Effect of Microscopic Pore Structure on the Mechanical Properties of Raw Phosphogypsum–Basalt Fiber Cementitious Materials

This study investigates the mechanical properties and internal pore structure characteristics of raw phosphogypsum–basalt fiber (RPG-BF) cementitious materials with varying raw phosphogypsum (PG) replacement ratios. Specifically, six different PG addition levels (0%, 3%, 6%, 9%, 12%, and 15% by mass of cementitious materials) with a constant basalt fiber dosage of 0.1% (by volume of concrete) were adopted. The mechanical properties of RPG-BF cementitious materials were evaluated by testing the 7-day and 28-day compressive strengths, 28-day split tensile strength, and 28-day flexural strength. Meanwhile, the pore distribution characteristics of the RPG-BF cementitious materials were systematically analyzed using liquid nitrogen adsorption (LNA) tests and scanning electron microscopy (SEM) observations. The experimental results indicate the following: (a) With an increase in PG content, the mechanical properties of RPG-BF cementitious materials exhibit a significant downward trend: the 28-day compressive strength, split tensile strength, and flexural strength decrease by 49%, 44%, and 43%, respectively. (b) The internal pores of the RPG-BF cementitious materials possess excellent fractal characteristics, with fractal dimensions ranging from 2.52 to 2.62. As the PG content increases, the pore structure becomes more intricate and less homogeneous, which is a microstructural factor associated with the degradation of mechanical properties. (c) There exists a strong Pearson’s linear correlation (R > 0.82, with R2 ranging from 0.67 to 0.94) between the pore fractal dimension of RPG-BF cementitious materials and their 7-day/28-day compressive strength, split tensile strength, and flexural strength. (d) SEM observations show that the quantity of micropores and microcracks in the RPG-BF cementitious materials increases with increasing PG content, further confirming deterioration of the material microstructure.