This paper aims to enhance the effective utilization of construction solid waste renewable brick powder (RBP) and circulating fluidized bed fly ash (CFBFA), addressing the issues of resource consumption and environmental pollution associated with these two types of solid waste. It employs CFBFA to synergistically activate RBP for the preparation of solid waste-based earthwork subgrade backfill. This research examines the impact of RBP and CFBFA content on the performance of earthwork subgrade backfill (ESB), while the microstructure of the paste test block was investigated using XRD, SEM, FTIR, and TG-dTG techniques. The synergistic mechanism of multisolid waste was examined at the micro level, and the appropriate ratio of solid waste-derived lowcarbon ESB was thoroughly assessed. The findings indicate that an increase in the CFBFA content generally enhances the mechanical strength of the paste. At the experimental ratio of RBP: CFBFA: coarse-grained soil = 8: 32: 60, the 28-day unconfined compressive strength (UCS), California Bearing Ratio (CBR) value, rebound modulus value, shear strength value, and compression modulus value of the sample attain their maximums, measuring 5.3 MPa, 41.9 %, 71.9 MPa, 10.5 KPa, and 15.76 MPa, respectively, all exceeding the standard values. The hydration products of cementitious materials based on RBP and CFBFA mostly consist of C-S-H gel, ettringite (AFt), and calcite. The robust honeycomb gel structure, created by the staggered interconnection of C-S-H gel and ettringite, is the primary contributor to mechanical strength. The modified cementitious material, composed of RBP-CFBFA, exhibits effective cementation and solidification properties for heavy metals, achieving leaching concentrations that comply with Class III water standards as outlined in the Chinese standard GB/T 14848-2017.

In this essay, by summarizing the research progress and achievements of various scholars at home and abroad in recent years on the material properties and corrosion resistance of magnesium phosphate cement (MPC), we review the factors influencing on the properties of MPC, and analyze the effects of raw materials, retarders, and admixtures on the properties of MPC. Two different hydration mechanisms of MPC are discussed, and finally the research progress of MPC in the field of anti-corrosion coatings for steel and ordinary concrete (OPC) is highlighted, and suggestions and prospects are given.

Red mud (RM) is a strongly alkaline waste residue produced during alumina production, and its high alkali and fine particle characteristics are prone to cause soil, water, and air pollution. Phosphogypsum (PG), as a by-product of the wet process phosphoric acid industry, poses a significant risk of fluorine leaching and threatens the ecological environment and human health due to its high fluorine content and strong acidic properties. In this study, RM-based cemented paste backfill (RCPB) based on the synergistic curing of PG and ordinary Portland cement (OPC) was proposed, aiming to achieve a synergistic enhancement of the material's mechanical properties and fluorine fixation efficacy by optimizing the slurry concentration (63-69%). Experimental results demonstrated that increasing slurry concentration significantly improved unconfined compressive strength (UCS). The 67% concentration group achieved a UCS of 3.60 MPa after 28 days, while the 63%, 65%, and 69% groups reached 2.50 MPa, 3.20 MPa, and 3.40 MPa, respectively. Fluoride leaching concentrations for all groups were below the Class I groundwater standard (<= 1.0 mg/L), with the 67% concentration exhibiting the lowest leaching value (0.6076 mg/L). The dual immobilization mechanism of fluoride ions was revealed by XRD, TGA, and SEM-EDS characterization: (1) Ca2(+) and F- to generate CaF2 precipitation; (2) hydration products (C-S-H gel and calixarenes) immobilized F- by physical adsorption and chemical bonding, where the alkaline component of the RM (Na2O) further promotes the formation of sodium hexafluoroaluminate (Na3AlF6) precipitation. The system pH stabilized at 9.0 +/- 0.3 after 28 days, mitigating alkalinity risks. High slurry concentrations (67-69%) reduced material porosity by 40-60%, enhancing mechanical performance. It was confirmed that the synergistic effect of RM and PG in the RCPB system could effectively neutralize the alkaline environment and optimize the hydration environment, and, at the same time, form CaF2 as well as complexes encapsulating and adsorbing fluoride ions, thus significantly reducing the risk of fluorine migration. The aim is to improve the mechanical properties of materials and the fluorine-fixing efficiency by optimizing the slurry concentration (63-69%). The results provide a theoretical basis for the efficient resource utilization of PG and RM and open up a new way for the development of environmentally friendly building materials.

