Research on the activity of dry-discharge coal-fired slag of different particle sizes and their performance as precursors of alkali-activated backfilling cementitious materials

As the largest coal producer globally, China’s annual output accounts for about 45.44% of global coal production (Ouyang et al., 2022). In 2022, China’s raw coal production was 4.56 billion tons, up 10.5% year-on-year, while the national coal energy consumption reached 3.03 billion tons of standard coal, with the power industry consuming 55% (National Bureau of Statistics, 2023; Suvitha et al., 2024). Thermal power generation is responsible for about 96% of global CO₂ emissions and 20% of global solid waste, including about 80–90% fly ash and 10–20% coal-fired slag (CFS, also known as coal bottom ash) (Ju et al., 2023; Meena et al., 2023). At present, despite the widespread use of fly ash, applications for CFS remain limited (Mudgal et al., 2021; Yang et al., 2023). The CFS is primarily a mixture of completely burned ash and incompletely burned cinder (Lauzurique et al., 2021; Mohammed et al., 2021; Zhang et al., 2023). According to incomplete statistics, each burning of 1 ton of coal will produce 20%–30% of the amount of ash, of which CFS accounts for about 20% of the total. Currently managed by storage and landfill, which consumes substantial land resources, causes groundwater and air pollution, increases the difficulty of environmental control and subsequent treatment cost, and constantly heightens people’s concern about the ecological environment (Al Biajawi et al., 2023; Argiz et al., 2018; Singh et al., 2018; Jeon et al., 2023; Kang et al., 2023). Therefore, regulatory-compliant utilization of CFS is imperative to achieve green and low-carbon development (Shan et al., 2025).

In recent years, many scholars have conducted research on CFS and found that CFS contains active silica and alumina, which is a kind of pozzolanic material, and is mainly used as aggregate in concrete or as a partial cement replacement (Hamada et al., 2022; Tamanna et al., 2023). Researches have shown that the active silica minerals contained in concrete can trigger alkali-silica reaction, causing expansion (Figueira et al., 2019; Leemann et al., 2024). Researchers therefore began to investigate the use of pretreated CFS to replace some of the cement and to explore its potential as cementitious materials. For instance, Mangi et al. (2019) examined influence of CFS’s fineness (processed for 20 h and 30 h) as an auxiliary cementitious material on the properties of concrete in seawater, observing reduced seawater effects with 10% CFS substitution, though fineness showed minimal impact. Kang et al. (2024) reported enhanced hydration activity in composites using 25% CFS pretreated with diethanol isopropanolamine as a cement substitute. Similarly, Guan et al. (2023) noted a 12.66% increase in compressive strength in cement-based materials containing 30% CFS (processed for 40 min) when cured at 192 °C compared to 20 °C, showing that finer CFS particles and higher temperatures can significantly improve performance. While prior research has focused primarily on the content of CFS in cement materials and its beneficial impact, there remains a gap concerning the influence of different CFS particle sizes on material performance. This area merits further exploration to optimize the use of CFS in enhancing cement material properties. Due to variations in raw coal quality and calcination process, CFS exhibits lattice distortion and amorphous phase structure, resulting in a more complex composition and particle size variability than fly ash, and is subject to greater fluctuations in coal quality and combustion process (Gong et al., 2024). Studying its fundamental influencing factors is ultimately directly reflected in particle size and morphology. There are significant differences in external morphology, internal structure, chemical composition, mineral phase composition and pozzolanic activity of CFS with different particle sizes. As a cement admixture, it will indirectly affect the performance and stability of cementitious materials (Feng et al., 2024; Żyrkowski et al., 2016; Shanthakumar et al., 2008). Therefore, this paper proposes the grading treatment of CFS and conducts a comprehensive investigation into the hydration activity of CFS across different grades. Additionally, leveraging regional advantages, CFS is utilized as a precursor for the development of coal-fired slag-based cementitious materials (CFS·C) by using smelting magnesium slag (Liu et al., 2021a) —a highly reactive alkaline solid waste—to activate CFS, exploring the application potential of CFS·C. For instance, Ruan et al. (2023b) demonstrated the feasibility of using modified magnesium slag to entirely replace cement, producing cementitious materials with strengths ranging from 5.60 to 19.30 MPa. He et al. (2025) further studied the long-term carbonation effects on the properties of such materials. Thus, the synergistic utilization of modified magnesium slag and CFS realized the high-value and large-scale use of CFS, completely replacing the use of cement and reducing CO₂ emissions.

This paper first categorizes CFS, followed by a systematic examination of the physicochemical properties of CFS across all grades, establishing the relationship between CFS components and their pozzolanic activity. Subsequently, modified magnesium slag (MMS) is employed to activate graded CFS, leading to the preparation of CFS·C, and the mechanical properties, hydration process and microstructure of CFS·C were investigated to assess the feasibility of CFS as a precursor in cementitious materials. Finally, the correlation between major oxide content, loss on ignition (LOI), pozzolanic activity, cumulative heat release and 28 d thermogravimetric total mass loss with compressive strength were investigated. This research clarifies the fundamental sources of CFS activity, offering a reference for its high-value and large-scale utilization and contributing to mitigating the environmental challenges associated with CFS disposal.