MgO Cement Research Articles

MgO activated calcined marine clay and its carbonation

Marine clay, a common waste product abundantly available in coastal regions, can serve as an alternative silica source after activation due to its pozzolanic reactivity. In this study, a binder comprising reactive MgO cement and thermally activated marine clay (AMC) was developed. It was found that the developed MgO-AMC binder with a mass ratio of MgO to AMC of 0.4 to 0.6 achieved a 28-day compressive strength of 20.4 MPa under ambient curing. Through accelerated carbonation curing, the highest 28-day compressive strength increased by 45%, and the highest carbon sequestration ratio reached 11.8%. The phase evolution and microstructural development of hydrated/carbonated binder were comprehensively investigated by TG-IR, XRD, NMR, and SEM-EDS. In addition, thermodynamic modelling was performed to further understand the phase assemblage of the hydrated binder, and indicated that M-A-S-H phase was thermodynamically more stable in the developed binder.

Comparison and mechanism analysis of MgO, CaO, and Portland cement for immobilization of heavy metals in MSWI fly ash

The significant production of municipal solid waste incineration fly ash (MSWI FA) underscores the importance of developing efficient solidification materials. This study employed MgO and CaO for immobilizing MSWI FA (with a 70% fly ash incorporation), and the immobilization effect was compared with that of Portland cement (PC). Experimental findings revealed that MgO exhibited the most effective stabilization for heavy metals (Cd, Cu, Pb, and Zn) compared to CaO and PC. XRD, FTIR, TG, and SEM analysis indicated that the principal hydration products in MSWI FA binders solidified with MgO, CaO, and PC were Mg(OH)2, CaCO3, and C-S-H gel, respectively. Mg(OH)2 efficiently immobilized heavy metals through chemical complexation and surface adsorption mechanisms. The MgO-treated MSWI FA demonstrated the highest residual fractions and the lowest easily leachable fractions. Moreover, the leaching characteristics of heavy metals were significantly influenced by the pH level, so MgO-treated MSWI FA with a leachate pH of 9.18 achieved the precipitation and stabilization of most heavy metals. In summary, this study provided an effective material selection for MSWI FA immobilization and presented a novel strategy for MSWI FA management.

Improving the Hydration and Carbonation of Reactive MgO Cement with Amino Acids and the Influencing Mechanisms

Improving the Hydration and Carbonation of Reactive MgO Cement with Amino Acids and the Influencing Mechanisms

Effect of carbonation on development of reactive MgO-based pervious concrete

Pervious concrete, developed to solve urban flooding and overloaded drainage issues, has limited strength due to its large void ratio. Reactive MgO cement (RMC) is a sustainable cement gaining strength through carbonation, which can have a more pronounced carbonation-induced enhancement in mechanical properties compared with Portland cement. However, its carbonation degree was limited due to the dense microstructure. To address both issues, pervious concrete using RMC was developed, and its void ratios were leveraged to facilitate the carbonation of RMC. Experimental results revealed that compared with that of samples cured under ambient environment (i.e. 4.7–15.7 MPa), the compressive strength of RMC-based pervious concrete cured under accelerated carbonation (11.2–39.7 MPa) and water/CO2 cycles (10.3–29.6 MPa) was enhanced by 77–278%. As the design void ratio increased from 0.15 to 0.30, the permeability coefficient of pervious concrete increased from 0.1 to 2.3 mm/s, while the carbonation of RMC-based pervious concrete also benefited from high void ratio, with the highest 28-day carbon sequestration ratio reaching 12.7%, as indicated by XRD, TGA tests and SEM observations. Furthermore, a mathematical model was established to estimate the thickness of cement paste on aggregates in pervious concrete, which was less than 1 mm and decreased as the design void ratio increased.

Evaluation of the technical properties of reactive-MgO cements produced by solar calcination of magnesite in a fluidized bed reactor

Magnesium oxide (MgO)-based cements display very interesting technical properties and environmentally-friendly features. The novel idea investigated in this study is to synthesize MgO cements, using as raw material natural magnesite calcined in fluidized bed heated by concentrated solar energy. Calcination was performed in a lab-scale system equipped with a concentrated solar simulator, operated under different process conditions. The most reactive MgO was mixed with 3% by weight of MgCO3 (nucleation agent) and four different solutions containing magnesium acetate or chloride. The binders were hydrated in air or 20% CO2 atmosphere (accelerated carbonation conditions) until 28 days. X-ray diffraction, differential-thermal and mercury intrusion porosimetry analyses, and compressive mechanical strength tests, were performed on the hydrated systems. Solar calcination produced a highly reactive MgO. The performance of the cement pastes improved at higher curing times, and when using magnesium acetate as hydration agent, as also witnessed by the application of a kinetic model. Accelerated carbonation conditions further enhanced the mechanical properties of the cements thanks to the formation of nesquehonite, allowing to reach a mechanical strength comparable to that of ordinary Portland cements class 32.5. The achieved outcomes encourage the production of low-CO2 magnesite cements from solar calcined magnesite, boosting the green aspect of the entire process.

