Effect of Recycled Powder from Construction and Demolition Waste on the Macroscopic Properties and Microstructure of Foamed Concrete with Different Dry Density Grades

Foamed concrete has been widely applied in construction engineering; however, the performance requirements vary across different structural components. Its production typically involves a substantial consumption of cement, which imposes both environmental and economic burdens. Therefore, this study examined foamed concrete with dry apparent densities of 500–1000 kg/m3, in which cement was partially replaced (0–30%) by recycled powder from construction and demolition waste. Macroscopic performance was evaluated through drying shrinkage, compressive strength, softening coefficient, carbonation coefficient, and thermal conductivity, while microstructural analysis was conducted to clarify the underlying mechanisms. The results indicate that the internal composition of the recycled powder primarily consists of SiO2, CaCO3, and C-S-H gel. When recycled powder is used to replace cement, the microstructure of the resulting paste gradually deteriorates compared to that of the control group without recycled powder, and a significant amount of inert SiO2 is introduced. As the replacement ratio of recycled powder increases, the compressive strength of foamed concrete across various density grades exhibits a gradual decline. Notably, when the replacement ratio reaches 30%, the reduction in mechanical performance becomes more substantial. However, the incorporation of recycled powder can effectively mitigate the drying shrinkage of foamed concrete. Moreover, the incorporation of recycled powder exerts minimal influence on the thermal conductivity and porosity of foamed concrete, demonstrating its favorable compatibility and potential for application in foamed concrete systems.