Study On The Basic Properties And Application Of High Temperature Carbonation Of Recycled Concrete Powder

Rapid urban processes produce a large amount of construction and demolition waste every year including recycled aggregate and recycled concrete powder（RCP）.The accelerate carbonation of RCP is one of the effective ways to improve the sustainable development of concrete in construction industry.Flue gas contains 20%CO₂,which can be used for carbonation of RCP.In addition,the high temperature of flue gas may promote the carbonate efficiency of RCP.A high temperature carbonation method with high efficiency and low energy consumption is proposed in this study to realize the recycling of RCP and provide theoretical basis for cement plant flue gas carbonation.In order to improve the CO₂ uptake,this project carried out a series of studies on the parameter optimization of high-temperature carbonation and the application of high-temperature carbonated products.Firstly,three reaction parameters（temperature,water-solid ratio,carbonation time）were used as influencing factors,and the coupling effect of the above reaction parameters on the CO₂ uptake was investigated by response surface method.High temperature carbonation parameters were optimized to predict CO₂ uptake.Then,the physical and chemical properties of RCP carbonated at different temperatures（20℃,60℃,100℃,140℃）were analyzed to study the influence of high temperature carbonation on the specific surface area,particle size dist ribution,mineral composition and microscopic morphology.RCP was used to partially replace cement to study its influence on the hydration,fresh and mechanical properties of cement-based materials.Finally,the pozzolanic activity of the RCP was characterized and the feasibility of using it as supplementary cementitious materials was compared with other materials.The following conclusion can be drawn:（1）The CO₂ uptake was affected by the coupling effect of temperature and water-solid ratio.High temperature can improve CO₂ diffusion efficiency,but resulted in a rapid water evaporation,which prohibit or even prevent the carbonation reaction.According to the model prediction,the optimum reaction parameter of high temperature carbonation of RCP was at 100℃with water-solid ratio of 0.3 and carbonation time of 30 min.Under optimum condition,the maximum amount of CO ₂sequestration at 16.02%can be achieved.（2）High temperature carbonation increase d the specific surface area and decreased the porosity of RCP.It also increased the particle size distribution,and made the particle size distribution more uniform.As temperature increased,the crystalline of calcium carbonate transformed from vaterite to calcite,forming calcium carbonate with better crystallinity and more stable thermodynamics.Nano-calcium carbonate was formed on the surface of the RCP and become larger as temperature increase d.The carbonation temperature affects the cement properties by affecting the carbonation degree and crystallinity of RCP.The higher the carbonation degree（100℃and 140℃）,the better the performance as cement.The nucleation and chemistry effect of calcium carbonate shorten the setting time of cement paste and promote hydration.（3）Another carbonation product,that is silica gel with high polymerization degree possessed a good pozzolanic activity.The activity is about half of that of fly ash,but its pozzolanic reaction speed is faster.The strength activity index of RCP after carbonation is greater than 75,which indicates it can be used as a supplementary cementitious material in concrete,meeting the requirements of pozzolan materials.