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The characteristics of reactive magnesia (MgO)-carbonated silt in respect to long-term stability have not been well understood in severely cold climate despite the usage of reactive MgO in enhancing the engineering performances. Under the binder content of 15% and initial water content of 25%, MgO-admixed silt specimens were carbonized for 3 h and 6 h and then subjected to different numbers of freezing-thawing (F-T) cycles. After different F-T cycles, the physico-mechanical properties of MgO- carbonated silt were analyzed in comparison with Portland cement (PC)-stabilized silt through physical and unconfined compression tests. Besides, a series of micro tests on MgO-carbonated specimens was performed including X-ray diffraction (XRD), scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests. The results demonstrate that both mass change ratio and moisture content of carbonated/stabilized silt decrease, and these values of MgO-carbonated silt are significantly lower while the density is higher compared to PC-stabilized silt. The strengths and moduli of MgO-carbonated silt are still two times higher than those of PC-stabilized specimens and the strength change ratio of keeps above 0.8 after F-T cycles. There is no visible transformation between nesquehonite and dypingite/hydromagnesite, although the XRD peaks of nesquehonite decrease and the bonding and filling effects weaken slightly. After 6 and 10 F-T cycles, the pore-size characteristics changed from a unimodal distribution to a three-peak and bimodal distribution, respectively. The total, macro and large pore volumes increase obviously while the medium and small pore volumes decrease except for intra-aggregate pore. The findings show better F-T durability of MgO-carbonated silt, which would be helpful for facilitating the application of MgO carbonation in the soil treatment.

The innovative carbonation technique based on reactive MgO and CO2 has been identified as an environmentally friendly and efficient method in the improvement of weak soils. Previous laboratory studies have indicated that carbonated MgO-admixed soils had significant improvement in mechanical properties. However, there are to date limited investigations on the soft-soil field application of this technique. In this study, a field trial was conducted to ascertain the feasibility of the MgO mass carbonation technique in improving shallow soft-soil subgrades. A series of field tests, including temperature, dynamic cone penetrometer, and light-weight deflectometer tests, were undertaken. The results indicated that compared with uncarbonated soil layers, there was two to three times increase in dynamic resilient moduli and soil resistances of carbonated MgO-admixed soils. The outcomes of this field investigation will contribute to the utilization of the combined stabilizer of MgO and CO2 and the mass carbonation technology in subgrade improvement.

Carbonation of reactive magnesia (MgO) has recently received increasing attention in the area of soil stabilization and ground improvement. However, as a critical parameter in terms of long-term seepage behavior in the geotechnical analysis, the hydraulic conductivity of carbonated reactive MgO-stabilized silt has not been fully studied. In this context, the effect of water-MgO ratio (ratio of initial water content to MgO content, w0/c) and carbonation time on hydraulic conductivity (or permeability) characteristics was systematically investigated. Serial microstructural tests including mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) analyses were applied to elucidate the intrinsic mechanisms. The obtained results indicate that as the initial water-MgO ratio decreases, the void ratio gradually decreases and the reduction of hydraulic conductivity becomes less prominent because of the little presence of flow paths. The hydraulic conductivity of carbonated MgO-admixed silt similar to that of PC-treated silt is mainly governed by the porosity, and its correlation with void ratio is proposed in the article. The variations of permeability with void ratio are consistent with those of the cumulative pore volume from MIP results in general, and the medium pores (3–30 μm) are substantiated to be the primary contributor in controlling the permeability. SEM and DSC analyses reveal that the cementation of soil particles and filling of hydrated magnesium carbonates marginally reduce the voids and permeability. The reasons for changes of permeability behaviors have been confirmed by the pore-size distribution and microstructure characteristics.

