IntroductionThe use of steel slag as a cementitious material is the most likely field to achieve large-scale engineering utilization. However, the inherent defects of low activity and poor grindability caused by the high iron oxide content of steel slag limit its dosage in cement. One important direction to improve the activity and grindability of steel slag is to obtain a product similar to granulated blast furnace slag (GBFS), mainly composed of glass, by melting and reducing iron oxides in steel slag. This GBFS-like residue exhibits lower early activity due to its lower alkalinity coefficient (K=n(CaO)+ n(MgO)/n(SiO₂)+ n(Al₂O₃)). In order to improve the early activity of the residue, this paper focuses on the effect of CaO content on the formation of clinker minerals. Coal gangue is used as a reducing agent to reduce iron oxides in steel slag for making full use of industrial solid waste.MethodsBased on the principle of complete reduction of iron oxides in steel slag and coal gangue, the appropriate ratio of coal gangue and steel slag is calculated according to the chemical composition of steel slag and the fixed carbon content of coal gangue. CaO was obtained by calcining and analytical reagent CaCO₃ at 1500 ℃. Different amounts of resultant CaO (10, 20, 30, 40 g) were incorporated into 100 g mixture of steel slag and coal gangue, respectively. The reference sample without CaO and the above four samples are named as C0, C10, C20, C30 and C40, and the K of samples are 1.05, 1.33, 1.61, 1.89, 2.17, respectively. After thoroughly mixing each sample, 200 g mixture containing CaO was placed into a corundum crucible and calcined at 1500 ℃ for 30 min. After calcination, the crucible was take out from the high temperature furnace for water quenching. Iron alloy particles are obtained by crushing, peeling, and magnetic separation from water quenched slag. The remaining water quenched residue (WQR) is used for mineral phase analysis, cement mortar strength, soundness, and other tests.Results and discussionIron alloy particles in C0, C10, C20, C30 sample can be easily peeled off. But it is difficult for C40 sample and some small metal particles can be observed at the crucible bottom. This is because the increase of K leads to an increase in the viscosity of the melt, making it difficult for iron particles to sedimentation and aggregation.The analysis of the chemical composition of the WQR after stripping iron particles shows that the reduction rate δ and recovery rate η of the reference sample C0 are about 92%. For C10 (K=1.33), the η and δ of Fe reach maximum values. As the CaO content continues to increase, the η and δ gradually decrease. But, these two parameters of C20 still exceed that of C0. The f-CaO content and soundness of the WQR meet the requirements of relevant national standards, although these two values will slightly increase with the increase of dosage of CaO.The results of XRD Rietveld refinement for quantification indicate that C0 and C10 samples are mainly consist of glass and contain a small amounts of Gehlenite, spinel et al. With the increase of CaO content, the diffraction peak of the crystalline phase gradually increases, while the amorphous envelope peak gradually weakens. For C20, the main crystalline phases are C₂S and bredigite, and diffraction peaks of periclase can be observed. In the C30 sample, the main mineral phases are C₂S, C₃A, iron, and periclase. Meanwhile, C₃S diffraction peak is relatively low. The C₃S diffraction peak in the C40 sample is significantly enhanced, with a content of 50%. FTIR shows that as the CaO content increases from 20 g to 40 g, the absorption band of β-C₂S decreases, while the absorption band of M3 C₃S gradually increases.BSE image show that the C10 sample is mainly composed of glass matrix, with a small amount of C₂S grain, serrated-like fine stuff formed at the edge of C₂S grains, snowflake like or dendritic substance surrounding the C₂S, and iron particles. EDS results show that the order of Al content in several phases is C₂S < fine stuff < dendritic substance < matrix. The order of Ca and Si content is C₂S >fine stuff > dendritic substance > matrix.In the C30 sample, sharp edged plate-like particles with a Ca/Si of about 3 formed, indicating that C₂S has begun to transform into C₃S. The formation of C₃S and C₂S not only significantly reduces the Si content in the matrix, but also leads to an increase in the relative content of Al in the intermediate matrix. When the dosage reaches 40%, C₃S has already formed in large quantities. The Ca/Al ratio of the intermediate phase is about 1.53, indicating the formation of C₃A.ConclusionsThe main conclusions of this paper are summarized as following. When K value of WQR is less than 1.6, increasing the CaO content appropriately can improve the reduction rate and recovery rate of iron. The C10 sample is mainly composed of glass. When K is 1.3, a small amount of β-C₂S is formed in the WQR, which gradually grows by absorbing CaO and SiO₂ from the glass matrix. With the increase of K, Periclase begins to form in the zone where silicate minerals are more abundant. When the CaO dosage reaches 40 g, the amount of C₃S continues to increase to 50%, and C₃A form simultaneously. Although high alkalinity leads to the formation of C₃S and C₂S minerals in the system, it also results in the formation of periclase. To avoid the potential long-term unsoundness, it is more appropriate to control K value at around 1.3.