Reactive Magnesia Research Articles

Strength and leaching behavior of CaO- and MgO-treated Cd-contaminated soils subjected to partial and full carbonation

Cadmium (Cd)-contaminated soils may pose a significant threat on human health. They are often treated with quick lime (CaO) and reactive magnesia (MgO), but these treatments often result in low strength. Hence, in this study, partial and full carbonation are used to enhance the stabilization/solidification of CaO- and MgO-treated Cd-contaminated soils, aiming to achieve CO2 sequestration, strength improvement, and Cd immobilization. Performance of treated contaminated soils is evaluated through unconfined compressive strength (UCS), leaching, X-ray diffraction (XRD), and thermogravimetric analysis (TGA) tests. The results indicate that carbonation significantly enhances the UCS of both CaO- and MgO-treated Cd-contaminated soils. After carbonation, MgO-treated soils exhibit higher UCS than CaO-treated soils. Partial and full carbonation yield similar UCS in CaO-treated soils, while full carbonation results in higher UCS in MgO-treated soils. For CaO-treated soils, partial carbonation keeps Cd leachability below the 1 mg/kg limit, but full carbonation increases it beyond this limit. In contrast, fully carbonated MgO-treated soils maintain Cd leachability below the limit, though partial carbonation leads to higher leachability. Formation of Ca and Mg carbonates contributes to the strength improvement of soils. Cd(OH)2 and its complex, as well as CdCO3 exist in partially and fully carbonated soils, lowering leached Cd concentration. Overall, partial carbonation is better for CaO-treated soils, while full carbonation is preferable for MgO-treated soils.

An efficient bio-stabilization technology with bio‑carbonation of reactive magnesia for soil improvement in cold regions

Low curing temperature conditions (5–15 °C) in cold regions pose major challenges for soil improvement using conventional binders, underscoring the urgent need for solutions to enhance soil strength and ensure engineering safety. This study investigated the feasibility and temperature-dependent behaviors of bio‑carbonation of reactive magnesia (BCRM) technology for soil improvement in cold regions. Unconfined compressive strength tests were conducted to explore the effects of curing temperature (T) and curing age (t) on strength enhancement. Combined with macro- (water content and dry density) and micro- (mineral composition and microstructure) analysis, the underlying mechanisms were elucidated. Experimental results showed that low T retarded the bio‑carbonation reaction of reactive magnesia, resulting in longer t required to obtain stable ultimate strength. However, despite lower increase rates, bio‑carbonized samples achieved higher ultimate strength and secant modulus at lower T. It was primarily attributed to the preferential formation of hydrated magnesia carbonates with higher content and crystallinity at low T, which enhanced the bridging and bonding performance. Comparative analyses with ordinary Portland cement (OPC) highlighted the superior efficiency of BCRM technology in stabilizing soil at low T, showing higher ultimate strength and shorter curing age. Notably at 5 °C, the ultimate strength of the bio‑carbonized sample cured for 12 days was up to 2.94 times that of the OPC-reinforced sample cured for 28 days. This study provides an efficient solution for soil improvement in low-temperature conditions. It is expected to enhance soil stability and hold significant implications for preventing and mitigating geological and geotechnical risks associated with soil deterioration in cold region engineering.

Long-term mechanical performance of fly ash-incorporated reactive magnesia cements

In this research, after producing reactive magnesia directly from calcination of magnesite, reactive magnesia specimens with fly ash were prepared and cured in two different environments to investigate their long-term mechanical performance by carbonation. To investigate the effect on their microstructures of carbonation, the XRD profiles of the samples were also obtained and investigated. It is found that the strength of reactive magnesia samples is highly affected by the addition rate of reactive magnesia and the curing regime. In spite of the presence of the nesquehonite (MgCO3.3H2O) in the reactive magnesia cements having no portland cement, any type of magnesium carbonate crystal was not found for the cements with portland cement in carbon dioxide curing. The strength increase in reactive magnesia cements can be due to the incorporation of Mg2+ ions to the calcite crystals.

