Coal gangue is one of the industrial solid wastes that may harm the human body through the ecosystem for a long time. Using coal gangue in geopolymer preparation can effectively reduce cement output and meet the sustainability requirements. In this paper, the physical and chemical characteristics, including the heavy metal content, of coal gangue from different producing areas are described. Then, the mechanism of physical activation (mechanical and thermal activation), chemical activation, and compound activation of coal gangue are illustrated. The machinability, as well as the mechanical, microscopic, and toxicity consolidation properties of geopolymers prepared from coal gangue, are summarized and analyzed. The results indicate that the coal gangue geopolymers can have higher mobility and mechanical strength than cement-based composites by adjusting high calcium element material, alkali activator content, Na₂SiO₃ modulus, and curing condition. After physical activation, coal gangue is used in geopolymer preparation with a chemical activator (alkali excitation agent), which effectively forms a three-dimensional silicon aluminate polymer network. The pore structure is dense, the physical fixation and chemical bonding are strengthened, and the solidification and adsorption of heavy metal ions are improved. Further, it can also be applied to solidifying radioactive waste, which is following the future development direction.

Coal gangue is a kind of industrial solid waste with serious ecological and environmental implications. Producing concrete with coal gangue aggregate is one of the green sustainable development requirements. This paper reviews the properties and preparation methods of Chinese gangue aggregate, studies the influence of gangue aggregate on concrete properties and the prediction model of gangue concrete, and summarizes the influence of modified materials on gangue concrete. The studies analyzed in this review show that different treatments influence the performance of coal gangue aggregate concrete. With the increase in the replacement ratio of coal gangue aggregate in concrete, the concrete workability and mechanical performance are reduced. Furthermore, the pore structure changes lead to decreased porosity, greatly affecting the durability. Coal gangue is not recommended for producing high-grade concretes. Nevertheless, pore structure can be improved by adding mineral admixtures, fibers, and admixtures to the coal gangue concrete. Hence, the working properties, mechanical properties, and durability of the concrete can be improved effectively, ensuring that coal gangue concrete meets engineering design requirements. Adding modified materials to coal gangue concrete is a viable future development direction.

In this study, high concentration wastewater from ready-mixed concrete plants was used to replace potable water as mixing water of concrete, with replacement rates of 0%, 25%, 50%, 75%, and 100%, by weight. The solid content of the wastewater was 12%. Five groups of C20 concrete mix proportions were designed. Different concrete properties, including workability, compressive strength and durability under freeze–thaw cycles, carbonization, and drying conditions, were studied, and the effect of the increase in the proportion of wastewater as a replacement for potable water was investigated. The microstructural attributes of the developed C20 concrete were studied through X-ray diffraction (XRD), thermal analysis (TG-DSC), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), and mercury intrusion porosimetry (MIP). Finally, the economic benefits of replacing potable water with wastewater were analyzed. The results indicate that using wastewater for concrete mixing reduces workability and a superplasticizer is needed to ensure adequate concrete workability. At the ages of 7, 28, and 56 days, with the increase in the proportion of wastewater as a replacement for potable water (0, 25%, 50%, 75%, 100%), the compressive strength of concrete shows a trend of first decreasing, then increasing, and then decreasing. When the proportion of wastewater replacing potable water is 75%, the concrete compressive strength is the highest. The microstructure showed that the main products of wastewater-mixed concrete are calcite (CaCO₃), portlandite (Ca(OH)₂), ettringite (Aft), and calcium silicate hydrate (C-S-H). Adding wastewater to concrete does not lead to the formation of new products in the concrete. Wastewater can fill the concrete pores well, thus optimizing the pore structure. When the proportion of wastewater replacing potable water is 75%, C20 concrete has the densest microstructure, lower porosity, and better pore structure. Durability properties further indicate that 25%, 50%, and 75% of wastewater replacing potable water can improve the concrete’s frost resistance. However, there is a negative impact on the carbonation resistance of wastewater. Wastewater replacing 75% potable water by weight can improve the drying shrinkage of concrete. The recycling of wastewater is not only green and environmentally friendly but also has good economic and environmental benefits.

The research into the road performance of coal gangue is of great significance for the consumption of coal gangue and reducing pollution. In this paper, the coal gangues were prepared by separation and crushing processes, and their gradations were also optimized. Aiming to identify the possible problems of coal gangue as a pavement base, an unconfined compressive strength test, a splitting test, a freeze–thaw test, and a drying shrinkage test of cement stabilized gangue with varying cement amounts were carried out, and the test results were compared and analyzed. The test results showed that the maximum dry density and optimum moisture content (OMC) of the optimized cement stabilized gangue and cement stabilized macadam increased with cement content. The maximum dry density and OMC of cement stabilized macadam were larger than that of cement stabilized gangue with the same cement content. The optimized 7-day unconfined compressive strength of cement stabilized gangue can meet the requirements for a secondary and lower highway base and subbase. The OMC and cement content are the critical factors affecting the compressive strength loss rate of cement stabilized gangue after freeze–thaw cycles. The smaller the OMC of cement stabilized gangue and the larger the cement content, the lower the compressive strength loss rate. With an increase in cement content, the drying shrinkage strain of cement stabilized gangue increased. The results show that a cement content of 4% is optimal for the cement stabilized coal gangue, which can be used for the light traffic base and heavy traffic subbase of class II and below highways. It provides a basis, guide, and reference for the application of coal gangue materials in a high-grade highway base.

The Grey Relation Entropy (GRE) theory is used to analyze the sensitive pore size that affects the compressive strength of concrete. The relationship between the strength and pore structure is revised based on the sensitivity coefficient. The revised model is used to calculate the compressive strength of concrete. In order to verify the validity of the proposed model, the calculated results are compared with experimental ones, showing satisfactory agreement with a larger correlation than existing methods.

In order to study the mechanical properties and effect of a regenerant on a cold recycled mixture with asphalt emulsions (CRMEs), the moisture susceptibility, high-temperature performance, low-temperature performance, dynamic mechanical properties and durability of CRMEs were analyzed and evaluated by immersion splitting strength tests, freeze-thaw splitting strength tests, rutting tests, semi-circle bending tests, uniaxial compression dynamic modulus tests and indirect tensile tests. Scanning electron microscopy (SEM) was used to analyze the micromorphology of CRMEs modified with regenerant. Finally, a comprehensive evaluation system of five different CRMEs was established based on the efficacy coefficient method to quantitatively analyze the comprehensive performance of the CRMEs. The test results showed that the regenerant can significantly improve the water immersion splitting strength, freeze-thaw splitting strength fracture energy density, and fatigue resistance of CRMEs. However, the addition of regenerant affected the high-temperature performance of the cold recycled mixture. The dynamic modulus of the CRMEs first increased and then decreased with regenerant content increasing. When the regenerant content was 8%, the dynamic modulus of the CRMEs was the highest. Adding styrene-butadiene rubber (SBR) latex can improve the high-temperature performance of CRMEs, but the moisture susceptibility, low temperature performance and fatigue resistance of the cold recycled mixture were not significantly improved, and the dynamic modulus of the mixture was reduced. Based on the efficacy coefficient method, the optimal content of regenerant is 8%. Regenerant are potential modifiers for cold recycled mixture that they can significantly improve the dynamic mechanical properties and durability.