was controlled at 19 min,and the final setting time was controlled at 23 min.When curing at 0℃,the 2 h compressive strength of the mortar was 8.5 MPa,the 4 h compressive strength was 14.3 MPa,the 6 h compressive strength was 15.5 MPa,and the 1 d compressive strength was 32.3 MPa.