Investigating key factors for superior CRMC-based mortars cured under CO2 pressurized environment

An exploratory programme was undertaken to individually test the influence of reactive magnesia to waste-based material (biomass fly ash) ratio, water to solids in binder ratio, static compaction pressure, accelerated carbonation curing period, and curing temperature, on the compressive strength results of Carbonated Reactive Magnesia Cement (CRMC)-based mortars. Subsequently, the observed optimal conditions of each parameter were combined to assess their influence when applied together. A total of twenty-four mixture labels were designed, with specimens moulded under static compaction pressure and subjected to a pressurized accelerated carbonation curing under controlled conditions. The devised mortars were evaluated through compressive strength tests, and the microstructure was carried out through TG-DTG, ATR-FTIR, XRD, SEM-EDX, and MIP analyses. The results demonstrated that none of the tested influencing factors can be exclusively considered as the sole determinant for the compressive strength outcomes. Thus, this study highlights the importance of thoroughly considering any modifications in both mixture design and accelerated carbonation curing conditions to achieve the optimal properties of CRMC-based mortars. Additionally, the devised CRMC-based mortars exhibited favourable binding properties with biomass fly ashes, allowing for its secondary use and contributing to waste circularity by avoiding landfill disposal, supporting the development of the waste circularity and CO2 sequestration through mineralization in carbonated materials.

Investigation of non-uniform carbonation in strain-hardening magnesia composite (SHMC) and its impacts on fiber-matrix interface and fiber-bridging properties

Unlike Portland cement, the reactive magnesia cement (RMC) develops its strength via carbonation. Non-uniform microstructure and properties due to the varied carbonation degree across cross-section depth have thus been observed. However, the existing micromechanics design theory employs uniform material properties (cementitious matrix and fiber-matrix interface characteristics) to design strain-hardening composites. This research investigated such non-uniform carbonation in a strain-hardening magnesia composite (SHMC) and its impacts on fiber-matrix interface and fiber-bridging properties. Results showed that mineral phase composition (of the cementitious matrix) and fiber-matrix interface properties across cross-section depth are highly non-uniform in SHMC due to varied carbonation degrees. The outer layer exhibited a much higher carbonation degree with significantly stronger fiber-matrix friction, while the inner core showed negligible carbonation with distinct fiber-matrix properties. Insights gained from the micro-scale investigation were used to calculate the fiber-bridging constitutive law of SHMC with non-uniform carbonation across cross-section depth and to assess its strain-hardening performance. This investigation highlights the importance and necessity of considering non-uniform carbonation in the modeling and design of SHMC. The study presents a framework for the consideration of non-uniform micromechanical parameters in the design of strain-hardening cementitious composites.

Investigation of the properties of Mg(OH)2 extracted from magnesium-rich brine via the use of an industrial by-product

Compared with magnesium oxide (MgO) produced through the dry route, the wet route production can lead to MgO with a high purity and reactivity. During this process, the use of chemical alkalis to precipitate the precursor, magnesium hydroxide (Mg(OH)2), is the highest contributor to cost and energy consumption. This study proposes a low-cost and sustainable way to recover Mg(OH)2 from simulated reject brine using concrete wash water (CWW). Mg(OH)2, with a purity of 94 % and a recovery degree of 98 % was synthesized at room temperature. Higher stirring speed and use of low pH CWW increased the impurity (CaCO3) content. The synthesized Mg(OH)2 showed a similar reactivity to MgO synthesized from reject brine with chemical bases due to its high specific surface area (102.8 m2/g). The calcination of the synthesized Mg(OH)2, led to MgO with a higher reactivity than similar ones obtained through the wet-route. Furthermore, the textural properties of Mg(OH)2 synthesized under different conditions were evaluated to establish a correlation between the reaction conditions and reactivity of Mg(OH)2. Increase in reaction temperature led to a small crystalline size, whereas the aging process resulted in the agglomeration of the synthesized Mg(OH)2, influencing its reactivity. The findings presented in this study can pave the way for the sustainable recovery of Mg(OH)2 and its direct use as a binder or as a precursor for the production of reactive MgO.