Mucky soft soil with weak properties was usually improved by the chemical stabilization owing to the high cost of the situ replacement, and the main chemical binder used was Portland cement (PC). However, with the needs of environmental protection, energy conservation and emissions reduction, the research of low-carbon and high-efficiency soft soil treatment technology will be the development trend of modern engineering construction. The combined binder of reactive magnesia (MgO) and carbon dioxide (CO2) was regarded as a new curing agent instead of traditional PC, and the mass carbonation technology as an innovative idea was put forward to reinforce soft soil. In this view, the field mass stabilization was carried out and the single-point test was analyzed. Research results show that the temperature from the carbonation reaction is higher than that from the hydration reaction, and the temperature, strength and carbonation degree of the upper foundation soils increase significantly compared with the lower foundation soils, while the water content and pH value of the upper foundation soils are less than those of the lower foundation soils; the smaller the distance from the CO2 gas supply is, the higher the carbonation temperature, resilience modulus, penetration resistance, strength and carbonation degree are; the pipe-cover mass carbonation–stabilization technology can realize the carbonation and reinforcement of the on-site weak soils, but it should be noted that the CO2 should be injected at the bottom. The maximum ventilation radius should be 60 cm and the distance between the CO2 pipes should be less than 1.2 m in practical application. The bearing capacities of all regions are above 200 kPa, and the resilient moduli are about 20 MPa. The experimental research of mass stabilization based on MgO-CO2 carbonation technology has groundbreaking scientific significance and application prospects in the emission reduction and sustainable development of civil engineering.

The natural soil will have a significant strength improvement when admixed with reactive magnesia (MgO) and subjected to CO2 carbonation, which has been identified as an innovative and environment-friendly technique in the domain of soil treatment. MgO activity has been revealed to have a significant influence on the treatment effectiveness during the carbonation process. With this in view, the effect of MgO activity index on physicochemical, electrical and strength properties of carbonated silt was investigated. Results show that the MgO activity index and initial water-MgO ratio play crucial roles in controlling the aforementioned properties. With the initial water-MgO ratio reducing or MgO activity index increasing, the mass increment ratio, growth rate of unit weight, pH, resistivity, strength and CO2 sequestration increase to different degrees, while the volume increment ratio, water content, specific gravity, porosity and saturation degree decrease. The unconfined compressive strength shows a better linear relation with resistivity, indicating the applicability of resistivity method in the strength evaluation of carbonated MgO-admixed soil. Moreover, the thermal and microstructural analyses have explained the changing mechanism of physicochemical, electrical and strength properties. Finally, the analysis of the CO2 sequestration indicates that the carbonated MgO-admixed silt could achieve a high carbonation degree when the initial water-MgO ratio is less than 2.0, showing the feasibility of MgO carbonation in the CO2 sequestration.

Nowadays, the cement industry is under pressure to reduce the carbon footprint and energy demands of cement-based construction materials. Carbonation technology is a process that the Ca-/Mg- alkaline materials can react with CO2 to form stable carbonates, thus it is regarded as a potentially attractive technology of CO2 sequestration, including mineral carbonation, cement/concrete carbonation, and carbonation of reactive MgO-stabilized material. This paper investigates the CO2 uptake of reactive MgO-stabilized soils after accelerated carbonation under the high CO2 concentration (99.9%). The CO2 uptake is examined through the HNO3 acidification test and thermal analysis technology. Key results revealed that the carbonation of reactive MgO-stabilized soils could consume large amounts of water and CO2 to produce expansive carbonation products. The CO2 uptake is observably affected by several factors such as MgO content, water content, carbonation time and CO2 pressure. After the several hour’s carbonation (about 3–6 h), the CO2 uptake can basically achieves about 0.8–1.0 times matter quantity of reactive MgO.

The carbonation of reactive MgO-treated soil are considered as an innovative treatment technology in the realm of soft ground improvement. With this in view, the indoor mixing column model tests were carried out under different initial water content and CO2 pressure through the artificial dig-hole column method. The study shows that the temperature of mixing column firstly increases and then decreases, and the temperature can reach the highest in less than 2 h. The temperature peak is the highest at the initial water content of 20%, the second at 15% and the lowest at 30%. In addition, the temperature peak increases with CO2 pressure. The strength decreases with increasing initial water content or decreasing CO2 pressure, and it decreases exponentially with the increase of water content. Nesquehonite peaks are slightly higher at water content of 30% than that at 15%. The peaks of nesquehonite and dypingite or hydromagnesite are relatively higher at 200 kPa than those at 25 kPa. At low water content of 15%, there are amounts of prismatic nesquehonite and few flake dypingite/hydromagnesite. At the high water content of 30%, there are amounts of carbonation products with loose connection. At low CO2 pressure of 25 kPa, the pores are larger, and there are lots of flocculent brucite and few nesquehonite. Based on XRD and SEM analyses, the existences of nesquehonite hydromagesite/dypingite are the main reason for strength development.

