The significant difference in water-to-binder ratio, activator type and concentration between alkali-activated slag (AAS) paste/mortar/concrete and AAS-based cemented paste backfill (AAS-CPB) means that previous results related to the properties and mix optimization of AAS materials cannot be directly translated to AAS-CPB. This study statistically identifies the effect of key influential variables such as silicate modulus, slag fineness and activator concentration on 3- and 28 day unconfined compressive strength (UCS) of AAS-CPB by central composite design (CCD) established in response surface methodology (RSM). In this study, the prominence of independent variables and their relations are investigated by using ANOVA (analysis of variance) having a significant level of 0.05. ANOVA results certify that there is a strong link between the level of variable contribution on UCS performance of AAS-CPB and curing age. Obviously, silicate modulus and activator concentration are the most major variables influencing UCS at 3 and 28 days, respectively. Increased fineness of slag and higher pH of pore solution enhance 3 day UCS, but restrain the further hydration of unreacted slag and subsequent the gain in strength at advanced curing ages. The combination of independent variables of silicate modulus (0.295), slag fineness (12630.2), activator concentration (0.45) gives the optimum responses.

Research focusing on waste management and CO₂ mineralization simultaneously has been a popular topic in the mining community, and a common approach is to mineralize CO₂ with coal-based solid waste (CSW, e.g., gangue (CG), fly ash (FA), coal gasification slag (CGS)) produced by mining activities. Despite the understanding of CO₂ mineralization by cementitious materials, the mineralization capacity of alkali-activated CSWs remains unknown. Therefore, the mineral composition evolution and mineralization capacity of different alkali-activated materials (prepared with CG, FA, CGS, and sodium hydroxide (which works as the alkali-activator), respectively) are investigated with the adoption of Gibbs Energy Minimization Software (GEMS). The results indicate that the abovementioned three alkali-activated CSWs are majorly composed of calcium silicate hydrate, magnesium silicate hydrate, kaolinite, sodium zeolite, and liquid. Due to the difference in the chemical composition of different CSWs, the amount of hydration products varies. Specifically, the alkali-activated CSWs made with CGS have the maximum calcium silicate hydrate (C-S-H), while those prepared with FA enjoy the lowest porosity. In addition, the CO₂ mineralization process will result in the formulation of carbonate and, theoretically, the maximum quantity of mineralized CO₂ is less than 20% of the binder used. Furthermore, compared with CG and CGS, FA is characterized with the highest mineralization capacity. The findings in this study contribute to the understanding of CO₂ mineralization with alkali-activated CSWs.

During the extraction and utilization of coal resources, a large amount of CO₂ and coal-based solid wastes (CBSW), such as coal gangue, are generated. To reduce the carbon and waste emissions, an effective approach is to mineralize the CO₂ with the CBSW and then backfill the mineralized materials into the goaf area. However, efficient CO₂ mineralization is challenging due to the low reactivity of coal gangue. To this end, mechanical activation was used for the modification of coal gangue, and the mechanical activation mechanism of coal gangue was revealed from a microcosmic perspective by dry powder laser particle size testing (DPLPST), X-ray diffractometer (XRD) analysis, Fourier-transform infrared spectrometer (FTIR) analysis, and scanning electron microscopy (SEM). The results showed that compared with the unground coal gangue, the average particle size of coal gangue after 0.5 h, 1 h, and 1.5 h milling decreases by 94.3%, 95%, and 95.3%, respectively; additionally, the amorphous structures of the coal gangue after milling increase, and their edges and corners gradually diminish. After the pressure mineralization of coal gangues with different activation times, thermogravimetric (TG) analysis was performed, and the CO₂ mineralization effect of the mechanically activated coal gangue was explored. It is found that the carbon fixation capacity of the coal gangue after 0.5 h, 1.0 h, and 1.5 h mechanical activation is increased by 1.18%, 3.20%, and 7.57%, respectively. Through the XRD and SEM, the mechanism of CO₂ mineralization in coal gangue was revealed from a microcosmic perspective as follows: during the mineralization process, alkali metal ions of calcium and magnesium in anorthite and muscovite are leached and participate in the mineralization reaction, resulting in the formation of stable carbonates such as calcium carbonate.

Coal gangue can be used for CO₂ mineralization and then be backfilled into the goaf, which can simultaneously address the long-term management of coal gangue and CO₂. Mechanical and composite activation methods were employed to enhance the activity and mineralization potential of coal gangue, and ion leaching experiments were conducted to leach active ions from mechanically-activated coal gangue (MACG) and composite-activated coal gangue (CACG) in this study. Additionally, the evolution of the mineralization potential of MACG and CACG samples under different conditions was explored by inductively coupled plasma-optical emission spectrometry (ICP-OES) tests. The results show that with the extension of milling time, the leaching rates of Ca²⁺, Mg²⁺, and K⁺ of MACG samples gradually increase, while the leaching rate of Ca²⁺ of CACG samples gradually decrease, and leaching rates of Mg²⁺ and K⁺ of CACG samples gradually increase. The activation mechanism of MACG samples was analyzed by the dry powder laser particle size analysis (DPLPSA) and Brunauer-Emmett-Teller (BET). The results show that when the milling time is less than 2 h, the average particle size of MACG samples decreases significantly and the specific surface area increases significantly with the extension of the milling time. However, when the milling time increases to 4 h, the changes in particle size and specific surface area become less significant, leading to an increase in the non-uniformity coefficient and a reduction in porosity. Thermogravimetry (TG) analysis was used to analyze the material components of MACG and CACG samples. It was found that due to the combustion of organic matter, thermal activation after mechanical activation can effectively improve the activity of coal gangue. After mechanical activation, the activity improvement of coal gangue is gradually reduced due to the advance of the transition temperature from metakaolinite to mullite. Finally, MACG and CACG samples before and after mineralization were analyzed by TG tests. The mineralization capacity of MACG samples with a 2-hour milling time was found to be 0.43%, whereas CACG samples achieved a higher mineralization rate of 0.53%.