Effects of Carbonation Conditions and Sand-to-Powder Ratio on Compressive Strength and Pore Fractal Characteristics of Recycled Cement Paste–Sand Mortar

This study investigates the influence of carbonation duration and sand-to-powder ratio on the compressive strength and pore structure of recycled cement paste–sand (RCP-S) mortar. Specimens incorporating four different sand contents were subjected to carbonation for 1 and 24 h. Fractal dimensions, ranging from 2.60159 to 3.86742, indicated increased pore complexity with extended carbonation exposure. Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were employed to characterize pore features, including volume, surface area, and diameter. A Menger sponge-based fractal model was applied to compute the fractal dimensions and investigate their relationships with microstructural parameters and mechanical performance. Results showed that prolonged carbonation markedly reduced macropores and large capillary pores, enhanced fine pore content, and improved overall pore connectivity. Fractal analysis revealed that Segments I and IV exhibited the most significant fractal characteristics. The fractal dimension demonstrated exponential correlations with pore diameter; quadratic relationships—with superior statistical performance—with porosity, surface area, and pore volume; and a power–law relationship with compressive strength. These findings highlight the potential of fractal parameters as effective indicators of pore structure complexity and mechanical performance. This study offers a quantitative basis for optimizing pore structure in recycled cementitious materials, promoting their sustainable application in construction.