| Owing to the less content of C3S and high content of RO phase in the mineral composition, the weak cementitious activity and the unstable soundness, the steel slag is in difficult utilization with large number in the building materials industry. Our group put forward the reconstruction method of steel slag, that is, conditioning components are added to make chemical reaction with the molten slag under the high temperature entironment when steel slag is dumped from the converter, so the composition and structure of steel slag would be changed, and the steel slag would become a kind of high cementitious activity admixture.Based on the basic theory, we think that there are two measures to enhance the cementitious activity of the steel slag. First, RO phase should be changed into C4AF. Another, the iron should be separated from the slag by reduction. Two methods are also needed to improve the content of C3S. The two different reconstruction methods were studied in this paper in order to find a universal conditioning component and reconstruction method. XRD,SEM-BEI and petrographical analysis were used to study the mineral composition of reconstruction slag, activity index, SEM and TG-DTA were used to measure and analyse the cementitious activity of reconstruction slag, the chemical reaction occurred and its rule of the reconstruction process were analyzed, and the relationship between composition and performance of reconstruction slag was discussed.The industrial waste and natural minerals were chosen as the conditioning components. With reference to the chemical composition of cement clinker, the conditioning component materials and its content were directly determined by theoretical calculation, reconstruction test was developed by simulating clinker calcinations. The results showed that RO phase was decomposed, which tends to be more complete when CaO content increase. And C2F and C4AF, Mg produced mainly existed in free MgO crystal; Lime, as a kind of conditioning component, showed a great impact on reconstruction; Reconstruction could produce a lot of C3S minerals and improve cementitious activity of steel slag, the activity index of it could achieve 95%.Lime was shosen as the single conditioning component and the reaction mechanism of reconstruction was studied. The results showed that: (1) The first reaction in reconstruction process was CaO + SiO2→C2S, then, RO phase was decomposed under the action of the CaO, the reaction was CaO + RO→CF + MgO, C2S+CaO→C3S could react only if isolated Fe2O3 was fully saturated by CaO and then produced C2F, C2(A,F),C4AF. (2) The mainly minerals of reconstruction slag were CF, C2F, C2(A,F), C4AF, C2S, C3S, MgO. With the increase of lime content, Fe2O3 respectively existed in Fe3O4 and CF, C2F and C2(A,F), C2F and C4AF. MgO mainly existed in free periclase and minority existed in silicate minerals and liquid phase as solid solution and liquid phase. Al2O3 produced C2(A,F) or C4AF. Minerals grew and developed well when alkalinity was high, the shape of C3S was six-plate and long cylindrical, and the shape of C2S was round granular, when the content of liquid pahse is high, it mainly contained calcium ferrite but when RO phase did not decompose completely in the low alkalinity, and liquid content reduced, the shape of C2S was irregular. (3) Lime reconstruction reaction was "the follow-up reaction" of slagging process, and reconstruction process was a "second slagging" reaction in a relatively stable environment.Then, according to the reconstruction reaction and mineral composition of reconstruction slag, slag-lime saturation factor KHs= [CaO-1.1Al2O3-0.7Fe2O3] /[2.8SiO2] was proposed to characterize the activity of slag, BG, LG, WG steel slags were reconstructed by lime at KHs 0.9 , the mineral composition of reconstruction slag calculation which used slag-lime saturation was put forward, the relationship of composition- structure - performance of reconstruction slag was discussed. The results showed that: (1) The mainly liquid of three kinds of reconstruction slag were C2F and C2(A,F), MgO mainly existed in free periclase, silicate minerals were C2S, C3S. WG reconstruction slag had highest cementitious activity because it contented more C3S, its activity index was 85%. (2) It was not enough to use KHs as the single parameter to consider the amount of lime in reconstruction and effect of reconstruction. The control parameters of lime reconstruction slag should be further elaborated, e. g. to use KHs and SM simultaneously.The iron reduced reconstruction slag was also studied in this paper. The experimental furnace installed nitrogen protection was transformed, direct reduction method and smelting reduction method reconstructed slag were used. The results showed that: (1) RO phase and Fe2O3 in liquid could be reduced to iron by direct reduction method, iron and slag could not be separated because the slag did not melt, the iron reduced well-distributed between silicate minerals and liquid in steel slag as iron beads which size were 20μm-200μm, cementitious activity of reduced slag was increased, but its grindability got worse. (2) the iron reduction rate closed to 100% by smelting reduction method, iron and slag could be separated easily. The molten slag were taken to cool with the furnace, cool by copper tile, cool by water quenching. Composition of the slags cooled with the furnace and cooled by copper tile were mainly crystalline , it contend C2S, Ca2Al2SiO7, Ca3MgSi2O8 and Ca2Mg2Si2O7, which did not have cementitious activity. The slag processed by water quenching contained a lot of vitreous body, and because its composition and structure were similar to blast furnace slag, its had high cementitious activity, whose activity index could achieve more than 85%. Smelting reduction method could completely utilize steel slag and create high economic value, and it was a new technology of steel slag utilization and had a great sense of popularization.
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