| Iron tailings are solid wastes discharged in the process of smelting iron and steel.With the vigorous development of China’s iron and steel industry,its hoarding has increased year by year,but the comprehensive utilization rate is no more than 30%.The accumulated iron tailings occupy land resources and pollute the natural environment.Applying the abandoned iron tailings to the construction materials industry and reducing the consumption of cement and natural river sand can effectively reduce the energy consumption and carbon emission of the cement industry,alleviate the pressure of the shortage of non-renewable resources.It is of great significance to the resource utilization of solid wastes of iron tailings and the development of civil construction materials.In this paper,low-cost and high-performance iron tailings powder mortar was prepared by taking the waste iron tailings with low activity as the research object.The hydration mechanism of iron tailing powder composite Portland cement cementing material and the microstructure characteristics of iron tailings powder mortar were analyzed,and the mechanism of controlling mechanical properties of iron tailing powder mortar by mortar replacement method was expounded.The main research contents and results of this paper are as follows:(1)taking the fine aggregate replacement method(only replacing river sand)and paste replacement method(replacing water and cement at the same time)as a reference,it highlights the advantages of low cost and high mechanical properties of iron tailings powder mortar prepared by mortar replacement method(replacing water,cement and river sand at the same time).When the water cement ratio is 0.5 and 0.6,the curing age is 28 days and the substitution rate is 20%,the compressive strength of mortar replacement method is 34.57% and 56.29% higher than that of pure cement mortar.Under the mortar replacement method,the economic cost per unit mass of iron tailings powder mortar is saved by 19.98%,and the carbon dioxide emission per unit volume of iron tailings powder mortar is reduced by 16.4%.(2)Based on the compressive strength combined action principle of Martin Cyr,the effects of cement dilution effect,filling effect,crystal nucleus effect and chemical effect on the compressive strength of iron tailings powder composite Portland cement cementitious material are analyzed.The hydration characteristics and hydration reaction degree of iron tailings powder composite Portland cement cementitious material were studied by X-ray diffraction analysis,thermogravimetric analysis and hydration kinetics analysis.The researches show that the improvement of compressive strength of iron tailings powder composite Portland cement cementitious material mainly comes from the filling effect and crystal nucleus effect of iron tailings powder.The nucleation effect of iron tailings powder promotes the hydration of aluminum rich clinker in cement,generates more ettringite and fills the early pore structure.The reaction of calcite on the surface of iron tailings powder with aluminum rich clinker will produce more carboaluminates.Besides,compared with inert quartz powder composite Portland cement cementitious material,low activity iron tailings powder increases the hydration reaction rate in nucleation,crystal growth stage and phase boundary reaction stage,and improves the degree of cement hydration reaction as a whole.(3)The distribution characteristics of calcium and silicon in hydration products at the interface transition zone between iron tailings powder and cement slurry were analyzed by BSE-EDS.The result shows that the interface of iron tailings powder is wrapped by CSH hydration products with high calcium and low silicon.Since the iron tailings powder is negatively charged in alkaline solution,it has the ability to capture calcium ions in pore solution,which promotes the adhesion of calcium ions and the nucleation and growth of high calcium and low silicon CSH,and optimizes the interface transition zone.(4)The pore characteristics of mortar were observed and analyzed by mercury injection method and scanning electron microscope.The elastic modulus and bulk density of hydration products at the interface transition zone between iron tailings powder and slurry were measured based on nano indentation test.The distribution characteristics of river sand aggregate in mortar were observed by metallographic microscope.The research shows that the mortar replacement method optimizes the micro pore structure of iron tailings powder mortar,reduces the average pore diameter,total porosity and pore volume,increases the fractal dimension,and makes the pore structure network denser and more complex.There are more CSH gel with high bulk density and high modulus of elasticity in the interface transition zone between iron tailings powder and cement paste.In addition,iron tailings powder increases the viscous resistance of river sand aggregate in mortar,improves the uneven distribution of river sand aggregate,and alleviates the concentrated distribution of micro cracks at the interface of river sand aggregate.(5)Based on the hypothesis of powers and Brownyard,the theoretical total porosity of mortar is calculated,a semi theoretical and semi empirical compressive strength formula of iron tailings powder mortar is established,and the evolution relationship of hydration degree,initial water volume and substitution rate on total porosity and compressive strength is explored.The results show that under the mortar replacement method,the increase of hydration degree,the decrease of initial water volume and the increase of substitution rate all lead to the decrease of total porosity of mortar and the improvement of compressive strength.Therefore,the optimization of pore structure,the improvement of matrix strength at the interface transition zone and the uniform distribution of river sand aggregate are the main reasons for improving the mechanical properties of iron tailings powder mortar by mortar substitution method.Figures 126 Tables 47 References 134... |
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