## 1\. Introduction The production of municipal solid waste (MSW) is on the rise as a result of ongoing socio-economic and urbanization developments. While landfilling has historically been the primary method for MSW disposal, it has been gradually supplanted by municipal solid waste incineration (MSWI) in many countries due to its advantages of significant mass (ca. 70%) and volume (ca. 90%) reduction [\[1\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib1), [\[2\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib2), [\[3\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib3), energy recovery in the form of electricity [\[4\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib4), and high temperature sterilization. Fig. 1 depicts a comparison of regional MSW disposal and incineration quantities in China from 2018 to 2021 [\[5\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib5). Since 2018, China has witnessed a steady annual growth rate of approximately 6% in MSW disposal, resulting in a total of 249 million tonnes by 2021. Meanwhile, the proportion of incinerated MSW to the total disposed MSW has increased from 45.1% to 72.6%. Nevertheless, as the by-product of the incineration process, MSWI fly ash (MSWI-FA) is classified as hazardous waste due to its high levels of heavy metals, chlorides, and dioxins [\[6\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib6), [\[7\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib7). The amount of MSWI-FA generated is equivalent to 3-5 wt% and approximately 12 wt% of the MSW combusted in the grate furnace and fluidized bed, respectively [\[8\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib8). This signifies that China alone produces over 10 million tonnes of MSWI-FA each year [\[9\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib9). Treatment methods for MSWI-FA commonly involve solidification/stabilization (S/S) [\[10\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib10), [\[11\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib11), [\[12\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib12), and thermal treatment [\[13\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib13), [\[14\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib14). The heavy metal concentrations in raw MSWI-FA and leachate before and after various treatments are depicted in Fig. 2 a, Fig. 2 b, and Fig. 2(c-d), respectively. Evidently, the heavy metal concentration of leachate from S/S methods employing chemical fixation and physical encapsulation ( Fig. 2 c) is significantly lower than that from the raw MSWI-FA ( Fig. 2 b) [\[15\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib15). Nevertheless, cement-based S/S method still poses a considerable risk of heavy metal leaching under strongly acidic conditions and exhibits poor long-term performance [\[13\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib13), [\[16\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib16). Moreover, the production process of cement, an adhesive material commonly used in S/S, entails a substantial carbon footprint. Additionally, organic matter (e.g., humic acid powder) in the soil may interfere with the cement hydration process, thereby hindering the immobilization efficiency of heavy metals [\[17\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib17). Thermal treatment, encompassing high-temperature processes (melting and sintering, 1100-1500 ℃) and hydrothermal treatment (250-300 ℃), presents a more efficient method for heavy metal immobilization compared to S/S, as illustrated in Fig. 2 d. Moreover, increased temperatures lead to the breakdown of dioxins in MSWI-FA and a significant decrease in product volume. Notably, the primary chemical constituents of MSWI-FA comprise CaO, SiO2, and Al2O3, as shown in Fig. S1 ( Supporting Information), capable of generating glassy phases that enhance the pozzolanic activity after high-temperature treatment like melting. The glassy phases generated enable the potential application in the production of various building materials, including low-carbon cementitious products, ceramsites, and bricks, thus facilitating the valuable utilization of MSWI-FA [[27]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib27), [\[28\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib28). Nevertheless, the significant economic investments and energy requirements associated with high-temperature processing pose obstacles to its extensive implementation [\[28\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib28), [\[29\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib29). It is important to note that thermal treatment processes generate secondary pollutants, such as volatile heavy metals in high-temperature thermal treatment and wastewater during hydrothermal treatment. Consequently, there exists a pressing necessity to develop an environmentally effective strategy for handling MSWI-FA. Studies have demonstrated that the co-treatment of MSWI-FA with alumina-/silica-containing waste (ASW) can potentially address the limitations associated with treating MSWI-FA alone, such as inadequate immobilization of heavy metals and significant energy demands during thermal treatment [[32]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib32), [\[40\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib40). ## 3\. Roles of ASW in thermal co-treatment Thermal treatment methods, such as melting [\[37\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib37), [\[132\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib132), sintering [\[133\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib133), [\[134\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib134), and hydrothermal treatment[\[135\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib135), [\[136\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib136), have demonstrated efficacy in the elimination of dioxins (i.e., polychlorinated dioxins/furans, PCDD/Fs), immobilization of heavy metals, and the production of treated products suitable for utilization as building materials. However, the energy-intensive nature of thermal treatment processes, particularly at elevated temperatures, may contribute to an increased overall carbon footprint associated with the disposal of MSWI-FA, potentially diminishing the benefits of carbon reduction achieved through the utilization of MSWI-FA as a building material. Huber _et al._ conducted environmental assessments for various MSWI-FA treatment methods over two distinct time scales: 100 years and an indefinite period. The results of their study indicate that direct thermal treatment and cement-based S/S exhibit the most significant environmental impacts, while the co-disposal of MSWI-FA with combustible waste emerges as a more promising alternative [\[137\]](https://www.sciencedirect.com/science/article/pii/S0304389424022568?utm_source=chatgpt.com#bib137).