Solidification of sodium sulfate saline loess by biomineralization of reactive magnesium oxide binder

This study aims to solidify sodium sulfate saline loess by biomineralization of reactive magnesium oxide (MgO) binder. The impact of Na2SO4 concentration on the viability and urease activity of S. pasteurii, the mechanism and products of biomineralization of MgO, and the effectiveness of the biomineralization-MgO binder in solidifying saline loess with varying salt content (1%, 3%, and 5%) were investigated. Results showed that low and moderate concentrations of Na2SO4 favored bacterial proliferation. However, the presence of Na2SO4 inhibited bacterial urease activity, and the inhibition was more significant at higher Na2SO4 concentrations. In addition, low and moderate concentrations of Na2SO4 decreased the specific urease activity, whereas high concentrations of Na2SO4 significantly increased specific urease activity. S. pasteurii was able to use carbonate ions formed by urea hydrolysis for the mineralization of MgO and to form magnesium carbonate minerals dominated by rosette-like dypingite and hydromagnesite crystals. The primary mechanism involves microbial cells and extracellular polymeric substances leading to partial dehydration of Mg2+ ions from the Mg2+-H2O complex and allowing for further association with carbonate anions to from Mg-bearing carbonates. Unconfined compressive strength tests conducted on the saline loess samples after 7 days of curing revealed a significant influence of urea concentration on the strength of the solidified soil. The optimal urea concentration to obtain a better 7-day UCS ranged from 4 mol/L to 5 mol/L. Furthermore, solidified soil with 5% salinity yielded the highest 7-day UCS and soil with 3% salinity exhibited the lowest 7-day UCS at the same urea concentration. XRD and SEM analysis of the solidified soil samples indicated that the formation of magnesium carbonate minerals in the soil matrix by the biomineralization-MgO binder was responsible for the UCS enhancement. The remarkable 7-day UCS of saline loess solidified with biomineralization-MgO binder demonstrates the effectiveness of this material in curing saline loess.

Effect of internal moisture content (IMC) on the CO2 sequestration efficiency of hollow natural fiber (HNF)-reinforced reactive magnesia cement (RMC) composites

The incorporation of minor hollow natural fibers (HNF) can significantly accelerate the CO2 diffusion in reactive magnesia cement (RMC). The CO2 diffusivity in HNF is highly related to its internal moisture content (IMC), which subsequently affects the carbonation process of HNF-RMC, but this is outside the scope of the existing studies. Therefore, this work investigates the effect of IMC on carbonation, strength, and microstructure of HNF-RMC composites. HNF-RMC mortar specimens are dried to different levels of nominal IMC (0–100%), and cured under high CO2 concentration for carbonation. Following the curing, the matrix composition was characterized with varying techniques (QXRD, TGA, FTIR, and acid digestion) for quantifying the CO2 sequestration; the compressive strength at different depths was determined with uniaxial compressive test; the microstructure was examined with SEM/ESEM. The results show that the carbonation depth is increased by three to four times by reducing the IMC to 0%. The effects of varying IMC on local carbonation and strength are distinct in the shallow (0–20 mm) and middle/deep region (20–100 mm). In the shallow region, inadequate CO2 dissolution is the limiting factor, these performances decreased with the increasing desiccation. In the deeper regions, moisture absorption by HNF decreased the CO2 diffusivity, hence the CO2 availability is the limiting factor, and these performances increased with the desiccation. These observations suggest that the understanding that carbonation maximizes at a medium moisture level is not valid in HNF-RMC–controlling the moisture can be important for improving the CO2 sequestration and consequent mechanical strength.

