Study On The Mechanical Properties And Durability Of Coal Gangue Cementitious Material

Coal gangue is a kind of associated solid waste produced in the process of coal mining and washing.China is rich in coal,but lacks oil and gas.It determines that coal is the dominant position of energy consumption.The characteristics make China become the world’s largest coal-producing country,and thus produces a large amount of coal gangue.The accumulated coal gangue not only occupies a large amount of land,but also leads to geological disasters and land degradation.Under the action of oxidation and rain,it also pollutes the air and water quality,seriously endangering the local ecological security.Therefore,in order to improve the environmental problems caused by improper treatment of coal gangue,the comprehensive treatment and utilization of coal gangue receives more and more attention.According to the existing research results of the research group and relevant research at home and abroad,the composite activation method is used to improve the pozzolanic activity of the coal gangue in the Ordos area because of the characteristics of high silica and alumina.Also,the mechanism of composite activation is clarified.Then,alkali activated coal gangue mortar are prepared by100%of the composite activated coal gangue.After that,the effects of slag content,alkali content and water-binder ratio on the basic properties,drying shrinkage properties and freeze-thaw resistance of alkali-activated coal gangue mortar are systematically studied,and the mechanism of action is clarified.Finally,the effects of different kinds of admixtures on the drying shrinkage of alkali activated coal gangue mortar are analyzed from three aspects:reducing drying shrinkage,limiting drying shrinkage and micro expansion offsetting drying shrinkage,and its action mechanism is clarified.The main conclusions of this paper are as follows:（1）The composite activation method used in this paper can improve the pozzolanic activity of coal gangue.In thermal activation,increasing calcination temperature and calcination time can remarkably improve the activity of coal gangue.The strength and Ca（OH）₂ consumption of coal gangue samples calcined at 700℃for 2h are 1.83 times and 2.23 times higher than those of raw coal gangue.But,the calcination temperature above 700℃and the calcination time over2h have limited effect on its activity.（2）The mechanical properties and impermeability of alkali-activated coal gangue mortar are better than ordinary Portland cement.Its flexural strength and compressive strength are respectively 56.18%and 22.60%higher than those of cement,and its impermeability is 1.15times that of cement.With the increase of slag content,the flexural strength,compressive strength and impermeability of alkali-activated coal gangue mortar gradually increase.With the increase of alkali content,the mechanical properties of mortar show a trend of increasing firstly and then decreasing,reaching the maximum when the alkali content is 12%.Increasing the water-binder ratio reduces the mechanical properties and impermeability of the mortar.（3）The freeze-thaw resistance of alkali-activated coal gangue mortar is better than that of ordinary Portland cement.After 100 freeze-thaw cycles,the mass loss rate,flexural strength loss rate and compressive strength loss rate of alkali-activated coal gangue mortar are respectively 23.73%,59.31%and 26.36%lower than those of cement.Increasing the slag content can improve the freeze-thaw resistance,while increasing the alkali content and the water-binder ratio reduce the freeze-thaw resistance.In addition,fractal dimension can quantitatively characterize the damage degree of alkali-activated coal gangue mortar under freeze-thaw cycles.（4）With the increase of slag content,alkali content and water-binder ratio,the mass loss rate and drying shrinkage of alkali-activated coal gangue mortar increase gradually.Both shrinkage reducing admixture and expansion agent can remarkably reduce the drying shrinkage.When adding 4%shrinkage reducing admixture and expansion agent,the drying shrinkage of mortar at 180d decrease by 42.25%and 16.09%.The effect of modified basalt fiber on the drying shrinkage of mortar is not obvious.When 3%modified basalt fiber is added,the dry shrinkage of mortar at 180d is reduced by 5.62%.