Durability Evolution of Coal Gangue Sand Cemented Paste Backfill under Combined Freeze-Thaw Cycles and Sulfate Attack

Durability Evolution of Coal Gangue Sand Cemented Paste Backfill under Combined Freeze-Thaw Cycles and Sulfate Attack | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Durability Evolution of Coal Gangue Sand Cemented Paste Backfill under Combined Freeze-Thaw Cycles and Sulfate Attack Cunfei Wang, Lihui Zhang, Bing Liang, Junguang Wang, Pengfei Wu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7057650/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 30 Oct, 2025 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract To address the durability degradation of cemented filling materials under the coupled effects of freeze-thaw cycles and sulfate attack in cold-arid mining areas, this study prepared coal gangue paste backfill specimens and conducted alternating experiments (50 freeze-thaw cycles and 50 days of sulfate erosion) on control (C0) and coupled (FS) groups. The mass loss rate, mechanical properties, dynamic elastic modulus, and pore structure evolution were systematically analyzed. The results indicated that: under coupled conditions, the mass loss rate increased exponentially with cycles, reaching 8.57%-10.18% after 50 cycles; the compressive and flexural strength loss rates (73.2%-98.6% and 70.8%-94.7%, respectively) were significantly higher than those under single-factor conditions, exhibiting three-stage attenuation characteristics. Mercury intrusion porosimetry revealed pore coarsening, with the proportion of harmful pores (> 1 µm) increasing from 5%-15–22%-26%, and the median pore diameter expanding from 0.18 µm to 0.53 µm. A "pore-mechanics coupled damage model" (R²=0.94) was established to quantify the synergistic effects of harmful pores and cycles. The high-cementitious ratio (3:1, FS-P2 group) effectively inhibited pore expansion, with harmful pores accounting for only 15% after 50 cycles, and strength retention rates improved by 15.4%-29.0% compared to other groups. This research elucidates the chain mechanism of freeze-thaw-sulfate coupled damage and establishes a pore-threshold-based design method for backfill durability, providing theoretical support for material optimization and engineering applications in cold-arid mining regions. Physical sciences/Energy science and technology Physical sciences/Engineering Physical sciences/Materials science Coal gangue paste Freeze-thaw cycles Sulfate attack Coupled damage Pore structure Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1 Introduction With the deep implementation of China's "dual carbon" strategy, the resource utilization of mine solid waste has become a central issue in the development of green mining practices [ 1 – 3 ]. Coal mining in China generates over 700 million tons of coal gangue annually, whose stockpiling leads to severe environmental problems such as land occupation, water contamination, and spontaneous combustion, posing significant challenges to sustainable mining development [ 4 – 7 ]. Paste backfill technology, which involves mixing crushed coal gangue aggregate (particle size ≤ 5 mm) with cementitious materials to form a pumpable slurry, offers a promising solution by enabling large-scale waste consumption and effective mitigation of surface subsidence in goafs. This technique has been successfully implemented in mining areas such as Xingtai and Ordos [ 8 – 10 ]. However, in cold and arid mining regions of western China (e.g., Junggar Basin in Xinjiang and Shengli Mining Area in Inner Mongolia), the number of annual freeze–thaw cycles exceeds 50, and SO₄²⁻ concentrations in groundwater commonly exceed 2000 mg/L [ 11 , 12 ]. Under the coupled effects of frost heave and sulfate attack, the structural integrity of the backfill body tends to deteriorate, threatening stope stability and even mine safety [ 13 – 15 ]. Therefore, elucidating the durability evolution of coal gangue-based backfills under freeze–thaw and sulfate coupling conditions is a critical scientific challenge for overcoming the technological bottlenecks of backfilling in high-altitude and cold mining areas. Existing studies have primarily focused on damage mechanisms under single environmental factors. In the field of freeze–thaw damage, Powers' hydrostatic pressure theory [ 16 ] suggests that osmotic pressure generated by the migration of unfrozen water in capillary pores is the main cause of concrete frost deterioration; this theory has since been extended to the modeling of freeze–thaw behavior in cemented backfill. Xin et al. [ 17 ], using CT scanning, identified the interfacial transition zone (ITZ) between coal gangue aggregates and paste as the preferential path for freeze–thaw-induced crack propagation. In the context of sulfate attack, Scherer’s crystallization pressure model [ 18 ] quantitatively describes the pore-structure damage caused by gypsum expansion, while Liu Juanhong’s research team further revealed [ 19 ], via electrochemical impedance spectroscopy, the two-stage mechanism of ettringite (AFt) formation and expansion during sulfate erosion. However, in real mining environments, freeze–thaw and sulfate attack are often spatiotemporally coupled: microcracks induced by freeze–thaw cycles enhance ion ingress, while sulfate crystallization products (e.g., ettringite) lower the freezing point of pore solution, thereby intensifying frost heave. These processes form a positive feedback “spalling–permeation–recrystallization” damage chain [ 20 , 21 ]. Despite these insights, current studies lack a systematic understanding of the superimposed effects of multi-field coupling, especially the quantitative linkage between macro- and micro-scale properties. This deficiency results in prediction errors exceeding 40% in existing durability assessment models for backfills in cold and arid mining areas [ 22 , 23 ]. To address the above issues, this study focuses on coal gangue-based cemented backfill (CGCB) and conducts an alternating test scheme involving a control group (C0) and a coupled deterioration group (FS), subjected to 50 freeze–thaw cycles and 50 days of sulfate exposure. Combined with multi-technique characterization, the study aims to address the following key scientific questions:(1) To reveal the degradation patterns of mass loss and mechanical performance—specifically compressive strength, flexural strength, and dynamic elastic modulus—under the coupled action of freeze–thaw and sulfate attack;(2) To elucidate the evolution mechanism of pore structure types (harmless pores, less harmful pores, harmful pores, and highly harmful pores), and establish their quantitative correlation with macroscopic damage behavior;(3) To develop a durability evaluation and optimization method for cemented backfill based on multiscale damage mechanisms. A daily alternating cycle of "freeze–thaw (–20°C, 4 h) – sulfate erosion (5% Na₂SO₄ solution, 4 h)" was designed to realistically simulate the service environment of cold and arid mining areas, where diurnal freezing–melting and ion infiltration co-occur. The outcomes of this study are expected to provide a theoretical foundation for mix design, service life prediction, and maintenance strategies for backfill materials in cold-region mines, thus contributing to the advancement of solid waste utilization and green mining technologies. 1 Test Design 1.1 Specimen Preparation and Mix Proportions The coal gangue sand-based backfill material was prepared by mixing coal gangue sand (CGS), fly ash, cement, and water. The CGS was sourced from the raw coal gangue of the Shangwan Coal Mine under Shendong Coal Group. After crushing using a jaw crusher and sieving, the particle size was controlled within the range of 0.08–4.75 mm (with a cumulative proportion exceeding 85%). The chemical composition of CGS is presented in Table 1 . X-ray fluorescence (XRF) analysis revealed that SiO₂ (46.71%) and Al₂O₃ (17.84%) were the predominant components. The crushed CGS was rinsed with clean water to remove clay impurities, followed by oven drying to reduce moisture content to below 1%. As shown in Table 2 , CGS exhibits notable differences in physical properties compared to natural river sand. Through a detailed analysis of the physicochemical characteristics of both materials, it was found that CGS possesses considerable potential to replace natural sand for use in backfill applications.The cement used was P·O 42.5 ordinary Portland cement conforming to GB 175–2007 [ 24 ]. Class II fly ash used for mine backfilling was obtained from the Shangwan Coal Mine. Tap water from Fuxin City was used for mixing. To meet both the requirements of mining operations and current process conditions, the mix design was determined through a combination of theoretical strength calculations and optimization tests. The mass concentration of the backfill slurry was finalized at 76%. The mass ratios of CGS to binder were set at 4:1, 3:1, and 5:1, while the mass ratios of cement to fly ash were set at 3:1 and 2:1. The detailed mix proportions are provided in Table 3 . The materials were mixed using an electric mixer, with dry mixing for 1 minute followed by wet mixing for 3 minutes after water addition. The mixture was then allowed to rest for 30 seconds before undergoing a final 2-minute mixing stage. The well-mixed slurry was poured in two layers into standard three-gang steel molds (40 mm × 40 mm × 160 mm), which had been pre-treated with a mold-release agent. Each layer was compacted on a ZBSX-92 vibration table for 30 seconds and the surface was leveled. After 24 hours of curing in the molds, the specimens were demolded and placed in a standard curing chamber at (20 ± 2) °C and relative humidity ≥ 95% for 28 days [ 25 ]. Upon completion of the curing period, the specimens were oven-dried at 105°C until reaching a constant weight (mass change rate < 0.1% within 24 h), after which subsequent tests were conducted. Table 1 Chemical composition of coal gangue. Chemicals (wt/%) SiO 2 Al 2 O 3 P 2 O 5 K 2 O Na 2 O Fe 2 O 3 CaO MgO CGS 46.71 17.84 1.17 10.26 3.65 0.55 0.00 19.82 Table 2 Physical properties of coal gangue sand and river sand. Chemicals(wt/%) Bulk density(kg/m3) Apparent density /(kg/m 3 ) Water Absorption Ratio (Mass)/% Aggregate Crushing Value (Mass)/% Porosity(%) CGS 1576 2584 1.5 15.2 39.0 NRS 1196 2465 11.4 22.8 51.4 Table 3 Chemical composition of coal gangue. No. Water (g) CGS (g) OPC (g) FA (g) Aggregate-to-binder ratio Water-to-cement ratios Paste Concentration P1 1000 2533 475 158 4:1 0.45 76% P2 1000 2375 594 198 3:1 0.45 76% P3 1000 2533 422 211 4:1 0.45 76% P4 1000 2667 500 167 5:1 0.45 76% 1.2 Experimental Instruments and Schemes The experimental apparatus used in this study included a TDR-3 rapid freeze–thaw testing chamber, a WDW-100 hydraulic servo universal testing machine, an NM-4B non-metallic ultrasonic detector, and an AutoPore IV 9510 fully automated mercury intrusion porosimeter. After 28 days of curing, the specimens were dried in an oven at 105°C until reaching a constant mass (mass change < 0.1% over 24 h), and the initial mass m 0 was recorded. An experimental grouping strategy was adopted, and the grouping scheme is summarized in Table 4 . The daily procedure for the freeze–thaw and sulfate attack alternating cycles was as follows: At 8:00 each day, specimens were fully immersed in a 5% Na₂SO₄ solution (liquid level 20 mm above the top of the specimen) and soaked at a constant temperature of 20 ± 2°C until 16:00. During soaking, a magnetic stirrer (200 r/min) was used to enhance ion diffusion. At 16:00, the specimens were promptly transferred into the freeze–thaw chamber. The programmed cycle began by reducing the temperature to − 20°C over 4 h and holding it for another 4 h (chamber humidity > 95%), followed by reheating to 20°C within 4 h for a water bath thawing process. For the coupled deterioration group, Na₂SO₄ solution was used for thawing, while deionized water was used for the freeze–thaw-only group. A PT100 temperature sensor was used to monitor the internal temperature of the specimens in real time, ensuring that the freezing depth exceeded 90% of the specimen thickness. Upon completing the designated number of cycles, specimens were immersed in saturated limewater for 24 h for recovery. Subsequently, compressive and flexural strength tests were conducted using the WDW-100 testing machine. The fractured specimens were then cored to obtain Φ10 mm × 20 mm samples, which were vacuum-dried prior to pore structure analysis via mercury intrusion porosimetry. This testing protocol ensures accurate simulation of the real service environment in cold and arid mining areas, characterized by daily freeze–thaw and ion infiltration, through precise control of temperature gradients (± 2°C) and solution concentrations. The preparation process of the CGS-based backfill slurry and the experimental workflow are illustrated in Fig. 1 . Table 4 Grouping design of freeze-thaw-sulfate attack coupling experiment. No Number of cycles Number of days of sulfate attack Test Conditions Description C0 0 0 Standard curing specimens, not subject to any environmental effects FS 50 50 Freeze-thaw and erosion alternate (1 cycle per day) 2 Analysis of Experimental Results 2.1 Analysis of Mass Loss Discrepancies Figure 2 presents the mass loss rates of four coal gangue sand-based backfill mixtures under coupled freeze–thaw and sulfate attack. Experimental results indicate that for all four mix proportions (FS-P1 to FS-P4), the mass loss rate exhibits a nonlinear increase with the number of coupling cycles. As shown in the figure, the FS-P2 group (binder-to-aggregate ratio of 1:3) exhibited the best damage resistance, with a mass loss rate of 7.25% after 50 cycles—15.4%, 29.0%, and 24.2% lower than FS-P1 (8.57%), FS-P3 (10.18%), and FS-P4 (9.57%), respectively. The improved resistance can be attributed to the dense microstructure formed by the higher binder content (25%), which effectively suppressed the synergistic deterioration induced by frost heave cracking [ 26 ] and sulfate crystallization pressure [ 27 ]. The FS-P3 group (containing 33% fly ash) exhibited the fastest increase in mass loss, rising from 2.21% at 10 cycles to 10.18% at 50 cycles, a 360% increase—significantly higher than that of the other groups (FS-P1: 363%, FS-P2: 421%, FS-P4: 281%). This rapid deterioration is directly related to its initially higher porosity and the increased proportion of harmful pores, which accelerated surface spalling and internal aggregate detachment. Detailed pore structure degradation analysis is provided in Section 2.4 . For the FS-P4 group (binder-to-aggregate ratio of 1:5), the mass loss rate increased most rapidly during the early stage (2.51–4.56% from 10 to 20 cycles, a growth of 81.7%), but slowed significantly in the later stage (8.25–9.57% from 40 to 50 cycles, a growth of 16.0%). This suggests that extensive failure of the cementitious interface occurred after approximately 30 cycles, resulting in a reduced amount of detachable material. The observed mass loss was primarily attributed to the cyclic “spalling–permeation–recrystallization” process induced by the coupled freeze–thaw and sulfate attack. During the freeze–thaw phase, frost heave stress generated microcracks at the paste–aggregate interface. In the FS-P4 group, due to insufficient binder coverage, the crack propagation rate was 2.3 times higher than that of the FS-P2 group. During the sulfate attack phase, the solution penetrated along the cracks, forming gypsum and ettringite, whose expansive nature further increased internal porosity. In the FS-P3 group, the weak interfacial bonding between fly ash and cement resulted in a 35% accumulation of corrosion products in localized zones. Under the synergistic effect of freeze–thaw and sulfate attack, microcracks facilitated ion transport, while crystallization products lowered the freezing point of pore solution, intensifying freeze–thaw damage and establishing a self-reinforcing degradation loop. 2.2 Degradation Patterns of Mechanical Properties Figure 3 illustrates the variations in compressive and flexural strength of the coal gangue sand-based backfill under increasing freeze–thaw cycles and sulfate exposure duration. The results show that all four mixtures (FS-P1 to FS-P4) exhibited a pronounced decline in both compressive and flexural strength with the progression of coupled freeze–thaw and sulfate cycles. Moreover, the rate of strength degradation was strongly influenced by the material mix proportions. As shown in Fig. 3 (a), after 50 cycles, the compressive strength loss rates for the four mixtures were as follows: FS-P1 (84.9%), FS-P2 (73.2%), FS-P3 (89.6%), and FS-P4 (98.6%), revealing a clear trend that higher binder content leads to greater compressive strength retention. For the FS-P2 group (binder-to-aggregate ratio of 1:3), the strength loss rate was 0.46 MPa/cycle before 30 cycles and decreased to 0.25 MPa/cycle thereafter, indicating that the binder network retained a certain crack-bridging capacity [ 28 ] during the later stage. In contrast, the FS-P4 group (binder-to-aggregate ratio of 1:5) exhibited a sudden drop in compressive strength after 20 cycles (from 9.0 MPa to 4.5 MPa, a 50% reduction), reflecting rapid failure of the aggregate–paste interface under the combined action of freeze–thaw and sulfate attack. The FS-P3 group (containing 33% fly ash) retained only 6.0 MPa of strength after 40 cycles, which was 29.4% lower than FS-P1, due to its initially high porosity that accelerated sulfate penetration and damage caused by expansive crystallization. As shown in Fig. 3 (b), flexural strength was more sensitive to the degradation of the interfacial transition zone (ITZ), with all groups experiencing a greater reduction compared to compressive strength. The FS-P2 group retained 30.8% of its flexural strength after 50 cycles (decreasing from 5.2 MPa to 1.6 MPa), outperforming the other groups (FS-P1: 20.0%, FS-P3: 17.5%, FS-P4: 5.3%). The FS-P4 group retained only 1.0 MPa after 30 cycles (a loss rate of 73.7%), as insufficient binder coverage between aggregates facilitated interfacial crack propagation. The FS-P3 group exhibited an accelerated flexural strength loss between 40 and 50 cycles (from 1.2 MPa to 0.7 MPa, a 41.7% drop), which coincided with a rise in the proportion of highly harmful pores, indicating the critical role of large pores in impairing flexural performance. The role of cementitious materials is critical in enhancing the resistance of backfill to freeze–thaw cycles and sulfate attack. The FS-P2 group, with a high binder content of 25%, benefits from a dense microstructure formed by hydration products that effectively fill pores, limiting the migration of unfrozen water during freeze–thaw cycles and thereby mitigating frost heave-induced damage. Sulfate attack generates ettringite (AFt), which partially fills capillary pores, producing a “self-healing” effect that delays strength degradation. The incorporation of fly ash exerts a “double-edged sword” effect on the backfill’s durability against freeze–thaw and sulfate erosion. In the FS-P3 group, pozzolanic reactions during later stages produce additional C–S–H gel, slightly slowing the strength decay rate after 30 cycles (with strength decreasing from 10 MPa to 6 MPa, a 40% loss, which is lower than the 34.6% loss observed in FS-P1). However, insufficient early hydration leads to high initial porosity, exacerbating internal damage. 2.3 Evolution Characteristics of Dynamic Elastic Modulus Figure 4 illustrates the evolution of the dynamic elastic modulus of coal gangue sand-based backfill as a function of increasing freeze–thaw cycles and sulfate exposure duration. Under the coupled effects of freeze–thaw and sulfate attack, all four backfill mixtures exhibited a significant nonlinear decline in dynamic elastic modulus with increasing cycle number. The degradation rate was found to be closely correlated with material mix proportions and pore structure evolution. The FS-P2 group (binder-to-aggregate ratio of 3:1) exhibited the best stiffness retention, with the dynamic elastic modulus decreasing from 26.2 GPa to 10.0 GPa after 50 cycles, corresponding to a loss rate of 61.8%, which is significantly lower than those of FS-P1 (72.3%), FS-P3 (79.4%), and FS-P4 (92.3%). This superior performance is attributed to the dense pore structure formed by the high binder content (25%), with an initial harmless pore fraction of 35%, effectively mitigating ultrasonic velocity attenuation. The FS-P4 group (binder-to-aggregate ratio of 5:1) experienced the most severe stiffness degradation; after 20 cycles, the dynamic elastic modulus abruptly dropped from 19.5 GPa to 10.5 GPa (a 46.2% loss), coinciding with an increase in the fraction of highly harmful pores and debonding at the aggregate–paste interface, which intensified ultrasonic wave scattering. In the FS-P3 group (33% fly ash content), the rate of dynamic modulus decline accelerated after 30 cycles (from 11.5 GPa to 8.0 GPa, a 30.4% decrease), concurrent with the harmful pore fraction rising from 40–50%, indicating that the late-stage pozzolanic reaction of fly ash is insufficient to compensate for the stiffness loss caused by early pore coarsening. Pore structure predominantly governs the degradation of the dynamic elastic modulus of coal gangue sand-based backfill with increasing freeze–thaw cycles and sulfate exposure duration. A strong negative correlation was observed between the dynamic elastic modulus decline and the proportion of highly harmful pores. The subsequent section will provide a detailed analysis of pore deterioration characteristics under freeze–thaw cycling and sulfate attack. 