Inhibition Of Steel Slag On The Early-age Hydration Of Cement: Mechanisms And Applications

Using steel slag as a kind of mineral admixture to prepare concrete is an important way to recycle steel slag.Retardation is one of the critical issues restricting the practical application of cement-steel slag composite cementitious materials.Retardation is closely related to the early-age hydration.Revealing the inhibition mechanisms of steel slag on the early-age hydration of cement is important to understand the retardation of cement-steel slag composite cementitious materials.It provides a basis for the large-scale resource utilization of steel slag as the mineral admixture in cement and concrete.This study was carried out according to the research idea of phenomenon-mechanism-application.Firstly,the early-age hydration kinetics and the evolution of solid phases,aqueous species and microstructures in cement-steel slag composite cementitious materials were deeply characterized to explore the influence of steel slag on the early-age hydration of cement.Also,the effects of steel slag,fly ash,blast furnace slag and quartz powder on the early-age hydration of cement were compared.Then,the inhibition mechanisms of steel slag on the early-age hydration of cement were investigated.The main component which plays a major role in inhibiting the early-age hydration of cement as well as its mechanisms of action were revealed.Besides,the contributions of each mineral phase to the hydration heat release of cement-steel slag composite cementitious materials were revealed by both experimental quantitative methods and kinetic/thermodynamic simulation.The trend of the hydration heat release curve was explained,and the mechanism of low heat release was revealed.Finally,the scientific utilization of steel slag inhibiting the early-age hydration of cement was explored.The main conclusions are listed as follows.（1）Steel slag significantly inhibits the early-age hydration of cement.Steel slag leads to a significantly delayed second exothermic peak（i.e.,the significantly prolonged induction period）,the markedly reduced peak value and the obviously lower cumulative heat release.Steel slag significantly inhibits the precipitation of CH and C-S-H.The effects of steel slag on the early-age hydration of cement are quite different from those of common mineral admixtures such as quartz powder,slag and fly ash.Only steel slag shows the characteristic of significantly inhibiting the early-age hydration of cement.（2）C₁₂A₇in steel slag plays a major role in inhibiting the early-age hydration of cement.The hydration of C₃S is inhibited by the aluminum and calcium ions provided by the rapid dissolution of C₁₂A₇.There is a critical content of C₁₂A₇in steel slag.When the C₁₂A₇ content in steel slag exceeds this critical content,the setting is determined by the hydration products of C₁₂A₇.The rapid hydration of a large amount of C₁₂A₇,which forms a lot of foil-like or flake-like hydration products with the large specific surface area(C₂AH₈and CAH₁₀)in a short time,causes the fast setting.But the hydration of C₃S is still strongly inhibited by C₁₂A₇.When the C₁₂A₇content in steel slag is lower than this critical content,the setting is determined by the hydration products of C₃S,and the strong inhibition effect of C₁₂A₇ on the early-age hydration of C₃S leads to the retardation.（3）The reaction degree of C₃S decreases by about 40%in the cement-steel slag composite paste with 30%steel slag content compared with that in the plain cement paste at 1 d.The cumulative heat release curve of the cement-steel slag composite cementitious material shows a trend of sharp growth（once mixed with water）-slow growth（before 1 d）-rapid growth（after 1 d）.The sharp increase in the cumulative heat release once the cementitious material is mixed with water is mainly attributed to the heat release of the rapid dissolution of C₁₂A₇ in steel slag.The slow growth of the cumulative heat release within 1 d is attributed to the inhibition effect of C₁₂A₇ on the hydration of C₃S.The rapid increase in the cumulative heat release after 1 d is mainly attributed to the increase in the reaction degree of C₃S.（4）The inhibition effect of steel slag on the early-age hydration of cement can play a positive role if it is scientifically utilized.High-volume steel slag concrete may be used in large-volume concrete structures,which can significantly reduce the temperature peak.Steel slag may be used with ultra-fine slag to prepare composite admixtures,which can solve the problem that ultra-fine slag always causes large temperature rise and shrinkage of concrete.