Hu Huang

Red mud (RM), a waste residue from alumina extraction, poses serious environmental impacts on water resources, land resources, and ecological systems due to its large production, high alkalinity, and low resource utilization. To enhance the overall utilization rate of RM solid-waste materials, this study focuses on RM, blast furnace slag (BFS), and fly ash (FA) cementitious materials as the research objects. Through mechanical tests and microstructural analysis, the optimal mix ratio of the ternary RM-based cementitious material is determined, and a systematic study of its microstructural evolution is conducted. Concurrently, fractal theory was used to quantify the microstructure of the material, revealing the evolution laws of the mechanical properties of ternary red-mud-based cementitious materials from a mesoscopic perspective. The results indicate that reducing the proportion of RM or slag alone to increase the FA content yields inferior modification effects compared to simultaneously reducing the proportions of both RM and BFS to increase FA content. Compared with the binary RM-based cementitious material made of RM and BFS, the 28-day compressive strength increases by approximately 25%, reaching 50 MPa. The incorporation of FA can reduce the volume of harmful pores in the cementitious matrix, providing ample reactive material for subsequent hydration reactions, promoting later hydration products, and improving the distribution of the internal pore structure. This leads to increases in both fractal dimensions, and a rational mix proportion can effectively improve the microstructure and mechanical properties of the ternary RM-based cementitious material.

To mitigate the adverse effects of particle agglomeration in alkali-activated coal gangue-based cementitious (AAM–CG) materials, ultrasonic treatment and fractal theory, combined with microscopic analysis techniques were employed to investigate the physical activity of coal gangue (CG) and the microscopic mechanisms of AAM–CG materials. The results indicate that ultrasonic treatment effectively enhances the mechanical properties of AAM–CG materials. With increasing ultrasonic duration, the compressive strength initially rises and then declines, whereas it shows a continuous upward trend with increasing ultrasonic power. The optimal dispersion of CG particles in AAM–CG materials was achieved under ultrasonic treatment at 840 W for 4 min, resulting in a peak compressive strength of 106 MPa. This represents a 28.8% enhancement compared to non-sonicated controls. Ultrasonic treatment effectively disperses agglomerated particles, fully activates CG reactivity, promotes the formation of cementitious phases, improves pore-filling effects, and optimizes the internal pore structure of the material. Compared to untreated samples, the fractal dimension of the pore structure increased after ultrasonic treatment, harmful pores decreased, and porosity was reduced by 32%. This study expands the application of ultrasonic technology in the preparation of alkali-activated geopolymers and provides an efficient activation method for the resource utilization of CG.

To address the detrimental effects of high mud content on cemented sand and gravel (CSG) materials, this study focuses on CSG materials with different contents of alkali-activated rice husk ash (RHA). The microscopic enhancement mechanism of mechanical properties of CSG materials with alkali-activated RHA were studied through the experiments. Based on the changes in microstructure, pore sizes, and mineral composition, the microstructural features of CSG materials are quantitatively characterized using pore characteristic parameters and fractal dimensions, revealing the mechanism by which alkali-activated RHA improves the mechanical properties of CSG materials. The results indicate that alkali-activated RHA effectively enhances the mechanical properties of CSG materials. As the RHA content increases, the compressive strength and elastic modulus initially increase and then decrease, while the failure strain first decreases and then increases. The failure mode transitions from splitting failure to shear failure, with the optimal mechanical performance at 5% RHA content. Microscopic experimental analysis found that adding an appropriate amount of RHA can promote the generation of cementitious substances, improve the internal pore structure, and increase the fractal dimension. However, excessive RHA can adsorb moisture, inhibit part of the hydration reactions and reduce the fractal dimension. Under the action of alkali activators, the activity of RHA is enhanced, generating more cementitious materials and significantly improving the pore-filling effect within the material, especially affecting the capillary pores.

In order to investigate the steel fiber parameters on the damage characteristics and crack evolution of cellular concrete materials, uniaxial compression–acoustic emission combined tests were carried out on steel-fiber-reinforced cellular concrete (SFRCC) with different steel fiber contents (0%, 0.5%, 1%, 1.5%, and 2%) and different porosities (10% and 20%). The material damage evolution characteristics were analyzed by acoustic emission parameters and IB values, and the crack types were identified using Gaussian mixture clustering method (GMM) pairs. The results show the following: the inclusion of steel fibers increased the compressive strength of cellular concrete by 19.8~46.3% at 10% porosity, and by 37.1~102.2% at 20% porosity; the addition of steel fibers significantly increased the density and intensity of the acoustic emission signals; the decreasing tendency of the IB value at the peak stress slowed down with the increase in the amount of steel fibers, and the steel fibers could effectively inhibit the crack development; crack classification results show that the proportion of shear cracks in all stages of cellular concrete increased significantly after the addition of steel fibers.

In this study, a multilayer composite rolling model consisting of a rolling wheel, a protective layer, a GCL, and a support layer was constructed by the discrete element method (DEM). Soil compaction and gravel migration, and their effects on the GCL, were analyzed from a fine viewpoint, and three key indexes for the safety assessment of the GCL were proposed: local elongation, gravel embedment value, and bentonite allotment number. The results show that the soil porosity and cumulative settlement do not decrease all the time with the number of rolling passes, and there exists an optimal number of rolling passes during the rolling process; the protective layer of gravel soil moves more frequently than the support layer; and the nearly rectangular and nearly elliptical gravels are more likely to rotate. The maximum local elongation of the GCL was 3.79% during the lapping process, and all gravels in contact with the upper boundary of the GCL extruded the GCL to varying degrees during the lapping process. The distribution of bentonite particles is closely related to the contact mode between gravel and GCL.