Effect Of Steel Slag On The Hydration And Shrinkage Characteristics Of Calcium Sulfoaluminate Cement

Calcium sulfoaluminate cement is a special cement with the advantages of gaining high strength at early age,hardening rapidly,low energy consumption and low CO₂ emissions.Steel slag is an industrial waste with weak cementitious property generated during steelmaking,which has not been effectively used comprehensively.In this paper,steel slag is used as supplementary cementitious material to replace part of the calcium sulfoaluminate cement.Based on the physical and mechanical properties,electrical resistivity,pore solution analysis,X-ray diffraction,mercury intrusion porosimeter,chemical shrinkage and autogenous shrinkage tests,the effects of steel slag on the hydration mechanism and shrinkage characteristics of calcium sulfoaluminate cement was studied.The results can be concluded as follows:The amount of steel slag less than 20%will reduce the fluidity of the calcium sulfoaluminate cement pastes,the fluidity of the pastes increased with the increase of steel slag.Steel slag prolongs the setting time of cement pastes.The compressive strength of the hardened cement pastes with more than 20%steel slag at 90 days and180 days have not been lost,the strength continued to increase during the curing age.The strength contribution coefficient can indicate the hydration activity of different amounts of steel slag at different ages.The steel slag content of 10%and 20%is conducive to the development of long age strength of calcium sulfoaluminate cement pastes.Ettringite is the main hydration product of calcium sulfoaluminate cement pastes with steel slag,and Ca（OH）₂ was not observed.When the content of steel slag is 40%,C₂ASH₈ is formed in the cement pastes at 28 days,which helps to improve the strength of the cement pates in the long age.The bound water in the cement pastes decreased as the amount of steel slag increased within 90 days,while the bound water in the cement pastes with 10%,20%,and 40%steel slag content exceeded the control group at 180 days,indicating that more hydration products were formed.The slow hydration of C3S and C2S in the steel slag will produce Ca（OH）2,which will increase the pH value of pore solution of cement pastes.The addition of steel slag changed the pore size distribution of the hardened cement pastes at 180 days,which reduced the pores smaller than 20 nm and increased the pores larger than 200 nm.The RO phase in the steel slag is chemically inert during 180 days.The chemical shrinkage of calcium sulfoaluminate cement pastes decreases with the increase of steel slag.When the amount of steel slag is not more than 20%,the chemical shrinkage of the cement pastes increased rapidly in the first 12 h.When the amount is more than 20%,the chemical shrinkage of the cement pastes changes slowly within 72 h.The autogenous shrinkage of the calcium sulfoaluminate cement pastes decreases with the increase of steel slag,and has a good linear relationship with the relative humidity of the pastes.The ratio of autogenous shrinking to linear chemical shrinkage is low,most of the chemical shrinkage exists as unfilled pores.The development trend of dry shrinkage and autogenous shrinkage is similar,but the shrinkage is larger than autogenous shrinkage.The electrical resistivity of calcium sulfoaluminate cement pastes gradually decreases with the increase of steel slag.According to the differential curve of electrical resistivity,the cement pastes hydration process is divided into a dissolution period,an induction period,an acceleration period,a deceleration period,and a stable period,respectively,which has a good corresponding relationship with the temperature evolution of the pastes.The setting time,chemical shrinkage,and autogenous shrinkage of calcium sulfoaluminate cement pastes with steel slag are linearly related to the development of electrical resistivity,and the compressive strength is linearly related to the logarithm of electrical resistivity.When the curing temperature increased,the compressive strength of the calcium sulfoaluminate cement in the control group decreased at 28 days,but the strength of the cement pastes with steel slag continued to increase.The bound water at 40℃and60℃is higher than that of 20℃,and the change in pH of the pore solution at 60℃for 6 h to 3 days was much higher than 20℃.The chemical shrinkage of the calcium sulfoaluminate cement decreased with the increase of the curing temperature at 72 h,but the chemical shrinkage at different temperatures did not change much when the steel slag content was 40%.By studying the hydration and shrinkage mechanism of calcium sulfoaluminate cement pastes with steel slag,this paper provides theoretical basis and data reference for steel slag as an auxiliary cementing material of calcium sulfoaluminate cement,which helps to promote the comprehensive utilization of steel slag,achieve sustainable development.