Research On Red-mud Based Cementitious Paste Goaf Backfill Materials And Its Environmental Performances

Red mud is a solid waste residue generated from the digestion process of the bauxite ores with caustic soda for alumina production.It is a large amount of industrial solid waste with large emissions,serious impact on the environment and difficulty in large-scale resource utilization.In general,the utilization and safe disposal of the bulk amounts of red mud are currently needed.China is the largest producer and consumer of coal in the world,and the massive mining of coal resources can lead to the formation of goaf.The gradual settlement above goaf threatens the safety of surface buildings and has serious impacts on geological environments.In view of the large stock of red mud piles and the comprehensive treatment of coal mined areas,red-mud based cementitious paste goaf backfill materials were prepared by using red mud,fly ash and other industrial solid waste.It can not only consume a lot of solid waste,but also provides a new idea for the comprehensive treatment of goaf and surrounding environments,which has significant social and economic benefits.In this study,first,the optimum mix ratio range of red mud-based cementitious filling material was obtained through mix ratio design,and the influence of the change of raw material content on the working and mechanical properties was evaluated;Then,based on the principle of alkali and sulfate activation,the microscopic mechanisms of red mud-based cementitious materials by modern testing techniques(SEM,XRD,FTIR)were researched,and then the stabilization/solidifying mechanisms of heavy metals were analyzed;Finally,the leaching experiments of red mud-based cementitious filling material under different pH and E_hconditions were designed to investigate the effects of environment on the solidifying effect of pollutants by red mud-based cementitious materials,in addition,to simulate the actual condition of red mud-based cementitious filling material in contact with the goaf water after consolidation grouting in the goaf,the static soaking experiments of red mud-based cementitious filling material in goaf water and deionized water were also designed in this paper,and a preliminary attempt was made to study the treatment effect of materials on acid goaf water.The research results show that:(1)The suitable content of red mud is 55%.When the content of red mud was less than50%,the increase of red mud could help to enhance the degree of alkali activation of the system and promote the formation of more hydration products,which is helpful to improve the strength of the materials,however,the mechanical properties of the material will decrease when the content of red mud is more than 50%;The increase of cement and slag could significantly improve the mechanical properties.It is appropriate for the cement content to be controlled at 7.5%?10%and the slag powder content at 5%due to cost.The addition of desulfurized gypsum could produce AFt(ettringite crystal),which contributed to the improvement of the early strength of the material prominently,but when its content was more than 5%,it could lead to the decline of the mechanical properties of the material,so the appropriate content is 5%.(2)Because of the special hydration mechanism of red mud-based cementitious materials,the main hydration products are C-S-H(calcium silicate hydrate),C-A-H(calcium aluminate hydrate),AFt etc.These amorphous gels have large specific surface area,which can wrap up the particles such as red mud and fly ash in the system,improve the overall adhesion of the material,and enhance the mechanical properties of the material macroscopically;these amorphous gels have large specific surface area and ion exchange capacity,which enable the materials to physically absorb and solidify a variety of total metal ions.AFt could interspersed and filled in the gap of the material,which improves the overall connection and density of the material.The special structure of AFt also enables it to stabilize heavy metal ions in the gaps and channels inside the crystal column by chemical replacement.(3)The leaching experiments of red mud-based cementitious material under different pH and redox environments show that the acidic substances can dissolve a part of the polymer gels,thus,damaging the performance and stability of the materials,while the alkaline environment can enhance the degree of alkali activation of the system and produce an enhanced extent of hydration products,thus,improving the ability of the material to immobilize the pollutants to a certain extent.In the oxidizing environments,the red mud-based samples undergo the acid production process,the stronger is the oxidizability of the environment,the higher is the acidity,which leads to the release of the pollutants from samples,while the reducing environments can control the release of the pollutants to a certain extent.(4)The static soaking tests to simulate practical application progress of red mud-based cementitious filling material show that red mud-based cementitious filling material can effectively solidify its own pollutants and will not cause secondary pollution to the environment,which provides a theoretical basis for the feasibility of red-mud based cementitious paste goaf backfill materials.The experimental study on the removal of heavy metals from goaf water by red mud-based cementitious filling material shows that the powder samples have a significant removal effect on many kinds of heavy metals,and the removal rates of many kinds of heavy metals were more than 96%,which provides a new idea for the comprehensive treatment of goaf environment.