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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.

A new method for curing electrolytic manganese residue (EMR) using the biocarbonization of reactive MgO (r-MgO) is presented in this paper. This method uses microbial technology to provide CO2 to r...

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.

The strength and hydraulic conductivity of vertical cutoff walls consisting of reactive magnesia-activated ground granulated blast furnace slag (GGBS), bentonite, and soil (MSB) have been investigated in previous studies. However, there has been little comprehensive study of the influence of wet-dry cycles on the mechanical and microstructural properties of MSB backfills. In this paper, the durability of MSB backfills when exposed to wet-dry cycles is investigated. The variations in mass change, dry density, pH value, pore size distribution, and mineralogy are discussed. The results show that the mass change of ordinary Portland cement (OPC)-based and MSB backfills increases with respect to wet-dry cycles. The MSB backfills exhibit up to 8.2% higher mass change than OPC-based ones after 10 wet-dry cycles. The dry density, pH value, and unconfined compressive strength of MSB backfill decrease with the increasing number of wet-dry cycles. Increasing the GGBS-MgO content from 5% to 10% in MSB backfills results in 2.1–2.3 times higher strength, corresponding to a reduction of 2%–12% in cumulative pore volume; while increasing the bentonite content slightly reduces the strength of MSB mixtures, corresponding to an increase of cumulative pore volume by 4.6%–7.9%. The hydrotalcite-like phases and calcium silicate hydrate (C-S-H) are the primary hydration products in MSB backfills. Moreover, the continuous wet-dry cycles result in the precipitation of calcite and nesquehonite.

A new loess solidification method using biocarbonization of reactive MgO is presented in this paper. The proposed method was employed to solidify loess, and the study focused mainly on exploring the influence of the reactive MgO content on the moisture content, dry density, unconfined compressive strength (UCS) with deformation, and microstructure of the solidified loess. The results indicated that with the increase in reactive MgO content, the moisture content of the solidified loess decreased approximately linearly, and the dry density increased; the curing reaction was rapid that occurred mainly at an early stage. The UCS increased with the increase in reactive MgO content and also increased approximately linearly with the increase of the dry density. The ratio of deformation modulus E50 to peak strength qu varied between 30 and 110. The UCS of the biocarbonized sample was approximately seven times higher than that of the original loess sample. The solidification effect of biocarbonized loess was also significantly higher than that of cement-solidified loess. The SEM results demonstrated that with the increasing reactive MgO content, the filling and cementing effect of hydrated magnesium carbonates (HMCs) on the soil particles further improved, leading to a better solidifying effect and resulting in higher strength.

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.

This study investigated the long-term leachability of antimony (Sb) in a smelting residue (39519 mg/kg) solidified/stabilized by reactive magnesia (MgO). Different dosages of MgO (0% as control, 2%, 5%, and 10% on a dry basis) were compared, and the long-term performance was evaluated by an accelerated exposure test consist of 20 consecutive leaching steps with simulated strong acid rain (SAR, HNO3: H2SO4 = 1:2, pH = 3.20) as the extractant. Notably, the MgO treatments efficiently reduced the Sb leachability. Compared to the original slag (8.3 mg/L), the leaching concentrations based on a Chinese standard HJ/T299-2007 were reduced by 58%, 79%, 85%, and 86% at MgO dosages of 0%, 2%, 5%, and 10%, respectively. Because the studied slag was rich in oxides like SiO2, CaO, and MgO, the hydration reactions probably happened during the aging processes with oxic water. It was inferred that the formed hydration products have a self-solidification/stabilization function to suppress the Sb leaching from the solid phase. The mineralogical characterization results proved that the hydrated Mg(OH)2 played an essential role in the decrease of Sb leachability. Besides, the MgO addition promoted the hydration of this smelting slag and formed new hydrate gels that immobilize Sb in this slag. Our results confirmed that MgO-amended slags were resistant to continuous SAR corrosion. Compared to the control, the dosage of 5% MgO could effectively reduce the cumulatively released Sb by 57%, with only 0.46% of total Sb could be leached. The decomposition of Mg(OH)2 and hydrate gels determined the re-release of Sb in a long term. Our work has demonstrated that reactive MgO amendment could be potentially selected as an effective strategy for the treatment of Sb-containing smelting residues in field conditions.

This paper introduced a novel stabilization/solidification (S/S) method for the treatment of lead (Pb)- and zinc (Zn)-contaminated soils using reactive magnesia (MgO) and carbon dioxide (CO2). The MgO was first mixed with the Pb- and Zn-contaminated soils, and then the gaseous CO2 was introduced into the MgO-soil specimens for up to 120 h. The Portland cement (PC) was used as the control binder, and the PC-treated Pb- and Zn-contaminated soils were cured for 28 days. The treated contaminated soils were subjected to unconfined compressive strength test, one stage batch leaching test, mineralogical, and thermal analysis. The results indicated that the addition of Zn considerably slowed down the carbonation of MgO-stabilized soils, whilst Pb did not. For Pb-contaminated soils, MgO+CO2 yielded similar or higher strength compared to PC, but significantly reduced the Pb leachability. For Zn-contaminated soils, MgO+CO2 achieved similar Pb leachability compared to PC, but produced significantly higher strength. Magnesium carbonates were produced in the MgO+CO2-treated contaminated soils, which enhanced the strength of the soils. Additionally, lead carbonates and zinc carbonates were produced in the Pb- and Zn-contaminated soils, respectively, which contributed to the immobilization of Pb and Zn.

The thermodynamic equilibrium conditions of bicarbonate leaching were analyzed and used to develop a mathematical model for determining the parameters giving the highest extraction of MgO from the raw materials. Laboratory experiments on magnesite samples from Satkinsk deposit, Chelyabinsk Region, confirmed that the simulation results were adequate.