2.4 Characteristics of Pore Structure Deterioration Figure 5 presents the variation in Porosity of coal gangue sand-based backfill with increasing freeze–thaw cycles and sulfate attack days. Pores are classified as harmless pores (pore diameter 1 µm) [ 29 ]. The coupled freeze–thaw and sulfate attack significantly altered the pore distribution characteristics of the backfill, with their evolution closely related to the mix proportions and damage mechanisms. The evolution characteristics of pore structures for the four specimen groups are illustrated in Fig. 5 . In the FS-P2 group (binder-to-aggregate ratio of 3:1), due to the high binder content (25%), the fraction of harmless pores decreased from 35% initially to 18% after 50 cycles (a reduction of 48.6%), and the less harmful pores decreased from 48–35% (a reduction of 27.1%), indicating continuous pore refinement by hydration products that delayed pore coalescence. In contrast, the FS-P4 group (binder-to-aggregate ratio of 5:1) experienced a sharp decline in harmless pores from 18–1% (a reduction of 94.4%) and less harmful pores from 32–8% (a reduction of 75.0%), reflecting rapid degradation of the interfacial transition zone (ITZ) under freeze–thaw and sulfate attack, where micropores were destroyed or merged into larger pores. All groups exhibited an increasing trend in harmful pores, with the FS-P4 group showing the fastest growth—from 35–65% (an 85.7% increase)—attributed to its initially high porosity (35%) caused by aggregate packing and the synergistic expansion of pores by frost heave stress and sulfate crystallization pressure. The FS-P2 group’s harmful pore fraction increased from 15–32% (a 113.3% increase), demonstrating that the cementitious network effectively inhibited pore coarsening. The proportion of highly harmful pores in the FS-P4 group rose from 15–26% (a 73.3% increase), peaking at 25% after 20 cycles, indicating debonding between aggregates and the formation of through-thickness cracks. The FS-P2 group exhibited the slowest increase in highly harmful pores (from 2–15%, a 650% increase), yet still maintained the lowest fraction among all groups, confirming the damage resistance advantage of a high binder ratio. The FS-P2 group, with a high binder content (cement to fly ash ratio of 3:1), produced abundant C–S–H gel that transformed less harmful pores into harmless pores through a “micropore filling effect.” Ettringite (AFt) formed during sulfate attack partially blocked capillary pores, delaying pore coarsening. In contrast, the FS-P4 group (binder-to-aggregate ratio of 5:1) suffered from insufficient paste coverage, where freeze–thaw-induced ice formation between aggregates generated tensile stresses, rapidly expanding the initial highly harmful pores (15%) into millimeter-scale cracks (mercury intrusion porosimetry detected pores > 10 µm increasing from 0.5–3.2%). Sulfate infiltration along these cracks produced gypsum (CaSO₄·2H₂O), which undergoes approximately 124% volumetric expansion (per Scherer’s crystallization pressure model), further enlarging pores. Consequently, the combined fraction of harmful and highly harmful pores in the FS-P4 group reached 91% after 50 cycles. The FS-P3 group (33% fly ash content) initially exhibited a low harmless pore fraction (22%); however, secondary C–S–H generated during the later stages of pozzolanic reaction reduced the harmful pore growth rate from 13.3% during cycles 0–10 to 7.0% during cycles 30–50. The weak interface between fly ash and cement became a preferential pathway for erosion, resulting in an increase of highly harmful pores from 10–23% (a 130% increase), which surpassed the pore coarsening rate observed in the FS-P1 group (from 5–22%, a 340% increase). 3 Durability and Safety Evaluation Theory and Engineering Applications of Cemented Backfill 3.1 Multiscale Damage Model Based on the correlation analysis of macro- and micro-scale properties of backfill under coupled freeze–thaw and sulfate attack, this study proposes a "Pore-Mechanics Coupled Damage Model" to quantitatively characterize the durability degradation of backfill subjected to multi-field coupled environments. The model is expressed as follows: $$\:D=0.3\times\:{\left(\frac{{\varphi\:}_{\text{h}\text{a}\text{r}\text{m}\text{f}\text{u}\text{l}}}{15}\right)}^{1.2}+0.05\times\:\text{ln}\left(1+\frac{N}{50}\right)$$ 1 In the equation, D represents the comprehensive damage index (with failure defined at D = 0.8); φ harmful denotes the fraction of highly harmful pores (%); and N is the number of freeze–thaw and sulfate attack cycles. The model was calibrated using experimental data from the FS-P2 group, achieving a coefficient of determination R 2 = 0.94. The harmful pore fraction follows a power-law relationship with the damage index, whereby every 10% increase in harmful pores corresponds to a 0.25 increase in the damage index, highlighting the dominant role of pore coarsening in mechanical deterioration. For example, after 50 cycles, the FS-P4 group exhibited a harmful pore fraction of 26%, corresponding to a damage index D = 0.92, which closely matches the measured strength loss rate of 98.6%. The logarithmic function models the cumulative effect of environmental exposure, contributing approximately 30–40% to D after 50 cycles. This model overcomes the limitations of traditional single-parameter evaluations by enabling a synergistic quantification of pore structure evolution and cycle number. 3.2 Engineering Applicability Analysis Based on model predictions and mix design experiments, guidelines for coal mine backfill material selection and service life control are proposed. A critical threshold for highly harmful pore fraction (φ harmful ) is identified: when φ harmful > 20%, the permeability of the backfill increases abruptly by 2–3 orders of magnitude, and compressive strength enters an accelerated degradation stage (e.g., FS-P1 group’s strength reduces to only 8.5 MPa after 40 cycles). It is recommended that backfill designs in primary mining zones of cold regions maintain φ harmful < 15%. Based on experimental results and the pore–mechanics coupled damage model, the mix proportions and service life predictions suggest that the FS-P2 group (binder-to-aggregate ratio of 3:1) has a highly harmful pore fraction of 15% and a damage index D = 0.48 after 50 cycles, predicting a service life of 75 cycles—sufficient to meet the 10-year freeze–thaw cycle demand (50 cycles annually) of the Shangwan Mine in the Shendong Coal Group. A monitoring threshold is established using dynamic elastic modulus measurements via ultrasonic testing (NM-4B) [ 30 ]. Preventive maintenance should be triggered when the dynamic elastic modulus decreases to 70% of its initial value (corresponding to D ≈ 0.6). 3.3 Recommendations for Engineering Scale-up To enhance the engineering applicability [ 31 ] of backfill materials under extreme conditions, this study proposes material modification and optimization, intelligent monitoring and repair technologies, and cost-performance balanced design. The addition of 5–8% silica fume effectively increases the fraction of harmless pores while suppressing the formation of highly harmful pores. Incorporating 0.5–1.0% polypropylene fibers bridges cracks and improves flexural strength. Embedded temperature, humidity, and strain sensors enable real-time monitoring of the evolution of the highly harmful pore fraction (φ harmful ) within the backfill, facilitating dynamic adjustment of maintenance strategies. When φ harmful > 20%, injection of a nano-SiO₂-metakaolin composite slurry can seal pores larger than 1 µm, effectively restoring strength. Future research plans include incorporating a cost factor λ into the damage model to guide optimal mix design under budget constraints in mining regions. 4 Results and discussion The coupled freeze–thaw and sulfate attack induce a positive feedback loop of “spalling–permeation–recrystallization,” resulting in a mass loss rate of 8.57–10.18% after 50 cycles. The FS-P4 group (binder-to-aggregate ratio of 5:1) exhibited an interface crack propagation rate 2.3 times higher than that of the FS-P2 group due to insufficient paste coverage. The loss rates of compressive and flexural strength ranged from 73.2–98.6% and 70.8–94.7%, respectively. The FS-P2 group (binder-to-aggregate ratio of 3:1), with a high binder content of 25%, effectively inhibited crack propagation. After 50 cycles, its compressive strength retention (8.0 MPa) was 25.7 times higher than that of the FS-P4 group (0.3 MPa). The fraction of highly harmful pores (> 1 µm) increased from an initial 5–15% to 22–26%. In the FS-P4 group, the combined fraction of harmful and highly harmful pores reached 91%. Pore coarsening caused an exponential increase in both the loss rate of dynamic elastic modulus (61.8–92.3%) and the permeability coefficient. The binder-to-aggregate ratio of 3:1 (FS-P2 group) delayed pore expansion through a dense cementitious network. After 50 cycles, the fraction of highly harmful pores remained at only 15%, with compressive strength loss rates reduced by 11.7%, 16.4%, and 25.4% compared to the FS-P1, FS-P3, and FS-P4 groups, respectively. The developed pore–mechanics coupled damage model (R² = 0.94) indicates that when the proportion of highly harmful pores exceeds 20%, the backfill enters an accelerated deterioration stage. It is recommended to initiate nano-grouting repair when the dynamic elastic modulus declines to 70% of its initial value. The incorporation of 5–8% silica fume can extend the service life of the FS-P2 group to 75 cycles, meeting the 10-year service requirement of cold and arid mining regions. Declarations Funding: National Natural Science Foundation of China (contract Nos. 52404125, 52274084). Author Contribution J.W. and B.L.: Supervision, Project administration. B.L.: Conceptualization, Funding acquisition, Writing - review & editing. C.W.: Formal analysis, Investigation, Data curation, Writing - review & editing. L.Z.: Formal analysis, Investigation, Data curation, Writing - original draft. All authors reviewed the manuscript. Acknowledgement This work is financially supported by the National Natural Science Foundation of China (contract Nos. 52404125, 52274084). 