Magnesia carbonation can be adopted as a soil solidification technology for geotechnical engineering. Recent studies have shown that urea decomposition under the catalyzation of ureolytic bacteria can provide a carbon source for magnesia carbonation. Although many related studies have been reported, the mechanical behaviour of the magnesia solidified soil, especially its durability and long-term performance, still require further deep investigations. Besides, the use of plant urease instead of bacteria for magnesia carbonation is also of research interest and requires further studies. In this study, we used crude soybean urease for the catalyzation of urea decomposition in order to provide carbon source for magnesia carbonation (soybean urease intensified magnesia carbonation, SIMC). The mechanical behaviour and durability of SIMC solidified soil under drying-wetting and soaking conditions in acid rain solution were investigated. For SIMC samples, the addition of urea and urease as internal carbon sources led to a much higher strength compared with those without them. The optimum urea concentration was 2 mol/L, and higher concentrations could have negative impact on the strength. As for magnesia, the highest strengths were obtained when the addition was 8 %. During the drying-wetting cycles and soaking tests with acid rain water, there was a generally moderate decreasing trend in strength for the SIMC samples with more drying-wetting cycles or soaking durations. However, the strength reduction ratio, which was defined as the long-term strength in acid environment to that in neutral environment, was much higher compared to the PC samples, implying a much stronger resistance to acid rain water. The mineralogical analysis revealed that hydrated magnesium carbonates were the major effective cementing materials.

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.

To address the low utilization rate of construction waste soil and the environmental impact of traditional cement solidification, this study investigates the effect of desulfurized gypsum and silica fume in synergy with cement for construction waste soil. The effects of solidifying material dosage, liquid-to-solid ratio, and mixing ratio on mechanical properties were analyzed. Optimal performance was achieved with the dosage of solidifying material was 20%, the liquid-to-solid ratio was 0.2, and the mixing ratio of desulfurized gypsum, silica fume, and cement was 2:1:1, meeting the requirements of the technical specification for application of road solidified soil (T/CECS 737-2020). This formulation is referred to as FS-C type solidified soil. A self-fabricated carbonation device was employed to assess carbonation methods, time, and curing age on the mechanical properties of solidified soil. Carbonation for 6 h post-molding significantly enhanced strength, while carbonation in a loose state led to strength reduction. SEM analysis revealed a denser microstructure in carbonated samples due to calcium carbonate and silica gel formation. Compared to traditional cement solidification, FS-C type solidified soil reduces cement consumption by 15%, decreases CO2 emissions by 299.25 g/m(3), and sequesters 85 g/m(3) of CO2. These findings highlight the potential of FS-C type solidified soil as an environmentally friendly alternative for construction applications.

The low chemical reactivity of bauxite residue (BR) has significantly limited its effective utilization, leading to widespread disposal and severe environmental pollution. In semi-arid regions, dispersive soils threaten the stability of silt retention dams, which are vital for controlling erosion. To enhance the utilization of BR and mitigate the dispersibility of dispersive soil, this study employed thermal activated BR to treat both artificially prepared and natural dispersive soils. The stabilizing effects of thermal activated BR on dispersive soil were evaluated using dispersibility identification tests, mechanical tests, and particle size analyses. The stabilization mechanisms were further examined through chemical, microscopic, and mineralogical tests. Results indicate that incorporating 2 % thermal activated BR effectively controls the dispersibility of both dispersive soils, increasing their unconfined compressive strength (UCS) and Brazilian tensile strength (BTS) by up to 426 % and 167 %, respectively. During the initial reaction period (0-3 days), the abundant calcium and aluminum ions precipitated by the BR rapidly reduce the thickness of the water film around the clay particles, while the BR powder fills the voids between soil particles. As the reaction progresses, hydroxide ions are continuously released, peaking at 7 days (pH 11.33), triggering a vigorous carbonation reaction. The resulting calcium carbonate fills and cements the soil particles. In the later stages of the reaction (14-28 days), carbonation and hydration reactions occur simultaneously, binding numerous particles <= 0.075 mm into sand-sized particles, thereby significantly enhancing the soil strength and water stability. The validation tests of naturally dispersive soils in this study provide guidance for the resource utilization of BRs and the improvement of dispersive soils.