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.

Reactive magnesium oxide (RMO) and circulating fluidised bed combustion (CFB) slags were used to prepare magnesium silicate cements using sodium hexametaphosphate (NaHMP) as a water reducer. The effects of curing condition and the initial levels of RMO and silica fume (SF) were studied for up to 90 d. The mechanical properties of the prepared pastes were evaluated through measurements of compressive strength. Mercury intrusion porosimetry (MIP) was employed to investigate the pore structure. X-ray diffraction, thermogravimetric analysis, mercury penetration analysis and scanning electron microscopy (SEM) were carried out to investigate the reaction products and final products. The results indicated that the final products were mainly magnesium silicate hydrate, hydrotalcite phases and hydromagnesite. Steam curing yielded higher strength, increased reaction products and closure of the macropores. SEM analysis showed that the product after steam curing exhibited abundant nanolattice structures (length ≤100 nm). MIP analysis showed that steam curing led to lower porosity and fewer macropores. The specimen prepared with 64 wt% CFB slag, 16 wt% SF and 20 wt% RMO exhibited the highest compressive strength (105 MPa).

Stabilization/solidification (S/S) is commonly applied to treat heavy metal-contaminated soils through the use of lime and ordinary Portland cement (OPC). Recently, reactive magnesia (MgO) has emerged as a novel binder for S/S of heavy metal-contaminated soils; however, a comprehensive comparison between MgO, lime (CaO), and OPC for S/S application is still missing. This study compares the S/S efficiency of MgO, CaO, and OPC for soils contaminated by six individual heavy metals (Pb, Cu, Zn, Ni, Cd, and Mn) through unconfined compressive strength (UCS) test, one stage batch leaching test, and microstructural analysis. The addition of binders can transform soluble heavy metal salts to insoluble hydroxides and their complexes, and hence the leachability of heavy metals decreases. However, the level, to which the leachability can be reduced, is highly pH dependent. Contaminated soils treated with MgO have pH of 9–10.5, at which the leachability of Pb and Zn is much lower than that of OPC- or CaO-treated soils with pH of 10.5–13; for example, the leached Pb and Zn from MgO-treated soils are only 0.1%–3.3% and 0.1%–9.4% of those from OPC-treated soils, respectively. On the other hand, the leached Cd and Mn from OPC-treated soils are 0.1%–28.5% and 0.1–10.7% of those from MgO-treated soils, respectively, due to the high pH and the formation of calcium silicate hydrate (CSH) in OPC-treated soils. OPC and CaO are more effective than MgO in decreasing the Ni leachability at high original concentrations, but less effective at low original concentrations. For all soils except those contaminated by Zn, the OPC generally produces a much higher UCS, up to two orders of magnitude, than the CaO and MgO. The results of study indicate that no single binder can treat all types of heavy metal-contaminated soils perfectly, and the selection of binder is a site-specific problem. Nanyang Technological University The authors appreciate the grant (M4081914) from Nanyang Technological University, Singapore and the assistance from Shunli Shen, Jian Wei Lui, and Thiam Loong Lim.

Reactive magnesia (MgO)-activated ground granulated blast furnace slag (GGBS) is a newly developed binder for soil stabilization/solidification. It can be used as an alternative of conventional hydraulic binders for the stabilization/solidification of heavy metal-contaminated soils. Acid rain exposure has been found to weaken the heavy metal immobilization in solidified/stabilized soils in the long term. However, very limited studies have been conducted to investigate the effect of acid rain on the physical and strength properties of GGBS–MgO-stabilized heavy metal-contaminated soils. In this study, GGBS–MgO-stabilized lead (Pb)-contaminated soils are subjected to the semi-dynamic leaching test using the simulated acid rain (SAR) as leachant, which has pH values of 2.0, 3.0, 4.0, and 5.0. Deionized water (pH 7.0) is also used for comparison. Dry density, soil pH, needle penetration depth, and unconfined compressive strength (qu) of the solidified/stabilized soils are measured after the leaching test. The results show that exposure to SAR yields reduction in qu and soil pH, whereas the needle penetration depth increases and dry density changes only marginally. The qu of the solidified/stabilized soils increases noticeably with increased GGBS–MgO content. It is also found that the stabilized Pb-contaminated soil exhibits lower qu than its clean soil counterpart. Furthermore, a quantitative relationship is proposed to correlate the normalized needle penetration resistance with the normalized qu. Finally, a multiple linear regression is performed to statistically reveal the dependence of soil strength on three independent variables, which are SAR pH, Pb concentration and binder content.

Reactive magnesia-based cement (RMC) is an emerging group of alternative binder to Portland cement. Recently, the first fiber-reinforced RMC-based strain-hardening composites (SHC) have been developed by the authors. The current work investigated the feasibility of the PC-free RMC-based SHC formulations to engage autogenous healing. Results showed that crack sealing and significant mechanical recovery can be realized through proper environmental conditioning. The presence of water is necessary to engage autogenous healing and elevated CO2 concentration leads to the formation of HMCs that can seal larger crack. However, ample supply of CO2 results in fast sealing of crack on the near surface region, which blocks the pathway for further carbonation and healing of interior region of cracks. Microstructure analysis reveals that the healing products are hydrated magnesium carbonates (HMCs) and different conditioning regimes lead to different types of HMCs as the healing products. Ministry of Education (MOE) The authors would like to acknowledge the Ministry of Education, Singapore for the financial support of this research by the MOE Academic Research Fund Tier 2 project No. MOE2017-T2-1-087 (S).