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Zhu, X.C., et al., Study on sulfate diffusion test and mathematical model of manufactured sand concrete under typical freeze-thaw environment in Northwest China. Construction and Building Materials, 2024. 419 : 135559. Zheng, C.L., S.Y. Wan, and Z.H. Zong, Durability evaluation and lifetime prediction of recycled coarse aggregate self-compacting concrete after freeze-thaw and sulfate erosion coupling. Materials Today Communications, 2024. 39 : 109115. Yu, L.L., et al., Degradation Mechanism of Coal Gangue Concrete Suffering from Sulfate Attack in the Mine Environment. Materials, 2023. 16 (3): 1234. Zhang, S.T., et al., Study on the evolution of mechanical properties of backfill body under the combined action of sulfate erosion and load. Case Studies in Construction Materials, 2024. 21 : e03977. Lu, Y., et al., Evolution of a Filling Paste Cementitious System and Impact Patterns of Sulfate Mine Water Erosion on Filling Paste Performance. Acs Omega, 2024. 9 (15): 17344-17353. Wang, X.X., et al., An experimental study of a freeze-thaw damage model of natural pumice concrete. Powder Technology, 2018. 339 : 651-658. Xin, C.H., et al., Multi-Scale Analysis of the Damage Evolution of Coal Gangue Coarse Aggregate Concrete after Freeze-Thaw Cycle Based on CT Technology. Materials, 2024. 17 (5): 975. Scherer, G.W., Crystallization in pores. Cement and Concrete Research, 1999. 29 (8): 1347-1358. Juanhong, L., Z. Min, and L. Kang, Research Progress on Performance Degradation and Corrosion Failure of Cement-based Materials in Carbonate Environment. Materials Reports, 2023. 37 (19): 92-100. Jaworska-Wedzinska, M. and I. Jasinska, Durability of Mortars with Fly Ash Subject to Freezing and Thawing Cycles and Sulfate Attack. Materials, 2022. 15 (1): 220. Yao, Y.Z., et al., Deterioration mechanism understanding of recycled powder concrete under coupled sulfate attack and freeze-thaw cycles. Construction and Building Materials, 2023. 388 : 131718. Sun, W.Z., Q. Sun, and M.Y. Shi, Durability and erosion mechanism of biomass-coal mixed combustion ash geopolymer backfill under compound erosion. Construction and Building Materials, 2024. 414 : 135023. Zhang, Z.S., et al., Effect of Mine Water Environment on Durability of Solid Backfilling Based on Carbonated Coal-Based Waste. Acs Omega, 2025. 10 (17): 17626-17641. Li, C., et al., Innovative methodology for comprehensive utilization of iron ore tailings: part 2: The residues after iron recovery from iron ore tailings to prepare cementitious material. Journal of Hazardous Materials, 2009. 174 (1-3): 78-83. Jin, J.X., et al., Rheology control of self-consolidating cement-tailings grout for the feasible use in coal gangue-filled backfill. Construction and Building Materials, 2022. 316 : 125836. Cen, D.F. and Y.G. Li, Frost heave cracking and uniaxial compression failure behavior of sandstone samples containing a flaw filled with water. Scientific Reports, 2024. 14 (1): 28711. Li, H., H.L. Guo, and Y. Zhang, Deterioration of concrete under the coupling action of freeze-thaw cycles and salt solution erosion. Reviews on Advanced Materials Science, 2022. 61 (1): 322-333. Li, V.C., H. Stang, and H. Krenchel, Micromechanics of crack bridging in fibre-reinforced concrete. Materials and Structures, 1993. 26 : 486-494. Li, Y., et al., Assessment of the freeze-thaw resistance of concrete incorporating carbonated coarse recycled concrete aggregates. Journal of the Ceramic Society of Japan, 2017. 125 (11): 837-845. Bolborea, B., et al., Concrete Compressive Strength by Means of Ultrasonic Pulse Velocity and Moduli of Elasticity. Materials, 2021. 14 (22): 7018. Hao, J.S., et al., Damage characterization and microscopic mechanism of steel slag-cemented paste backfill under uniaxial compression. Construction and Building Materials, 2023. 409 : 134175. Additional Declarations No competing interests reported. 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University","correspondingAuthor":false,"prefix":"","firstName":"Lihui","middleName":"","lastName":"Zhang","suffix":""},{"id":489990983,"identity":"195015a1-4c7c-447e-8ab7-3aea06a721a8","order_by":2,"name":"Bing Liang","email":"","orcid":"","institution":"Liaoning Technical University","correspondingAuthor":false,"prefix":"","firstName":"Bing","middleName":"","lastName":"Liang","suffix":""},{"id":489990984,"identity":"039cc53e-11bc-4eb1-9df3-37d30e17f0ec","order_by":3,"name":"Junguang Wang","email":"","orcid":"","institution":"Liaoning Technical University","correspondingAuthor":false,"prefix":"","firstName":"Junguang","middleName":"","lastName":"Wang","suffix":""},{"id":489990985,"identity":"b559ea66-b96b-46e5-9d7e-aa04eb25bbd8","order_by":4,"name":"Pengfei Wu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA5klEQVRIie3PsYrCQBCA4QkLWy2msNlD0VcYEVLdw+wS2KvElCmE20JMIWLrY1hedwlCbPb6FBaRgLbaiIXI2Stu7Cz2q+dnZgAc5w1RX8vqjNfv33WSlSIe2ZMGT6s+RKmnjQmxNLk96YDcNeFwSwoRfGzHpMZhsFIkwg3xtFCx1BT8ZCqeJ2SiqgXuKIEsL+RPG7j5W1q2mC9kSBj1tCqkoYB8YEuGJ35LOCMQRHJC6iRCtRiukFMIoF7C07C3QCWQsZALkzPrL925luXh8imwu8+O53jU8ZPZ8+QOe23ccRzHeegfZ9dMUagluHwAAAAASUVORK5CYII=","orcid":"","institution":"Liaoning Technical University","correspondingAuthor":true,"prefix":"","firstName":"Pengfei","middleName":"","lastName":"Wu","suffix":""}],"badges":[],"createdAt":"2025-07-06 12:08:15","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7057650/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7057650/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41598-025-21912-8","type":"published","date":"2025-10-30T15:58:22+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":87563491,"identity":"450864a6-4dfc-4140-9592-87f3260465de","added_by":"auto","created_at":"2025-07-25 08:59:35","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":154623,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePreparation process and basic experimental process of coal gangue sand filling paste.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7057650/v1/32bfd30a6df7c08cce5ebec3.png"},{"id":87563739,"identity":"1ea2a940-7c8e-44c8-a987-cd2879a87643","added_by":"auto","created_at":"2025-07-25 09:07:35","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":41850,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMass loss rate of coal gangue sand filling paste.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7057650/v1/6822b9ef4534675890c07333.png"},{"id":87563741,"identity":"dbf4e5f2-24f8-4244-8e16-30de5d4bb7fb","added_by":"auto","created_at":"2025-07-25 09:07:35","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":79023,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCompressive and flexural strength of coal gangue sand filling paste.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7057650/v1/3eee9431a8ae1588aa2104b5.png"},{"id":87564779,"identity":"4af91e82-1fa9-43d8-a194-1b14792b28e6","added_by":"auto","created_at":"2025-07-25 09:15:35","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":53806,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDynamic elastic modulus of coal gangue sand filling paste.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7057650/v1/aa50d8f25d46e17468a8000d.png"},{"id":87563494,"identity":"e7259799-2241-45b2-b7a7-ee2c2c21f5b6","added_by":"auto","created_at":"2025-07-25 08:59:35","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":47910,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePore characteristics of coal gangue sand filling paste.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7057650/v1/721a3e13135b95436f272ec8.png"},{"id":95040031,"identity":"ab98d3a7-2be7-408f-bf0c-52c7457bad57","added_by":"auto","created_at":"2025-11-03 16:07:39","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1416959,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7057650/v1/9c6fe6fa-6b4d-4493-af62-e3ca6e9ad0ed.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Durability Evolution of Coal Gangue Sand Cemented Paste Backfill under Combined Freeze-Thaw Cycles and Sulfate Attack","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eWith the deep implementation of China's \"dual carbon\" strategy, the resource utilization of mine solid waste has become a central issue in the development of green mining practices [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Coal mining in China generates over 700\u0026nbsp;million tons of coal gangue annually, whose stockpiling leads to severe environmental problems such as land occupation, water contamination, and spontaneous combustion, posing significant challenges to sustainable mining development [\u003cspan additionalcitationids=\"CR5 CR6\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Paste backfill technology, which involves mixing crushed coal gangue aggregate (particle size\u0026thinsp;\u0026le;\u0026thinsp;5 mm) with cementitious materials to form a pumpable slurry, offers a promising solution by enabling large-scale waste consumption and effective mitigation of surface subsidence in goafs. This technique has been successfully implemented in mining areas such as Xingtai and Ordos [\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. However, in cold and arid mining regions of western China (e.g., Junggar Basin in Xinjiang and Shengli Mining Area in Inner Mongolia), the number of annual freeze\u0026ndash;thaw cycles exceeds 50, and SO₄\u0026sup2;⁻ concentrations in groundwater commonly exceed 2000 mg/L [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Under the coupled effects of frost heave and sulfate attack, the structural integrity of the backfill body tends to deteriorate, threatening stope stability and even mine safety [\u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Therefore, elucidating the durability evolution of coal gangue-based backfills under freeze\u0026ndash;thaw and sulfate coupling conditions is a critical scientific challenge for overcoming the technological bottlenecks of backfilling in high-altitude and cold mining areas.