Enhancing carbonation and thermal insulation of reactive magnesium oxide cement (RMC)-based 3D printable pastes with cenospheres

Reactive magnesium oxide cement (RMC) has been proposed as a sustainable alternative to ordinary Portland cement (OPC) due to its lower production temperature and ability to permanently sequester carbon dioxide (CO2) through carbonation. However, mixtures with only RMC can have significantly reduced CO2 absorption capacity due to limited CO2 diffusion, thereby compromising the performance of RMCs. This paper demonstrates the adoption of cenospheres, an industrial waste, as a lightweight filler in RMC-based 3D printable composites to enhance carbonation and improve thermal insulation. This work investigates the effect of incorporating different dosages of cenospheres on the rheological properties, compressive strength, thermal conductivity, and microstructural development of the RMC mixtures using X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The study reveals that substituting 30 % of RMC with cenospheres not only improves the printability and carbonation of the composite but also results in a 16 % reduction in density, achieving 1.72 g/cm³, and reduces the thermal conductivity by 59 % compared to the control without cenospheres to 0.63 W/(m·K). The findings provide valuable insights for developing lightweight 3D printable RMC-based composites.

Effect of hollow natural fiber (HNF) content on the CO2 diffusion, carbonation, and strength development of reactive magnesium cement (RMC)-based composites

Reactive magnesia cement (RMC) is an emerging class of green cement that hardens by sequestering CO2. However, CO2 diffusion into RMC is restricted to a few millimeters by the carbonation-induced dense microstructure on the outer layer, which severely slows down the strength growth and CO2 sequestration. To address this issue, this work employed hollow natural fibers (HNFs) to facilitate CO2 diffusion into the deep regions of RMC. The effects of HNFs contents on the mechanical strength development, holistic porosity, CO2 sequestration, CO2 diffusivity, and microstructure of RMC were investigated through different techniques. The findings revealed that the compressive strength could be more than doubled with the addition of adequate sisal fiber. Moreover, the CO2 sequestration and diffusivity could be continuously enhanced with the increasing HNFs content. However, overdosage of HNFs could induce a higher porosity and additional defects, which slightly compromises the mechanical strength. Finally, the durability of HNFs in simulated RMC and Portland cement (PC) environment was compared by accelerated aging test, showing that the alkaline-induced deterioration of HNFs could be almost eliminated in RMC. Therefore, this preliminary study reinforces the function of RMC as a carbon reservoir and lays the foundation for the large-scale utilization of HNFs in RMC.

The effect of depth-dependent carbonation on the interfacial strength between GFRP reinforcement bar and reactive magnesia cementitious composites

Reactive magnesia cement (RMC) is an emerging class of low-alkaline and CO2-sequestering binder, which can mitigate the deterioration of GFRP reinforcements induced by a high-alkaline environment, e.g., in Portland cement. This study investigated the slip behavior of GFRP rebar embedded in RMC composite, which varies with carbonation depth significantly. The variation of the interfacial bond was determined by a specially designed push-out test of the GFRP core; the variation of the carbonation degree and microstructure was examined by SEM-EDX, MIP, XRD, TGA, FTIR, and acid digestion tests. Both properties demonstrated a similar trend, decreasing rapidly with increasing depth. A new finite element model that considers the depth-dependency of the matrix compositions and the rebar-to-matrix interfacial bond is established. It can predict the constitutive bond-slip behavior of a long GFRP rebar embedded in an RMC composite with non-uniform carbonation.

Efficient stabilization of dredged sludge with high water content using an improved bio-carbonation of reactive magnesia cement method

This study proposed an improved bio-carbonation of reactive magnesia cement (RMC) method for dredged sludge stabilization using the urea pre-hydrolysis strategy. Based on unconfined compression strength (UCS), pickling-drainage, and scanning electron microscopy (SEM) tests, the effects of pre-hydrolysis duration (T), urease activity (UA) and curing age (CA) on the mechanical properties and microstructural characteristics of bio-carbonized samples were systematically investigated and analyzed. The results demonstrated that the proposed method could significantly enhance urea hydrolysis and RMC bio-carbonation to achieve efficient stabilization of dredged sludge with 80% high water content. A significant strength increment of up to about 1063.36 kPa was obtained for the bio-carbonized samples after just 7 d of curing, which was 2.64 times higher than that of the 28-day cured ordinary Portland cement-reinforced samples. Both elevated T and UA could notably increase urea utilization ratio and carbonate ion yield, but the resulting surge in supersaturation also affected the precipitation patterns of hydrated magnesia carbonates (HMCs), which weakened the cementation effect of HMCs on soil particles and further inhibited strength enhancement of bio-carbonized samples. The optimum formula was determined to be the case of T = 24 h and UA = 10 U/mL for dredged sludge stabilization. A 7-day CA was enough for bio-carbonized samples to obtain stable strength, albeit slightly affected by UA. The benefits of high efficiency and water stability presented the potential of this method in achieving dredged sludge stabilization and resource utilization. This investigation provides informative ideas and valuable insights on implementing advanced bio-geotechnical techniques to achieve efficient stabilization of soft soil, such as dredged sludge.