This study investigates production scaling up of MgO cement synthesized from reject brine and evaluates the mechanical properties and microstructures of the resulting MgO cement. Hydration process and carbonation degree of the resulting MgO mortar were investigated and compared with commercial MgO cement. Compressive strength, scanning electron microscopy (SEM), X-ray powder diffraction (XRD), particle size analysis, acid reactivity, and thermogravimetric and differential thermal analysis (TG/DTA) were engaged for sample characterization. Results showed that MgO cement synthesized from reject brine possesses higher mechanical strength of 32.5 MPa at the age of 28 days as compared to the commercial MgO cement (20.1 MPa). This is mainly attributed to the high reactivity of the synthesized MgO cement as revealed by the microstructure study. Master of Engineering (CEE)

In some porous unreinforced concrete products carbonation could be employed to deliver significant benefits, both in terms of mechanical strength and carbon footprint. While much work has focused on the carbonation of lime, recent studies have shown that magnesium-based cementitious systems, including slags and magnesia, can be employed as the main strength development process in carbonated reactive magnesia cement products. In this chapter, the design, performance and applications of carbonated reactive magnesia in different compositions and scenarios are presented and discussed. A number of hydrated magnesium carbonates form upon the carbonation of magnesia, with different controlling factors for their formation, resulting in different mechanical properties as well as CO2 sequestration capacity. Pressed semi-dry magnesia-based masonry blocks are the first application example presented. The results of both laboratory and full-scale production of a range of magnesia-based products are discussed in terms of the variable considered. Experimental work was complemented with computer simulations to model the mechanism of the carbonation process. The use of carbonated reactive magnesia was also considered for ground improvement applications. Forced carbonation of soils mixed with dry reactive magnesia was employed both in triaxial samples and with the use of a model auger that simulated field applications of the carbonation process. Blended magnesia mixes were investigated. In all the studies reported here, the investigations considered the dosage of the magnesia as well as the carbonation environment followed by mechanical and microstructural analyses. Different accelerated carbonation regimes were also considered. This chapter reports details of the experimental and field work carried out on carbonated reactive magnesia cements highlighting the carbonation mechanisms and resulting properties as well as potential benefits and environmental impacts.

Alkali-activated cement (AAC) is one of the sustainable alternatives to OPC due to the large proportion of waste materials in its mix composition. The reaction kinetics and mechanisms of the alkali and aluminosilicate materials are highly dependant on the chemical/mineral compositions of the raw materials, while the role of the inherent magnesium oxide (MgO) content in ground granulated blast-furnace slag (GGBS) has attracted much attention in the past decade. Meanwhile, the activation capability of using only earth alkali compounds for GGBS are proved feasible and more sustainable than these caustic alkalis. This chapter critically examined the role of inherent MgO and additive reactive (i.e., caustic) MgO in improving the mechanical performance and durability of AAC with different compositions. The hydration mechanisms and products formed using reactive magnesia as either a sole activator or an additive for GGBS-bearing AAC were discussed in detail. The state-of-the-art progress on the development of MgO-modified AAC and the effect of reactivity and dosage of MgO on the various properties of this type of cement and concrete products are reviewed. In addition, recent development of using sustainable MgO-GGBS blends in a number of geotechnical/geoenvironmental applications such as ground improvement, contaminated soil stabilisation/solidification and low permeability barriers are presented and discussed. It is concluded that more research is warranted to better understand the interaction between MgO and aluminosilicate materials and fully explore the merits of MgO-modified AAC in different applications.

Non-dispersible underwater concrete is a kind of construction material which is seeking its growing application in the offshore structures. However, the offshore structures are exposed to aggressive environment and harmful ions. Therefore, this study is conducted to investigate the mechanical, physical, microstructural, and durability-related properties of underwater concrete containing nano-silica and MgO. Nano-SiO2 was added at dosages of 0%, 1%, and 2%, and MgO was replaced at a rate of 0%, 5%, and 10% by weight of the binder. The effects of nano-silica and MgO particles was assessed via compressive strength test, Thermogravimetric analysis, X-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy. Chloride and carbonation resistance of the concrete specimens were also examined. The results suggest that the incorporation of nano-silica and MgO (at a dosage of 5%) improved the microstructure resulting in an elevated compressive strength and enhanced resistance to carbonation and chloride penetration. The addition of 10% MgO reduced the performance of concrete specimens, however, the incorporation of nano-silica compensated the reduction in performance.

Traditional PHC pipe pile in foundation engineering consumes high energy and has insufficient durability. A magnesium oxide carbonization test block is a new type of environmental protection block which bases on activated magnesium oxide cementation technology. The use of CO2 carbonation technology allows reactive magnesia to react to form basic magnesium carbonate to increase the compressive strength and durability of the block. Three kinds of different magnesium oxide powders were subjected to pressure test and determined the key technical parameters, such as optimal raw materials, sample preparation methods, carbonization environment and technology, and optimized design of pipe pile concrete material system.

This study presents a multi-scale evaluation of the effectiveness of two novel binders of reactive magnesia activated blast furnace slag-based binder named as GM and phosphate-based binder named as KMP to stabilize mixed zinc (Zn) and chloride (Cl) contaminated soil collected from an abandoned industrial electroplating company site. The stabilized soils cured at 1, 3, 7, and 28 days were subjected to soil pH, unconfined compressive strength, leachability and Zn speciation tests. The X-ray diffraction, scanning electron microscope and energy dispersive spectroscopy analyses were also performed to investigate the immobilization mechanisms of Zn and Cl in the stabilized soils. The test results showed that the unconfined compressive strength of the soils stabilized with 6% wt GM or KMP after 28 days curing was significantly improved, and the leached Zn and Cl concentrations were well below their corresponding remediation goals. In general, the KMP stabilized soil possessed superior performance in terms of higher unconfined compressive strength in early curing (7 days) and lower leached Zn concentration. While the GM exhibited superior Cl immobilization in the stabilized soil. The X-ray diffraction and energy dispersive spectroscopy results indicated that the Zn-bearing precipitates and Cl-bearing Friedel’s salt were the predominant mechanisms of immobilizing Zn and Cl by using GM, while the formation of phosphate-bearing precipitates containing Zn and Cl were the predominant mechanisms of immobilizing Zn and Cl by using KMP. The results demonstrate that both GM and KMP are promising binders in treating Zn and Cl-contaminated soils at electroplating industry sites.