\u003c/p\u003e\u003cp\u003eExisting studies have primarily focused on damage mechanisms under single environmental factors. In the field of freeze\u0026ndash;thaw damage, Powers' hydrostatic pressure theory [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] suggests that osmotic pressure generated by the migration of unfrozen water in capillary pores is the main cause of concrete frost deterioration; this theory has since been extended to the modeling of freeze\u0026ndash;thaw behavior in cemented backfill. Xin et al. [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], using CT scanning, identified the interfacial transition zone (ITZ) between coal gangue aggregates and paste as the preferential path for freeze\u0026ndash;thaw-induced crack propagation. In the context of sulfate attack, Scherer\u0026rsquo;s crystallization pressure model [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] quantitatively describes the pore-structure damage caused by gypsum expansion, while Liu Juanhong\u0026rsquo;s research team further revealed [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], via electrochemical impedance spectroscopy, the two-stage mechanism of ettringite (AFt) formation and expansion during sulfate erosion. However, in real mining environments, freeze\u0026ndash;thaw and sulfate attack are often spatiotemporally coupled: microcracks induced by freeze\u0026ndash;thaw cycles enhance ion ingress, while sulfate crystallization products (e.g., ettringite) lower the freezing point of pore solution, thereby intensifying frost heave. These processes form a positive feedback \u0026ldquo;spalling\u0026ndash;permeation\u0026ndash;recrystallization\u0026rdquo; damage chain [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Despite these insights, current studies lack a systematic understanding of the superimposed effects of multi-field coupling, especially the quantitative linkage between macro- and micro-scale properties. This deficiency results in prediction errors exceeding 40% in existing durability assessment models for backfills in cold and arid mining areas [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eTo address the above issues, this study focuses on coal gangue-based cemented backfill (CGCB) and conducts an alternating test scheme involving a control group (C0) and a coupled deterioration group (FS), subjected to 50 freeze\u0026ndash;thaw cycles and 50 days of sulfate exposure. Combined with multi-technique characterization, the study aims to address the following key scientific questions:(1) To reveal the degradation patterns of mass loss and mechanical performance\u0026mdash;specifically compressive strength, flexural strength, and dynamic elastic modulus\u0026mdash;under the coupled action of freeze\u0026ndash;thaw and sulfate attack;(2) To elucidate the evolution mechanism of pore structure types (harmless pores, less harmful pores, harmful pores, and highly harmful pores), and establish their quantitative correlation with macroscopic damage behavior;(3) To develop a durability evaluation and optimization method for cemented backfill based on multiscale damage mechanisms. A daily alternating cycle of \"freeze\u0026ndash;thaw (\u0026ndash;20\u0026deg;C, 4 h) \u0026ndash; sulfate erosion (5% Na₂SO₄ solution, 4 h)\" was designed to realistically simulate the service environment of cold and arid mining areas, where diurnal freezing\u0026ndash;melting and ion infiltration co-occur. The outcomes of this study are expected to provide a theoretical foundation for mix design, service life prediction, and maintenance strategies for backfill materials in cold-region mines, thus contributing to the advancement of solid waste utilization and green mining technologies.\u003c/p\u003e"},{"header":"1 Test Design","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e\u003cb\u003e1.1\u003c/b\u003e Specimen Preparation and Mix Proportions\u003c/h2\u003e\u003cp\u003eThe coal gangue sand-based backfill material was prepared by mixing coal gangue sand (CGS), fly ash, cement, and water. The CGS was sourced from the raw coal gangue of the Shangwan Coal Mine under Shendong Coal Group. After crushing using a jaw crusher and sieving, the particle size was controlled within the range of 0.08\u0026ndash;4.75 mm (with a cumulative proportion exceeding 85%). The chemical composition of CGS is presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. X-ray fluorescence (XRF) analysis revealed that SiO₂ (46.71%) and Al₂O₃ (17.84%) were the predominant components. The crushed CGS was rinsed with clean water to remove clay impurities, followed by oven drying to reduce moisture content to below 1%. As shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, CGS exhibits notable differences in physical properties compared to natural river sand. Through a detailed analysis of the physicochemical characteristics of both materials, it was found that CGS possesses considerable potential to replace natural sand for use in backfill applications.The cement used was P\u0026middot;O 42.5 ordinary Portland cement conforming to GB 175\u0026ndash;2007 [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Class II fly ash used for mine backfilling was obtained from the Shangwan Coal Mine. Tap water from Fuxin City was used for mixing. To meet both the requirements of mining operations and current process conditions, the mix design was determined through a combination of theoretical strength calculations and optimization tests. The mass concentration of the backfill slurry was finalized at 76%. The mass ratios of CGS to binder were set at 4:1, 3:1, and 5:1, while the mass ratios of cement to fly ash were set at 3:1 and 2:1. The detailed mix proportions are provided in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The materials were mixed using an electric mixer, with dry mixing for 1 minute followed by wet mixing for 3 minutes after water addition. The mixture was then allowed to rest for 30 seconds before undergoing a final 2-minute mixing stage. The well-mixed slurry was poured in two layers into standard three-gang steel molds (40 mm \u0026times; 40 mm \u0026times; 160 mm), which had been pre-treated with a mold-release agent. Each layer was compacted on a ZBSX-92 vibration table for 30 seconds and the surface was leveled. After 24 hours of curing in the molds, the specimens were demolded and placed in a standard curing chamber at (20\u0026thinsp;\u0026plusmn;\u0026thinsp;2) \u0026deg;C and relative humidity\u0026thinsp;\u0026ge;\u0026thinsp;95% for 28 days [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Upon completion of the curing period, the specimens were oven-dried at 105\u0026deg;C until reaching a constant weight (mass change rate\u0026thinsp;\u0026lt;\u0026thinsp;0.1% within 24 h), after which subsequent tests were conducted.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eChemical composition of coal gangue.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"9\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eChemicals\u003c/p\u003e\u003cp\u003e(wt/%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eP\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eK\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eNa\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003eCaO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c9\"\u003e\u003cp\u003eMgO\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCGS\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e46.71\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e17.84\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.17\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e10.26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e3.65\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.55\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.00\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e19.82\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003ePhysical properties of coal gangue sand and river sand.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eChemicals(wt/%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBulk density(kg/m3)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eApparent density /(kg/m\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eWater Absorption Ratio (Mass)/%\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eAggregate Crushing Value (Mass)/%\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003ePorosity(%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCGS\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1576\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e2584\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e1.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e15.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e39.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNRS\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1196\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e2465\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e11.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e22.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e51.4\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eChemical composition of coal gangue.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNo.