Study On CO₂ Mineralization Mechanism Of Waste Containing Ca And Mg And Hardening Performance Of Mineralized Product

The CO₂ mineralization of wastes containing calcium and magnesium to prepare products with cementitious potential is an important way to recycle wastes.In-depth exploration of the CO₂ mineralization mechanism of wastes containing calcium and magnesium,scientific measurement of potential on the mineralized product are the prerequisites for high-value and large-scale utilization of waste,and the coordinated reduction of CO₂ emissions.In this paper,calcium carbide slag,steel slag and concentrated seawater are selected as the research objects.The application potential and design optimization direction of mineralized products are studied from the aspects of reaction mechanism,kinetic model,structural characteristics of carbon mineralization products and environmental benefit evaluation.The main achievements are as follows:（1）During thermal CO₂ mineralization of carbide slag under the normal pressure,the newly formed calcium oxide phase forms and reacts with CO₂ to form a sintered calcium carbonate layer.The kinetic equation of the nucleation reaction of isothermal CO₂ mineralization reaction of carbide slag under the normal pressure,was established,and it was revealed that the reaction time,particle size,reaction temperature,CO₂partial pressure and ventilation flow rate were the main factors affecting the mineralization reaction.The kinetic equation of the non-isothermal heterogeneous mineralization of carbide slag based on the shrinking nucleation model and the crystal random nucleation growth model reveals that the more cycles,and the lower the mineralization reaction activity.The mineralized product of calcium carbide slag is mainly calcite-type calcium carbonate,which can be used as auxiliary cementitious material in the mineralized products.（2）Thermal CO₂ mineralization reconstitution promotes the reduction of f-CaO content in steel slag powder from 1.72%to 0.36%,and promotes the formation of highly active metastable states.The thermal mineralization reconstruction of steel slag powder and the accelerated mineralization curing of paste synergistically promote the content of steel slag in the product as high as 80%,the compressive strength at 24 h and72 h increased to 43.7 MPa and 47.5 MPa respectively,and the maximum CO₂ storage capacity reaches 37.4%,heavy metals（such as Cr,V and Ti）gradually transformed into inactive phases.A multi-component solid waste CO₂ mineralized product was constructed.When the compressive strength reached more than 15 MPa,the content of wastes reached up to 100%,of which the content of steel slag and calcium carbide slag were 20%and 6%,respectively.The particle accumulation mineralization hardening structure model of the multi-component cementitious material carbon-fixing products is proposed and calculated to have a porosity ranging from 19.06%to 21.11%.（3）The separation of calcium and magnesium in concentrated seawater can be achieved by adding an alkaline solution to adjust the p H value.For the mineralization process of Mg Cl₂-CO₂-Na OH system,when the reaction temperature is controlled within 45℃and the molar ratio of OH:CO₂ is within 1.8,nesquehonite is mainly formed.Nesquehonite thermally activated at 150～220℃can obtain good cementitious properties by pressing at standard atmospheric pressure and normal room temperature.The preferred thermal activation process is that with a temperature of 160℃;the preferred molding process is that with a water-solid ratio of 0.8.The compressive strength distribution range of the activated test block at 160℃is 0.7～3.5 MPa,and the bulk density is 698～863 mg/cm³.It may be that the morphological changes after hydration and hardening of amorphous nesquehonite positively contribute to its cementitious potential.（4）The life cycle inventory and model of CO₂ mineralization hardening products were constructed.Through LCA method,64 scenario analysis of multiply-solid wastes carbon-fixing products and 36 scenario analysis of Mg-based construction products were evaluated for environmental benefits,and the impact of key raw materials and production steps was quantitatively evaluated.The most preferred solution of these two products can achieve CO₂ emission reduction of 393.54 kg CO₂/m³,0.21 kg CO₂/kg.Based on LCA analysis,the optimization direction of these two products’process is proposed.