This study focuses on the development of a new strain-hardening composite (SHC) involving carbonated reactive MgO cement (RMC) and fly ash (FA) as the main binder. Rheological properties of the developed composites were investigated by varying FA and water contents to achieve desirable fiber dispersion. A suitable mix design, in which polyvinyl alcohol (PVA) fibers were introduced to provide tensile ductility, was determined. The effect of key parameters such as w/b ratio and curing age on the mechanical properties of carbonated RMC-SHC was evaluated. Adequate binder content and w/b ratio was necessary for desirable fiber dispersion. Lower water contents and longer curing ages contributed to the strength development of RMC-SHC by improving the fiber-matrix interface bond and enhancing the formation of a dense carbonate network. MOE (Min. of Education, S’pore) Accepted version

Nesquehonite is an attractive magnesium precipitation product as it sequesters CO2 and exhibits good filterability. Upon calcination, high-quality reactive magnesia is produced. As part of the hydrometallurgical processing of nickel, magnesia is used as a precipitating agent to recover nickel and cobalt in the form of a Mixed Hydroxide Precipitate (MHP). However, a waste solution with elevated concentration of magnesium is formed which can be an environmental liability. Chemical thermodynamic equilibrium modeling of magnesium carbonate precipitation from magnesium sulfate liquor predicted a high reaction extent of 99% could be achieved. Magnesium carbonate precipitation experiments were carried out by gradually mixing sodium carbonate solution and synthetic industrial liquor in a batch reactor at 25 °C, while monitoring solution concentrations and pH, used to estimate the supersaturation index. Calcination of the resulting solid at temperatures ranging from 450 to 650 °C produced magnesia with acetic acid activities ranging from 14 s to 41 s. The most reactive magnesia was obtained at the lower temperature of 450 °C with an activity of 14.7 ± 0.6 s, which meets industrial product specifications. Periclase was identified as the dominant magnesia mineral phase along with thenardite as a minor component.

This paper deals with research concerning the graphene-doped construction materials, namely with the optimization of the ideal content of the nanoadditive in the final composite. This research was conducted building on the previous studies on use of graphene in magnesium oxychloride cement (MOC). The evaluation of the effect of the graphene nanoplatelets based on their content was performed using various analytical methods as well as mechanical tests. Among the used methods are XRF, SEM, EDS, HR-TEM, XRD and OM. After 28 days of air curing, the mechanical, macro- and micro-structural parameters were studied. One of the main characteristics of the materials was their porosity, which was thoroughly studied in order to determine the influence of the graphene on its drop. This parameter is very important for the MOC-based materials, hence it is directly connected to their resistance to water. The compressive strength of the sample containing 1 wt% of graphene reached value of 99.7 MPa. This sample has shown a massive increase in both compressive (26.8 %) and flexural strength (14.9 %) as well as in the modulus of elasticity (73.1 %) in comparison with the reference sample which did not contain any additives. The developed composite materials show a promising route in the controlled improvement of reactive magnesia-based construction materials properties using carbon-based nanoadditives.

Previous research on reactive cementitious materials shows that, compared with traditional calcium-based cementitious materials, reactive magnesia-based cementitious materials (an alternative cementitious material) exhibit excellent mechanical properties; different hydration mechanisms, hydration products , and microstructures; and considerably high durability. This paper presents a comprehensive review of the raw materials, hydration products, phase transformation evolution, and crystal structures of reactive magnesia-based cementitious materials. Furthermore, developments in terms of the durability of reactive magnesia-based cementitious materials are summarized and analyzed based on the carbonation performance, water resistance, and brine corrosion resistance . Finally, considering the existing problems and advantages of reactive magnesia-based cementitious materials, potential future research directions are highlighted.

The development of alternative binders to Portland Cement has become a critical factor in decreasing the Portland cement industry’s carbon footprint, which nowadays represents about 7% of CO2 emissions worldwide. Thereby the seek for the development of alternatives binders led to a recent and growing interest in carbonate-based binders due to their ability to capture and store CO2 into their cementitious matrix and, consequently, leading to the rise of the research related to the scope of this work, Carbonated Reactive Magnesia Cement, which has as the main property the capability to adsorb CO2 into its cementitious matrix when subject to favourable carbonation curing conditions. Thus, this paper describes the Magnesia carbonation mechanism to subsequently review and enumerate the influencing factors over the carbonation curing of Carbonated Reactive Magnesia Cement-based materials. Afterwards, it summarizes recent works on this binding technology that don’t use Portland cement in their composition. Besides that, the carbonation curing conditions used in these studies are highlighted along with this work. Therefore, the main goal of this review is to bring a starting point of the carbonation curing influencing factors of Carbonated Reactive Magnesia Cement-based materials. Thereby, this review may help future research on this field and some issues to be overcome by this material group.