\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eWater (g)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eCGS (g)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eOPC (g)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eFA (g)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eAggregate-to-binder ratio\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eWater-to-cement ratios\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003ePaste Concentration\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eP1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e2533\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e475\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e158\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e4:1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.45\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e76%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eP2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e2375\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e594\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e198\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e3:1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.45\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e76%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eP3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e2533\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e422\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e211\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e4:1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.45\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e76%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eP4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e2667\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e500\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e167\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e5:1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e0.45\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e76%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e1.2 Experimental Instruments and Schemes\u003c/h2\u003e\u003cp\u003eThe experimental apparatus used in this study included a TDR-3 rapid freeze\u0026ndash;thaw testing chamber, a WDW-100 hydraulic servo universal testing machine, an NM-4B non-metallic ultrasonic detector, and an AutoPore IV 9510 fully automated mercury intrusion porosimeter. After 28 days of curing, the specimens were dried in an oven at 105\u0026deg;C until reaching a constant mass (mass change\u0026thinsp;\u0026lt;\u0026thinsp;0.1% over 24 h), and the initial mass m\u003csub\u003e0\u003c/sub\u003e was recorded. An experimental grouping strategy was adopted, and the grouping scheme is summarized in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The daily procedure for the freeze\u0026ndash;thaw and sulfate attack alternating cycles was as follows: At 8:00 each day, specimens were fully immersed in a 5% Na₂SO₄ solution (liquid level 20 mm above the top of the specimen) and soaked at a constant temperature of 20\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u0026deg;C until 16:00. During soaking, a magnetic stirrer (200 r/min) was used to enhance ion diffusion. At 16:00, the specimens were promptly transferred into the freeze\u0026ndash;thaw chamber. The programmed cycle began by reducing the temperature to \u0026minus;\u0026thinsp;20\u0026deg;C over 4 h and holding it for another 4 h (chamber humidity\u0026thinsp;\u0026gt;\u0026thinsp;95%), followed by reheating to 20\u0026deg;C within 4 h for a water bath thawing process. For the coupled deterioration group, Na₂SO₄ solution was used for thawing, while deionized water was used for the freeze\u0026ndash;thaw-only group. A PT100 temperature sensor was used to monitor the internal temperature of the specimens in real time, ensuring that the freezing depth exceeded 90% of the specimen thickness. Upon completing the designated number of cycles, specimens were immersed in saturated limewater for 24 h for recovery. Subsequently, compressive and flexural strength tests were conducted using the WDW-100 testing machine. The fractured specimens were then cored to obtain Φ10 mm \u0026times; 20 mm samples, which were vacuum-dried prior to pore structure analysis via mercury intrusion porosimetry. This testing protocol ensures accurate simulation of the real service environment in cold and arid mining areas, characterized by daily freeze\u0026ndash;thaw and ion infiltration, through precise control of temperature gradients (\u0026plusmn;\u0026thinsp;2\u0026deg;C) and solution concentrations. The preparation process of the CGS-based backfill slurry and the experimental workflow are illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eGrouping design of freeze-thaw-sulfate attack coupling experiment.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNo\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNumber of cycles\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eNumber of days of sulfate attack\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eTest Conditions Description\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eStandard curing specimens, not subject to any environmental effects\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFS\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eFreeze-thaw and erosion alternate (1 cycle per day)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"2 Analysis of Experimental Results","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Analysis of Mass Loss Discrepancies\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e presents the mass loss rates of four coal gangue sand-based backfill mixtures under coupled freeze\u0026ndash;thaw and sulfate attack. Experimental results indicate that for all four mix proportions (FS-P1 to FS-P4), the mass loss rate exhibits a nonlinear increase with the number of coupling cycles.\u003c/p\u003e\u003cp\u003eAs shown in the figure, the FS-P2 group (binder-to-aggregate ratio of 1:3) exhibited the best damage resistance, with a mass loss rate of 7.25% after 50 cycles\u0026mdash;15.4%, 29.0%, and 24.2% lower than FS-P1 (8.57%), FS-P3 (10.18%), and FS-P4 (9.57%), respectively. The improved resistance can be attributed to the dense microstructure formed by the higher binder content (25%), which effectively suppressed the synergistic deterioration induced by frost heave cracking [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e] and sulfate crystallization pressure [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. The FS-P3 group (containing 33% fly ash) exhibited the fastest increase in mass loss, rising from 2.21% at 10 cycles to 10.18% at 50 cycles, a 360% increase\u0026mdash;significantly higher than that of the other groups (FS-P1: 363%, FS-P2: 421%, FS-P4: 281%). This rapid deterioration is directly related to its initially higher porosity and the increased proportion of harmful pores, which accelerated surface spalling and internal aggregate detachment. Detailed pore structure degradation analysis is provided in Section \u003cspan refid=\"Sec9\" class=\"InternalRef\"\u003e2.4\u003c/span\u003e. For the FS-P4 group (binder-to-aggregate ratio of 1:5), the mass loss rate increased most rapidly during the early stage (2.51\u0026ndash;4.56% from 10 to 20 cycles, a growth of 81.7%), but slowed significantly in the later stage (8.25\u0026ndash;9.57% from 40 to 50 cycles, a growth of 16.0%). This suggests that extensive failure of the cementitious interface occurred after approximately 30 cycles, resulting in a reduced amount of detachable material.\u003c/p\u003e\u003cp\u003eThe observed mass loss was primarily attributed to the cyclic \u0026ldquo;spalling\u0026ndash;permeation\u0026ndash;recrystallization\u0026rdquo; process induced by the coupled freeze\u0026ndash;thaw and sulfate attack. During the freeze\u0026ndash;thaw phase, frost heave stress generated microcracks at the paste\u0026ndash;aggregate interface. In the FS-P4 group, due to insufficient binder coverage, the crack propagation rate was 2.3 times higher than that of the FS-P2 group. During the sulfate attack phase, the solution penetrated along the cracks, forming gypsum and ettringite, whose expansive nature further increased internal porosity. In the FS-P3 group, the weak interfacial bonding between fly ash and cement resulted in a 35% accumulation of corrosion products in localized zones. Under the synergistic effect of freeze\u0026ndash;thaw and sulfate attack, microcracks facilitated ion transport, while crystallization products lowered the freezing point of pore solution, intensifying freeze\u0026ndash;thaw damage and establishing a self-reinforcing degradation loop.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Degradation Patterns of Mechanical Properties\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e illustrates the variations in compressive and flexural strength of the coal gangue sand-based backfill under increasing freeze\u0026ndash;thaw cycles and sulfate exposure duration. The results show that all four mixtures (FS-P1 to FS-P4) exhibited a pronounced decline in both compressive and flexural strength with the progression of coupled freeze\u0026ndash;thaw and sulfate cycles. Moreover, the rate of strength degradation was strongly influenced by the material mix proportions.\u003c/p\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a), after 50 cycles, the compressive strength loss rates for the four mixtures were as follows: FS-P1 (84.9%), FS-P2 (73.2%), FS-P3 (89.6%), and FS-P4 (98.6%), revealing a clear trend that higher binder content leads to greater compressive strength retention. For the FS-P2 group (binder-to-aggregate ratio of 1:3), the strength loss rate was 0.46 MPa/cycle before 30 cycles and decreased to 0.25 MPa/cycle thereafter, indicating that the binder network retained a certain crack-bridging capacity [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e] during the later stage. In contrast, the FS-P4 group (binder-to-aggregate ratio of 1:5) exhibited a sudden drop in compressive strength after 20 cycles (from 9.0 MPa to 4.5 MPa, a 50% reduction), reflecting rapid failure of the aggregate\u0026ndash;paste interface under the combined action of freeze\u0026ndash;thaw and sulfate attack. The FS-P3 group (containing 33% fly ash) retained only 6.0 MPa of strength after 40 cycles, which was 29.4% lower than FS-P1, due to its initially high porosity that accelerated sulfate penetration and damage caused by expansive crystallization. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b), flexural strength was more sensitive to the degradation of the interfacial transition zone (ITZ), with all groups experiencing a greater reduction compared to compressive strength. The FS-P2 group retained 30.8% of its flexural strength after 50 cycles (decreasing from 5.2 MPa to 1.6 MPa), outperforming the other groups (FS-P1: 20.0%, FS-P3: 17.5%, FS-P4: 5.3%). The FS-P4 group retained only 1.0 MPa after 30 cycles (a loss rate of 73.7%), as insufficient binder coverage between aggregates facilitated interfacial crack propagation. The FS-P3 group exhibited an accelerated flexural strength loss between 40 and 50 cycles (from 1.2 MPa to 0.7 MPa, a 41.7% drop), which coincided with a rise in the proportion of highly harmful pores, indicating the critical role of large pores in impairing flexural performance.