Property Improvement Of Recycled Hardened Cement Powder From Construction And Demolition Waste By Mineral Carbonation With Low Concentration Of CO₂

To date,global anthropogenic emissions of atmospheric CO₂ are larger than at any other time in human history,around 36.3 Gt in 2021.The cement industry is the second industrial emitter of CO₂,which accounts for over 7%of man-made CO₂ emissions.The decomposition of Portland clinker and combustion of fossil fuels processes involved in the production of cement are a large source of CO₂emissions.Portland clinker substitution with recycled hardened cement powder generated from construction and demolition waste could play a greater role to reduce CO₂emissions from the cement production.However,the high porosity and inhomogeneous of recycled hardened cement powder may bring some negative impact on the performance and mechanical properties of Portland cement concrete,such as higher water absorption capacity,lower strength and workability.In the meantime,carbon capture,utilization and storage（CCUS）is a critical emissions reduction technology for cement manufacture.Mineral carbonation of cementitious materials in CCUS can be applied as a low-carbon cement and concrete production technology,in which CO₂ chemically reacted with calcium hydroxide and calcium silica hydrate in adhered old mortar,forming thermodynamically stable carbonate minerals to enhance performance and could provide a route for CO₂ permanent sequestration.There were few papers that summarized the mechanism of CO₂mineralization and not fully understood for the relationship between CO₂sequestration of recycled hardened cement powder and the workability,durability and mechanical performance of concrete incorporating with recycled hardened cement powderIn this work,the effects of different mineralization processes on the activity,water absorption,admixture adaptability and CO₂sequestration of recycled hardened cement powder were studied,and the mineralization mechanism of recycled hardened cement powder with low concentration of CO₂（CO₂ content of 19.98 vt.%）was investigated via XRF,XRD,SEM-EDS,FTIR,TGA-DSC and N₂-BET.And beyond that,this study aimed to elucidate the main mechanisms involved in the mineralization of CO₂ in Portland cement clinker and its hydration products of recycled cementitious materials,and factors affecting the efficiency of the mineral carbonation.The main findings from this paper were that the performance of recycled hardened cement powder by direct aqueous mineral carbonation was much better than those of gas-solid carbonation.The water requirement of recycled hardened cement powder was reduced by 33%,and the hydration activity and additive adaptability were increased by14%and 39%respectively.Property improvement of the recycled hardened cement powder by CO₂mineral carbonation was mainly due to the reaction of CO₂ reacted with calcium hydroxide,calcium silica hydrate and unhydrated cement clinker to form thermodynamically stable carbonate minerals to refine the pores or microcracks and accelerate cement clinker hydration,and enhanced the properties of water requirement,the hydration activity and additive adaptability.C₃S andβ-C₂S,as well as CH and C-S-H formed on hydration of C₃S or C₂S,are known the main CO₂ reactants.In recycled cement-based materials carbonation procedure,C₃A and C₄AF show minimal reactivity with CO₂ to form Ca CO_3.Finally,it can be concluded that CO₂ mineralization reinforced recycled hardened cement powder not only improved the recycling of construction and demolition waste as alternative raw material,but also a certain amount of CO₂was permanently stored by recycled hardened cement powder.