Magnesium potassium phosphate cement (MKPC) mortar as a construction repair material has been studied and applied for several years. In this paper, reactive magnesia sand is employed to replace conventional inert quartz sand as the fine aggregate of MKPC mortar, to adjust the M/P (i.e. MgO-to-KH 2 PO 4 ) molar ratio of cement paste during mortar formation, as well as to improve the mortar performance though strengthening the bonding capacity of the interfacial transition zone (ITZ) (i.e. the paste to aggregate interface). Experimental results showed that the compressive strength of the MKPC mortar with reactive magnesia sands could achieve a mean value of 36.65 MPa at 12 h, and outperformed about 27.30% than that with inert quartz sand at 28-day. The interfacial transition zone (ITZ) development in the MKPC mortar with magnesia sands was investigated using the on-site X-ray computed tomography (CT). The failure modes of specimens subjected to uniaxial compression were analyzed and visualized by means of image processing technique. In addition, the chemical interaction between the paste and the aggregate was theoretically and experimentally analyzed, explaining well the excellent volume stability of the MKPC mortar with reactive magnesia sands.

Accelerated carbonation of reactive magnesia (MgO)-treated soil is an innovative and sustainable method for improving ground through absorbing CO2. The strength and microstructural properties of various types of carbonated soils at varying initial water contents (w0) are explored, and liquid limit (wL), a parameter representing soil type, is highlighted in this paper. The strength properties of carbonated soils are investigated by unconfined compression tests. To explain the carbonation mechanism, the microstructural properties are obtained from different experimental tests: X-ray diffraction, scanning electron microscopy, thermogravimetric analysis and mercury intrusion porosimetry. The results indicate that large amounts of CO2 and water are consumed in the carbonation process, which results in the reduction of water content and the formation of carbonation products such as nesquehonite and hydromagnesite/dypingite. The strength of carbonated soils decreases with the increase in wL or water cont...

This paper investigated the performance and environmental impacts of reactive magnesia cement (RMC)-based formulations containing pulverized fuel ash (PFA) and ground granulated blast furnace slag (GGBS). Concrete samples, whose binder component was composed of RMC with 0–50% PFA and GGBS replacement were subjected to carbonation curing for up to 28 days. The performance of each sample was analyzed and compared to corresponding Portland cement (PC)-based samples via porosity, water sorptivity, compressive strength and thermal conductivity measurements. The performance results were supported with the assessment of the environmental impact of each sample throughout their production and utilization phases. Samples in which 50% of the binder component was replaced by PFA indicated the highest strength development, reaching strengths as high as 60 MPa at 28 days, which were 33% higher than those of the corresponding RMC control sample. The advantageous strength gain demonstrated by RMC-PFA samples was associated with a reduction in sample porosity due to the filler effect of PFA as well as the formation of strength providing phases through the hydration and carbonation reactions. The use of both PFA and GGBS decreased the environmental impacts of RMC formulations, which was reflected as lower CO2 emissions, as well as reduced damage on human health and eco-system quality when compared to RMC and PC samples. The environmental efficiency calculations involving a combination of the net CO2 emissions and mechanical performance of each sample revealed the benefits of supplementary cementitious materials within RMC formulations.

The reinforcement technology of carbonation based on reactive magnesia (MgO) and carbon dioxide (CO2) is a low-carbon and high-efficiency foundation treatment method. This paper investigates the compaction, mechanical and microstructural characteristics of carbonated reactive MgO-stabilized silt with varying MgO-soil ratios, carbonation time and water-soil ratios. The results indicate that the maximum dry density of uncarbonated reactive MgO-stabilized silt increases while the optimum moisture content reduces compared to the parent soil. The unconfined compressive strength of reactive MgO-stabilized soil was found to have increased after CO2 carbonation for several hours. With increasing MgO-soil ratio and carbonation time, the failure mode changes from elasticplastic to brittleness, and the failure strain of carbonated specimens mainly ranges between 0.8% and 1.6% and the ratio of the deformation modulus to unconfined compressive strength is about 30 to 200. The water-soil ratio has a slight influence on the evolution of strength. Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) analyses have indicated that the carbonation products facilitate the strength growth of reactive MgO-stabilized silt. Thermogravimetric analysis (TGA) shows that CO2 uptake increases with increasing carbonation time and achieves the highest under the MgO-soil ratio of 20%, carbonation time of 6 h, and water-soil ratio of 25%. The stabilization mechanism of carbonated reactive MgO-stabilized silt is proposed according to microstructure analyses, providing a deeper understanding of the application of the reactive MgO carbonation technology in the ground reinforcement.

Engineering properties of soft soils can gain great improvement through the addition of reactive magnesia (MgO) and further carbonation of substantial gaseous CO 2 absorbed. The paper studies the influence of MgO activity index on engineering properties of the carbonated silt with different water-MgO ratio. The engineering properties are investigated mainly through unconfined compression tests, and then the strength development are explained by the scanning electron microscopy (SEM). The results demonstrate that the mechanical properties of carbonated MgO-stabilized soils were greatly influenced by MgO activity index and water-MgO ratio, and the unconfined compressive strength of reactive MgO-stabilized soil has increased obviously after CO 2 carbonation. With increasing MgO activity index and reducing water-MgO ratio, the unconfined compressive strength increased, and the failure mode of carbonated specimens approximately changes from elastic-plastic to brittleness as well as their failure strain mainly ranges between 0.5% and 2.3%. The deformation modulus of carbonated silt generally increases with increasing unconfined compressive strength, and the ratio of the deformation modulus to unconfined compressive strength is about 35 to 150. A simplified equation with combining MgO activity index and water-MgO ratio is proposed for accurately predicting the unconfined compressive strength of carbonated MgO-stabilized soils. Moreover, the microstructural characteristics explain the strength gain of carbonated MgO-stabilized soils.

The durability of CO2-carbonated reactive magnesia (MgO)-admixed clayey soils against drying-wetting cycles has not been well understood now despite the fact that the engineering properties...

