Study On The Cementitious Properties Of Red Mud-coal Gangue-cement Composite

Red mud and coal gangue are industrial solid wastes produced by aluminum smelting industry and coal industry respectively.A large amount of red mud and coal gangue not only wastes land resources,but also pollutes the ecological environment and endangers human health.Concrete is one of the most widely used building materials in the world with its characteristics of good plasticity,high strength,and economical applicability.Red mud or coal gangue substitute cement to produce concrete cementing material,which can save cement production cost and reduce the stock of solid waste.This thesis taken Bayer red mud and coal gangue from Guizhou as the research object and investigated the influence of red mud and coal gangue powder content on the performance of concrete,the preparation of cementitious materials by activated coal gangue powder,and the influence of red mud-activated coal gangue powder-cement composite material on the performance of concrete.The main research results are as follows:（1）Bayer red mud and coal gangue contain a large amount of aluminosilicate minerals with high content of Si O₂ and Al₂O₃,which had potential pozzolanic activity.Red mud added less than 5%was beneficial to the strength of concrete.Red mud particles could improve the pore structure of concrete matrix and interfacial transition zone（ITZ）,enhance the compactness of matrix and reduce the width of ITZ.When the content of red mud exceeded 10%,the damage of cement hydration products to concrete performance was stronger than that of red mud pore filling to concrete performance,resulted in the decrease of concrete compressive and flexural strength.The water-binder ratio of concrete matrix and ITZ mixed with coal gangue was unevenly distributed,and the hydration of cement migrated to the vicinity of coal gangue particles.The cement hydration product calcium hydroxide grew in large quantities and preferentially oriented here,and the crystal particles were coarse and disorderly arranged,weakened the cementation performance at the matrix and ITZ.In the later stage of curing,the water molecules remaining near the coal gangue particles evaporated and left a large number of holes,which damaged the compactness of concrete structure,increased the area of weak links and reduced the hardness of ITZ,which was not conducive to the mechanical properties of concrete,resulted in the decrease of compressive and flexural strength of concrete with the increase of coal gangue content.（2）The good activation temperature of coal gangue was 800°C.At this time,kaolinite in coal gangue was transformed into metakaolinite,which could release active Si O₂ and active Al₂O₃.The dissolution of Al and Si elements increased,and the viscosity and water absorption of paste decreased.Compared with coal gangue raw materials,the effect of 800°C activated coal gangue as mineral admixture on the encapsulation and adsorption of water molecules was reduced,the water-binder ratio of matrix and ITZ was increased,and the crystal particles of cement hydration product calcium hydroxide were small.In addition,the active Si O₂ and active Al₂O₃ in activated coal gangue could react with calcium hydroxide to form C-S-H gel and C-A-H gel,reduced the average pore size of the matrix,improved the density of the matrix and ITZ.The flexural strength and compressive strength of concrete curing for 28 days and prepared by 800°C activated coal gangue were 7.47%and 20.17%higher than those of unactivated coal gangue concrete,respectively.When the activation temperature exceeded 900°C,the metakaolinite in coal gangue began to recrystallize into mullite,resulted in a decrease in the cementitious activity of coal gangue and the strength of the prepared concrete.（3）Red mud and activated coal gangue did not have a synergistic effect on the viscosity of slurry and the cementitious activity of composite materials but have a synergistic effect on the strength of concrete matrix and ITZ.The preparation of cementitious materials by red mud and activated coal gangue in a certain proportion could overcome the shortcomings of low pozzolanic activity and high water-binder ratio near coal gangue particles caused by red mud and activated coal gangue respectively to a certain extent.When the ratio of red mud:activated coal gangue:cement was 1:1:8,the good filling effect of red mud and the good pozzolanic activity of activated coal gangue had a synergistic effect.The hydration of cement was sufficient,the harmful pores of the matrix were reduced,the strength and compactness of the matrix were greatly improved,and the relative area of ITZ was reduced without obvious through cracks.The compressive strength of concrete cured for 28 days and prepared under this condition was 16.73%higher than that of concrete prepared by 20%red mud,10.52%higher than that of concrete prepared by 20%activated coal gangue,and also higher than the compressive strength of each test concrete with 90%cement content.