Study On Activation Characteristics And Environmental Safety Of Red Mud Multi-cement System

Red mud is a major by-product of alumina production from monohydrallite,and its properties include high alkalinity,potential toxin factors and complex composition make it difficult to improve its resource utilization ratio.Presently,the storage method of red mud in China is outdoor mostly,which take up a large area of land resources.Under natural conditions,red mud dust particles entrained in the air will affect the respiratory system of residents,and there is also the risk of leaching into the soil and underground with rain and snow to produce water pollution.Therefore,it is necessary to solve the problem of how to treat red mud efficiently and safely to increase the utilization ratio of it.To address the above problems,this research applies different red mud activation methods,including multifarious thermal activation of red mud and co-activation of red mud with other solid waste materials to investigate the link between its macroscopic properties,microscopic mechanisms,and environmental safety,which to discuss the freeze-thaw resistance of red mud-based cementitious materials.The main research contents and conclusions:(1)Thermal activation of red mudRed mud was subjected to thermal activation,thermoalkali activation,and thermocalcium activation,and the effects of different thermal activation methods on the mechanical properties,hydration mechanisms,and environmental risks of red mud-based cementitious materials were discussed and analyzed.The optimum temperature for the thermal activation of red mud varies in different ways.The optimum temperatures for thermal activation of red mud and thermal calcium activation were 900℃,and the flexural strength of cementitious samples under that condition at 14 d were 4.46 MPa and 4.35 MPa.The optimum temperature for thermal alkali activation of red mud was 1000℃,and the flexural strength of red mud 1000℃ thermal alkali activation cementitious samples at 28 d could reach 3.26 MPa,which met the single block flexural strength(>3 MPa)of the Class I pavement block of the Industrial Standard for Construction Materials of the People’s Republic of China-Concrete Pavement Bricks(JC/T446-2000).The red mud thermally activated cementitious samples at different temperatures showed different solidified effects on different heavy metal elements and alkali substances,and leaching concentration of heavy metal elements in the red mud thermally activated cementitious samples at 900℃ and the red mud 600～800℃ thermal alkali activation cementitious samples were lower than the limit values of Class III and Class IV water of surface water environmental quality.(2)Co-activation of red mudFive groups of red mud-based cementitious samples were prepared by adding steel slag,silicate cement,blast furnace slag cement,flue gas desulfurization gypsum,and fly ash with red mud as the main raw materials,and the effects of different additives on the mechanical properties,hydration mechanism and environmental risks of red mud-based cementitious materials were analyzed and discussed.The cementitious samples prepared by different additives with red mud formed similar hydration products,mainly hydrated calcium silicate(C-S-H),tobermorite and Ca(OH)2,along with the presence of a solid solution of tobermorite-Ca(OH)2.The flexural strength of the cementitious samples conformed to the single flexural strength value(≥ 3.0 MPa)of the Class I pavement block of the Industrial Standard for Construction Materials of the People’s Republic of China-Concrete Pavement Bricks(JC/T446-2000).The combination of alkali substances and heavy metal elements with the cementitious hydration products was by physical adsorption,chemical substitution and directional replacement,and the leachate concentrations of heavy metal elements were in accordance with the Class III water limit standard of surface water environmental quality.In Addition,the specific activity of radioactivity of cementitious samples met the radioactivity limit standard of building materials and decorative materials.(3)Freeze-thaw resistance of red mud-based cementitious materialsRed mud and steel slag were used as the main raw materials to prepare red mud-based cementitious samples as the comparison group,and the 900℃ red mud thermal activated cementitious materials and multiple groups red mud with additive co-hydrated cementitious materials were selected for 25 freeze-thaw cycles according to the slow freezing method in the Test Methods for Long-term Performance and Durability of Ordinary Concrete.The principles of freeze-thaw damage of red mud-based cementitious materials were discussed by analyzing load-displacement curves,mass and strength loss ratio,phase changes,microstructure,planar porosity,and CT scan images.The results showed that Ca(OH)2 crystals appeared in the thermal activated red mud-based cementitious materials at 900℃,while red mud-cement and red mud-slag cement-based cementitious materials produced more calcareous alumina crystals,and the columnar structure of calcareous alumina crystals had better freeze-thaw resistance than Ca(OH)2 crystals,so the freeze-thaw resistance of red mud-cement-based cementitious materials was better than that of cementitious materials of other systems.