Effects of dry–wet and freeze–thaw cycles on the mechanical properties and microstructure of quartz schist

• Pore characteristics of QS exposed to various conditions were studied. • Relationship between macro- and micro-deterioration mechanism of QS was revealed. • Water played a significant role in QS deterioration processes. • Alternating action of F-T and D-W caused serious damage to QS. To address the durability issues of quartz schist (QS) on the slopes of open-pit mines under complex environmental conditions in high-cold regions, this study investigates the effects of freeze-thaw (F-T), dry-wet (D-W), and alternating (D-W-F-T) cycles on the macro and micro characteristics of QS using a multiscale test design. Changes in mechanical strength, mineral composition, and pore distribution were analyzed using triaxial compressive strength (TCS) tests, X-ray diffraction (XRD), and nuclear magnetic resonance (NMR). A study of pore structure was conducted via scanning electron microscopy (SEM), with quantitative analysis of pore roundness, orientation, and fractal dimension. Statistical correlations between macro- and micro-scale parameters were established. The results showed that, at the macro scale, the compressive strength of QS decreased from 115.78 MPa to 54.77 MPa, 41.68 MPa, and 43.91 MPa following F-T, D-W, and D-W-F-T cycles, respectively. At the micro scale, F-T cycles disrupted the cementation between mineral particles, enlarged pore sizes, and formed large pores and cracks. D-W cycles altered the internal mineral structure of the QS, affecting its physicochemical properties. The porosity of the F-T, D-W, and D-W-F-T group specimens increased from 0.22% to 0.53%, 0.43%, and 0.54%, respectively. Combining macro and micro analyses revealed that during the early stages of F-T and D-W cycles, continuous water-rock interactions dissolved hydrophilic minerals and disintegrated clay minerals, increasing weak points in the QS. Prolonged cycles led to pore structure deterioration due to ice extrusion and water-absorbing expansion of mineral components, ultimately reducing the macroscopic strength of the QS.