Microscopic Mechanism Of CO₂ Mineralization Of Calcium Silicates And CO₂ Uptake Capacity Of Cement And Solid Wastes

Carbon capture,utilization and storage(CCUS)will be one of the key technologies to effectively achieve carbon removal goals in the coming decades.Among them,carbonation curing of cement and solid waste-based concrete is a promising technology to permanently fix CO2 while producing building products with some economic value.Due to the complexity of multi-calcium silicate based cement and solid waste systems,it is important to understand the microscopic mechanisms of mineralization at the micron and nano scales of different calcium silicates and to establish quantitative relationships between raw material compositions and CO2 sequestration properties and heat of mineralization reactions of cements and solid wastes for the development of carbonation curing concrete formulations,process optimization and further efficient and economic applications.In response to the above challenges,the following research has been carried out in this thesis.1.Mineralization experiments and reaction heat experiments of the main calcium silicate compositions in cement and solid waste were carried out.The microscopic mechanisms of CO2 mineralization on the micron and nanometer scale and the mechanisms of interaction effects of different calcium silicate minerals were revealed.In this thesis,the high-temperature sintering process of a variety of calcium silicate minerals was optimized,and CO2 mineralization experiments of typical mineral single compositions(tricalcium silicate,β-phase dicalcium silicate,y-phase dicalcium silicate,crystalline and amorphous phase calcium silicate,tricalcium aluminate,tetracalcium iron aluminate,etc.)and mixture of calcium silicate and y-phase dicalcium silicate were studied under three typical dispersion states.The results showed that in the flat dispersed state,the CO2 uptake capacity was mainly influenced by the reactivity and the actual CO2 uptake ratio of each mineral was nearly the same as the theoretical value.In the compacted state,the hydration products of hydrated active calcium silicate minerals hindered the diffusion of CO2 gas,while the hydrated inert minerals still had the pore structure after compaction;In the natural accumulation state,the CO2 uptake capacities were between the flat dispersed state and compacted states.The measured apparent molar heats of reaction of different calcium silicate minerals are lower than the theoretical standard molar enthalpy of reaction(the ratio to the theoretical standard molar enthalpy of reaction is about 45%-70%),and there are two possible reasons for the difference,the first is the heat absorption of water evaporation heat prompted by the exothermic mineralization reaction(the ratio is about 5%-20%),and the second is the exothermic heat of hydration reaction of calcium silicate samples before starting the mineralization reaction(the ratio is about 15%-35%),which is expected to provide a reference for the exothermic estimation of industrial processes.For the microscopic mechanism of CO2 mineralization,from the micron-scale perspective,the hydration-active calcium silicate minerals such as tricalcium silicate and β-phase dicalcium silicate form amorphous hydration product gels to connect different particles,while the hydration reaction of hydration-inert calcium silicate is very weak,and the transformation process from the original calcium silicate particles to the silica-rich phase shows the "pseudocrystal substitution.From the nano-scale perspective,during the mineralization of calcium silicate,some Si atoms maintain their original chain structure,and some are transformed into a higher degree of polymerization into a silica-oxygen tetrahedral mesh structure,or even ordered quartz SiO2 crystals.As the polymerization degree increases,it takes up less space and forms pores and cracks inside the "pseudocrystals".When calcium silicates with different reaction rates were mixed,the dispersion state had a significant effect on the carbon fixation performance of the mixed system.Due to the retention of the shell shape of calcium silicate with slower reaction kinetics,it played the role of pore support in the early stage,which facilitated the diffusion of CO2 gas,and gradually formed calcium carbonate calcite in the later stage,which promoted the denseness of pores and the improvement of mechanical properties.2.Three carbon fixation performance prediction models were developed,the first one based on the mineral compositions of cement,the second one based on the chemical compositions of fly ash and the third one based on the chemical compositions of steel slag and blast furnace slag.The quantitative relationship between raw material compositions and performance in a uniformly dispersed state was elucidated.Mineralization experiments of various cement,fly ash,blast furnace slag,and steel slag samples were conducted in the flat dispersed state(PCO2=0.5 MPa,25℃,w/s=0.4),and multivariate linear fitting of the correlation between compositions and CO2 uptake capacities were performed to predict the CO2 uptake ratios with different reaction durations.Based on the experimental results of four cement samples in 0-120 hours,a model of the correlation between mineral compositions of raw materials and CO2 uptake ratios was established,and the results showed that the overall fitting method was simpler but the goodness of fit was slightly worse,and the multi-component weighting fitting method had the best goodness of fit(R2>0.9956).The conversion of multiple mineral compositions in the mixed state is smaller than that in the individual state,which may be related to the "mineral locking effect.Based on the experimental results of eight fly ash samples in 0-48 hours,a model of the correlation between chemical compositions of raw materials and CO2 uptake ratios was established.Compared with the predicted values using the classical theoretical maximum CO2 uptake ratio formula,the fitting effect was significantly improved.Based on the experimental results of three blast furnace slag and three steel slag samples in 0-120 hours,a model of the correlation between chemical compositions of raw materials and CO2 uptake ratios was established.By introducing the Fe2O3 positive correlation term,the goodness of fit was much greater than that of the classical theoretical maximum CO2 uptake ratio formula.Although the model was fitted under specific optimization conditions,it can provide a preliminary prediction of the maximum CO2 uptake ratio of mineralized cement and solid wastes under different working conditions,and provide reference for the formulation development of carbonation cured cement and solid wastes building materials.3.A weight gain and oven-dry method with good applicability and operability and a new basis based on the revised mass of reactive compositions have been proposed.A relatively complete standardized evaluation system for CO2 mineralization of cement and solid wastes was formed.The principles and characteristics of different methods for measuring CO2 uptake mass and mass conversion bases were systematically compared.The error of the weight gain method mainly comes from the estimation of evaporated water,and the error of the local sampling method comes from the poor representation of local sampling to the overall sample.Although the accuracy of the gas metering method was relatively high,but it requires accurate measurement of temperatures and pressures.Therefore,a new weight gain and oven-dry method with good applicability and operability and a new basis based on the revised mass of reactive compositions have been proposed,which can directly reflect the reaction degree of calcium and magnesium and other active components in the raw material,while excluding the influence of water in the sample.Finally,a relatively complete standardized database and evaluation system for CO2 mineralized cement and solid wastes was developed,including modules for background information recording,basic physical and chemical property measurements,CO2 uptake performance analysis and prediction,and storage of original test files.In addition,examples of databases for raw material analysis and mineralization properties of all samples covered in this paper are provided.