Study On The Mechanical Properties Of The Composite Cementitious System Of Coal Fly Ash,red Mud And Gasification Slag

The massive stockpiling and emission of typical industrial solid wastes such as fly ash,red mud and gasification slag cause serious harm to the environment,and the preparation of composite cementitious materials using fly ash,red mud and gasification slag is an important way to promote the large-scale consumption of industrial solid wastes.There are many studies on the preparation of gelation blocks from fly ash and red mud,and there are reports on the intermixing of the two,but there is no systematic comparison of the differences in the cementing performance of different types of fly ash（fluidized bed fly ash and coal pulverized furnace fly ash）and red mud intermixing;meanwhile,the study on the cementing performance of enhanced fly ash-red mud cementing system with gasification slag as coarse aggregate has not been reported.Therefore,in this paper,through the experimental study and theoretical analysis of fly ash-red mud-gasified slag composite cementing system,the influence law of mechanical properties of fly ash-red mud-gasified slag composite cementing system is explored.The main research work and conclusions are as follows.（1）Using fly ash,red mud and cement as raw materials,the differences in mechanical properties of gelation blocks obtained by blending and pressing different types of fly ash（CFB/PC）and red mud with cement respectively were investigated,and the mechanical properties of gelation blocks obtained by pressing and forming pure cement were used as reference to comprehensively evaluate the mechanical properties of fly ash/red mud-based gelation blocks.The results show that the mechanical properties of gelation blocks are mainly related to their apparent density,water absorption rate and mineral composition.The higher apparent density and lower water absorption of gelation blocks indicate higher compactness,lower porosity,higher compressive strength and better mechanical properties.According to the analysis of 28 d compressive strength,the best mechanical properties were obtained by adding PC（76.2 MPa）,followed by RM（72.5 MPa）,and the worst by adding CFB（56.1 MPa）.Further,the comprehensive evaluation combined with the gelling activity index showed that RM had the strongest gelling activity（73.7%）,followed by PC（70.1%）,and CFB was the lowest（54.3%）.（2）Based on the difference of gelling activity of fly ash/red mud based gelling specimens,the effects of different fly ash and red mud intermixing ratios on the morphology,physical properties（apparent density,water absorption）,compressive strength and mineral composition of fly ash-red mud based gelling specimens were investigated to investigate the effect of fly ash and red mud intermixing ratios on the mechanical properties of gelling specimens.The results show that:the apparent density of the gelation blocks decreases with the decrease of red mud dosing（he increase of fly ash dosing）;the water absorption rate increases with the increase of CFB dosing,and increases and then decreases with the increase of PC dosing;the compressive strength increases with the increase of CFB dosing,and then decreases and then increases;the trend of increasing and then decreasing with the increase of PC dosing.Further analysis and comparison showed that the best ratio of cementitious material was fly ash:red mud=1:2.When CFB:RM=1:2,the compressive strength of gelation blocks reached 16.8 MPa,the apparent density was 1.90 g/cm³,and the water absorption rate was also small;when PC:RM=1:2,the compressive strength of gelation blocks reached 18.1 MPa,and the apparent density was 1.87 g/cm³,indicating that the compactness was low,indicating high compactness,small porosity and better mechanical properties.Combined with the XRD and SEM images,it can be seen that the gelling specimen made by adding CFB-RM consists of irregularly shaped particle agglomerates with pin-rod structure on the surface of the particles,presumably because a large number of calcium alumina crystals are generated,forming a dense network structure leading to higher strength;the gelling specimen made by adding PC-RM consists of irregular agglomerates and spherical particles,with a large amount of irregular material on the surface between the spherical particles adhesion,and the Ca（OH）₂ content in gelation blocks is low,indicating that a better synergistic effect between PC and RM occurs at this ratio,and the volcanic ash reaction consumes a large amount of Ca（OH）₂.（3）On the basis of the optimized fly ash and red mud intermixing ratio,the effect of gasification slag doping on the morphology,physical properties（apparent density,water absorption）,compressive strength and mineral composition of fly ash-red mud-gasification slag-based cementitious specimens was further investigated to explore the influence law of gasification slag doping on the mechanical properties of cementitious specimens.The results show that:with the increase of gasification slag dosing,the color of gelation blocks becomes darker and the surface becomes rougher,the 28 d apparent density of gelation blocks shows a rising trend,the water absorption rate shows a decreasing trend,and the compressive strength shows a rising,then falling,then rising and then falling trend.When fluidized bed fly ash was used,the compressive strength of gelation blocks made by adding70%gasification slag（36.5 MPa）was greater than that of gelation blocks made by adding30%gasification slag（23.2 MPa）;when coal fly ash was used,the compressive strength of gelation blocks made by adding 70%gasification slag（25.1 MPa）was different from that of gelation blocks made by adding 30%GS.The compressive strength（25.1 MPa）of the gelling specimens made with the addition of 70%GS is basically the same as that of the gelling specimens made with 30%GS（25.2 MPa）.Combined with the XRD and SEM plots,it can be seen that the strength formation at 30%addition is mainly due to the better synergistic effect of CFB/PC and RM,which generates more gel products;the strength formation at 70%addition is mainly due to the higher hardness of GS particles,which enhances the strength as an aggregate.Further analysis and comparison showed that the mechanical properties of the fly ash-red mud-gasified slag-based gelling specimens were the best when the dose of gasified slag was 30%,and the compressive strength exceeded the national standard MU 20 strength requirement,with smooth and dense surface appearance,high polymerization and good denseness of microstructure.The gelation blocks made of PC consists of irregular agglomerates,spherical particles of different sizes and shapes,and there are a lot of irregular substances adhering to the surface between the spherical particles.