Pore fractal characteristics of granulated blast furnace slag-carbide slag cementitious material and its effect on heavy metal adsorption

Coal has long occupied the main position of energy consumption structure in China. The problem of heavy metal pollution in mine water has become increasingly severe due to washing and processing as well as tunnel development activities. Cementitious materials have significant advantages in the treatment of heavy metal mine water, and their pore structure characteristics and fractal properties play a key role in the adsorption performance. In order to study the adsorption effect of pore structure and pore fractal characteristics of cementitious materials on heavy metal ions, solid waste cementitious materials (GCCM) were prepared from slag and carbide slag. Through experimental methods such as XRD, SEM, low-temperature N₂ adsorption/desorption, Cr⁶⁺ adsorption, and the FHH fractal model, the evolution of hydration products of GCCM, the characteristics of pore fractals and the adsorption performance of Cr⁶⁺ were studied. The results show that the hydration products of GCCM are mainly C−(A)−S−H gel. When the molar ratio of n(CaO)/n(SiO₂+Al₂O₃) is 0.635, the gel network is dense. The adsorption/desorption curve of low-temperature nitrogen belongs to Type IV and presents a hysteresis loop of type H₂, mainly mesoporous (2−50 nm). The pore structure characteristics of GCCM show obvious fractal behavior. The partial shape dimension within the pore (D₁) is greater than the fractal dimension of the pore surface (D₂), reflecting that the multi-level pore structure dominated by the hydration products of C−(A)−S−H gel and the material heterogeneity lead to higher internal spatial complexity. The fractal dimension D is positively correlated with compressive strength, specific surface area and pore volume, and negatively correlated with average pore diameter and n(CaO)/n(SiO₂+Al₂O₃). The adsorption process of Cr⁶⁺ by GCCM conforms to the Langmuir monolayer adsorption. D is positively correlated with the maximum adsorption capacity (Q_max) of Cr⁶⁺, and D1 has a more significant effect on Q_max (R²=0.964). The surface pores of GCCM provide direct contact sites for the adsorption of heavy metals. The leaching concentration of heavy metal ions in GCCM meets multiple limit requirements such as Class III water in GB 8978—1996 and GB 20426—2006. The research provides fractal theoretical support and proportioning optimization basis for the application of solid waste-based cementitious materials in the treatment of heavy metals in mine water.