分类: 矿山工程技术 >> 矿山地质学 提交时间: 2025-07-17
摘要: In China, annual generation of building-related construction and demolition waste (brCDW) exceeds 2 billion tons, with a recycling rate of less than 40%, significantly lower than the European average. The majority of unrecycled brCDW is either landfilled or stockpiled in suburban areas, leading to severe environmental pollution and resource wastage. Therefore, developing high-value utilization strategies is crucial to improving the overall recycling rate of brCDW. To address the aforementioned issues, this study developed a novel approach by synthesizing a brCDW-derived geopolymer to stabilize high liquid limit subgrade soil. The unconfined compressive strength (UCS) test, shear strength test, resilient modulus test, and permanent strain test were conducted to investigate the effects of brCDW-derived geopolymer dosage, curing time, stress state, and moisture condition on the engineering properties of geopolymer stabilized soil. Mechanistic-empirical models were employed to accurately estimate the stress-dependent resilient modulus and permanent strain of geopolymer stabilized soil at any given stress state. In addition, the Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) test were performed to investigate the strengthening mechanism of brCDW-derived geopolymer stabilization of subgrade soil. Finally, the sustainability of brCDW-derived geopolymer stabilization approach was assessed in terms of material production cost, carbon dioxide emission, and energy consumption. These test results demonstrated that increasing the geopolymer dosage effectively improved the UCS, shear strength and resilient modulus of stabilized soil and reduced the permanent strain of stabilized soil. The addition of 8% and 12% geopolymer showed significant improvement on the soil strength, while 4% geopolymer had negligible impact on the soil strength. The UCS test results indicated that 8% geopolymer provided the most economical improvement on the stabilized soil. Increasing the brCDW-derived geopolymer dosage effectively improved the resilient modulus of stabilized soil, but did not affect the stress-dependent behavior of stabilized soil. Increasing confining pressure or decreasing deviatoric shear stress still resulted in a higher resilient modulus for geopolymer stabilized soil. The resilient modulus of the geopolymer stabilized soil was sensitive to moisture condition. When the moisture content increased from optimum moisture content (OMC) to 1.15 OMC, the resilient moduli of geopolymer stabilized soil reduced approximately by 20%. Increasing the brCDW-derived geopolymer dosage reduced the accumulated permanent strain of stabilized soil. A mechanistic-empirical rutting model was used to predict the permanent strain of geopolymer stabilized soil at any given stress state. The high liquid limit soil stabilized by 8% geopolymer had sufficient resistance to permanent deformation. But when the moisture condition reached 1.15 OMC, the 8% geopolymer might not provide the stabilized soil an adequate resistance to permanent deformation. The SEM test results indicated that the porosity of stabilized soil was significantly decreased when the geopolymer dosage increased to 8%. The EDS test results demonstrated that the predominant gel types generated from the geopolymer stabilization might be Calcium-Aluminum-Silicate-Hydrate (C-A-S-H), Calcium-Silicate-Hydrate (C-S-H), and Calcium-Aluminate-Hydrate (C-A-H) gels. There was also a small amount of Sodium-Alumino-Silicate-Hydrate (N-A-S-H) gel detected in the geopolymer stabilized soil. Compared to the traditional soil stabilizers (e.g., conventional Portland cement and quick lime), the production of brCDW-derived geopolymer saved material cost by 31-36%, carbon dioxide emission by 44-55%, and energy consumption by 48-49%. In general, the utilization of brCDW-derived geopolymer was a sustainable approach for soil stabilization.