Carbonation of reactive magnesia (MgO) is employed for treating soft soils, which has received attention in the ground improvement as an innovative technology. However, no literature on the permeability of reactive MgO-carbonated soils has been studied. Based on the previous research, this paper focuses on the permeability coefficient of reactive MgO-carbonated soils and PC-treated soils. Through the laboratory permeability tests, the influence of reactive MgO content, carbonation time, initial water content and CO2 ventilation pressure on the permeability coefficient of carbonated silt and silty clay was systematically studied. Moreover, the permeability coefficient of PC-treated soils was used for comparison under the same conditions of MgO content and initial water content. The results show that: the permeability coefficient of reactive MgO-carbonated soils reduces with the MgO content increasing, it is the same magnitude with that of PC-treated soils at the same dosage; and the permeability coefficient of carbonated silt is obviously larger than that of carbonated silty clay. When both the MgO-stabilized silt and silty clay are carbonated for 6.0 h, the corresponding permeability coefficient could reach the minimum (10−6 m/s). Ventilation pressure has little effect on the permeability coefficient of reactive MgO-carbonated soils, which is slightly smaller when the ventilation pressure is 200 kPa. Therefore, the reactive MgO-carbonated soils have similar impermeability with PC-treated soils, and have a good prospect of popularization and application.

The use of the piezocone penetration test (CPTU) in a geotechnical site investigation offers direct field measurement on stratigraphy and soil behavior. Compared with some traditional investigation methods, such as drilling, sampling and field inspecting method or laboratory test procedures, CPTU can greatly accelerate the field work and hereby reduce corresponding operation cost. The undrained shear strength is a key parameter in estimation of the stability of natural slopes and deformation of embankments in soft clays. This paper provides the measurements of in situ CPTU, field vane testing and laboratory undrained triaxial testing of Lianyungang marine clay in Jiangsu province of China. Based on the literature review of previous interpretation methods, this paper presents a comparison of field vane testing measurements to CPTU interpretation results. The undrained shear strength values from both the field vane tests and cone penetration resistances are lowest at the mid-depths of the marine clay layers...

The use of the piezocone test (CPTU) in geotechnical site investigation offers direct field measurement on stratigraphy and soil behavior. Compared with some traditional investigation methods such as drilling, sampling, field inspecting method, or laboratory test procedures, CPTU can greatly accelerate the field work and thereby reduce corresponding operation cost. This article reviews previous interpretation methods of obtaining undrained shear strength, overconsolidation ratio, and coefficient of consolidation of soft soils from CPTU data, and introduces a new method, which the measured pore pressure into account. Piezocone tests with Vertek-Hogentogler CPTU truck have been complemented in a widely explored area in the soft marine deposit of Lianyungang City. Geotechnical design parameters (q t , f s , and u) obtained from CPTU and dissipation tests are evaluated for the marine clay in this area through methods based on cavity expansion (CE) and critical state (CS) theory proposed by Mayne, and compared...

Dry Jet Mixing (DJM) is a commonly used in-situ soil improvement approach for increasing the strength and reduction on compressibility of soft marine clayey soils. However, the possibility of DJM improved soil achieving designed strength is largely dependent on the mixing process in that strength distribution of DJM improved soils is often scattered. This discreteness has been attributed to less homogeneity characteristic of mixed soil mass during the procedure of DJM. Given the discrete strength distribution of mixed soils, the strength of DJM improved soil applied in design is usually much lower than that obtained from the laboratory tests so that the security of geotechnical engineering projects can be guaranteed. Recently, with modifications of traditional DJM drill machine, it has become more likely that higher mixing homogeneity be achieved. “Double-Pole and Bidirectional Mixing” technology is just one of these modifications through which the strength of improved cement soil column can be increased. In this study, strength characteristics of column body were investigated both in laboratory using unconfined compressive strength test and in the field using standard penetration test. The test results reveal that bidirectional DJM method tends to attain a higher homogeneity thus increase bearing capacity of soft sediment. On the other hand, the strength of the bottom of column and the quality of column shaft can be improved as well. Due to potential advantages of bidirectional DJM technology above, the prospect of its application is promising.