Study On Damage And Deterioration Mechanism Of Fly Ash Filling Material Under Chloride Salt Erosion

Because of the complex composition of underground mine water,the filling body in goaf will suffer from various ions erosion in mine water,especially chloride mine water widely distributed in various mining areas.The strong permeability of chloride ion erosion makes the filling body structure deteriorate gradually,and then leads to a sharp decline in the bearing capacity and durability of the filling body structure.In this paper,the mechanical properties,damage characteristics,permeability and micro-changes of filling under chloride erosion were studied,and the damage and deterioration mechanism was explored.Based on the Natural Science Foundation of Shandong Province,this paper uses theoretical analysis and laboratory experiments to study the mechanical properties,damage characteristics,permeability and microscopic of the filling body under chloride salt erosion in the chlorine salt environment of the filling body in the goaf.The change study studies the mechanism of damage degradation.The main conclusions are as follows:(1)In this paper,a kind of fly ash filling material for chloride-resistant fly ash-cement-slag-quicklime cementitious system is studied.The orthogonal experimental design is used to analyze the influence rules of cement to binder percentage A,binder to filling material percentage B,slurry mass concentration C and lime content D.It is found that factor C is the key factor affecting slurry transportation performance.Factor A and B are the main factors affecting the setting time and strength of filling materials.The final ratio is cement:slag:fly ash=6:4:90,lime content is 2%,mass concentration is 62%.(2)Powdery spalling occurs on the surface of the filling body after erosion.The compressive strength and modulus of elasticity increase to a certain extent in the early stage of erosion,then decrease significantly.After erosion,they decrease by 32.24%and 38.19%respectively.The stress-axial strain curves under different erosion ages were analyzed.The curves showed that the compression stage lasted longer,the elastic modulus decreased in the linear elastic stage,and the filling body softened and the supporting capacity of the filling body weakened when the crack propagation stage curve tended to be gentle.Based on the assumption of strain equivalence,the damage constitutive equation of filling body is constructed,and the damage variable-strain relationship curve is drawn to reflect the five damage development stages of filling body.With the increase of erosion age,the duration of initial non-damage stage decreases and the initial damage of filling body increases;the slope of damage cumulative stage curve increases,and the damage development accelerates;the damage rising trend weakens and the plasticity increases;the critical damage variable increases,and the peak strength decreases.(3)The permeability test results show that the penetration depth of chloride ion varies linearly with the increase of erosion age,and the apparent diffusion coefficient of chloride ion decreases rapidly at first and then tends to stabilize with the increase of erosion age.It is found that the apparent diffusion coefficient of chloride ion of fly ash filling material is tens of times higher than that of ordinary concrete material,and its permeability is very high.The chloride ion diffuses steadily to the deep part of the material in the chloride erosion environment,which makes the bearing capacity of filling body decrease,and is not conducive to maintaining good durability of filling body in the chloride mine water environment for a long time.(4)Combining with the macro and micro test results,the damage degradation of filling body is divided into three stages:the first stage is beneficial erosion stage,the strength performance of the material is improved,and the permeability is reduced.Because of the chloride intrusion,the increase of hydration products and the formation of Friedel’s salt "ion diffusion shield layer",which inhibits the erosion of chloride ions,chloride intrusion is considered as a "beneficial factor".In the second stage,the strength and elastic modulus of filling body begin to decrease,the plasticity increases,and the structural damage of filling body material is caused by physical crystallization and chemical reaction.Expansion products are called "expansive factors".They form local stress in the material,which results in small cracks in the material.In the third stage,the strength and elastic modulus decrease sharply in the deterioration stage.Under the combined action of chloride salt dissolution erosion,solubility erosion and expansive erosion,the pore and crack continue to expand and connect with each other.Large cracks result in loose internal structure and continuous expansion of deteriorated area,which reduces the bearing capacity and durability of filling body.