Preparation Of Carbon Fixing Cementitious Material And Its Coupling Mechanism Of Hydration And Carbonation

Based on the purpose of carbon reduction and carbon neutrality.In order to prepare low carbon,low emission,economic,high carbonation capacity,fast carbon fixation rate of cementitious materials.In this paper,carbon fixing cementitious materials was prepared by using limestone tailings and solid wastes on the basis of studying the mechanism of mineral phase hydration and carbonation with carbon fixing potential.The coupling mechanism of hydration and carbonation was studied and the carbon sequestration benefits were evaluated.The main research contents and results are as following:In hydration curing,carbonation curing,hydration-carbonation（HC）recycling three kinds of curing environment,studied the mineral phase C₃S,β-C₂S,γ-C₂S,C₃A,Ca（OH）₂and P.I,PLH,SS of hydration reaction,the carbonation reaction and hydration-carbonation interaction mechanism.Carbon sequestration capacity and carbon sequestration mechanism of different ore phases was clarified.The results show that the carbon fixation properties,physical and chemical properties and durability of the cementitious materials are determined by the shaping of the solid microstructure by the processes of carbonation and hydration.Different minerals have different effects on pore structure during carbonation and hydration.Different pore structures have different capacities for CO₂diffusion,adsorption and solidification.The formation of carbonation products and hydration products reduces the porosity and pore content of the matrix,and creates a new pore structure.In other words,the microscopic pore structure is identified,but it is found that the hydration reaction after carbonation will increase the matrix pore content in the cycle curing process.This is because the humidity in the carbonation environment is low,and the generation of carbonation products consumes the water inside the matrix,and cuts the connecting pores,forming a dense coating layer,resulting in the reduction of pore volume,the blockage of carbon dioxide diffusion channel,and the increase of pore solution after the transfer to the hydration environment.The superposition of hydration products breaks the coating of carbonation products and increases the porosity.Compared to the carbonation curing,cycle maintenance promotes C₃S,β-C₂S,γ-C₂S,C₃A,Ca（OH）₂and P.I,PLH,SS carbon reaction.The actual amount of carbon sequestration is increased with the increase of the number of alternating cycles.Hydration-carbonation cycle maintenance,more conducive to C₃S,β-C₂S,γ-C₂S,P.I,PLH and SS carbon reaction.The cycle curing of carbonation-hydration s more favourable to the occurrence of C₃A carbon fixation reaction.Based on the characteristics of each mineral phase,with limestone tailings and calcium carbide slag as the main raw materials,tailings carbon fixation cementitious（WK-CFCM）and solid waste carbon fixation cementitious（GF-CFCM）were designed and prepared respectively.By designing and adjusting the chemical composition of tailings and studying the calcination system,the actual carbon sequestration amount of prepared WK-CFCM reached 36.24%and the degree of carbon sequestration reached 61.74%after 28 days of carbonation.By adjusting the content of Ca（OH）₂,the mineral phase type and content of carbon fixing cementing material were regulated.The effects of Ca（OH）₂on the physical properties,carbonation properties and hydration and carbonation coupling mechanism of GF-CFCM were systematically studied.The results showed that the mineral phase Ca（OH）₂significantly improved permeability of GF-CFCM,increased the porosity of the matrix,and significantly enhanced the theoretical carbon sequestration,carbonation rate and compressive strength of GF-CFCM after carbonation.The actual carbon sequestration of cyclic curing HCHC samples was much higher than that of carbonizing for 28 days.At the same time of cyclic curing,the actual carbon sequestration amount of sample carbonation-hydration cyclic curing was lower than that of hydration-carbonation cyclic curing and carbonation curing.In order to further improve the carbonation rate of GF-CFCM,the pore structure was regulated and optimized.The results showed that by regulating the particle size of the mineral phase Ca（OH）₂,the capillary pores in the matrix before the reaction of carbon fixing cementing material was cut and refined,and nano-pores were constructed,and more CO₂was absorbed.Thus,carbon sequestration ability of GF-CFCM was enhanced,and the highest actual carbon sequestration amount reached 49.1%,and the carbon sequestration amount after HCHC cycle was always higher than that of CHCH cycle curing.Content of hydration products after CFCM reaction was regulated by steel slag,which changed the distribution of pore structure and enhanced the dissolution rate of CO₂diffusion.The higher the concentration of CO₂in the curing environment,the higher the CO₂saturation in the pore solution,and the faster the carbonation reaction.The actual carbon sequestration amount and reaction degree of CFCM were the best when the humidity was75±5%RH and the temperature was 10℃.The change of product structure greatly affects the solidification ability of GF-CFCM for CO₂,and the increase of porosity enhances the carbonation rate of GF-CFCM.The production of GF-CFCM saves a lot of energy and resources,and has excellent carbon fixation ability.After preparing 300kg/m³and 600kg/m³foamed concrete with CFCM,the cumulative carbon emission within the life cycle is-34.7kg and-31.7kg,respectively.Compared with P.I,its standard coal consumption is reduced by 51.2kg/t cementing material,resource consumption is reduced by 42.1%,CO₂,NOx and SO₂emissions are reduced by 42.1%.