CO2-sequestering ability of lightweight concrete based on reactive magnesia cement and high-dosage biochar aggregate

Lightweight concrete is widely used in non-structural components of buildings, e.g., partition wall bricks. Most current lightweight concretes engage Portland cement and lightweight aggregates. In this study, lightweight concretes based on reactive magnesia cement (RMC) and high amount of biochar aggregates (up to 508 kg/m3, 61 vol% of the concrete) are developed. The new material demonstrates the ability to sequestrate massive amounts of CO2 from ambient air, and thus has the potential to be used for greener non-structural applications. The CO2 sequestration during curing under high-concentration CO2 (10 %), which is mainly via the RMC carbonation, is determined by acid digestion. The CO2 sequestration during service under ambient CO2 concentration (0.1%, or 1000 ppm), which is via the continued carbonation of RMC and the adsorption of biochar, is determined by monitoring the removal of CO2 in situ; particularly, the CO2 sequestrations by the RMC and the biochar are determined separately. The effect of varying biochar content and CO2 curing time on the overall CO2 sequestering ability of the concrete are studied, together with the variation of the local CO2 sequestration with carbonation depth in the concrete. Based on the experimental results, the new concretes are seen to achieve adequate mechanical properties for non-structural application. They can permanently sequestrate a significant amount of CO2 during the curing phase (0.2–0.3 g per gram RMC), and constantly sequestrate noticeable amounts of CO2 during the service stage (1–12 mg per gram RMC, 1 mg per gram biochar). The incorporation of porous biochar not only decreases the material density, but also improves the RMC carbonation by accelerating CO2 penetration, while the RMC barely influences the biochar adsorption. However, the RMC carbonation ability permanently vanishes with curing age, whilst the biochar adsorption decreases marginally with the curing age.

Autogenous healing of the interface between hollow natural fiber (HNF) and reactive magnesia cement (RMC) matrix

The fiber-matrix interface healing determines the restoration of tensile strength of fiber reinforced cementitious composites (FRCCs). This work first-timely investigates the autogenous healing of debonded interface between hollow natural fiber (HNF) and reactive magnesia cement (RMC) matrix and subsequent tensile strength recovery. HNF-RMC single-fiber specimens are preloaded to induce interfacial debonding, and then conditioned under water-air cycles for healing. The healing-induced recovery of the interfacial properties in the HNF-RMC group are significantly higher than the control PVA-RMC and PVA-PC groups. Tensile test of notched FRCCs demonstrated that the tensile strength of HNF-RMC can be more than doubled after cracking-healing, and its healing efficiency is around 50% higher than that of PVA-PC and PVA-RMC. Based on microstructural characterization in situ and finite element modeling, the increased healing degree is attributed to two new mechanisms related to the special porous microstructure of HNF. First, the lumens and micropores within cell of HNF can be the pathway for CO2 and water penetration, which promotes the formation of healing products (e.g., hydrated magnesium carbonates) within fiber-matrix interface and hence the recovery of chemical and frictional bonds. Second, the healing products formed within lumens caused lateral fiber expansion, and thus prominently enhanced the frictional bond.

