A novel MgO-mixing column was developed for deep soft soil improvement, utilizing in-situ deep mixing of MgO with soil followed by carbonation and solidification via captured CO2 injection. Its low carbon footprint and rapid reinforcement potential make it promising for ground improvement. However, a simple and cost-effective quality assessment method is lacking. This study evaluated the electrical properties of MgO-mixing columns using electrical resistivity measurements, exploring relationships between resistivity parameters and column properties such as saturation, strength, modulus, CO2 sequestration and uniformity. Microscopic analyses were conducted to elucidate the mechanisms underlying carbonation, solidification, and electrical property changes. The life cycle assessment (LCA) was performed to assess its carbon reduction benefits and energy consumption. The findings reveal that the electrical resistivity decreases rapidly with increasing test frequency, remaining constant at 100 kHz, with the average electrical resistivity being slightly higher in the upper compared to the lower section. Additionally, electrical resistivity follows a power-law decrease with increasing saturation. Both electrical resistivity and the average formation factor exhibit strong positive correlations with unconfined compressive strength (UCS) and deformation modulus, enabling predictive assessments. Furthermore, CO2 sequestration in MgO-mixing columns is positively correlated with electrical resistivity, and the average anisotropy coefficient of 0.96 indicates good column uniformity. Microstructural analyses identify nesquehonite, dypingite/hydromagnesite, and magnesite as significant contributors to strength enhancement. Depth-related changes in electrical resistivity parameters arise from variations in the amount and distribution of carbonation products, which differently impede current flow. LCA highlights the significant low-carbon advantages of MgOmixing columns

As a prevalent problematic soil in geotechnical engineering, organic-rich soil exhibits inferior engineering characteristics that necessitate stabilization treatment in practical applications. Among various soil improvement techniques, chemical stabilization using Portland cement (PC) has gained widespread adoption due to its operational convenience. However, conventional PC involves not only environmental burdens associated with resource- and energy-intensive production processes and carbon emissions but also substantial interference from organic matter (OM) during its hydration process, inhibiting the formation of cementitious bonds. To address these challenges, this study proposes an innovative green stabilization approach using reactive MgO carbonation technology. A comprehensive investigation was conducted to evaluate the physicochemical evolution, mechanical behavior, and microstructural characteristics of organic soils under varying OM contents and carbonation durations. Key findings revealed that unconfined compressive strength demonstrated a linear inverse relationship with OM content while exhibiting time-dependent enhancement during carbonation. Strength development correlated positively with mass gain and dry density but inversely with water content. Microanalytical results indicated OM-dependent phase transformations, showing decreased nesquehonite crystallization and increased dypingite/hydromagnesite formation with ascending OM content. Mechanism analysis suggested that OM content regulated carbonation product speciation and aggregate morphology, thereby governing the coupled processes of particle cementation, pore structure refinement, and mechanical strengthening. This research demonstrates the technical viability of MgO carbonation for organic soil stabilization while contributing to sustainable geotechnical practices through carbon sequestration.

Reactive magnesium oxide (MgO) and ground granulated blast furnace slag (GGBS) are cementitious materials introduced into sludge solidification, which not only reutilizes solid waste but also reduces cement consumption. Through the carbonation of reactive MgO and GGBS, the strength of the solidified sludge is further improved and CO2 is stably sequestrated in carbonate minerals. This paper investigates the strength and microstructural development and CO2 uptake of solidified sludge with varying water content, binder content, and ratio of MgO to GGBS. According to unconfined compressive strength (UCS) tests, when the binder content is 20% and the ratio of reactive MgO to GGBS is 2 & ratio;8, the strength of carbonated samples increases the most, which is six times that of the sample without reactive MgO. With binder content, the CO2 uptake of sample increases up to 2.1 g. Scanning electron microscope (SEM), X-ray diffractometer (XRD), and thermogravimetry-differential thermogravimetry analysis (TG-DTG) tests were conducted to systematically elucidate the micromechanism of carbonation of sludge solidified by reactive MgO and GGBS. Various carbonation and hydration products enhance the soil strength through filling pores and integrating fine particles into bulk aggregates. As the ratio of reactive MgO to GGBS increases, dypingite and hydromagnesite were converted into nesquehonite with better morphological integrity, and thus strengthens the soil skeleton. Diverse calcium carbonate polymorphs from carbonated GGBS also promote sludge strength growth and CO2 sequestration. Test results indicate that the addition of reactive MgO further improves the hydration and carbonation properties of GGBS, so the CO2 uptake grows with the ratio of reactive MgO to GGBS. The synergistic effect of reactive MgO and GGBS increases the carbonation performance of the mixed binder, so likewise the compressive strength.

