Fundamental Research On Mineralized Gelling Mechanism And Regulation Techniques Of Microbial Carbonized Steel Slag Building Material Products

Steel slag is a solid waste discharged from iron and steel metallurgy industry,the large amount of abandoned steel slag will not only occupy farmland and pollute the environment,but the heavy metal elements in the steel slag will also infiltrate the soil to pollute underground water,posing a potential threat to national lives.Moreover,as the largest consumer of coal and cement worldwide,China is facing tremendous pressure about CO₂emission reduction.Inspired by the phenomenon of microbial induced carbon cycling and carbonation process in nature,a type of microorganism which can increase the rate of hydration of CO₂ by 10⁷ times through the secreted enzymes was finded.In this essay,the microorganism was adopted to increase the efficiency and velocity of CO₂ sequestration.The CO₂ was sequestrated in the form of minerals,meanwhile the minerals would have certain cementing properties.A novel bio-carbonized steel slag brick and some other building material products with high strength,fine volume stability and excellent durability were prepared based on enzyme catalysis of microorganisms.First,the feasibility study of preparing microbial carbonized steel slag building material products were carried out in both liquid and solid systems to demonstrate the feasibility of microbial accelerated mineralization in both systems.The law of recovery,growth and survival of bacterial powder was explored in the highly alkaline liquid supernatant formulated with steel slag powder.Microscopic testing technology was utilize to coMPare the calcium carbonate deposited by chemical and microbial methods.The results indicated that the two types of calcium carbonate owns the same crystal form but both morphology and degree of crystallinity of them differ.The calcium carbonate produced based on chemical method is mainly non-cohesive cube-type calcium carbonate,while that of microbial method is mainly spherical calcite,accoMPanied by square,layered type ones.There are also some colloid and microorganism gathering around the particles,microorganism acting as crystal nuclei,then the calcium carbonate produced based on microbial method is cemented to each other.In the mortar system,phenolphthalein staining method and Image J quantification method were used to characterize the accelerating slag mineralization of microorganisms,and it was concluded that the addition of microorganisms can significantly increase the carbonation rate and carbonization degree during the carbonization process.The optimum microbial dosage was1.0%of the cementitious material?dry mass basis?.The accelerating effect of microorganisms was most obvious at 4h carbonation time,and flexural strength of the microbial carbonized steel slag brick were1.26 times and 1.52 times those of regular carbonized ones when carbonized for 3 hours,respectively.The moisture content is critical to the preparation of the microbial steel slag building material products.The concept of residual water-cement ratio?R_e?was adopted here to characterize the moisture content of bricks and the temperature of carbonization was measured by temperature sensors.The degree of color reaction,mass gain,CO₂ sequestration efficiency,and microstructure were utilized to explore the effect of moist content on the bio-carbonized steel slag bricks including the product,reaction rate and degree of carbonization.The result was that as R_e decreased,the transportation of CO₂ inside the sample accelerated,the internal temperature increased gradually caused by the carbonization and the whole carbonization accelerated.However,the acceleration of micrograms was weakened and the final degree of carbonization was lowered with the decrease of R_e.Hence,the compressive and flexural strength first increased rapidly and then decreased smoothly.When R_e was 0.20,it is not only beneficial to the transmission of CO₂,but also conducive to the hydration of CO₂to increase the carbonization reaction rate and carbonization degree.The rate of mass gain was 21%after 4h of carbonation,the content of brick pore whose size is from 0 to 50 nm and140 to 180nm was reduced significantly to improve the durability and the compressive strength and flexural strength was up to 32.9MPa and 5.93MPa.Furthermore,every ton of carbonized steel slag bricks can fix 63.1kg of CO₂.According to the annual output of 1 billion standard bricks?200mm×100mm×55mm?,it consumes about 10 thousand tons of CO₂ gas per year.Meanwhile,how to maximize resource utilization of internal calcium source of the steel slag in the microbial carbonized steel slag building material products was one of research priorities in this essay.The effect of calcium sources on the carbonation of microbial carbonized steel slag products was studied by utilizing a calcium source or weak acids to accelerate the dissolution of the internal calcium sources in steel slag.It could be concluded that the adding calcium source?Ca?OH?₂?has little effect on the acceleration of microorganisms,but it could effectively increase the crystallinity,yield and strength of calcium carbonate when R_e was 0.20,on the other hand,the accelerating effect of microorganisms is more obvious with the increase of the concentration of acetic acid coMPared to regular carbonized ones.Synthesize the analysis XRD,SEM,BET,TG-DSC the conclusion could be made that acetate ions migrate to the outer layer of the brick to form a dense layer,hindering the following carbonation of the interior of the sample.In this case,there was a hoop effect so that the flexural strength of the brick body increased significantly.The compressive and flexural strength of the bricks with 0.5 mol of acetic acid and 4 hours of carbonation are 28.58MPa and 5.95MPa,respectively.Finally,the microbial carbonized steel slag brick was poured into a square model of 200×100×55mm and then pressed to molding.The R_e of the brick was 0.20 and the time of carbonation was 8 hours.The brick prepared had excellent property,for example,2days the compressive strength was no less than 25MPa,2days the flexural strength was up to 7.86MPa and the water absorption was no more than 6%.The volume stability microbial carbonized steel slag brick was fine and the specific strength was up to 0.017,meeting the requirements of MU10 building bricks.