Effect And Mechanism Of Microorganism On Volume Stability And Utilization Efficiency Of Steel Slag Cementitious Materials

As a by-product of steelmaking process,steel slag has a huge emission and has not been used effectively for a long time.The huge amount of steel slag not only occupies the limited land resources and pollutes the environment.On the one hand,steel slag contains a large amount of cementitious minerals,which can be used to prepare building materials,but the hydration activity of these material is very low,it is difficult to use them efficiently.On the other hand,the stability problem caused by the high content of free calcium oxide and free magnesium oxide in steel slag makes its useing in building materials exist serious safety problem.According to the above problems,this paper proposed a method of preparing steel slag cementitious material by microbial technology.The method could accelerate the dissolution rate of calcium and magnesium ions,and transforme these ions into carbonate mineralization products.At the same time,this microbiological technology could solve the stability problem of steel slag caused by free calcium oxide and free magnesium oxide,the stable and efficiently utilization of steel slag as building materials is realized.This article innovative proposed a microbial technology to prepare steel slag cementitious materials.Around the safe and efficient use of mineralized steel slag cementing material,analyzed the stability and conversion efficiency of calcium-magnesium minerals.For the problems existing in the single mineralized microbe steel slag cementitious material,the composite microbial technology was proposed to prepare steel slag cementitious material.The kinetics and mineralization products of single mineral(f-Ca O?f-Mg O??-C₂S?C₃S)under the action of composite microorganisms was studied.The mechanism of composite microorganisms in pore solution of steel slag cementitious material was analyzed.Combined with the industrial production of road paving materials from microbial mineralized steel slag,the formulation and preparation technology of composite microbial steel slag cementitious materials were optimized,the properties and microstructure of microbial steel slag cementitious materials were systematically studied,and the control index and application method of microbial admixtures suitable for steel slag cementitious materials were formed.The main research conclusions and innovative achievements obtained include the following four aspects:(1)The special mineralized microorganism which is suitable for steel slag cementitious material was selected.The compressive strength of microbial mineralized steel slag cementing materials could reach to 41.7MPa and increased by 52%compared with carbonization steel slag cementing materials;the reaction rate of free calcium oxide increased from 81%to 94%,the reaction rate of free magnesium oxide increased from 62%to 76%;the utilization rate of calcium and magnesium in steel slag minerals increased from 37.3%to 59.8%.After the compression and evaporation experiment,the linear expansion rate of microbial mineralized steel slag cementitious material decreased to 0.53‰,more than 50%lower than that of carbonization group,but the expansion rate was still higher than the national standard of 0.5‰,the stability problem of microbial mineralized steel slag cementitious material remain to be solved.In order to thoroughly solve the problem of steel slag stability and improve the utilization efficiency of steel slag,It is still necessary to further improve the reaction rates of f-Ca O?f-Mg O??-C₂S?C₃S and explore the mechanism of microbial mineralization in steel slag.(2)In order to solve the problem of single mineralized microbe steel slag cementitiousmaterial,a composite microbe technique was proposed.The study on the mineralization process and mechanism of single minerals of f-Ca O and f-Mg O show that the apparent activation energy of f-Ca O under the action of composite microorganisms is 1.65k J/mol,and the apparent activation energy of f-Mg O is 7.89k J/mol,which were significantly lower than that of carbonizationreaction.The reaction type of f-Mg O changes from chemical reaction control process to diffusion control process.The volume stability of steel slag from five steel factories under the action ofcomposite microorganisms all conform to the national standard.For the steel slag which thecontents of f-Ca O less than 7%and the content of f-Mg O less than 7%,the composite microbial technology could effectively solve the stability problem.For the steel slag of baosteel withdifferent fineness,when the linear expansion rate is below 0.5‰,the fineness with composite microbe could be reduced from 600m²/kg to 360m²/kg,which has significant energy saving benefits for steel slag grinding.Microbial mineralization products have smaller grain size,larger particle size and denser microstructure.The apparent activation energy of?-C₂S under the action of composite microorganisms is 15.3k J/mol,and the apparent activation energy of C₃S is13.7k J/mol,which were significantly lower than that of carbonization reaction.The reaction type of?-C₂S and C₃S changed from mixed control process to diffusion control process.The mainproducts of mineralized?-C₂S and C₃S are calcium carbonate,and the main crystal types arecalcite and pellet aragonite.The amorphous phase in the mineralized product is mainly amorphous silica and a small amount of C-S-H.Through the analysis of grain size,microstructure andadhesion force,it is found that the mineralized product has smaller grain size,stronger adhesion force and compact microstructure.Microbial mineralization products have smaller grain size,stronger adhesion and compact microstructure(3)In order to further reveal the mechanism of composite microorganisms,the germination and growth of microorganism and mineralization crystallization process in the pore solution of steel slag were analyzed in depth:after the microorganisms were excited and added,the dissolution rate of calcium and magnesium ions in the steel slag cementitious material was greatly improved,and the concentration of calcium ions is nearly ten times,which of magnesium ions is nearly five times,which provides abundant positive ions for the progress of the mineralization reaction;the p H of the slag pore solution is significantly reduced,which provides suitable conditions for the germination and growth of mineralized microorganisms,and the germination percentage is higher than 80%at200min,which is similar to the germination percentage in deionized water.The mineralized microorganism enzyme km at 30?is the smallest,which is 1.12×10^-2mol.At this time,the efficiency of the mineralized enzyme on the carbon dioxide substrate reached the maximum value.The Zeta potential test shows that the surface of the microbial cells can be used as a nucleation site to accelerate mineralization;and mineralized microbial enzymes can significantly increase the rate of mineralized products.Ca CO₃ deposition rate constant f can reach 5.75×10^-2h^-1 when the enzyme activity is 0.41 U/ml.It is 0.03×10^-2h^-1 when there is no microorganism,which is a nearly 200 times increased.Using XRD,EDS,SEM,and TEM analysis methods,it was confirmed that the calcite crystals induced by microbial method were spherical or irregular spherical aggregates.There were also many bacterial imprints on the calcite formed by bacterial mineralization.The particle size of biological calcium carbonate was larger than that of the chemical calcium carbonate.The decomposition temperature of biological calcium carbonate is higher than that of chemical calcium carbonate.(4)Combined with the industrial production of microbial mineralized steel slag road pavement materials,the formula and preparation process of the composite microbial steel slag cementitious material were optimized.The prepared composite microbial mineralized steel slag road pavement product has stable products,good volume stability,high strength and excellent durability.And the control index and application method of microbial admixture is formed to use for steel slag cementitious materials.The composite microbial steel slag cementitious material with optimized preparation process and ratio can reach a maximum strength of 55.7MPa,which is more than doubled compared with the strength of carbonized steel slag;the linear expansion ratio is 0.2‰,which is far lower than the national standard for the linear expansion of cement products,and it is80%lower than that of carbonized steel slag.Through XRD and TG analysis,the reaction ratio of silicate minerals,free calcium oxide,and free magnesium oxide in the composite microbial steel slag cementitious material increased significantly,and the utilization ratio of calcium and magnesium minerals was more than doubled compared to carbonization.By quantitative characterization of the pore structure,the pore structure of the composite microbial steel slag specimen was more uniform,and the porosity decreased.