Study On Mechanical Properties Of Alkali-activated Material Cured At Sub-zero Temperature For Road Use In Cold Region

With rapid development in terms of roads and bridges construction in China,researches on road materials have attracted widely attentions and many road materials used for various working conditions have been emerged.Multiple latitudes are located in China region.The application of traditional road material frequently faces to the safety and construction quality problems during winter construction period in cold areas.The hydration of Portland cement is stopped and the volume of hardened cement behaves frost heave at sub-zero temperature.Although the methods of adding early strength accelerator and antifreeze can solve the above problems that are caused by low temperature,it increases the complexity and technical difficulty of practical construction.The activator of alkali-activated materials is alkaline and it can effectively reduce freezing point of liquid phase in the system.Compared with traditional Portland cement-based materials,alkali-activated materials possess obvious advantages at sub-zero temperature.It is greatly scientific significance and engineering value to study the early strength improvement of alkali-activated materials for road at sub-zero temperature.Lime-fly ash bound macadam（LFBM）is the most widely used for the pavement base in China because of high strength in later period,small shrinkage coefficient and low price.However,LFBM delays the construction progress due to its the characteristics of low early strength and slowly hardening rate.In order to improve it,LFBM was modified by ordinary Portland cement（OPC）,Ca SO₄·2H₂O and ground granulated blast-furnace slag（GGBS）in this study,and the effects on mechanical strength,shrinkage deformation and internal microstructure were investigated by incorporating OPC,Ca SO₄·2H₂O and GGBS,thus obtaining the optimal mix of CLFBM.Moreover,the related mechanism was revealed as well.In order to make alkali-activated slag（AAS）mortar have higher early mechanical strength and faster setting rate for rapid repair of pavement in cold area,the effects of potassium silicate and sodium silicate on the mechanical strength,hydration characteristics,hydration products and microstructure of AAS cured at the sub-zero temperature were studied,and the related mechanism was analyzed as well.Compared with sodium silicate,AAS mortar incorporated with potassium silicate as activator had higher mechanical strength and hydration heat release and the amount of C（-A）-S-H gel in the product was also increased.It showed that AAS mortar prepared by potassium silicate as alkali activator has more advantages in winter construction.In order to further improve the early mechanical strength of AAS mortar at sub-zero temperature,the effects of Ca O and OPC on the fluidity,setting time,mechanical strength,hydration characteristics,hydration products and microstructures of AAS were studied.The research results showed that the addition of Ca O and OPC can shorten the setting time,improve the early strength,improve the hydration heat release rate and total hydration heat release amount of AAS,the amount of C（-A）-S-H gel in the product also greatly increased,thus the microstructure of AAS mortar was optimized.However,the addition of Ca O and OPC can reduce the fluidity of AAS mortar,and OPC had the negative effect on the development of the later strength.The mechanism of Ca O and OPC improving AAS mortar can be attributed to the elevated heat in the system and the increased ion concentration required for polymerization reaction,and the double effects jointly promoted the reaction process of AAS at sub-zero temperature.When the curing temperature is too low,AAS mortar will not meet the mechanical strength of repairing pavement,and the freeze damage will occur.The enhanced the reaction rate and early mechanical strength of AAS mortar at sub-zero temperature can be achieved by appropriately prolonged pre-curing time.The pre-curing time of mechanical strength of AAS mortar meeting the requirements for pavement repair was shortened by adding Ca O and OPC.Pre-curing not only promoted the early strength of AAS mortar,but also benefited the later strength growth.In addition,pre-curing not only made the mechanical strength of AAS mortar at sub-zero temperature meet the repair requirements,but also inhibited the freezing failure behavior.The finite element simulation method was used to predict the temperature field.It turned out that the prediction of early compressive strength of AAS mortar specimens at sub-zero temperature was feasible because the deviation between the calculated results and test results was small and the trend was consistent.Then,the factors affecting the temperature field and early compressive strength of AAS mortar specimens at sub-zero temperature were analyzed and discussed.The key control factors in the process of heat preservation and maintenance in winter construction were obtained as well.