This study investigates efficient dehydration and solidification techniques for waste mud generated from loess pile foundations during highway construction in Lanzhou, Northwest China. The waste mud, characterized by high viscosity (85% moisture content) and alkalinity (pH 11.2), poses environmental risks if untreated. Dehydration experiments identified an optimal composite flocculant mixture of 3.5 g polyaluminum chloride (PAC) and 22 mL anionic polyacrylamide (APAM) per 500 mL waste mud, accelerating sedimentation and reducing the supernatant pH to 8.65, compliant with discharge standards. Solidification tests employed a composite curing agent (CG-T1+cement), demonstrating enhanced mechanical properties. The California Bearing Ratio (CBR) of the solidified sediment reached 286%, and the unconfined compressive strength (UCS, 7-day) exceeded 2.0 MPa, meeting roadbed specifications. The combined use of PAC-APAM for dehydration and CG-T1-cement for solidification offers an eco-friendly and economically viable solution for reusing treated waste mud in construction applications, addressing regional challenges in mud disposal and resource recovery.

The mechanical behavior of expansive soil in geotechnical engineering is significantly sensitive to loading rates, hydration, confining pressure, etc., where most engineering problems are attributed to the existence of montmorillonite in expansive soil. Here, the hydration, confining pressure, and loading rate effect on the mechanical behavior of montmorillonite were investigated through the triaxial tests and molecular dynamics (MD) simulation method, revealing their fundamental mechanism between the microscale and macroscale. The average basal spacing of hydrated montmorillonite system, the diffusion coefficient and density distribution of interlayer water molecules were calculated for the verification of MD model. The experimental results indicated that the stress-strain relationship of montmorillonite was the strain-hardening type. The failure stress did not increase monotonously with the increase in loading rate, and there were two obvious critical points. The failure stress of the soil sample increased with the increase of the confining pressure, and the decrease of the water content, where their fundamental mechanism between microscale and macroscale were adequately discussed. Furthermore, the stress-strain response, total energy evolution, deformation evolution of atomistic structure, and broken bonds evolution were analyzed to deeply understand the fundamental deformation mechanism at the microscale. The multi-scale studies could effectively examine the macroscopic mechanical behavior of expansive soil and elucidate its microscopic mechanisms.

In this study, the effects of vertical strain and hydration time on the mechanical behavior and microstructure of expansive soils are examined, addressing the challenges they pose to engineering structures due to moisture-induced swelling pressure and deformation. Conducting hygroscopic expansion tests on soils with varying initial dry densities, the study explores the relationship between swelling pressure and vertical strain. Additionally, the effect of different hydration times on these properties is assessed. Using mercury intrusion porosimetry, the soil specimens are dissected into top and bottom layers to observe microstructural changes over varying hydration periods. The results indicate a decline in swelling pressure and expansion rate with increased strain; at 1% strain, there is a 54% decrease in vertical swelling pressure and a 41% reduction in lateral pressure. Expansion rate attenuation is more significant, with an 83% decrease in vertical and 92% in lateral rates. The research concludes that the hydration process under limited strain consists of two stages: the initial strain stage, with pronounced top layer expansion, and a subsequent constant-volume stage, where the top layer undergoes compression and the bottom layer expands significantly.

Currently, traditional vertical barrier materials are associated with large carbon footprints and high costs (in some regions) due to the widespread use of Portland cement and sodium-based bentonite materials. In recent years, a new technology of Carbonized Reactive Magnesia Cement (CRMC) has gradually been developed to sequester CO2 using Eco-cement. The application prospects of CRMC in vertical barrier materials are explored in this study. The changes in flowability of Reactive Magnesia Cement (RMC) slurry and the unconfined compressive strengthen (UCS) and permeability characteristics of CRMC treated soils are investigated. The results show that the fluidity of RMC slurry decreases further with the increase of MgO substitute cement content. For RMC slurry meeting the fluidity requirements, UCS increased rapidly in the early period (3 h) after carbonization, reaching 348.33 kPa, and the hydraulic conductivity k decreased (k < 1 x10(- 7) cm/s) in the later period (14d), and the final hydraulic conductivity reached 6.13 x 10(- 8) cm/s (28d). The pores of the material are filled with a large number of hydration products and carbonates, which alters the pore size distribution structure of the material. This is the reason for the mechanical properties and permeability performance of CRMC treated soils. The overall results of this study well demonstrate that CRMC treated soils, as a new, environmentally friendly, and cost-effective material, have great potential in the construction of vertical barriers.

