Pore structure and mechanical characteristics of CRS mortar based on NMR and fractal theory

Crushed rock sand (CRS) is increasingly used as a substitute for natural sand in mortar aggregates, and the mixing ratio and pore structure of CRS mortar affect its macromechanical performance. This study used five CRSs with different grain sizes to formulate five single-graded and their synthetic-graded mortars with the same paste film thickness by regulating the aggregate-cement ratio. Utilizing nuclear magnetic resonance (NMR) technology in conjunction with fractal theory, the evolution characteristics of pore structure parameters (porosity, pore fractal dimension and permeability coefficient) and mechanical properties are analyzed, and the correlation between pore structure and mechanical parameters is established. The results indicate that, at a constant paste film thickness, the total porosity (phi total) of the single-graded mortars increases with the augmentation of the aggregate-cement ratio, while the total pore fractal dimension (Dtotal), compressive strength (sigma c), and compressive modulus (Ec) progressively decrease. The complexity of the pore structure in mortars tends toward homogeneity with an increasing number of distinct pore structures. The synthetic-graded mortar exhibits a higher pore structure fractal dimension than the single-graded mortars, indicative of a more intricate pore structure. Furthermore, a significant correlation is observed between the pore structure fractal dimension of the mortar and its porosity, permeability coefficient, sigma c, and Ec, revealing the interplay among the internal pore structures of the material and their control over the macroscopic properties.