Coastal regions often face challenges with the degradation of cementitious foundations that have endured prolonged exposure to corrosive ions and cyclic loading induced by environmental factors, such as typhoons, vehicular traffic vibrations, and the impact of waves. To address these issues, this study focused on incorporating Nano-magnesium oxide (Nano-MgO) into cemented soils to investigate its potential impact on the strength, durability, corrosion resistance, and corresponding microstructural evolution of cemented soils. Initially, unconfined compressive strength tests (UCS) were conducted on Nano-MgO-modified cemented soils subjected to different curing periods in freshwater and seawater environments. The findings revealed that the addition of 3% Nano-MgO effectively increased the compressive strength and corrosion resistance of the cemented soils. Subsequent dynamic cyclic loading tests demonstrated that Nano-modified cemented soils exhibited reduced energy loss (smaller hysteresis loop curve area) under cyclic loading, along with a significant improvement in the damping ratio and dynamic elastic modulus. Furthermore, employing an array of microscopic analyses, including nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and scanning electron microscopy (SEM), revealed that the hydration byproducts of Nano-MgO, specifically Mg(OH)2 and magnesium silicate hydrates, demonstrated effective pore space occupation and enhanced interparticle bonding. This augmentation markedly heightened the corrosion resistance and durability of the cemented 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.

Effect of cement, Ground Granulated Blast Furnace Slag (GGBS), GGBS:magnesia (MgO) and GGBS:MgO:cement were studied as agents on stabilisation of a clay soil contaminated with glycerol solution. The contaminated soil was mixed with 5, 10 and 15% of the above agents. Atterberg limits and compaction tests were conducted on these mixtures. Additionally, strength and durability tests were performed on prepared samples at different curing times. The strength of soil contaminated with 4, 8, and 12% glycerol was reduced by 23.5, 30.1, and 36.5%, respectively, compared to the natural soil. By adding 5% cement to the soil contaminated with 4% glycerol, its strength after 7, 14, and 28 days of curing time was increased to 1581, 1984.5, and 2343.4 kPa, respectively. All the selected agents increased the strength of the contaminated soil and its increase was dependent on the percentage of the agent and curing time. It was revealed that GGBS:MgO:cement was more effective in increasing the strength than the other used agents. Durability tests also showed that the weight loss of the samples at different conditions was less than 10%. SEM results showed that the increase in strength of the soil results from the interaction between soil and agent.

Carbonation technology using MgO and CO2 has been considered a rapid, effective, and environmentally friendly method for improving weak soils, mainly applied in shallow foundation treatments. This study introduced a novel MgO-carbonated composite pile (MCP) technique developed by injecting CO2 through a gas-permeable pipe pile into a MgO-mixing column for carbonation and solidification and its applications in weak subgrade treatments. Several field tests were carried out to study the characteristics of MCP as well as the performance of the MCP-reinforced foundations, including carbonation reaction temperature monitoring, pore-water pressure monitoring, standard penetration tests (SPTs), unconfined compressive strength (UCS) tests, static load tests, and subgrade deformation monitoring. Results showed vigorous and uniform carbonation within the MgO-mixing column, confirming the feasibility of constructing large-diameter MgO-mixing columns. The distribution, evolution, and affected zone of excess pore-water pressure induced by MCP installation were determined. The MCP exhibited good pile quality, with average SPT blow count and UCS value of 39 and 1021 kPa, respectively. MCP's bearing capacity was superior to prestressed high-strength concrete pipe piles, with ultimate vertical and lateral bearing capacities of 1920 and 119 kN, respectively. The MCP-reinforced foundation exhibited a small settlement of 54.5 mm under embankment loads. Life cycle assessment indicated significant carbon reduction benefits for MCP, with 44.7% lower carbon emissions compared to traditional composite piles.