The ionic soil stabilizer (ISS) can synergistically enhance the mechanical properties and improve the engineering characteristics of iron tailings soil in conjunction with cementitious materials such as cement. In this paper, the influence of ISS on the cement hydration process and the charge repulsion between iron tailings soil particles was studied. By means of Isothermal calorimetry, X-ray diffraction (XRD), Scanning electron microscope (SEM), and Low-field nuclear magnetic resonance microscopic analysis methods such as (LF-NMR), X-ray photoelectron spectroscopy (XPS), Non-evaporable water content and Zeta potential were used to clarify the mechanism of ISS-enhanced cement stabilization of the mechanical properties of iron tailings soil. The results show that in the cement system, ISS weakens the mechanical properties of cement mortar. When ISS content is 1.67%, the 7 d compressive strength of cement mortar decreases by 59.8% compared with the reference group. This retardation arises due to carboxyl in ISS forming complexes with Ca2+, creating a barrier on cement particle surfaces, hindering the hydration reaction of the cement. In the cement-stabilized iron tailings soil system, ISS has a positive modification effect. At 0.33% ISS, compared with the reference group, the maximum dry density of the samples increased by 6.5%, the 7 d unconfined compressive strength increased by 35.3%, and the porosity decreased from 13.58% to 11.85%. This is because ISS reduces the double electric layer structure on the surface of iron tailings soil particles, reduces the electrostatic repulsion between particles, and increases the compactness of cement-stabilized iron tailings soil. In addition, the contact area between cement particles increases, the reaction energy barrier height decreases, the formation of Ca(COOH)2 reduces, and the retarding effect on hydration weakens. Consequently, ISS exerts a beneficial effect on augmenting the mechanical performance of cement-stabilized iron tailings soil.

Magnesium phosphate cement (MPC), renowned for its rapid hardening, low water demand, low-temperature hydration capability, and excellent wear resistance, is an ideal construction material for the extreme lunar environment, characterized by high vacuum, low gravity, and severe temperature fluctuations. In this study, by-product B-MgO from lithium extraction in salt lakes was utilized to develop four types of phosphate cement systems: ammonium magnesium phosphate cement (MAPC), sodium magnesium phosphate cement (MSPC), calcium magnesium phosphate cement (MCPC), and potassium magnesium phosphate cement (MKPC). Through a comparative analysis of the physical and mechanical properties of these systems at varying calcination temperatures of MgO, MKPC was identified as the most suitable for lunar construction. Further investigations examined the influence of the water-to-binder ratio (W/B) and the mass ratio of raw materials (M/P) on MKPC performance, alongside a detailed analysis of its phase composition and microstructure. The results revealed that the optimal MKPC performance is achieved at an MgO calcination temperature of 1000 degrees C, an M/P ratio of 1:1 to 2:1, and a W/B ratio of 0.2 to 0.25. Additionally, MKPC was employed as a cementitious material to produce MKPC-simulated lunar regolith concrete with regolith contents of 30 %, 53 %, and 70 %. The fabricated concrete met the required mechanical properties and 3D printability standards under lunar environmental conditions. Even at high regolith content, the concrete maintained satisfactory mechanical performance. These findings provide an efficient and reliable material solution for lunar infrastructure construction. (c) 2024 Published by Elsevier B.V. on behalf of COSPAR.

During the improvement and reinforcement of peat foundation soils, cement hydration alters the pH of the subsurface water-soil ecosystem. This change negatively impacts humus acid, the main component of organic matter in peat soils, thereby deteriorating the engineering properties of peat foundations. Tests simulated the subsurface alkaline environment by using cement treat peat soils in actual projects. The objective is to understand the dynamic processes of cement hydration affecting peat environments and to investigate the dissolution properties of humus acid in peat soil under alkaline environment during cement hydration. Results indicate that peat soil environment transforms into an alkaline environment under cement hydration, where humus acid in peat soil exhibits dissolution properties under alkaline environment. Humus acid undergoes dissolution and reacts in alkaline environment. As the pH of the environment stabilizes, the dissolution of humus acid practically ceases. As humus dissolves, the pores inside peat soil expand, and the skeleton structure becomes less compact, reducing the soil's compactness connectedness, leading to significant strength loss. The dissolution of humus acid can significantly damage the peat soil structure. study provides valuable insights into engineering issues arising from humus acid dissolution in peat soil under alkaline environment induced by cement hydration.