The intrinsic mechanical properties of hydromagnesite, Mg5(CO3)4(OH)2·4H2O, a key phase of reactive MgO carbonate cement

To potentially enable CO2 sequestration, reactive MgO carbonate cement is emerging as an alternative binder to Portland cement. Understanding the mechanical properties of its binding phase is critical for understanding the strength development and performing materials design for reactive MgO cement systems; however, the intrinsic mechanical properties of hydromagnesite (Mg5(CO3)4(OH)2·4H2O), a key binding phase, remain unexplored. The present study utilized synchrotron-based high-pressure X-ray diffraction to determine the unit cell-scale, intrinsic mechanical properties of hydromagnesite for the first time. Up to hydrostatic loading of 7.7 GPa, the bulk modulus of hydromagnesite was determined as 59 GPa or 71 GPa fitted using the second-order or third-order Birch-Murnaghan equation of state, which we contextualize with binding phases in various cement systems. The experiment results are applicable in materials design of low-carbon concrete and valuable for the validation and calibration of atomistic models.

Comparison of magnesium oxysulfate (MOS) cement and reactive magnesia cement (RMC) under different curing conditions: Physical properties, mechanisms, and environmental impacts

The potential of MgO cement as a low-carbon alternative is widely acknowledged. However, there has been no systematic comparison of the physical properties, mechanism and environmental impact between reactive MgO cements (RMC) and MOS Cement. This study investigated the hydration and carbonization processes of RMC and MOS cement within the range of reactive MgO/MgSO4 molar ratio from 7 to 350. Results showed that different curing conditions had different impacts on RMC and MOS. Specifically, the density of both materials exhibited an increase, but the strength of RMC was enhanced while that of MOS decreased. Although the carbonate and 517 phases can coexist in MOS specimens under carbonization curing, their combination resulted in a notable strength decrease. Carbon footprint analysis showed that RMC cement had certain advantages in carbon emissions over the entire life cycle. Overall, these research outcomes provided valuable guidance for the design of RMC and MOS cement.

Passivation of reinforcing steel in reactive MgO cement blended with Portland cement

Reactive MgO cement (RMC) has an ability to gain strength by carbonation. One of the main concerns in RMC systems is the potential corrosion of reinforcing steel in structural applications. This study evaluated the feasibility of replacing small proportions of RMC with Portland cement (PC) to promote the passivation of reinforcing steel. Reinforcing steels embedded in RMC with different proportions of PC were investigated by electrochemical measurements and microstructural analysis. Pore solutions extracted from these pastes were evaluated for their chemical compositions. Inclusion of ≥2% PC enabled the passivation of steel. Passive film forming in RMC-PC blends consisted of Fe3O4 and Mg(OH)2, which was thicker than those in pure PC mixes. The formation of passive film was mainly attributed to the increased pH of the pore solution, which reached over 12 in RMC mixes containing only 2% PC. These findings highlighted the potential of using RMC mixes in reinforced concrete applications.

Mechanical properties and microstructure of glass fiber reinforced polymer (GFRP) rebars embedded in carbonated reactive MgO-based concrete (RMC)

Reactive MgO cement (RMC) is an emerging low-carbon material which can be a sustainable alternative to ordinary Portland cement (OPC) due to its capability to sequester CO2. However, the application of carbonated RMC concrete in conventional steel reinforced concrete infrastructure is restricted by its inherent low alkalinity (∼10.4), which cannot protect steel from rusting. To address this issue, glass fiber reinforced polymer (GFRP) rebars can be employed. In this study, the mechanical properties and microstructures of RMC-GFRP exposed to various temperatures (23, 40 or 60 °C) for 21, 45 and 90 days were investigated. Experimental results showed that the low-alkalinity carbonated RMC concrete was less aggressive than the normal concrete (NC) to GFRP rebars, remarkably enhancing the tensile strength retention of GFRP rebars from 73.6% to 90.4% after 90 days of exposure at 40 °C. Micromorphology observation indicated that the higher tensile strength retention was ascribed to the reduced GFRP degradation (i.e., matrix cracking, fiber erosion and fiber/matrix interfacial debonding) under the low-alkalinity condition. Additionally, the flexural strength of 90-day aged RMC- and NC-GFRP samples at 60 °C were close to that of unconditioned GFRP samples (791 MPa and 788 MPa vs. 791 MPa), which was attributed to the unchanged glass transition temperature and stable elastic modulus of GFRP. The RMC- and NC-GFRP samples also showed very little change in inter-laminar shear strength as compared to the unconditioned GFRP samples, as the middle layer of GFRP rebars was well-protected from external water and chemicals. Based on the empirical degradation model and activation energy derived from test results at different temperatures, the predicted tensile strength retention of RMC-GFRP after an extended period (∼100 years) under Canadian climate is still sufficient. This study will shed light on the understanding of degradation mechanism and practical application of GFRP reinforced RMC in infrastructure construction.