Traditional disposal methods such as landfilling and land reclamation are insufficient to mitigate the environmental impact of construction spoil, making non-sintered blocks a promising approach for resource utilization. This study investigates the production and performance of steel slag soil blocks as an alternative to conventional cement-based materials for non-sintered blocks. The optimal manufacturing parameters were identified as a sodium silicate solution with 6% Na2O, 30% steel slag content, a liquid/solid ratio of 0.18, and a forming pressure of 10 MPa, achieving a peak compressive strength of 14.46 MPa. Further, the synergistic combination of alkali activation and carbonation enhanced compressive strength to 17.4 MPa, attributed to the development of a compact microstructure characterized by a honeycomb-like C-(A)-S-H gel and well-crystallized, triangular-shaped aragonite. However, durability tests under freeze-thaw and wet-dry cycles revealed that carbonation can detrimentally affect performance. The transformation of C-(A)-S-H gel into calcium carbonate, with relatively weaker cementitious properties, led to internal cracking and surface detachment. Micro-CT analysis confirmed ring-like patterns under freeze-thaw conditions and diagonal cracks during wet-dry cycling, whereas reference blocks incorporating 30% ordinary Portland cement maintained superior compactness with no cracks. These findings suggest that although the alkali activation and carbonation process enhances early strength, further optimization is necessary to improve long-term durability before broader application can be recommended.

There is an increasing demand for innovative low-carbon alternatives to effectively improve soil properties to promote sustainability and achieve carbon neutrality. However, both the bio-carbonation of reactive magnesia cement (RMC) and enzymatically induced carbonate precipitation (EICP) had limitations, including inadequate strength and solidification inhomogeneity despite demonstrated effective for sand solidification. Therefore, the combination bio-carbonation of RMC and EICP was proposed to address their respective drawbacks. In addition to the combined treatment, other treatment methods (e.g., pure RMC hydration, bio-carbonation, and EICP) were also utilized to compare treatment effects under different treatment conditions (e.g., varying RMC contents, urea concentrations, and treatment cycles). Results showed that the combined treatment could effectively address the issue of insufficient precipitation resulting from low RMC concentrations or excessive CO2 levels, thereby both reducing the permeability of treated sand and enhancing its strength to improve the overall treatment efficacy. With one treatment cycle, the combined treated sample with 20 % RMC and 3 M urea concentration exhibited a higher strength, while the sample with 15 % RMC had better solidification effects after two treatment cycles. Compared to the bio-carbonation treatment, the combined treatment resulted in higher proportions of artinite, while obtaining lower proportions of nesquehonite, demonstrating an influence of calcium addition on the mineralogy of magnesium precipitates. The combined treatment can achieve both strength enhancement and homogenization of solidification as a low-carbon and highly efficient solidification method, showcasing significant application potential in geotechnical engineering and material engineering fields.

The production of traditional building materials like cement, lime, and common fired bricks consumes considerable energy and resources and causes atmospheric pollution. Thus, it's essential to develop more eco-friendly materials for new construction. This research focuses on an earth-straw mixture stabilized hybridly with cement and active MgO. Three aspects scaled from material mix design and mechanical performance to building energy- saving simulation were examined. Three types of earth were considered, and the effects of MgO on M-ME were studied through compression strength, thermal conductivity and TGA tests. The best compressive strength achieved was 12.5 MPa (about 167 % of the standard for non-burned bricks and 125 % of the standard for minimum fired bricks), and the best thermal conductivity was 0.371 / (m & sdot;K) (only 44.2 % of that of common fired bricks). Using Design Builder software, energy load differences between M-ME and fired clay brick walls were simulated under given conditions, and the indoor thermal environment was analyzed. Based on the amount of wall earthwork used in the project, the M-ME wall (YC3) can theoretically capture approximately 12.80 kg/ m3 of carbon from the air under natural curing conditions, mean while reducing heating energy consumption by 9.49 %. Overall, the utilization of soil and the presence of plant straw give M-ME advantages in carbon footprint and thermal performance over sintered and concrete bricks. As a new energy-saving material, M-ME significantly contributes to carbon reduction in production and operation phases, possessing great potential in decarbonizing the emission of the building sector.

Stabilizing weak clayey soils with lime is an effective method for improving the mechanical properties of soil. However, lime production is an energy-intensive process producing significant CO2 emissions in lime-stabilized soils, which can be counteracted through accelerated carbonation that enhances its engineering performance. The present study evaluates accelerated carbonation of lime-treated soils by adding gaseous (CO2-rich gas), liquid (water-CO2 mixture), and solid (sodium bicarbonate) CO2 sources. Results indicated that samples carbonated with gaseous CO2 exhibited 100% lime carbonation, while samples treated with solid and aqueous sources of CO2 had a mean lime carbonation of 60% and 40%, respectively. All lime-treated-carbonated samples exhibited a mean 50% increase in unconfined compressive strength compared to the untreated samples after a 7day curing period. Durability evaluation through cyclic wetting and drying indicated that the carbonated samples had higher durability than the untreated samples. X-ray computed tomography showed that adding solid and liquid sources of CO2 facilitated the flocculation of montmorillonite, reducing the porosity. However, a higher dosage of solid CO2 induced clay dispersion, increasing the porosity. X-ray diffraction and thermogravimetric analysis verified CO2 sequestration through the formation of calcite, a thermodynamically stable polymorph of calcium carbonate.