Study On The Carbon Dioxide Fixation Mechanism And Application Of Cement-based Materials

The global warming caused by greenhouse gas emissions is becoming more and more serious,and the resulting weather anomalies have threatened the human living environment.China has adjusted the economic development strategy in appropriate time and implemented the economic model of low-carbon development.Traditional building material industry has the disadvantages of high energy consumption,large emission and serious pollution.Particularly in cement industry,it has high carbon emissions due to the fuel combustion and the decomposition of carbonate minerals in the production process.However,based on the all life-cycle theory,the carbon footprint of cement in the all life-cycle will be reduced,if carbon dioxide can be absorbed by cement during the using phase.As a kind of carbon capture and storage technology,the carbonation curing of cement-based material can be used to fix the CO₂ during the use of cement.Through carbonation curing,cement-based materials not only fix CO₂,but also improve the performance of materials.However,there are still many unsolved problems in the mechanism of carbonation,like carbon dioxide fixation process,carbonation products and strength enhancing mechanism.It is of great significance to study the carbon dioxide fixation mechanism and application of cement-based materials.Which is beneficial to the low carbonization and greening of building materials.This study based on the National Key Basic Research Program of China?973 program?-long service life of concrete material and structure under harsh environment as well as the National Natural Science Foundation of China-structural parameters of hydraulic concrete nanoparticles diversity research and performance control.Using micro-calorimeter,transmission electron microscopy?TEM?,scanning electron microscope?SEM?,high resolution solid state nuclear magnetic resonance?MAS NMR?,thermogravimetric analysis?TG?,X-ray diffraction?XRD?,energy spectrometer?EDS?,and other comprehensive testing technologies to study the carbon dioxide fixation mechanism of cement-based materials,the effect of carbonation mixing on the CO₂ uptake and strength of cement-based materials was studied,the properties of carbonation curing concrete and the application of carbon dioxide fixation in industrial residues.The main conclusions are as follows:?1?Through self-designed test device,the thermodynamics process of tricalcium silicate?C3S?and beta dicalcium silicate??-C2S?paste carbon dioxide fixation reaction reaction was quantitative description.The exothermic data of corresponding carbonation reaction were measured.Under the conditions of 0.15 water to binder ratio and 0.5 bar CO₂ pressure,the maximum heat flow of the carbon dioxide fixation reaction was 0.54 W/g,and the accumulative carbon dioxide fixation reaction heat was 116.7 kJ/mol.With the same water to binder ratio and in the same time,the maximum heat flow of carbon dioxide fixation reaction was 200 times than that of the hydration,and the cumulative heat release of carbon dioxide fixation reaction was 3.5 times than that of the hydrated reaction.Under the same conditions,P-C2S paste maximum heat flow of carbon dioxide fixation reaction was 0.15 W/g,and the accumulative of carbon dioxide fixation reaction heat was 26.4 kJ/mol.With the same water to binder ratio and in the same time,P-C2S paste maximum heat flow of carbon dioxide fixation reaction was 800 times than that of the hydration,and the cumulative heat release of carbon dioxide fixation reaction was 300 times than that of the hydrated reaction.In the carbonation curing condition of 0.15 water to binder ratio,4 bar CO₂ pressure and 8 MPa molding pressure,the 2-hour age strength of C3S paste was 27.5 MPa,and the 24-hour age strength was 62.9 MPa.Under the same carbonation curing conditions,the strength of 2-hour and 24-hour of P-C2S paste were 18.6 MPa and 48.5 MPa respectively.The 24-hour strength of carbonated C3S and P-C2S pastes both exceeded that of hydrated pastes with 28 days of standard water curing,which indicates that by carbonation curing,cement-based materials can obtain higher strength quickly.In terms of the result of CO₂ uptake,C3S paste with 2-hour and 24-hour carbonation curing was 17.2%and 26.3%respectively.In P-C2S case,the CO₂ uptake was 18.9%and 25.4%respectively.In order to determine the maximum CO₂ uptake of C3S and P-C2S paste,through water addition and carbonized over several times the maximum CO₂ uptake of C3S and P-C2S reached 51.11%and 33.13%respectively.It confirmed that the carbonation products of C3S and ?-C2S were amorphous SiO₂ and calcite crystals by high spatial resolution TEM-EDS,and combined with XRD and 29Si MAS NMR,and based on the analysis of carbonation reaction product molecular structure.?2?The results prove that CO₂ can be fixed in the mixture in the form of CaCO3 crystal during the carbonation mixing process.The CO₂ uptake was more than 3%after 240 seconds carbonation mixing.Compared with the preparation process of normal mixing and carbonation curing,the CO₂ uptake of cement paste increased more than 30%by the preparation process of carbonation mixing and carbonation curing.It was found that the strength of the cement paste decreased due to carbonation mixing process.Therefore,this study proposed a hybrid mixing process?continuous normal mixing after carbonation mixing?,which could reduce the adverse effect of carbonation mixing on strength.?3?The compressive strength of carbonation curing concrete was more than 20%higher than that of normal curing concrete with the same mix proportion at the same age.In a short period after carbonation curing,the pH value of carbonation curing concrete was lower than that of normal curing concrete at the surface area.This was because the carbonation product of CaCO3 replaced the hydration products of Ca?OH?2,which reduced the pH value of the concrete.Through the subsequent normal curing,the pH value of carbonation curing concrete at the surface area has risen to more than 12,which is basically consistent with the pH value of normal curing concrete.The electric flux of carbonation curing concrete was less than that of normal curing concrete,showing a good resistance to chloride ion permeability.The properties of anti-abrasion of carbonation curing concrete at different impinging angles?90°,60° and 30°?were all better than that of normal curing concrete.Especially at small angles?30°?,when the cutting action is dominated,carbonation curing concrete showed a better performance.The pore structure optimization and micro-hardness improvement of the cement paste after carbonation curing are the reasons for the improvement of the chloride ion permeability and the anti-abrasion performance of carbonation curing concrete.?4?Pb in the municipal solid waste incineration?MSWI?fly ash is effectively solidified by carbonation curing method,which can reduce leaching toxicity of the fly ash.When cement and MS WI fly ash were mixed with 1:1,the strength of the paste can reach 20.4 MPa after 12 hours carbonation curing and subsequent 27 days normal curing,and the CO₂ uptake reached 11.6%.After carbonation curing,MSWI fly ash cement-based materials has harmless and low-carbon properties.MSWI mixed slag mortar with carbonation curing has the same compressive strength as that of reference mortar?without MSWI mixed slag?with normal curing.This indicated the negative effect of MSWI mixed slag on mortar compressive strength can be compensated by carbonation curing method.Carbonation curing can also suppress expansion of mortars with MSWI mixed slag.The CO₂ uptake of mortar with partly mixed slag substitution after carbonation curing exceeded 15%.?5?The steel slag bricks are prepared by carbonation curing method,which can not only benefit waste but also fix carbon dioxide.Three factors and three levels orthogonal test results showed that the effect order of water to binder ratio,carbonation curing time and the molding pressure on the strength of carbonation curing steel slag brick were:water to binder ratio>molding pressure>curing time.The compressive strength of steel slag brick reached 34.2 MPa under the conditions of water to binder ratio of 0.15,carbonation curing time of 8 hours and the molding pressure of 8 MPa.The strength of the samples can be further improved by subsequent hydration after carbonation curing.The reason for the cracking of hydration steel slag sample was that the f-CaO delayed hydration in hardened paste,the volume expansile of hydration product of Ca?OH?2 cracked the sample.By carbonation curing,the f-CaO can reacted before the paste hardened.Thus the cracking of the sample has been effectively inhibited.