Hydration, carbonation and strength development of reactive MgO cement blended with lime (CaO) under different curing conditions

This paper investigated the effect of lime added as substitution (by up to 30 wt%) of magnesia on the performance, phase composition and CO2 sequestration capacity of blended reactive MgO cement (RMC) under two different curing conditions. A relationship between the strength development and formation of hydration and carbonation phases was also established. The results showed that the incorporation of lime promote the setting and initial heat generation of RMC. More lime replacement led to higher CO2 absorption capability under both ambient and accelerated carbonation conditions, mainly due to the improved calcite. The presence of uncarbonated portlandite was found to have negative impact on strength development of blended RMC before and after water exposure. Carbonation curing enhanced the conversion of portlandite and brucite into calcite and nesquehonite, respectively, which effectively reduced the porosity and improved the denseness of blended RMC. The improvements in carbonates content, morphologies and microstructure in samples containing 20% lime resulted in ∼30% higher 28-day compressive strength than the plain samples.

Investigation of the properties of reactive MgO-based cements and their effect on performance

Reactive MgO cement (RMC) is a promising alternative cementitious material benefiting from a relatively low calcination temperature during its production and strength development in concrete formulations linked with its CO2 sequestering capacity. One of the main challenges with RMC is the variations in its performance in line with the significant differences observed in the properties of the main phase, MgO. To identify and analyze the effects of these properties on the performance of RMC, this study presents a detailed characterization of 9 commercial RMC powders from different sources and precursors and an investigation of their performance in terms of reaction mechanisms and strength development. The results showed that the progress of hydration was highly dependent on the reactivity of RMC, whilst the early stages of the reaction were influenced by the purity. Additionally, agglomeration ratio revealed a strong correlation with the strength after 7 days of carbonation curing and 28 days of hydration. Finally, a regression analysis was employed to propose a model for the prediction of strength based on the initial properties of the RMC powder. The results emerging from this study can serve as a guideline for the selection of most suitable RMC-based binders for various building applications.

Recent advances in magnesium-based materials: CO2 sequestration and utilization, mechanical properties and environmental impact

Cement industry is one of the main sources of greenhouse gases, which accounts for about 7% of global CO2 emissions. Sequestration of CO2 in cement-based materials is regarded as an effective alternative since it can convert CO2 into stable carbonates with relatively low additional energy consumption. This paper presents a comprehensive review on recent research and advances on carbon sequestration in magnesium-based binders with a focus on reactive MgO cement (RMC) and magnesium oxychloride cement (MOC). The carbon sequestration mechanism, the influence of carbonation on mechanical performance, and key parameters that control the carbonation process are summarized. In addition, a quantitative analysis of carbon sequestration in RMC-based materials is presented, demonstrating the effectiveness of offsetting carbon emissions via the use of alternative binder systems. Furthermore, a comparison of the environmental impact of Portland cement, RMC and MOC production is provided, emphasizing the need for enhancing the sustainability of cement production procedures. Overall, this paper presents a roadmap for emerging carbonation techniques that improve the mechanical performance and sustainability of magnesium-based binders.

New frontiers in sustainable cements: Improving the performance of carbonated reactive MgO concrete via microbial carbonation process

The low conversion of reactive MgO cement (RMC) into hydrated magnesium hydroxy carbonates (HMHCs) due to limited CO2 diffusion towards the sample core leads to inefficient RMC use and low strength development in RMC concrete. This study proposed a novel technique to enhance hydration and carbonation at both the exterior and core sections of RMC concrete via a synergistic combination of microbial carbonation process (MCP) with nucleation seeding (S). The production of CO32− ions via the introduction of urease-producing bacteria to catalyze the hydrolysis of urea enabled self-carbonation of RMC, leading to the formation of HMHCs under ambient conditions. Improvements in CO2 dissolution and formation of brucite with a low crystallinity further stimulated HMHC formation under accelerated carbonation conditions. The simultaneous use of MCP and S resulted in dense microstructures composed of HMHCs with improved morphologies, translating into strengths that were >3 times of the control (62 vs 20 MPa).