Impact of nano-dopants on the mechanical and physical properties of magnesium oxychloride cement composites – Experimental assessment

The importance of the reduction of the World's CO2 emissions due to their non-negligible contribution to climate change is inevitable. As the Portland cement (PC) industry is one of the main contributors to the overall volume of CO2 emissions produced over the World, a sustainable PC substitute must be implemented. Previous scientific studies suggest that magnesium oxychloride cement (MOC) – an eco-friendly binder based on reactive magnesia – could be a suitable alternative. This contribution studies the influence of various nano-dopants on the mechanical and physical properties of MOC-based composites. The nano-dopants introduced to the MOC composite were the following: multi-walled carbon nanotubes, oxidized multi-walled carbon nanotubes, graphene oxide, graphene nanoplatelets, and alumina nanosheets. Ahead of the composite preparation, the nano-dopants were analysed with a wide spectrum of analytical methods to study their composition and microstructure in detail. The used amount of nano-dopant was 1.0 wt%, relative to the cement paste weight. After a curing period of 28 days, the prepared composite samples underwent a comprehensive analysis of their microstructure, phase, and chemical composition, as well as micro- and macrostructural parameters and mechanical properties. Additionally, the research studied the effects of nano-dopants on the hygric properties and water resistance of the MOC composites after 24-h immersion in water. The findings revealed a noteworthy increase in mechanical parameters with the addition of nano-dopants. The highest compressive strength of 85.4 MPa was obtained for the MOC composite enriched with the addition of oxidized multi-walled carbon nanotubes. MOC doped with alumina nanosheets exhibited the highest value in flexural strength (20.99 MPa) and dynamic modulus of elasticity (38.85 GPa) measurements. The resistance of MOC composites to deterioration in water was also improved by all the implemented nano-dopants. The softening coefficient determined for samples immersed in water for 24 h varied from 62.7 to 74.8%. The best resistance to water degradation was obtained for MOC with alumina nanosheets, which showed a softening coefficient value about 23.2% higher than that of the reference sample. Thus, the incorporation of nanomaterials as functional nano-dopants in MOC-based composites shows promising opportunities in the improvement of both mechanical properties and water resistance, contributing to the development of environmentally sustainable building materials.

Experimental investigation into the self-carbonation mechanism of magnesium oxide carbon sequestration foamed concrete

A novel technique involves the preparation of foamed concrete utilizing MgO as a substitute for Portland cement (PC) and CO2 foam in place of air foam. This technique is referred to as “magnesium oxide (MgO) carbon sequestration foamed concrete (MCFC)" that enables self-carbonation of Reactive MgO cement-based concrete under ambient conditions without any accelerated carbonation curing conditions. The developed technology enhances the carbonation efficiency of MgO-based materials by combining a foaming agent with CO2 curing. The even mixing of CO2 foam with MgO, referred to the uniform distribution and incorporation of CO2 throughout the sample, facilitated by the foaming agent, leads to the production of ions within the cement, promoting the carbonation of dissolved Mg2+ ions and the formation of hydrated magnesium hydroxyl carbonates (HMHCs). Eight sample groups were prepared to explore the potential of MCFC in CO2 sequestration. XRD, TG-DTA, and SEM techniques were employed to investigate the mineral phase and carbonation products in MCFC. The pore structure, pore-size distribution, and porosity of MCFC under different curing conditions were investigated using X-ray and NMR. The self-carbonation enabled by CO2 foams improved the microstructure and mechanical performance of MgO-based samples. Notably, the morphology, microstructure, and carbonate content improvements resulted in a significant 100% increase in the 28-day compressive strengths (2 MPa) compared to the control sample cured under ambient conditions.

Study on hydration process of alkali-activated slag cement activated by weakly alkaline components

Alkali-Activated Slag Cement (AASC) activated by weak alkaline components has good workability and safety. Sodium carbonate (Na2CO3, SC), sodium sulfate (Na2SO4, SS), reactive magnesium oxide (R-MgO), and calcium hydroxide (Ca(OH)2, CH) were used to prepare the AASC. The hydration process, phase composition and hydration kinetics of AASC were studied. The results showed that the pH value of AASC slurry activated by weak alkaline components ranging from 10.8 to 13.2. CH could significantly affect the pH of AASC slurry. When the dosage reached more than 5%, the pH value of AASC slurry reached more than 12.0. The hydration heat release of AASC activated by weak alkaline components was mainly concentrated in 1 day. Two hydration heat release peaks appeared. The initial hydration peak appeared within 0.5 h-1 h, and the acceleration hydration peak appeared within 1 h-8 h. When the content of CH was 5%, and the content of SS and SC was 7% and 4%, the acceleration peak of AASC was much higher than that of others. At the same time, the Krstulovic-Dabic model was used to study the hydration kinetics of AASC. The increase of SS content prolonged the NG-I and I-D processes. CH accelerated the hydration rate of AASC and shortened the hydration process of AASC. The main hydration products of AASC activated by weakly alkaline components were C-(A)-S-H gel, calcite, ettringite, hydrotalcite and gaylussite. R-MgO was favorable for the formation of hydrotalcite, while SS was favorable for the formation of ettringite.