Loess has poor engineering performance and needs to be improved for engineering applications by adding a large amount of lime or cement, which is not consistent with the goal of carbon peaking and carbon neutrality. In this study, nano-SiO2 (NS) and nano-MgO (NM) were applied to improve the engineering performance of lowdosage lime/cement- stabilized loess. The improvement mechanisms of each binder on loess were analyzed by X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive spectrometer (SEM-EDS) tests. The impact of binder dosage and curing time (T) on unconfined compressive strength (UCS), resilient moduli (MR), California bearing ratio (CBR), internal friction angle (phi), cohesion (c), and compression coefficient (a1-2) of each stabilized loess were also explored by conducting a range of laboratory experiments. The results show that the addition of NS did not result in the formation of new substances. However, the formation of MH was noted with the addition of NM. The combination of lime and NS can significantly enhance the UCS, CBR, MR, and c of the stabilized loess, followed by the combination of cement and NS. With the increasing NM content, the above mechanical indices first increased and then decreased for the stabilized loess. Both the binder content and type caused a lesser impact on the phi and a1-2 than on other mechanical indices. Moreover, the mix ratio and feasibility of each stabilized loess applied in various engineering fields were analyzed based on relevant standards and the construction requirements of lime and cement. Finally, estimation models were established for the above mechanical indices of lime-NS stabilized loess, which can provide a reference for engineering design and quality control.

The ultimate goal of Mars exploration is to construct a Mars base. In particular, it is necessary to prepare fibres by using Martian soil as a raw material. High-strength Martian glass fibres can be used to reinforce composite materials to meet the requirements of high-strength functional materials in base construction. Owing to the wide variation in MgO content in Martian soil, in this study, the effects of MgO on the structure, strength and acid resistance of Martian glass and glass fibres were investigated. The preparation conditions and mechanical properties of simulated Martian soil fibre (SMSF) were studied via DSC, XRD, Raman, NMR, FT-IR and hightemperature rotational viscometry. The corrosion behaviour of MgO-SMSF in H2SO4 solution was subsequently studied via SEM/EDS. The results showed that MgO reduces the spinnability window and prevents the fibres from stretching continuously, and a threshold appears to exist at 10.89 % MgO. The viscosity of the melt decreased significantly, and the crystallization trend increased with MgO above the threshold. The fibre tensile strength showed a nonlinear relationship with a 22.85 % increase in the fibre tensile strength at 9.24 % MgO. SEM/EDS revealed that the surface of the SMSF formed a gel layer, and the mass retention of the MgO-SMSF in the H2SO4 solution reached 90.17 %. The corrosion of SMSF under acidic conditions was controlled by ion diffusion, with Mg+ and Ca+ diffusing to the fibre surface, resulting in nonuniform corrosion. Raman-based statistical structure-property modelling further explains the impact of MgO-induced structural changes on the tensile strength and elastic modulus, with good agreement between the model predictions and measured values.

The existing literature suggests that natural aggregate concrete demonstrates the least shrinkage, followed by recycled aggregate concrete (RAC) prepared using natural sand, with RAC prepared using recycled sand (RS) from the weathered residual soil of granite demonstrating the greatest shrinkage. Internal incorporation of a MgO expansion agent (MEA) effectively compensates for the excessive shrinkage of the latter; however, the influence of the MEA on the strength development of RAC prepared using RS after natural curing, rather than accelerated carbonation curing, remains unclear. In this study, compression tests of RAC prepared using RS at different stages of natural curing were performed and the corresponding material compositions of RAC were determined and quantified via X-ray diffraction and thermogravimetry-differential thermogravimetry. The soluble carbonate content in RS was determined by ion chromatography, and the morphology of RAC was observed using scanning electron microscopy. The mechanism of strength development of RAC during aging was determined. Furthermore, compressive tests of recycled lump-aggregate concrete (RLAC) were performed to investigate the influencing degree of RAC as fresh concrete on the compressive properties of RLAC. The following key results were noted: (a) the MEA impairs the compressive strength of concrete, but the degree of impairment decreases with curing, and this is attributed to the transformation of Mg(OH)2 to MgCO3. (b) The presence of soluble carbonates in RS (7.2 %) is the main source of carbonate in the conversion of Mg(OH)2 to MgCO3. Mg(OH)2 particles adhere to the surface of RS particles and react with soluble carbonate to generate MgCO3. (c) At 56 days of curing, the addition of 6 % MEA or increasing the replacement ratio of RS impaired the compressive strength of RLAC to a certain extent. However, even with 100 % RS, the compressive strength and elastic modulus of RLAC were impaired by only 7.4 % and 5.8 %, respectively. With 6 % MEA, the impairments were even smaller and negligible.