Various innovative research in the cement industry is looking into improving its environmental sustainability. Sodium carbonate-activated slag/fly ash (NC-SF) binders has recently evolved as a potentially more sustainable binding materials than both Portland cement and conventional alkali activated materials such as sodium silicate and sodium hydroxide activated materials. The reaction mechanism and some microstructural properties of NC-SF cements have been a major area of research recently. However, very few studies have scaled up the investigation of these binders into concrete specimens. This paper, therefore, provides new insight into the strength development and the durability performance of NC-SF concrete and MgO-modified NC-SF concrete. Concrete testing included measurements of compressive strength, split tensile strength, water absorption, depth of carbonation, sulphate exposure, acid exposure and elevated-temperature exposure. Microstructure studies were conducted using Powder X-Ray diffraction (PXRD), Thermogravimetric analysis (TGA) and Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR). It is concluded that NC-SF concrete mixes develop acceptable mechanical strength and demonstrate high resistance to sulphate attack. They also showed higher resistance to acid attack than the control mix based on sodium silicate-activated slag concrete. Here, emphasis is placed on the potential of developing NC-SF concrete with excellent performance and less complicated production methods as well as a low carbon footprint. It is also found that the use of reactive MgO enhanced the strength development of NC-SF concrete as well as its resistance to acid and carbonation.

Loess stabilization is an effective approach to solve the deformation and subsidence problems in loess areas. Unconsolidated-undrained direct shear tests were conducted to investigate the i...

Elevated soil lead (Pb) concentrations are a global concern owing to the toxic effects of this heavy metal. Solidification/stabilization (S/S) of soils using reagents like Portland cement (PC) is a common approach for the remediation of Pb contaminated sites. However, it has been reported that under long-term field conditions, the performance of PC treatments can diminish significantly. Therefore, novel reagents that provide longer-term stabilization performance are needed. In this study, four magnesium oxide (MgO) products of different reactivity values were applied (5 wt%) to a Pb contaminated clayey soil. The short-term (1–49 days) and long-term (25–100 years) temporal stabilization effects were investigated by laboratory incubation and accelerated ageing methods, respectively. The concentration of Pb in Toxicity Characterization Leaching Procure (TCLP) leachate was ~14 mg/L for the untreated soil; ~1.8 times higher than the TCLP regulatory level (5 mg/L). Only one day after treatment with MgO, the leachate concentration was reduced to below the regulatory level (a reduction of 69.4%–83.2%), regardless of the MgO type applied. However, in the long-term accelerated ageing experiments, only treatments using the most reactive MgO type could provide leachate concentrations that were consistently below the TCLP threshold throughout the 100 years of simulated ageing. The soil treated with the MgO of lowest reactivity was the first to exceed the regulatory level, at simulated year 75. It is thus demonstrated that MgO reactivity has a significant effect on its long-term effectiveness for contaminated soil stabilization. This is attributed to differences in their specific surface area and readiness to carbonate, which may facilitate the immobilization of Pb in the long term. It is also noteworthy that compared to PC, reactive MgO is more environmentally friendly owing to lower energy consumption and reduced CO2 emissions during its manufacture. Keywords: Soil pollution, Lead contamination, Remediation, Reactive magnesia, Reactivity, Accelerated ageing

The effect of chloride attack on carbonated reactive MgO cement (RMC) samples containing fly ash (FA) and ground granulated blast-furnace slag (GGBS) under cyclic wetting and drying was studied. Porosity, sorptivity and chloride profile observations were carried out in parallel to the XRD, TGA and SEM analyses performed before and after exposure. Inclusion of SCMs increased the resistance of RMC samples by lowering the porosity and leading to the formation of various phases such as bischofite, hydrotalcite and calcite, in addition to Mg-carbonates. Mg-carbonate phases such as hydromagnesite and artinite remained relatively stable, whereas a small reduction in nesquehonite content was observed after exposure. Resulting formulations presented improved durability and lower environmental impacts.

Hydrated lime (Ca(OH)2)- and reactive magnesia (MgO)-activated ground granulated blast-furnace slag (GGBS) have shown advantages in geotechnical applications compared with Portland cement (PC). To apply these two novel binders in slurry form to field applications, the properties of their grouts need to be investigated, which is the objective of this study. The heat of hydration, bleeding, viscosity, and setting behavior of Ca(OH)2-GGBS and MgO-GGBS grouts were investigated and compared with PC and pure GGBS grouts in this study. All grouts indicated a decrease in bleeding and setting time, but an increase in viscosity with decreasing the water/cementitious material (W/C) ratio. Compared with the PC grout, the pure GGBS grout with the same W/C ratio demonstrated higher bleeding and setting time, and lower heat rate and viscosity. Bleeding and setting time decreased and peak heat rate and viscosity increased with an increase in the activator (Ca(OH)2 or MgO)/GGBS ratio within GGBS-based grouts. The peak heat rate in Ca(OH)2-GGBS grouts was higher than that of pure GGBS or Ca(OH)2 grout, indicating that Ca(OH)2 was effective on activating hydration of GGBS. The peak heat rate in MgO-GGBS grouts was significantly lower than MgO grout and the heat rate was limited after 8 h. The bleeding and setting times of MgO-GGBS grouts were significantly lower than those of Ca(OH)2-GGBS grouts, which was primarily attributed to the hydration of MgO. Nanyang Technological University The support from Nanyang Technological University (M4081914.030), Singapore, is appreciated.

This study focused on the stabilization of lead glass sludge (LGS) using reactive magnesia (MgO) via the fabrication of lightweight building bricks. Two types of MgO with different reactivities were prepared by the thermal treatment of magnesium carbonate at 800 °C and 1200 °C (MgO-800 and MgO-1200, respectively). The fabrication of bricks and Pb stabilization were performed by wet mixing LGS with MgO followed by humidity incubation. Results showed that the Pb immobilization and performance of the produced bricks were strongly affected by MgO reactivity, curing time, and LGS–MgO weight ratios. Pb immobilization was performed by the transformation of soluble lead into an insoluble hydrocerussite phase, particularly in hydrated mixtures with high MgO content (> 25 wt%). Pb immobilization inside a magnesium silicate hydrate skeleton is the main mechanism in the hydrated samples containing 25 wt% MgO. To achieve “sustainability,” we recommend the use of a hydrated mixture containing 75 wt% of LGS and 25 wt% of MgO-800 in the production of building bricks because this mixture exhibits high compressive strength, high Pb immobilization, low energy demand, and low environmental pollution.

