Mechanism Research On Co₂ Mineral Carbonation Curing Of Building Materials Based On Non-hydraulic Cementitious Materials

Carbon dioxide capture,storage and utilization（CCUS）technology is considered to be the most effective method for controlling anthropogenic carbon emissions and mitigating climate change.CO₂ captured by industrial carbon capture technology requires large-scale and economic utilization technologies.CO₂ mineral carbonation curing technology for building materials is based on the direct gas-solid reaction between the early hydrated concrete materials and CO₂ to achieve carbon sequestration.It is expected to realize large-scale emission reduction of greenhouse gases and obtain the high-added value building material product simultaneously.In particular,the use of CO₂ mineral carbonation curing for prefabricated concrete when it replaces the existing high-energy steam curing or natural curing processes can shorten the curing time,reduce the production energy consumption,and optimize the material performance.Mineral carbonation curing technology is still at the stage of functional research and material development.Thus the optimized design of raw materials and the comprehensive understanding on kinetics and microscopic reaction mechanisms are urgently needed.In light of these problems,based on the study of mass transfer characteristics and transformation mechanism of mineralization process,this research explores the role of mineral carbonation in shaping microstructure,and finally clarifies the relevance among the mineralization reaction process,the microstructure changes,the macroscopic performance and the environmental benefits.Firstly,the effects of pressure,temperature and water-cement ratio on the mineral carbonation curing of calcium silicate cementitious system（Portland cement）were analyzed,as well as the change mechanism of mineral phases.CO₂ uptakes around 13¹8 wt.%are obtained by mineralization,and accelerated carbonation promote the densification of microscopic pore structure of cement paste.The densification of the structure further improves the mechanical performance of samples,and the compressive strength of cement paste after 2-hour mineral carbonation curing could reach 51.5 MPa,which is 10%higher than the strength of 7-day natural curing cement paste.Meanwhile,the quantitative analysis of gas permeability was used to determine the correlation mechanism between mineralization kinetics and pore water migration in the cementitious system.Based on the rate change of cement paste mineralization and gas-solid reaction diffusion control equation,an apparent mass transfer model was presented,and the mechanism of progressive product-layer diffusion control was proposed.The apparent rate constants under different curing conditions were also determined.On the basis of the diffusion control mechanism,the inert mineral（dolomite,limestone and silica）doping method was further designed to enhance the gas diffusion.The binary binder materials can reduce the cement use（5²5%）while improving the rate of CO₂ uptake,optimizing the mechanical properties through mineral carbonation curing.For the development of non-hydraulic cementitious materials,amorphous wollastonite and natural wollastonite（calcium silicate in crystal form）were used to partially replace cement to form cementitious materials with low calcium-to-silica ratio.The impacts of the crystal phase,blending ratio and reaction parameters on the microstructure of the material,mineral phases,microscopic pore structure and gas permeability properties were systematically studied.Results show that amorphous wollastonite mainly enhances the reaction by diluting cement particles and promoting the gas permeation through the pore structure.For the natural wollastonite,the structure change in blending materials is mainly affected by the pore-creating effect from pore water evaporation in the early stage and then dominated by the filling effect caused by the carbonation reaction of different silicate minerals in the middle and late stages of the reaction.The blending wollastonite takes effect in all stages.The advantage of non-hydraulic cementitious materials is not limited to the emission reduction benefit of raw material（reducing the use of traditional cement）.The staged control of mineralization process through mineral design should also be considered（e.g.,in the early stage,the gas diffusion and carbon sequestration are promoted,and the later densified structure improves the performance）.The natural wollastonite blending material showed a significant improvement of compressive strength when compared to the Portland cement,and the maximum compressive strength could reach 80MPa.The common microscopic kinetic mechanisms of non-hydraulic and hydraulic mineral particles are studied in depth.For the first time,the surface water coverage control mechanism of mineralization was proposed on the microscopic particle scale（the microscopic mineralization rate-limiting step is the migration and evaporation of surface water on mineral particles and the reduction rate of surface water coverage）.Obtained models were verified by the experimental results.Based on this mechanism,deep mineralization of wollastonite under higher reaction temperature（<100°C）can be achieved（secondary activation after the reaction is stopped）,and calcium carbonate products in form of calcite or aragonite can be controlled.Considering the influence of pore structure on the diffusion-reaction at different positions inside the cementitious system,the influencing mechanism of gaseous diffusion-reaction characteristics and internal pore water migration at non-particle scale were also obtained by the CT scan technology and section analysis.Lastly,the life cycle inventory and life cycle assessment study on the CO₂-cured building materials were also finished.The environmental benefits of seven novel CO₂-cured building materials and corresponding production crafts were evaluated,taking into account the specific impact of different raw materials and production steps.Results show that the formulation of wollastonite blending cementitious material proposed in this paper displayed the best scores in the aspects of carbon emission,energy consumption and other environmental impact indicators.The use of mineral carbonation curing instead of steam curing can achieve over 30%of the life cycle CO₂ emission reduction of building materials,and further optimization is expected to achieve more than 60%CO₂ reduction.Based on the environmental benefit evaluation,the optimization formula and process of mineral carbonation curing technology was proposed.