Activation Of Granular Coal Gangue And Its Application In Cement-based Materials

Coal gangue?CG?is a solid waste produced in the process of coal mining and coal preparation,accounting for 10-15%of the total solid wastes,and it has become the largest industrial solid waste in China.According to statistics,the accumulation of CG in China has exceeded 5 billion tons,and the annual growth is 150-200 million tons,while the annual utilization rate of CG is only about 60%,and most of them were used in backfill and subgrade cushion,lower added value of utilization.In this paper,calcined coal gangue?CCG?powder material used in cement was prepared by granular CG in Huaibei.In view of the requirements of cement-based materials for pozzolanic admixtures,the changes of coal content in CG before and after calcination were analyzed in detail.The intrinsic reason for the activity of CCG was revealed,and the activity of CCG was evaluated by rapid alkali-activated evaluation method.The color transformation mechanism of CCG was explored,and the method of quantifying and evaluating the color of CCG powder was provided,which lays a foundation for its better application in cement-based materials.The fluidity,mechanical properties,water adsorption and chloride ion penetration resistance of mortar mixed with CCG powder were systematically studied.The composite cement of CCG powder-fly ash was optimal designed using response surface methodology.CG with different mineral compositions was separated through the difference of color after CG was calcined,and its physicochemical properties was characterized in detail.Then the effect of sorted coal gangue on hydration mechanism and macroscopic properties of cement-based materials was investigated.The main conclusions drawn from the above research are as follows:?1?With the calcination temperature of CG increased,the removal of coal components and hydroxyl of kaolinite in CG was more sufficient.The K of specimens mixed with CG calcined at 650?,750?was higher,and its activity was the best.Similar conclusions were obtained by various activity evaluation methods.After CG was calcined,the structure of[SiO₄]and the coordination number of Al will be changed.When CG was calcined at750?,The relative contents of[SiO₄]of Q³?-100.26?structure and Al[5]in CG can reach66.19%and 43.62%respectively.The increase of the relative content of both made the activity of CCG increased greatly,and the relative content of both has a good correlation with the activity of CG.In the rapid alkali-activated evaluation method,the strength of the specimens develops rapidly and the source of strength was single,which avoided the shortcomings of other methods.Its operation was simple and the price was low.It can not only get the results quickly,but also quantify the activity of the material.It can be used as a rapid method to evaluate the activity of CCG.?2?The residual coal,decomposition and oxidation of siderite in CCG were the main reasons for the variability of the color of CCG.When oxygen was sufficient,the coal component was removed and siderite was oxidized to hematite,which made the color of CCG powder redder.The color of CCG powder can be effectively improved by controlling the holding time,calcination atmosphere and the particle accumulation mode,but it should be noted that the loss on ignition of CCG can not be large.Changing the particle accumulation mode can control the amount of oxygen intruding into the accumulation body and reduce the R value of calcined gangue powder?R is the color representing the red channel?to 90.Based on the color evaluation model of CCG powder,the color coordinate of coal gangue powder calcined by changing the particle accumulation mode was closest to the reference cementitious material,and the effect of color control was the best.?3?The coal and organic matter contained in CG will seriously affect the working performance of cement-based materials.Considering the activity of CCG,the energy consumption and the influence of CCG on the working performance of cement-based materials,the optimal calcination temperature should be 650-750?,and the holding time should be 6-8h.The initial absorption coefficient,secondary absorption coefficient and electric flux of mortar mixed with 30%coal gangue calcined at 750?were 44.6%,75.6%and 84%lower than control group,respectively.It shown that the pore structure of cement-based materials can be effectively improved by adding CCG powder.The compressive strength of mortar mixed with 30%admixture that CCG powder with optimal activity compounded with fly ash can reach the composite Portland cement standard of42.5 strength grade.?4?CG with different mineral compositions can be separated through the difference of color after CG was calcined.The content of quartz in sandy coal gangue was higher than that of in yellow-white and red coal gangue,and the content of metakaolin in yellow-white and red coal gangue was higher than that of in sandy coal gangue.The rapid alkali-activated evaluation method showed that the activity of sandy coal gangue was lower than that of yellow-white and red coal gangue.Yellow-white and red coal gangue can react with CH produced by cement hydration after it was mixed with cement,thus increasing the chemically combined water content of cement paste and improving the pore structure of cement paste.Both of them have little effect on the fluidity of cement mortar,and the activity index of them can reach more than 85%at 28 days.