Impact of Low-Activity Coal Gangue on the Mechanical Properties and Microstructure Evolution of Cement-Based Materials

With the increasing global demand for sustainable building materials, coal gangue, as a potential supplementary cementitious material (SCM), has attracted widespread attention. Coal gangue is primarily composed of clay minerals, among which the kaolinite content can significantly enhance its cementitious properties after activation. However, there are various grades of coal gangues, which restrain their application, especially for the low kaolinite content coal gangue. This paper investigates the feasibility of using iron-rich coal gangue with low kaolinite content as a cement substitute through high-temperature activation treatment. In the current study, activated coal gangue replaced cement clinker at proportions of 10%, 15%, and 20%, which was further mixed with limestone powder to form a new cementitious material system. The mechanical attributes of the systems were assessed using compressive strength and microhardness tests. The influence of hydration products and microstructural changes on system performance was further explored through electrochemical impedance spectroscopy (EIS) and quantitative X-ray diffraction (XRD) analysis. The findings suggest that a well-balanced addition of coal gangue can effectively substitute for cement clinker, thereby enhancing both the mechanical properties and microstructure of the systems. These results demonstrate that through appropriate activation treatments, coal gangue can be utilized as an effective SCM. While traditional SCMs like fly ash (FA) and ground granulated blast-furnace slag (GGBFS) have near-zero allocated carbon footprints, their global supply is diminishing and increasingly unreliable. In contrast, our approach valorizes a vast industrial waste stream, aligning with circular economy principles and offering a scalable, sustainable, and low-carbon alternative for the construction industry.