Hydration-induced strengthening and progressive failure of grouted sandstone under water immersion

Water immersion commonly causes mechanical weakening and deterioration of intact sandstone, whereas the continued hydration of cementitious grout may induce a contrasting strengthening response in grouted sandstone. To quantify this distinct water–grout interaction, cyan sandstone specimens with a central grouting structure were prepared and subjected to uniaxial compression tests coupled with digital image correlation (DIC) observations after immersion for 0, 5, 10, 15, and 20 days. The results show that, within the investigated 0–20 d immersion period, grouted sandstone exhibited hydration-induced strengthening rather than the conventional water-induced weakening observed in intact sandstone. The peak strength and elastic modulus increased progressively with immersion time, from 86.79 MPa to 117.52 MPa and from 10.95 GPa to 12.43 GPa, respectively. This strengthening trend is consistent with a contribution from continued hydration of the cementitious grout during immersion. Energy evolution analysis reveals that the turning point of dissipated energy effectively marks the transition from compaction to stable crack propagation. DIC observations demonstrate a systematic transition in failure mode from axial tensile splitting to tensile–shear composite failure and ultimately to X-shaped conjugate shear failure with increasing immersion duration. Based on these findings, a hydration-strengthened Drucker–Prager constitutive model is established by introducing hydration and elastic modulus strengthening coefficients, which successfully captures the time-dependent evolution of yield strength and elastic stiffness. The proposed model provides a quantitative framework for describing the early-stage mechanical evolution of grouted sandstone subjected to water immersion within the investigated 0–20 d period.