This study investigated the feasibility of producing reactive MgO cement from reject brine obtained from a desalination plant and evaluated its use as a binder in comparison to a commercial MgO. The mechanical performance of the samples cured under carbonation conditions for up to 28 days were further supported by x-ray diffraction (XRD), thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) and field emission scanning electron microscopy (FESEM) analyses. Samples involving synthetic MgO revealed higher strengths than those with commercial MgO, in line with the higher reactivity of the former. The increased dissolution of synthetic MgO led to its enhanced hydration and carbonation, resulting in samples with denser structures and improved performances. The findings highlighted issues regarding the large-scale production of MgO from reject brine and CO2 emissions of this process. The major environmental impact was associated with the production of the alkali base (NaOH), which could be improved via the identification of sustainable alkali sources used during production. Overall, the production of MgO from reject brine can save natural resources and pave the way for the sequestration of CO2 as stable carbonates in cement-based mixes.

This study investigated the healing efficiency and phase formations in pre-cracked reactive magnesia-based binders under CO2/water conditioning. Pre-loaded samples with various crack widths were subjected to a healing regime involving 10% CO2 and water for 10 cycles. The recovery of samples was assessed by resonance frequency measurements and optical microscopy observations. A relationship between crack width and type of healing phases was established via microstructural analysis and carbonation depth measurements. All samples achieved a complete stiffness recovery after a few healing cycles. The healed cracks revealed a higher stiffness than the surrounding matrix under reloading. The recovery of sample stiffness was associated with the densification of sample microstructure via the formation of hydrated magnesium carbonates (HMCs). Depending on the crack width and depth, two different types of HMCs (nesquehonite and hydromagnesite) were observed within the healed cracks, whose formation was influenced by pH, determined by the diffusion of CO2.

With lower calcination temperature, reactive magnesia cement (RMC) can be a potential alternative to the Portland cement. However, RMC concrete requires accelerated carbonation curing from external sources which greatly hinder the wider applications of RMC. This study proposed a bacteria-based method for the strength gain of RMC through internal carbonation. Sporosarcina pasteurii, urea, and yeast extract were used as a carbonation agent for internal carbonation of RMC pastes. Results showed that the flowability of the fresh bio-RMC paste increased by 20% while the initial setting time remained unchanged. Besides serving as the CO2 provider, urea can also function as superplasticizer to reduce the water demand of the bio-RMC pastes. The resulting bio-RMC pastes showed a continuous strength gain with time, demonstrating the feasibility of bacteria-induced internal carbonation of RMC. Microstructure analysis revealed abundant formation of hydrated magnesium carbonates in the bio-RMC pastes, which is responsible for the strength gain of the bio-RMC pastes. Ministry of Education (MOE) The authors acknowledge the financial support from the Singapore MOE Academic Research Fund Tier 2 (MOE2017-T2-1-087 (S)).

Low hydration and high water demand of MgO limits strength gain in MgO cement-based formulations. This study enhanced the microstructure and performance of MgO concrete mixes by increasing hydration and lowering water demand via the introduction of hydration (HA) and dispersion (DA) agents. Hydration and carbonation mechanisms were evaluated by isothermal calorimetry, TGA, XRD, FTIR and SEM. HA increased MgO dissolution and brucite precipitation. DA reduced water content and resulted in denser microstructures by increasing CO2 diffusion. Formation of dense carbonate networks with improved morphologies resulted in 28-day strengths of 45 MPa, which were >50% higher than the control sample.

Reactive magnesia (MgO) cements are proposed as a potentially sustainable binder due to their lower production temperatures (∼800 vs. 1450 °C) than Portland cement (PC) and ability to fully carbonate and gain strength during setting. Reactive MgO is mainly produced via the calcination of magnesite. Environmental implications of reactive MgO production must be analyzed before any final conclusions can be made regarding their contribution to the sustainability of the cement industry. This study evaluates the environmental impacts of reactive MgO production and provides a comparison with PC production using a life-cycle assessment (LCA) approach. The advantages of MgO production with respect to radiation, ozone layer, eco-toxicity, acidification/eutrophication, minerals and fossil fuels outweigh the disadvantages when compared to PC production. MgO has a lower impact on the overall ecosystem quality and resources than PC, but poses a larger damage to human health due to the high coal usage by most plants. The decomposition of magnesite releases a higher amount of CO2 than limestone (∼1.1 vs. 0.78–0.83 t/t), creating a higher climate change score for MgO despite its lower production temperatures. However, when the carbonation capability of MgO cements is considered, their net CO2 emissions are ∼73% lower than PC. The introduction of a strength ratio modification to MgO production results in ∼13% and ∼32% lower CO2 intensity and damage impact on human health when compared to PC production, respectively. The influence of energy type and amount on different impact and damage categories associated with MgO production was revealed by scenario modelling. The quantity of raw materials had a negligible effect on the overall environmental impact of reactive MgO production, whereas increasing the emissions led to an increase in climate change. Energy was identified as the key parameter with the highest influence on the environmental burdens of reactive MgO production. Results indicate that the use of alternative fuels could further improve the overall sustainability of reactive MgO cement production.