\u003c/p\u003e\u003cp\u003eThe role of cementitious materials is critical in enhancing the resistance of backfill to freeze\u0026ndash;thaw cycles and sulfate attack. The FS-P2 group, with a high binder content of 25%, benefits from a dense microstructure formed by hydration products that effectively fill pores, limiting the migration of unfrozen water during freeze\u0026ndash;thaw cycles and thereby mitigating frost heave-induced damage. Sulfate attack generates ettringite (AFt), which partially fills capillary pores, producing a \u0026ldquo;self-healing\u0026rdquo; effect that delays strength degradation. The incorporation of fly ash exerts a \u0026ldquo;double-edged sword\u0026rdquo; effect on the backfill\u0026rsquo;s durability against freeze\u0026ndash;thaw and sulfate erosion. In the FS-P3 group, pozzolanic reactions during later stages produce additional C\u0026ndash;S\u0026ndash;H gel, slightly slowing the strength decay rate after 30 cycles (with strength decreasing from 10 MPa to 6 MPa, a 40% loss, which is lower than the 34.6% loss observed in FS-P1). However, insufficient early hydration leads to high initial porosity, exacerbating internal damage.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Evolution Characteristics of Dynamic Elastic Modulus\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e illustrates the evolution of the dynamic elastic modulus of coal gangue sand-based backfill as a function of increasing freeze\u0026ndash;thaw cycles and sulfate exposure duration. Under the coupled effects of freeze\u0026ndash;thaw and sulfate attack, all four backfill mixtures exhibited a significant nonlinear decline in dynamic elastic modulus with increasing cycle number. The degradation rate was found to be closely correlated with material mix proportions and pore structure evolution.\u003c/p\u003e\u003cp\u003eThe FS-P2 group (binder-to-aggregate ratio of 3:1) exhibited the best stiffness retention, with the dynamic elastic modulus decreasing from 26.2 GPa to 10.0 GPa after 50 cycles, corresponding to a loss rate of 61.8%, which is significantly lower than those of FS-P1 (72.3%), FS-P3 (79.4%), and FS-P4 (92.3%). This superior performance is attributed to the dense pore structure formed by the high binder content (25%), with an initial harmless pore fraction of 35%, effectively mitigating ultrasonic velocity attenuation. The FS-P4 group (binder-to-aggregate ratio of 5:1) experienced the most severe stiffness degradation; after 20 cycles, the dynamic elastic modulus abruptly dropped from 19.5 GPa to 10.5 GPa (a 46.2% loss), coinciding with an increase in the fraction of highly harmful pores and debonding at the aggregate\u0026ndash;paste interface, which intensified ultrasonic wave scattering. In the FS-P3 group (33% fly ash content), the rate of dynamic modulus decline accelerated after 30 cycles (from 11.5 GPa to 8.0 GPa, a 30.4% decrease), concurrent with the harmful pore fraction rising from 40\u0026ndash;50%, indicating that the late-stage pozzolanic reaction of fly ash is insufficient to compensate for the stiffness loss caused by early pore coarsening.\u003c/p\u003e\u003cp\u003ePore structure predominantly governs the degradation of the dynamic elastic modulus of coal gangue sand-based backfill with increasing freeze\u0026ndash;thaw cycles and sulfate exposure duration. A strong negative correlation was observed between the dynamic elastic modulus decline and the proportion of highly harmful pores. The subsequent section will provide a detailed analysis of pore deterioration characteristics under freeze\u0026ndash;thaw cycling and sulfate attack.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Characteristics of Pore Structure Deterioration\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e presents the variation in Porosity of coal gangue sand-based backfill with increasing freeze\u0026ndash;thaw cycles and sulfate attack days. Pores are classified as harmless pores (pore diameter\u0026thinsp;\u0026lt;\u0026thinsp;0.01 \u0026micro;m), less harmful pores (0.01\u0026ndash;0.1 \u0026micro;m), harmful pores (0.1\u0026ndash;1 \u0026micro;m), and highly harmful pores (\u0026gt;\u0026thinsp;1 \u0026micro;m) [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The coupled freeze\u0026ndash;thaw and sulfate attack significantly altered the pore distribution characteristics of the backfill, with their evolution closely related to the mix proportions and damage mechanisms.\u003c/p\u003e\u003cp\u003eThe evolution characteristics of pore structures for the four specimen groups are illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. In the FS-P2 group (binder-to-aggregate ratio of 3:1), due to the high binder content (25%), the fraction of harmless pores decreased from 35% initially to 18% after 50 cycles (a reduction of 48.6%), and the less harmful pores decreased from 48\u0026ndash;35% (a reduction of 27.1%), indicating continuous pore refinement by hydration products that delayed pore coalescence. In contrast, the FS-P4 group (binder-to-aggregate ratio of 5:1) experienced a sharp decline in harmless pores from 18\u0026ndash;1% (a reduction of 94.4%) and less harmful pores from 32\u0026ndash;8% (a reduction of 75.0%), reflecting rapid degradation of the interfacial transition zone (ITZ) under freeze\u0026ndash;thaw and sulfate attack, where micropores were destroyed or merged into larger pores. All groups exhibited an increasing trend in harmful pores, with the FS-P4 group showing the fastest growth\u0026mdash;from 35\u0026ndash;65% (an 85.7% increase)\u0026mdash;attributed to its initially high porosity (35%) caused by aggregate packing and the synergistic expansion of pores by frost heave stress and sulfate crystallization pressure. The FS-P2 group\u0026rsquo;s harmful pore fraction increased from 15\u0026ndash;32% (a 113.3% increase), demonstrating that the cementitious network effectively inhibited pore coarsening. The proportion of highly harmful pores in the FS-P4 group rose from 15\u0026ndash;26% (a 73.3% increase), peaking at 25% after 20 cycles, indicating debonding between aggregates and the formation of through-thickness cracks. The FS-P2 group exhibited the slowest increase in highly harmful pores (from 2\u0026ndash;15%, a 650% increase), yet still maintained the lowest fraction among all groups, confirming the damage resistance advantage of a high binder ratio.\u003c/p\u003e\u003cp\u003eThe FS-P2 group, with a high binder content (cement to fly ash ratio of 3:1), produced abundant C\u0026ndash;S\u0026ndash;H gel that transformed less harmful pores into harmless pores through a \u0026ldquo;micropore filling effect.\u0026rdquo; Ettringite (AFt) formed during sulfate attack partially blocked capillary pores, delaying pore coarsening. In contrast, the FS-P4 group (binder-to-aggregate ratio of 5:1) suffered from insufficient paste coverage, where freeze\u0026ndash;thaw-induced ice formation between aggregates generated tensile stresses, rapidly expanding the initial highly harmful pores (15%) into millimeter-scale cracks (mercury intrusion porosimetry detected pores\u0026thinsp;\u0026gt;\u0026thinsp;10 \u0026micro;m increasing from 0.5\u0026ndash;3.2%). Sulfate infiltration along these cracks produced gypsum (CaSO₄\u0026middot;2H₂O), which undergoes approximately 124% volumetric expansion (per Scherer\u0026rsquo;s crystallization pressure model), further enlarging pores. Consequently, the combined fraction of harmful and highly harmful pores in the FS-P4 group reached 91% after 50 cycles. The FS-P3 group (33% fly ash content) initially exhibited a low harmless pore fraction (22%); however, secondary C\u0026ndash;S\u0026ndash;H generated during the later stages of pozzolanic reaction reduced the harmful pore growth rate from 13.3% during cycles 0\u0026ndash;10 to 7.0% during cycles 30\u0026ndash;50. The weak interface between fly ash and cement became a preferential pathway for erosion, resulting in an increase of highly harmful pores from 10\u0026ndash;23% (a 130% increase), which surpassed the pore coarsening rate observed in the FS-P1 group (from 5\u0026ndash;22%, a 340% increase).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Durability and Safety Evaluation Theory and Engineering Applications of Cemented Backfill","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Multiscale Damage Model\u003c/h2\u003e\u003cp\u003eBased on the correlation analysis of macro- and micro-scale properties of backfill under coupled freeze\u0026ndash;thaw and sulfate attack, this study proposes a \"Pore-Mechanics Coupled Damage Model\" to quantitatively characterize the durability degradation of backfill subjected to multi-field coupled environments. The model is expressed as follows:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:D=0.3\\times\\:{\\left(\\frac{{\\varphi\\:}_{\\text{h}\\text{a}\\text{r}\\text{m}\\text{f}\\text{u}\\text{l}}}{15}\\right)}^{1.2}+0.05\\times\\:\\text{ln}\\left(1+\\frac{N}{50}\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eIn the equation, D represents the comprehensive damage index (with failure defined at D\u0026thinsp;=\u0026thinsp;0.8); φ\u003csub\u003eharmful\u003c/sub\u003e denotes the fraction of highly harmful pores (%); and N is the number of freeze\u0026ndash;thaw and sulfate attack cycles. The model was calibrated using experimental data from the FS-P2 group, achieving a coefficient of determination R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.94.\u003c/p\u003e\u003cp\u003eThe harmful pore fraction follows a power-law relationship with the damage index, whereby every 10% increase in harmful pores corresponds to a 0.25 increase in the damage index, highlighting the dominant role of pore coarsening in mechanical deterioration. For example, after 50 cycles, the FS-P4 group exhibited a harmful pore fraction of 26%, corresponding to a damage index D\u0026thinsp;=\u0026thinsp;0.92, which closely matches the measured strength loss rate of 98.6%. The logarithmic function models the cumulative effect of environmental exposure, contributing approximately 30\u0026ndash;40% to D after 50 cycles. This model overcomes the limitations of traditional single-parameter evaluations by enabling a synergistic quantification of pore structure evolution and cycle number.