The Effects of Sand Incorporation on the Pore Structure, Strength, and Fractal Characteristics of Alkali-Activated Slag Cementitious Materials

Sand content plays a critical role in regulating the structural compactness and strength development of alkali-activated slag cementitious materials. In this study, three types of specimens—pure slag paste, standard sand mortar, and fine sand mortar—were prepared to investigate the effects of sand incorporation on pore structure and fractal characteristics. Mechanical properties, pore structure, and micro-morphology were systematically evaluated at different curing ages. Mercury intrusion porosimetry (MIP) was employed to measure porosity, pore size distribution, and the threshold pore diameter, while fractal dimensions were calculated to quantify pore complexity and compactness. The results showed that the pure slag paste achieved the highest compressive strength at all ages but posed environmental concerns due to high resource consumption. In contrast, sand-incorporated mortars exhibited stable strength development and continuous pore structure refinement. Notably, the use of fine sand in Group C reduced slag content by approximately 5.6% compared to Group A, contributing to lower CO₂ emissions and enhanced sustainability. Fractal analysis revealed a strong correlation between fractal dimension, pore compactness, and compressive strength. A higher fractal dimension indicated a more complex and interconnected pore network, promoting matrix densification. At 90 days, Group C achieved the highest fractal dimension and lowest porosity, attributed to the micro-filling effect of fine sand, which facilitated the formation of a denser and more continuous gel network. These findings provide a theoretical foundation for the multiscale characterization of alkali-activated cementitious systems and support the design of more sustainable mix formulations.