| In view of the urgent problems of large stock of solid waste and low resource utilization rate in the steel industry,the preparation and hydration mechanism of steel slag-iron ore tailings-slag-desulfurized gypsum based composite cementitious materials were studied with steel slag,iron ore tailings,tailings waste stone and other steel solid wastes as the main research objects.On this basis,the green low-carbon concrete with all solid wastes suitable for civil engineering was developed,which overcame the problem of low early strength of existing solid waste concrete.The following works were carried out:First,the mechanical chemical coupling activation method was used to activate iron ore tailings and steel slag,and the grinding characteristics and activity of both were studied.Secondly,on the basis of coupling activation of raw materials,the preparation and properties of steel slag-iron ore tailings-slag-desulfurized gypsum based composite cementitious materials were studied by orthogonal test and step grinding method,and the hydration and hardening mechanism of the composite cementitious materials was revealed by hydration heat analysis,XRD（X-ray diffraction）,SEM（scanning electron microscope）and other test methods.Finally,steel slag-iron ore tailings-slag-desulfurized gypsum based composite cementitious material,iron ore tailings fine aggregate,iron ore tailings coarse aggregate were used to prepare all solid waste green low-carbon concrete,and the comprehensive properties of all solid waste green low-carbon concrete was studied.The following conclusions are drawn:（1）Through the study on the grinding characteristics and activation of raw materials,it is concluded that the fineness of iron ore tailings with particle size less than 0.15 mm after screening can reach 525 m2/kg after 60 minutes mechanical grinding,and the 28-day activity index can reach 86.4%.The fineness of crushed steel slag with particle size less than 7 mm can reach 455 m2/kg after 45 minutes mechanical grinding,and the 28-day activity index can reach 77.3%,both of which can be used for active admixture.The fineness modulus of iron ore tailings is 2.55,belonging to the range of medium sand,and63.44%of its particle size is distributed in the range of 0.30～1.18 mm.The grading of iron ore tailings waste rock belongs to continuous grading,and the particle size of 63.54%waste rock is in the range of 16～26.5 mm.（2）Through single factor and orthogonal experiment,the optimal ratio of steel slag-iron ore tailings-slag-desulfurized gypsum composite cementitious material is obtained as follows:the mass ratio of steel slag to iron ore tailings is 2:3,the total content of steel slag and iron ore tailings is 36%,the content of slag is 51.5%,the content of desulfurized gypsum is 12.5%,the content of Na2SO4 and Na2Si O3·9H2O respectively accounts for1.2%of the total cementitious material,the cement sand ratio is 1:3,and the water cement ratio is 0.58;The steel slag,iron ore tailings,slag and desulfurized gypsum are grinded in three stages by step grinding.When the combined time is 22+30+40min,the 28-day compressive strength of the composite cementitious material obtained is 7.8%higher than that of the cementitious material obtained by single grinding and mixing,and the 28-day flexural strength and compressive strength of the mortar specimen are 7.8 MPa and 38.5MPa,respectively.The standard consistency water consumption of composite cementitious material is 34.7%,the initial setting time is 168 minutes,the final setting time is 345 minutes,and the stability is qualified.（3）The hydration mechanism of steel slag-iron ore tailing-slag-desulfurized gypsum composite cementitious material was studied by XRD,SEM and hydration heat analysis.It was found that the hydration degree of the cementitious material prepared by step grinding optimization was greater than that of the cementitious material prepared by single grinding mixing.With the increase of curing age,the hydration products of the steel slag-iron ore-tailings slag-desulfurized gypsum composite cementitious material system are mainly C-S-H gel,ettringite（AFt）and a small amount of cordierite.The contents of tricalcium silicate（C3S）and dicalcium silicate（C2S）are decreasing,and the minerals that do not participate in the reaction are left in the system.The hydration products interweave with inert minerals that do not participate in the reaction to form dense aggregates,which improves the strength of the gelling system.（4）The total solid waste green low carbon concrete prepared is 520 kg/m3 for composite cementitious material,726 kg/m3 for iron ore tailings,1044 kg/m3 for iron ore tailings waste rock,0.3 for water cement ratio,41%for sand ratio,6.24 kg/m3 for Na2SO4,6.24 kg/m3 for Na2Si O3·9H2O,1.5%for polycarboxylate based superplasticizer（accounting for the amount of composite cementitious material）,and 95 mm for the slump of the total solid waste green low carbon concrete The 7-day and 28-day compressive strengths are 32 MPa,43.6 MPa and 51.2 MPa respectively. |
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