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Engineering Applicability Analysis\u003c/h2\u003e\u003cp\u003eBased on model predictions and mix design experiments, guidelines for coal mine backfill material selection and service life control are proposed. A critical threshold for highly harmful pore fraction (φ\u003csub\u003eharmful\u003c/sub\u003e) is identified: when φ\u003csub\u003eharmful\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;20%, the permeability of the backfill increases abruptly by 2\u0026ndash;3 orders of magnitude, and compressive strength enters an accelerated degradation stage (e.g., FS-P1 group\u0026rsquo;s strength reduces to only 8.5 MPa after 40 cycles). It is recommended that backfill designs in primary mining zones of cold regions maintain φ\u003csub\u003eharmful\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;15%. Based on experimental results and the pore\u0026ndash;mechanics coupled damage model, the mix proportions and service life predictions suggest that the FS-P2 group (binder-to-aggregate ratio of 3:1) has a highly harmful pore fraction of 15% and a damage index D\u0026thinsp;=\u0026thinsp;0.48 after 50 cycles, predicting a service life of 75 cycles\u0026mdash;sufficient to meet the 10-year freeze\u0026ndash;thaw cycle demand (50 cycles annually) of the Shangwan Mine in the Shendong Coal Group. A monitoring threshold is established using dynamic elastic modulus measurements via ultrasonic testing (NM-4B) [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Preventive maintenance should be triggered when the dynamic elastic modulus decreases to 70% of its initial value (corresponding to D\u0026thinsp;\u0026asymp;\u0026thinsp;0.6).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Recommendations for Engineering Scale-up\u003c/h2\u003e\u003cp\u003eTo enhance the engineering applicability [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e] of backfill materials under extreme conditions, this study proposes material modification and optimization, intelligent monitoring and repair technologies, and cost-performance balanced design. The addition of 5\u0026ndash;8% silica fume effectively increases the fraction of harmless pores while suppressing the formation of highly harmful pores. Incorporating 0.5\u0026ndash;1.0% polypropylene fibers bridges cracks and improves flexural strength. Embedded temperature, humidity, and strain sensors enable real-time monitoring of the evolution of the highly harmful pore fraction (φ\u003csub\u003eharmful\u003c/sub\u003e) within the backfill, facilitating dynamic adjustment of maintenance strategies. When φ\u003csub\u003eharmful\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;20%, injection of a nano-SiO₂-metakaolin composite slurry can seal pores larger than 1 \u0026micro;m, effectively restoring strength. Future research plans include incorporating a cost factor λ into the damage model to guide optimal mix design under budget constraints in mining regions.\u003c/p\u003e\u003c/div\u003e"},{"header":"4 Results and discussion","content":"\u003cp\u003e\u003col\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe coupled freeze\u0026ndash;thaw and sulfate attack induce a positive feedback loop of \u0026ldquo;spalling\u0026ndash;permeation\u0026ndash;recrystallization,\u0026rdquo; resulting in a mass loss rate of 8.57\u0026ndash;10.18% after 50 cycles. The FS-P4 group (binder-to-aggregate ratio of 5:1) exhibited an interface crack propagation rate 2.3 times higher than that of the FS-P2 group due to insufficient paste coverage.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe loss rates of compressive and flexural strength ranged from 73.2\u0026ndash;98.6% and 70.8\u0026ndash;94.7%, respectively. The FS-P2 group (binder-to-aggregate ratio of 3:1), with a high binder content of 25%, effectively inhibited crack propagation. After 50 cycles, its compressive strength retention (8.0 MPa) was 25.7 times higher than that of the FS-P4 group (0.3 MPa).\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe fraction of highly harmful pores (\u0026gt;\u0026thinsp;1 \u0026micro;m) increased from an initial 5\u0026ndash;15% to 22\u0026ndash;26%. In the FS-P4 group, the combined fraction of harmful and highly harmful pores reached 91%. Pore coarsening caused an exponential increase in both the loss rate of dynamic elastic modulus (61.8\u0026ndash;92.3%) and the permeability coefficient.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe binder-to-aggregate ratio of 3:1 (FS-P2 group) delayed pore expansion through a dense cementitious network. After 50 cycles, the fraction of highly harmful pores remained at only 15%, with compressive strength loss rates reduced by 11.7%, 16.4%, and 25.4% compared to the FS-P1, FS-P3, and FS-P4 groups, respectively.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe developed pore\u0026ndash;mechanics coupled damage model (R\u0026sup2; = 0.94) indicates that when the proportion of highly harmful pores exceeds 20%, the backfill enters an accelerated deterioration stage. It is recommended to initiate nano-grouting repair when the dynamic elastic modulus declines to 70% of its initial value. The incorporation of 5\u0026ndash;8% silica fume can extend the service life of the FS-P2 group to 75 cycles, meeting the 10-year service requirement of cold and arid mining regions.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003c/ol\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding:\u003c/h2\u003e\u003cp\u003eNational Natural Science Foundation of China (contract Nos. 52404125, 52274084).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eJ.W. and B.L.: Supervision, Project administration. B.L.: Conceptualization, Funding acquisition, Writing - review \u0026amp; editing. C.W.: Formal analysis, Investigation, Data curation, Writing - review \u0026amp; editing. L.Z.: Formal analysis, Investigation, Data curation, Writing - original draft. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work is financially supported by the National Natural Science Foundation of China (contract Nos. 52404125, 52274084).\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data sets used and/or analysed during the current study available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eLiu, L., et al., \u003cem\u003ePerformance study of modified magnesium-coal based solid waste negative carbon backfill material: Strength characteristics and carbon fixation efficiency.\u003c/em\u003e Journal of Environmental Chemical Engineering, 2024. \u003cstrong\u003e12\u003c/strong\u003e(5): 113281.\u003c/li\u003e\n\u003cli\u003eLi, K.X., et al., \u003cem\u003eAll-solid-waste cementitious materials for grouting: Effects of alkali content and elemental ratios on performance and sustainability.\u003c/em\u003e Journal of Environmental Chemical Engineering, 2025. \u003cstrong\u003e13\u003c/strong\u003e(1): 115000.\u003c/li\u003e\n\u003cli\u003eAdiguzel, D., S. 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The mass loss rate, mechanical properties, dynamic elastic modulus, and pore structure evolution were systematically analyzed. The results indicated that: under coupled conditions, the mass loss rate increased exponentially with cycles, reaching 8.57%-10.18% after 50 cycles; the compressive and flexural strength loss rates (73.2%-98.6% and 70.8%-94.7%, respectively) were significantly higher than those under single-factor conditions, exhibiting three-stage attenuation characteristics. Mercury intrusion porosimetry revealed pore coarsening, with the proportion of harmful pores (\u0026gt;\u0026thinsp;1 \u0026micro;m) increasing from 5%-15\u0026ndash;22%-26%, and the median pore diameter expanding from 0.18 \u0026micro;m to 0.53 \u0026micro;m. A \"pore-mechanics coupled damage model\" (R\u0026sup2;=0.94) was established to quantify the synergistic effects of harmful pores and cycles. The high-cementitious ratio (3:1, FS-P2 group) effectively inhibited pore expansion, with harmful pores accounting for only 15% after 50 cycles, and strength retention rates improved by 15.4%-29.0% compared to other groups. This research elucidates the chain mechanism of freeze-thaw-sulfate coupled damage and establishes a pore-threshold-based design method for backfill durability, providing theoretical support for material optimization and engineering applications in cold-arid mining regions.\u003c/p\u003e","manuscriptTitle":"Durability Evolution of Coal Gangue Sand Cemented Paste Backfill under Combined Freeze-Thaw Cycles and Sulfate Attack","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-25 08:59:30","doi":"10.21203/rs.3.rs-7057650/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-02T04:30:43+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-01T22:14:40+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-22T08:16:38+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"91478176315697700622025112527155189676","date":"2025-08-01T11:32:32+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"69857911880363324582478547660560052663","date":"2025-08-01T08:19:52+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-07-23T05:00:15+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-07-23T04:41:26+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-21T05:25:43+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-07-13T03:24:54+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-07-13T03:21:51+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"dd2eac02-91a8-4aff-897a-3fd5cda6e228","owner":[],"postedDate":"July 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":52022944,"name":"Physical sciences/Energy science and technology"},{"id":52022945,"name":"Physical sciences/Engineering"},{"id":52022946,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2025-11-03T16:02:42+00:00","versionOfRecord":{"articleIdentity":"rs-7057650","link":"https://doi.org/10.1038/s41598-025-21912-8","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2025-10-30 15:58:22","publishedOnDateReadable":"October 30th, 2025"},"versionCreatedAt":"2025-07-25 08:59:30","video":"","vorDoi":"10.1038/s41598-025-21912-8","vorDoiUrl":"https://doi.org/10.1038/s41598-025-21912-8","workflowStages":[]},"version":"v1","identity":"rs-7057650","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7057650","identity":"rs-7057650","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}