Influence of supercritical carbon dioxide (scCO2) curing on carbonation and strength development of brucite recovered from desalination reject brine

Brucite is a potentially sustainable alternative to reactive magnesium oxide (MgO) cement (RMC) with no calcination requirement. This study investigated the effect of elevated CO2 curing concentrations on CO2 sequestration, the formation of carbonate phases, and the compressive strength of compacted pellets of brucite nanopowders synthesized from reject brine. It was revealed that the compressive strength of the compacted brucite pellets with optimum water-to-binder ratio (w/b) enhanced by 45% within 4 h under supercritical CO2 (scCO2) conditions compared to the maximum strength observed under 20% CO2 and 80% relative humidity (RH) at 30 ºC conditions. The scCO2 curing enabled the formation of a high-density nesquehonite phase primarily responsible for the strength development of the brucite pellets. In addition, the CO2 sequestration in the scCO2-cured brucite pellets reached up to 66% of their maximum storage capacity, indicating their excellent CO2 absorption capability.

Influence of natural fibers on hydration and carbonation of reactive magnesium oxide cement (RMC)

This study offers insights into the influence of cellulose fibers at 0, 0.25, and 0.5 vol% on the hydration and carbonation of reactive magnesium oxide cement (RMC). The cumulative heat increased with the fiber content, attributed to the nucleation and possibly internal curing effects of the fibers. The reinforcing action provided by the fibers also improved the mechanical performance of the air-cured samples, yet the compressive strength under 20% CO2 and 80% RH was maximized at 0.25 vol%. At 0.5 vol%, the contents of brucite and carbonates were reduced due to confinement and crack control, which restricted the growth of brucite and CO2 diffusion. The carbonation phase was mainly poorly crystalline or amorphous, marked by a fused structure and with a thermal decomposition peak at approximately 405 °C. The cellulose fibers with a porous cell wall and cavity provided a pathway for enhanced CO2 diffusion, resulting in concentrated carbonates at the fiber-matrix interface.

Durability of geopolymer cutoff wall backfill incorporating reactive MgO particles under dry-wet cycles

Vertical cutoff wall has been employed for decades to control groundwater flow and subsurface contaminant transport. The barrier performance of cutoff wall material may significantly deteriorate under dry-wet cycles due to precipitation, drought, and groundwater fluctuation. This study adopted active magnesium oxide (MgO) particles to enhance the durability of geopolymer cutoff wall backfill (GCWB) under dry-wet cycles. The influence of particle size and proportion of MgO on the unconfined compressive strength (UCS), hydraulic conductivity, self-healing capacity and durability of GCWB were comprehensively examined. The results indicate that the incorporation of active MgO particles hindered the geopolymerization process of GCWB to a certain extent, leading to an increase in hydraulic conductivity and a decrease in UCS. But the overall performance of GCWB was still favorable. The hydraulic conductivity of samples with MgO gradually increased to more than 1 × 10−8 m/s after at least two dry-wet cycles. In the subsequent cycles, the samples with MgO exhibited a pattern of first gradually declining then ascending in hydraulic conductivity. This is due to hydration and carbonization of the remained MgO in matrix during the dry-wet cycles, which converts into oxygen-rich carbonates, forming well-ramified networks and reducing the proportion of macropores. Thus, the durability of GCWB with MgO is significantly more favorable than that devoid of MgO. In particular, large particle size MgO can exert more persistent self-healing effect, endowing the samples with superior durability under the prolonged dry-wet cycles.