Because the calcination temperature of magnesia cement is significantly lower than that of the Portland cement, and previous researchers thought that the magnesium hydroxide could re-absorb carbon dioxide just as calcium hydroxide did, the reactive magnesia cement and magnesium silicates-based cement were labelled as low carbon emission cement. The life cycle carbon emission of magnesia cement is assessed in this paper, comparing with that of Portland cement. The results show that the reactive magnesium oxide cement lead to 79–395 kg/ton more direct CO2 emission than Portland cement during producing. Moreover, the thermodynamic analysis also indicates that the magnesium hydroxide cannot be carbonated in the reactive magnesia cement system or in the magnesium silicate cement system, i.e., the magnesia cement cannot absorb carbon dioxide from the service environment as people expected, whereas Portland cement can absorb more than 250 kg CO2 per ton. Also, the magnesia cement has lower performance than Portland cement. So magnesia cement has a bigger life-cycle carbon footprint than Portland cement. Therefore, it is misleading to develop magnesia cement as low carbon cement or carbon-negative cement.

The reactivities of the binary and ternary mixtures of reactive magnesia (MgO≡M), hydratable alumina (Al2O3≡A) and microsilica (SiO2≡S) micropowders were investigated by calorimetric method and other analytical techniques (X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), differential thermal/thermogravimetric (DTA-TG) analysis and scanning electron microscope (SEM) observations). Effect of water/solid mass ratio and temperature on the hydration behavior of the M–A, M–A–S, M–S, A–M–S and A–S hydratable binders were established. The methodological considerations on both in situ and ex situ mixing techniques have also been taken. The initial mixing peak was due to wetting of the binder particles and initial dissolution reactions. A second, extensive heat peak was associated with the formation of crystalline and noncrystalline hydration products, i.e., brucite Mg(OH)2, magnesium aluminate hydrate (MAH; H≡H2O)-like phase, magnesium silicate hydrate (MSH)-like phase and magnesium aluminum silicate hydrate gels (MASH). Negligible heat was evolved during the hydration reaction of A–S mixture, and the second reaction peak was not observed. Nevertheless, the presence of Al2O3 and SiO2 in other combinations contributes to the consumption of Mg(OH)2 which was formed during the initial stages of hydration and leads to the formation of cementitious products, like M–A–H, M–S–H and M–A–S–H.

Steel-making processes associate with highly corrosive slag and aggressive reactions giving rise to erosive conditions. In-situ spinel (MgAl 2 O 4 ) shows good slag resistance in Al 2 O 3 –MgO–C (AMC) secondary refining ladles because spinel has an excellent cation absorption capacity from the slag due to the void spaces in its crystal structure. The aim of our research work was the optimization of in-situ spinel formation in AMC refractory and to minimise the crack and rupture through residual expansion. Mechanically activated reactive magnesia was added up to 10 wt% in different proportions into the starting raw material batches, which comprised of graded fused alumina and flake graphite. A resol type resin was used as a thermosetting binder, and a combination of Al-B 4 C fine powder was used as an antioxidant. Prepared briquettes by the cold pressing of these batches were first cured in an oven at resin transition temperature for its polymerization. The samples were then coked at 1400 °C. The quantitative crystallinity was studied using X-ray diffraction patterns. The coefficient of thermal expansion was determined by dilatometric analysis and evaluation of microstructural cracks was examined by SEM.

Reactive magnesia (MgO)-activated ground granulated blastfurnace slag (GGBS), with fixed GGBS dosages but varying MgO/GGBS ratios, was used for stabilization of two soils and compared with brucite (Mg(OH)2)-activated GGBS and hydrated lime (Ca(OH)2)-activated GGBS. A range of tests, including unconfined compressive strength testing, X-ray diffraction, and scanning electron microscopy, was conducted to study the mechanical, chemical, and microstructural properties of the stabilized soils, and then to investigate the mechanism of MgO–GGBS soil stabilization. Results indicate that the Mg(OH)2had a minimal activating efficacy for GGBS-stabilized soil, while the reactive MgO yielded a higher activating efficacy than the Ca(OH)2. The activator–soil reactions in the stabilized soil slowed down the activating reaction rate for GGBS; this effect was less significant in MgO–GGBS-stabilized soil than in Ca(OH)2–GGBS-stabilized soil, and hence the GGBS hydration rate in the former was less reduced by the soil than the latter. The Mg2+and OH−ions produced from MgO dissolution participated in the GGBS hydration reactions without precipitating Mg(OH)2. The common hydration products in all GGBS-stabilized soils were calcium silicate hydrate–like compounds. Additionally, hydrotalcite and calcite could be produced in MgO–GGBS- and Ca(OH)2–GGBS-stabilized soils, respectively, especially with a high activator/GGBS ratio.

A reactive magnesia (MgO) was used to stabilize a natural soil; the MgO-stabilized soil was subjected to forced carbonation with pressurized gaseous CO2 in a triaxial cell set-up. The change of physical properties, including bulk density, moisture content, dry density, specific gravity, and porosity, of the stabilized soil during carbonation was studied. The mechanical and microstructural properties of the carbonated MgO-stabilized soil were also investigated through unconfined compressive strength (UCS) test, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP). The results indicated that the carbonation of MgO-stabilized soil consumed CO2 and water, and produced expansive carbonation products; this consequently increased the dry density, and reduced the moisture content, specific gravity, and porosity of the stabilized soil. After being carbonated for only 1.5 h, the MgO-stabilized soil yielded remarkable strength, with UCS higher than that of the 28 day ambient cured Portland cement–stabilized soil, mainly due to the high binding effect of carbonation products and the low porosity of carbonated MgO-stabilized soil. The carbonated MgO-stabilized soil achieved a high degree of carbonation in a few hours (≤12 h), with the maximum CO2/MgO ratio in a range of 0.76–1.07.