Study On The Performance Improvement Mechanism Of Magnesium-based Concrete With Recycled Aggregates Under CO₂ Curing

Reactive Magnesia Cement（RMC）is a low-carbon cement that has gained attention due to its low carbon footprint during production.At the same time,the use of Recycled Concrete Aggregates（RCA）to make concrete has received widespread attention due to the scarcity of natural aggregates,which can also help further reduce carbon emissions.Currently,there is a lack of comprehensive research on the use of RCA in magnesia-based concrete.This paper intends to clarify the different types of recycled aggregates（（1）untreated recycled aggregate;（2）carbonized recycled aggregate;（3）Mg（OH）₂soaked/carbonation treatment of recycled aggregate）on the carbonation process of magnesium-based concrete,so as to optimize the strengthening process of recycled aggregate and the mix ratio of recycled magnesium-based concrete.The main conclusions are as follows:（1）CO₂curing can improve the strength of magnesium-based concrete.Although the degree of carbonation is reduced,it is beneficial to the development of strength due to the refinement of pore structure and lower water-cement ratio;without sacrificing strength,using RCA can improve the strength and CO₂curing capacity of magnesium-based concrete;（2）Compared with RCA magnesium-based concrete,the hydration degree of RMC in the sample added with CRCA is higher,but the denser microstructure inhibits the transport of CO₂in the sample,At the same time,the mechanical properties of CRCA magnesium-based concrete are better,which may be due to the small water absorption of CRCA,thereby enhancing the workability and mechanical properties of concrete;the degree of carbonation is not the only factor that determines the strength growth of magnesium-based concrete.On the contrary,the pore structure,especially the large pore volume,is more critical for its strength growth;（3）In the A-RCA magnesium-based concrete,the hydration degree of the magnesium-based concrete is low,which may be due to the presence of a large amount of Mg O fine powder in the ITZ of the recycled aggregate,and the water absorption process of Mg O micropowder reduces the free water in the slurry,resulting in a decrease in the degree of carbonation and concrete strength.At the same time,the number and size of large pores（such as trapped air pores）increase significantly due to the reduction of concrete workability.However,in general maintenance,soaking/carbonizing recycled aggregates with Mg（OH）₂can improve the mechanical properties of magnesium-based concrete.Based on the above research results,it is expected that recycled magnesium-based concrete can be used in different engineering scenarios（components,blocks,etc.）.This type of concrete product realizes the efficient sealing of CO₂in concrete while effectively utilizing recycled aggregates,thereby helping to achieve the goal of reducing pollution and carbon in the construction field.