Introduction Using as supplementary cementitious materials (SCM) is one of the most important research topics in the resource utilization field of steel slag. However, the iron oxide content in steel slag is greater than that of other cementitious materials (i.e., clinker, granulated blast furnace slag, and fly ash). The solid solution formed by FeO and CaO-MnO-MgO (RO phase), hematite, wustite, Ca2Fe2O5(C2F) and other Fe-bearing phases have little contribution to the activity of ground steel slag. Some researchers use reductants such as H2, CO, anthracite, silicon carbide, and carbon powder to reduce iron oxides from molten steel slag into ferroalloys. Aluminosilicate melt is quenched and resultant residual used as SCM. These methods have a challenge of high cost of reducing agents and difficulty in industrial utilization. This paper used residual carbon in coal gangue as a reductant to reduce iron oxides of steel slag. The remaining aluminosilicate was quenched to prepare slag with a high hydration activity, which could achieve the synergistic disposal of two types of industrial solid waste.Methods Based on the chemical composition of steel slag and coal gangue, as well as the fixed carbon content of coal gangue, the amount of coal gangue required for the sufficient reduction of iron oxide in the two raw materials was determined. After mixing ground coal gangue and ground steel slag thoroughly, the mixture was heated to 1 200, 1 300, 1 350, 1 400, 1 450 ℃ and 1 500 ℃ at 40 ℃/min in an argon atmosphere (3 L/min), and then quenched with water after holding for 30 min. The resultant strip off iron particles, and the remaining aluminum silicate is called water quenched residue (WQR). The iron recovery rate was determined according to the chemical composition of the WQR. The mineral phases were determined by an X-ray diffractometer (XRD, Rigaku Co., Japan), and the glass content in water quenched residue was calculated using the Rietveld quantitative analysis method.The paste and mortar samples were prepared by adding 30% (in mass fraction) WQR into the reference cement. The cross-sectional morphology was determined by a model GeminiSEM 500 scanning electron microscope (Zeiss Co., Germany). The elements distribution of the paste samples was analyzed. The heat release curve of hydration for the first 72 h was recorded with a model TAM-Air-8microcalorimeter (TA Instruments Co., USA). The thermal behavior of the pure slurry after hydration for 3 d, 7 d and 28 d were analyzed by a model TGA/DSC1/1600 thermogravimeter (TG, Mettler Toledo Co., USA). The mortar specimens with a dimension of 40 mm×40 mm×160 mm were used for testing flexural and compressive strength after standard curing to the specific age.Results and discussion The reduction temperature is an important factor affecting the iron recovery rate in steel slag and the vitrification of WQR. When the reduction temperature exceeds 1 300 ℃, the reduction rate significantly increases. When the temperature exceeds 1 350 ℃, the Fe2O3 content decreases from 20% in the steel slag raw material to less than 2% in the WQR. The main mineral phases in the reduced residue at 1 100 ℃ are merwinite and gehlenite. When the temperature rises to 1 350 ℃, the diffraction peaks of gehlenite disappear. At 1 450 ℃, the diffraction peaks of merwinite almost disappear. When the reduction temperature is less than 1 300 ℃, the glass content in WQR is less than 60%. When the temperature reaches 1 350 ℃, the glass increases rapidly, approaching 80%. The glass exceeds 90% with further increasing the reduction temperature to 1 450 ℃. The fusion temperature is an important factor for the degree of vitrification of WQR.The hydration heat release curve of cement paste shows that the second exothermic peak of sample prepared with WQR generated at 1 500 ℃ is similar to the peak of the reference cement, but the exothermic peak attributed to the formation of the second ettringite decreases. The second exothermic peak of cement with WQR at 1 400 ℃ is 1 mw/g lower than that of WQR cement at 1 500 ℃. A lower heat release rate is due to the presence of crystalline phases in the WQR at 1 400 ℃. After 72-h hydration, a higher cumulative heat of the paste prepared with WQR at a higher melting temperature occurs. The cumulative heat of WQR cement at 1 400 ℃ and 1 500 ℃ is 71.65% and 85.64% of the reference cement, respectively. This is due to the high reduction temperature leading to a high degree of vitrification, thereby improving the hydration activity of the WQR.The XRD and TG results indicate that the Ca(OH)2 content in the paste gradually decreases with the increase of steel slag reduction temperature and the hydration time of WQR cement. This is because the glass content and pozzolanic activity of WQR increase with the increase of reduction temperature. Consequently, the Ca(OH)2 content of WQR cement decreases.The strength results of the mortar show that the 3-d flexural strength of cement with 30% WQR is almost equivalent to that of the reference cement. The growth rate of flexural strength of WQR cement is higher than that of reference cement with curing time, and its strength reaches 130% of reference cement strength at 28 d. The compressive strength of WQR cement at 3 d is 60% of the reference cement. The compressive strength development rate of WQR cement is higher than that of reference cement. After 28-d hydration, the compressive strength of cement prepared with WQR at 1 400 ℃ reaches 90% of the reference cement strength.Conclusions The recovery rate of Fe could reach 63%~94% by using carbon in coal gangue as a reducing agent to reduce iron oxide of steel slag. The reduction rate increased as the reduction temperature increased. When the reduction temperature was below 1 300 ℃, the mineral phases of the WQR mainly consisted of merwinite and gehlenite. The crystalline phase gradually decreased as the temperature increased. When the reduction temperature was >1 450 ℃, the glass content maintained at 91%. The glass phase content and the pozzolanic activity in WQR increased as the reduction temperature increased. The cement prepared with WQR had good mechanical properties. After 28-d hydration, the flexural strength and compressive strength of WQR cement reached 130% and 90% of the reference cement, respectively.