Fine-aggregate source can alter both the load-bearing response and surface scaling of mortar under freezing and thawing, but these responses are often reduced to a single durability ranking. This study compared coal gangue sand mortar (CGM), river sand mortar (RSM), desert sand mortar (DSM), and standard sand mortar (StSM) after 0, 25, and 50 freeze–thaw cycles (FTCs). Compressive strength and mass loss were measured using three replicate specimens per quantitative condition, while post-compression fragments were examined by scanning electron microscopy (SEM). The primary integrative analysis was a parameter-free two-dimensional damage-trajectory map that retained absolute compressive strength and mass loss as separate measured axes; a weighted coupled index was retained only as an auxiliary sensitivity check. Before cycling, the compressive strengths of StSM, RSM, DSM, and CGM were $68.13\pm 0.48$, $53.90\pm 0.39$, $19.47\pm 0.33$, and $6.49\pm 0.07$ MPa, respectively. After 50 FTCs, StSM retained the highest absolute strength ($33.61\pm 0.39$ MPa) and the lowest mass loss ($0.21\pm 0.02\%$), whereas DSM retained $9.91\pm 0.19$ MPa and exhibited the highest mass loss ($17.46\pm 0.05\%$). The StSM trajectory moved primarily toward lower strength with negligible surface-material loss, while DSM moved toward both low residual strength and severe scaling. RSM showed substantial strength reduction followed by later-stage surface loss. CGM followed an atypical trajectory in which measured strength increased to $11.13\pm 0.13$ MPa while mass loss reached $9.67\pm 0.04\%$; because age-matched non-frozen controls were unavailable, this apparent gain cannot be separated from continued hydration and specimen-age effects. The SEM images suggested pores, interfacial discontinuities, cracking, and matrix loosening, although some defects may have been induced or widened by compression. The trajectory representation exposed distinct deterioration modes without arbitrary weighting, whereas the calculated ordering of CGM and RSM in the auxiliary index changed with weighting and normalization choices. The results support reporting absolute residual strength and surface loss jointly when screening alternative fine aggregates for cold-region mortar.

To mitigate the shortage of natural river sand in northwest desert regions, utilize local desert sand resources, and address structural performance under harsh winter conditions, this study investigates the flexural behavior of freeze–thaw conditioned desert sand concrete beams (DSCBs) through rapid freeze–thaw and flexural testing. The investigated variables included desert sand replacement ratios (0%, 20%, 40%, 60%) and freeze–thaw cycles (0, 25, 50, 75). Failure modes, load–concrete strain curves, load–deflection relationships, and load–longitudinal reinforcement strain were analyzed. The results indicate that the crack development and failure modes of DSCBs are similar to those of normal concrete beams, and the plane-section assumption remains valid after freeze–thaw cycles. After 75 freeze–thaw cycles, specimens with the same replacement ratio exhibited the poorest mechanical properties—compared to unfrozen specimens, the ultimate capacity decreased by up to 17.5%, reinforcement strain increased by up to 31.9%, and failure deflection decreased by up to 62.0%. Under all freeze–thaw conditions, the 20% replacement ratio yielded the best performance, with ultimate capacity up to 5.3% higher, reinforcement strain up to 18.2% lower, and failure deflection up to 37.5% higher than those of ordinary concrete beams. Finally, correction factors for desert sand replacement ratio and freeze–thaw cycles were introduced to establish predictive equations for cracking moment and ultimate flexural capacity. The predictions are in good agreement with experimental results, providing a theoretical basis for engineering applications.

Alternative fine aggregates are often assessed using compressive strength at a single reference age, which may conceal an early maximum followed by later strength loss. This preliminary screening study compared mortars containing river sand (RS), standard sand (StS), desert sand (DS), soil sand (SS), coal gangue sand (CGS), and metamorphic rock sand (MRS) under one nominal mixture design. For each aggregate, one mortar batch was prepared and nine 70.7 mm cubes were cast, with three specimens tested at 3, 7, and 28 d. RS, StS, DS, and SS continued to gain strength. Within the single CGS batch, strength decreased from 15.56 ± 0.31 MPa at 7 d to 11.68 ± 0.15 MPa at 28 d; within the single MRS batch, it decreased from 21.63 ± 0.48 MPa to 15.75 ± 0.20 MPa. The corresponding losses were 24.95% and 27.18%, and exploratory within-batch Tukey tests yielded p < 0.001. These statistics describe specimen-level variation within the tested batches and do not establish batch-to-batch reproducibility. Representative 28 d SEM fields, raw-aggregate EDS, and qualitative XRD provide contextual observations but lack the temporal and spatial resolution needed to reconstruct a defect-formation process between 7 and 28 d. Because aggregate moisture state, absorption, flow, air content, and compaction were not independently controlled, the results identify a screening signal that requires independent-batch validation rather than a general material mechanism.

Electrical resistivity is a sensitive indicator of rock damage, yet the quantitative link between pore-structure change and resistivity response under varied hydro-thermo-mechanical conditions remains poorly constrained. In this study, triaxial compression tests were conducted on saturated limestone at confining pressures of 5–20 MPa, pore pressures of 0–9 MPa and temperatures of 20–80 °C, with synchronous measurement of electrical resistivity and pore volume; in situ X-ray computed tomography (X-CT) was performed on miniature specimens at six characteristic loading stages to resolve the accompanying mesoscale pore-structure evolution. Resistivity tracked pore volume closely, rising to at most 1.16 times its initial value as pre-existing microcracks closed, then falling once crack propagation restored fluid connectivity, with the turning point occurring at approximately 50% of the peak deviatoric stress. A resistivity change rate per unit pore volume (ρ′) is introduced to separate the two contributions: its stage-averaged magnitude during unstable crack propagation is about 8.5 times that during linear compression, showing that crack opening alters resistivity far more efficiently than closure of an equivalent pore volume. Consistently, the cementation exponent decreased monotonically throughout loading while the electrical tortuosity reversed from increasing to decreasing at crack initiation, indicating that deformation reorganizes the topology of the conductive network rather than merely its volume. X-CT reconstruction shows pore orientation migrating progressively toward the loading axis, with 93% of the pore volume concentrated within the 18–27° interval after failure, matching the macroscopic shear angle. Crack connectivity, rather than pore fluid content, therefore governs resistivity in damaged limestone, providing a quantitative basis for resistivity-based early warning of rock mass instability in underground engineering.