分类: 矿山工程技术 >> 矿山地质学 提交时间: 2025-07-17
摘要: The existing studies estimated that the waste concrete accounts for over 60% of the total CDW, thus improving the utilization rate of waste concrete is crucial to alleviate the burden of CDW disposal. In recent years, some scholars found that the recycled concrete powder (RCP) contained a high content of silicon (Si), aluminum (Al), and calcium (Ca) elements, which is a suitable precursor material for synthesizing geopolymer. However, a primary challenge in RCP derived geopolymer is to improve its mechanical strength suitable for engineering practice. This is attributed to the low content of amorphous substances in RCP, with most Si, Al, and Ca existing in the form of crystalline minerals. These minerals are less soluble in alkaline activators, inhibiting geopolymerization and resulting in reduced gel and lower strength. This study proposed a thermal-alkaline activation method to enhance the activity of recycled concrete powder (RCP) and then to produce high-strength geopolymers. The thermal decomposition behavior of RCP was analyzed using thermogravimetric-differential scanning calorimetry (TG-DSC) to investigate its phase decomposition characteristics across different temperature ranges, thereby determining the optimal calcination temperature range for RCP. The microstructural morphology and phase composition changes of RCP before and after calcination were further compared through scanning electron microscopy (SEM) and X-ray diffraction (XRD)analysis, revealing the influence of the calcination process on its microstructure. Based on these findings, a series of geopolymer specimens derived from RCP under various calcination conditions were prepared. The effects of calcination conditions on the unconfined compressive strength (UCS) growth of RCP-derived geopolymers were evaluated through UCS testing. Additionally, energy consumption under different calcination conditions was calculated to achieve a balance between mechanical performance enhancement and energy efficiency, ultimately leading to the identification of the optimal calcination parameters. Within the temperature range of 650°C to 800°C, significant mass loss was observed, indicating intense mineral decomposition. The calcined RCP samples with NaOH exhibited a diffuse peak between 32° and 35°, demonstrating a higher content of amorphous substances compared to those without NaOH. RCP contained substantial amounts of Si, Al, and Ca elements, predominantly in low-reactivity crystalline forms. Calcination with preloaded sodium hydroxide (NaOH) significantly altered the microstructure and phase composition of RCP. These observations suggest that a portion of Si, Al, and Ca elements in RCP transformed from their initial low-reactivity crystalline states into highly reactive amorphous forms, thereby enhancing the reactivity of RCP. The UCS of calcined RCP-derived geopolymers gradually increased with increasing calcination time. However, when the calcination temperature rose from 650°C to 800°C, the UCS initially increased and then decreased. The optimal condition for RCP-derived geopolymers was determined to be 750°C calcination temperature for 45 minutes, achieving a 7-day UCS of 17.6 MPa—almost five times greater than that of uncalcined RCP-based geopolymers. Considering the balance between UCS growth and energy consumption, the optimal calcination scheme was determined to be 750°C for 15 minutes. The uncalcined RCP-derived geopolymers exhibited a porous microstructure with loose gel formation and numerous unreacted RCP particles after 28 days of curing. In contrast, the calcined RCP-derived geopolymers displayed a dense structure with minimal observable pores and fewer unreacted RCP particles, indicating improved reactivity due to calcination.
分类: 矿山工程技术 >> 矿山地质学 提交时间: 2025-07-17
摘要: Road trenchless grouting is widely used for subgrade remediation due to its convenient application, low-carbon, and low-disturbance features. This study focuses on the circulating fluidized bed fly ash (CFBFA) - ground granulated blast furnace slag (GGBS) grouting material, researching its preparation and remediation effect of subgrade distress via laboratory tests. Results show that as CFBFA content rises, the grouting material's fluidity increases, bleeding rate decreases, setting time prolongs, and compressive and flexural strengths decrease. Through a self-designed test, the subgrade soil type, distress type, grouting pressure, and material compatibility are explored. The grouting material significantly affects the remediation of subgrade voids and slurry-soil interfaces. For example, at a 10 mm loose thickness and 1.5 MPa grouting pressure, the optimal remediation effect can reach 2.66 times of the strength of 96% compacted sandy clay of low liquid limit (CLS) subgrade soil, with a 59.10% increase in shear strength. Clayey sand (SC) has better loose remediation and interface shear strength than CLS subgrade soil. Based on data analysis, suitable grouting materials and pressures are recommended for different soil and distress types. Further X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS) tests are conducted. XRD pattern shows a weak geopolymerization reaction between grouting material and subgrade soil, with SC having stronger reactivity. SEM and EDS results demonstrate that the grouting material effectively binds subgrade soil particles and fills voids, with the combined effect of soil particle participation in the reaction and cementation significantly enhancing the bonding efficiency. This study classifies the grouting remediation effect into filling, slurry-soil reaction, cation exchange, and bonding effect, systematically explores the compatibility of subgrade factors with grouting materials and pressures, and provides new methods and ideas for improving the remediation effect.