The Hydration Mechanism And Long-term Properties Of Steam-cured Ternary Portland Cement Blends Containing Steel Slag And Metakaolin

Steel slag(SS)is a by-product of the steel-making process.According to the making process,SS can be classified into converter SS,electric SS and flat furnace SS.The annual emission of SS in China exceeds 80 million tons,while the utilization rate is only 22%.As a result,a large amount of untreated SS is discarded at random,occupying a lot of land resources and leading to serious environmental pollution.Therefore,it is in emergent need to be resolved about how to use SS scientifically and rationally.Recently,SS has been used as a mineral admixture,which has become one of the main research directions for the comprehensive utilization of SS.Under the background of building industrialization,steam curing,served as a method of high temperature activation,can promote the early hydration of cement and SS,as well as enhance productivity.However,steam curing coarsening the pore structure of concrete is harmful for the durability.To improve the mechanical properties of steam-cured SS cement-based materials and solve the durability problem,SS and metakaolin(MK)are used as mineral admixtures in cement-based materials.The basic physical properties,mechanical properties,hydration mechanism,drying shrinkage and sulfate attack resistance of the SS-MK-cement ternary composite cementitious material system under steam curing are studied.The main results are as follows:(1)When replacement of SS is less than 30%,the soundness of blended cement remains qualified,and the water requirement of normal consistency has no significant change.The hydration activity of SS is quite low,thus its incorporation will prolong the setting time of cement paste.Compared to pure cement paste and SS blended cement paste,the water requirement of normal consistency of SS-MK blended cement paste is increased,while the setting time is significantly shortened.(2)The addition of SS has a negative impact on the mechanical properties of cement mortar.Early steam curing may alleviate the adverse effects of SS to a certain extent,while the cooperation of MK can compensate for the decrease of mechanical properties caused by the replacement of SS,and obviously improve the mechanical properties of steam-cured SS-MK cement mortar.Considering the basic physical properties,mechanical properties and economic benefits,the optimal ratio of MK to SS(mass fraction)is 1:1.Under this optimal ratio and 30%replacement of cement,the compressive and flexural strength of steam-cured SS-MK cement mortar can be equal to that of pure cement mortar.(3)The addition of MK under steam curing accelerates the hydration process of blended cement,promotes the formation of ettringite,as well as the nucleation of C-S-H gel.As a result,the inhibitory effect of SS on cement hydration is lightened.Compared to pure cement paste and SS blended cement paste,the Ca(OH)₂ content of SS-MK blended cement is sharply reduced,while the content of C-S-H and C-A-S-H gels,the amount of chemically bound water,and the content of carboaluminate phase increase.Owing to the pozzolanic effect of metakaolin is more obvious under steam curing,the hydration products are densely packed and interwoven.Moreover,the pore structure is obviously refined,and the microstructure is more homogeneous and denser.(4)The drying shrinkage of cement mortar increases with the cooperation of SS under steam curing,but the sulfate corrosion resistance of SS blended cement mortar is slightly better than that of pure cement mortar.Because the addition of MK can optimize the pore structure,consume Ca(OH)₂and promote the formation of carboaluminate phases,the drying shrinkage of steam-cured cement mortar containing SS and MK is decreased.Furthermore,the sulfate corrosion resistance of SS blended cement mortar is greatly improved.