Strength, Permeability and Multiscale Pore Characteristics of Cement-Based Materials with High-Volume Fly Ash Contents and Different Water-Binder Ratios

Fly ash (FA) contents and water-binder ratio (w/b) are the important factors influencing the mechanical characteristics of cement-based materials, meanwhile the porous structure of material plays a vital role. The compressive strength and water permeability of cement-based samples with high-volume FA contents (0%, 30%, 40%) and different w/b (0.3, 0.4, 0.5) were measured. The results demonstrated that higher FA contents or greater w/b led to lower compressive strength and higher water permeability at an early stage. The pore structure features of samples were obtained using mercury intrusion porosimetry and low-temperature nitrogen gas adsorption (LT-NGA). The pore size distributions (PSDs) and the matrix compressibility in cementitious materials were also analyzed, with the compression coefficient K_C ranging from 11.919 to 1.249 × 10⁻⁴ MPa⁻¹. Increasing the FA content and reducing the w/b resulted in an optimized pore structure of the samples, characterized by changing the main pore type from mesopores (10–50 nm) to micropores (< 10 nm). The pores and fractures in the samples also had distinct multifractal characteristics. The strongest negative correlation between the compressive strength and the width of the multifractal generalized spectrum was proved by Pearson correlation analysis, and the correlation coefficient (ρ) was − 0.93. Furthermore, the water permeability had the strongest positive correlation with porosity, with ρ of 0.91. The content of macropores (50–10 μm) had greater influence on its macroscopic properties. The dense structure accompanied by homogeneous PSDs is advantageous for enhancing the strength and anti-permeability properties of cement-based materials.