Studies On The Modification And Hydraulic Performance Optimization Of Steel Slags

Steel slag is the residual waste of steel production process and hasn't been used effectively so far. A large quantity of steel slags have occupied vast amount of infield and contaminated the environment due to the dust and various poisonous elements emission. In fact, the presence of C3S, C2S, C4AF and C2F endorses steel slag hydraulic property. The C3S content in steel slag is much lower than in portland cement, and steel slag can be regarded as a weak cement clinker. However, the chemical and mineral compositions of steel slag are highly variable and the soundness is bad. Thus, the application of steel slag in cement and concrete industry is restricted. In the past few decades, many researchers adopted different approaches to activate steel slags in order to improve the utilization efficiency. Founded by Project of the National Eleventh Five-Year Research Program of China (2006BAF02A24),in this study, a novel method was developed to modify the chemical and mineral composition by incorporating adjusting materials into steel slag at high temperature for increasing the hydraulic activity of steel slag.In this paper, there was systematically investigated technological parameters and hydraulic performance of the modification of steel slags with different basicity at high temperature with incorporation of other industrial solid wastes such as electric arc furnace slag(EAFS), ground granulated blast furnace slag (GGBFS) and so on. The steel slags studied in this paper are supplied by Wuhan Iron and Steel Group Corporation, Handan Iron and Steel Group Corporation and Ma'anshan Iron and Steel Company (named as WGSS, HGSS and MGSS in brief, respectively). The optimum technological parameters and principle of modification to steel slag are studied, which can provide theoretical instruction for industrial modification. The main work and results are summarized as follows:Firstly, the proper technological parameters of modification to the steel slag with a middle basicity and low alumina content were studied.The EAFS and GGBFS can be used as the adjusting materials so as to improve property of the steel slag. The results show that when the basicity of steel slag ranges from 2.12 to 2.13 by incorporating EAFS,the activity index of modified steel slag at 28d is improved by 10～15% compared with that of unmodified steel slag. When the basicity of steel slag varies from 1.82 to 1.89 by incorporating GGBFS, the activity index of modified steel slag at 28d is improved by 10～18% compared with that of unmodified steel slag. It is suggested that modified steel slag should be cooled in air appropriately. The degree of correlation between chemical composition and activity index of modified steel slag is ranged in a in a descending order as follows: SiO₂, CaO, Fe₂O₃, Al₂O₃.Secondly, the proper technological parameters of modification to the steel slag with a high basicity and low alumina content were studied using orthogonal tests. GGBFS, Coal Cinder and Metakaolin are used as the adjusting materials.The results show that the influence of different parameters on the modification is ranged in a descend order as follows: the adjusting material kinds, the content of adjusting materials and calcining temperature. When the basicity of steel slag ranges from 2.10 to 2.45 by incorporating GGBFS,the activity index of modified steel slag at 28d is improved by 19～22% compared with that of unmodified steel slag.Thirdly, The mineral composition of modified steel slag and morphology images of hardened cement with modified steel slag addition were identified by X-ray diffraction (XRD) and scanning electron microscopy with an energy disperse spectroscopy (SEM-EDS).The hydration activity and process of modified steel slags were characterized by hydration heat evolution, electrical resistivity and the amount of non-evaporable water of steel slag paste. The results show that the content of silicate minerals and hydration activity at early age increase after modification. The cumulative hydration heat for 3d of modified steel slag is increased by 1.5～6 times than that of unmodified steel slags. The electrical resistivity of cement paste incorporating with 30% modified steel slag raises 26%～56% than that of unmodified steel slags, and the hydration degree at 28d is much higher than the control sample.Finally, the modification to the high basicity steel slag with a high alumina content was investigated. A method of modification the steel slag to aluminate cement was developed by incorporating indurstrial pure alumina into it. The results show that when the aluminate coefficient (Am) of steel slag is within the range of 0.7-0.9, an aluminate cementitious material with a high early strength can be obtained by melting method modification. The main mineral compositions of modified steel slags are CA, CA2, MgAl₂O₄ and C2AS, and its compressive strength at the age of 1d, 3d and 28d is 24MPa, 32MPa and 40MPa, respectively, which is 4.7-7.7 times comparing with the control sample. The hydration degree of modified steel slag after 3h is much higher than unmodified steel slag, and the electrical resistivity of modified steel slag paste is 20 times larger than that of control sample.