Thermal-alkaline Activation Enhances the Mechanical Properties of Low-activity Recycled Concrete Powder-derived Geopolymers postprint

Abstract: 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.