Could reactive MgO cement be a green solution? The effect of CO2 mineralization and manufacturing route on the potential global warming impact

Reactive MgO (rMgO) is considered a promising alternative to Portland cement (PC), because its capacity to mineralize carbon dioxide (CO2) can be used to reduce the life cycle global warming intensity (GWI) of concrete. However, the greenhouse gases (GHG) emitted during the manufacture of rMgO may outshine its environmental benefits of CO2 mineralization. The objective of this study is to compare the life cycle GWI of traditional PC binder to rMgO binder and rMgO-PC blended binder systems. The life cycle assessment (LCA) considers the actual amount of CO2 mineralized by the binders and their 28-day compressive strength, as well as different manufacturing routes (dry and wet) and energy sources (fossil fuels and electricity) to produce rMgO. Based on these variables, best- and worst-case scenarios were identified and analyzed. The results reveal that the quantity of CO2 stored in hydrated Mg carbonates increases as the rMgO replacement level increases, while the quantity of CO2 stored in Mg-calcite decreases. Mortar with rMgO-PC as binder exhibits a lower net GWI per unit of strength than mortars with either 100% PC or 100% rMgO as binder in the best-case scenarios. Using low carbon energy sources (such as renewables and nuclear) for rMgO production and on-site waste CO2 would significantly reduce the overall GHG emissions (by 82%). In the optimal situation, a net GWI reduction of almost 50% per unit of strength can be achieved by the rMgO-PC mortar. The wet route to produce rMgO looks particularly promising for a future decarbonized construction sector since no CO2 is chemically released during its manufacture. The properties and GWI of concrete containing rMgO carbonated in flowing CO2-rich gas warrant future study.

Advances in the hydration of reactive MgO cement blends incorporating different magnesium carbonates

Hydration of reactive magnesia cement (RMC) is limited by the formation of a Mg(OH)2 surface-layer on unreacted MgO particles. This study improved RMC hydration by using magnesium acetate (MA), along with hydromagnesite (H) and magnesite (M) as RMC replacements. While MA accelerated hydration and resulted in the formation of needle-like artinite, inclusion of M led to rosette-like crystals. The accumulated nucleation and growth of low-crystallinity Mg(OH)2 on H particles in a bird nest-like arrangement was observed for the first time in literature. This low-crystallinity Mg(OH)2 could be prone to carbonation. The replacement of up to 40% RMC with M in the presence of MA improved the compressive strength of RMC samples by 240%. This performance enhancement was supported by microstructure densification via the compact formation of hydrate and carbonate phases, defining M as a feasible partial RMC substitute.

Investigation of chloride penetration in carbonated reactive magnesia cement mixes exposed to cyclic wetting–drying

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.

Use of microbial carbonation process to enable self‑carbonation of reactive MgO cement mixes

The low hydration and carbonation of reactive MgO cement (RMC) under ambient conditions causes prolonged setting and low compressive strengths (~4 MPa). This study proposed a unique technique which led to the enhancement of the hydration and carbonation processes via the synergistic combination of microbial carbonation process (MCP) with a hydration agent (HA) that enabled the self‑carbonation of RMC-based mixes without using of any special curing environments. Through hydrolysing urea (CO(NH2)2) using ureolytic bacteria, CO32− ions were produced to facilitate the carbonation of dissolved Mg2+ ions to form hydrated magnesium hydroxy carbonates (HMHCs). The self‑carbonation of RMC enabled by the MCP resulted in formation of brucite with a poor crystallinity and its rapid conversion into HMHCs, which improved the setting time and compressive strength of RMC-based samples. The simultaneous use of MCP with 2 M urea and HA revealed HMHCs with improved morphologies, resulting in the highest compressive strength (~15 MPa).

Effect of calcination temperature on the light burned MgO matrix and its physical properties

ABSTRACT Light-burned MgO cement has a lower calcination temperature than ordinary Portland cement. It has been widely studied as a measure to reduce carbon dioxide because of its property of absorbing carbon dioxide during curing. This study investigated the effects of calcination temperature on the physical properties of light hydrated magnesium carbonate and calcined MgO hydrated in moisture and CO2 at 25 °C and 60 °C. The crystal size of light-burned MgO increased with increasing calcination temperature, and carbonates were formed through carbonation curing. Further, nesquehonite and hydromagnesite were formed in the 25CC and 60CC specimens, respectively, and the carbonate formation reduced with increasing crystal size. The highest compressive strength of 3.5 MPa was obtained for the 25CC specimen in which nesquehonite was formed; however, hydromagnesite exhibited better CO2 sequestration capacity.

Production of reactive magnesia from desalination reject brine and its use as a binder

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.

Magnesia (MgO) Production and Characterization, and Its Influence on the Performance of Cementitious Materials: A Review.

This paper presents a literature review concerning the characteristics of MgO (magnesium oxide or magnesia) and its application in cementitious materials. It starts with the characterization of MgO in terms of production processes, calcination temperatures, reactivity, and physical properties. Relationships between different MgO characteristics are established. Then, the influence of MgO incorporation on the properties of cementitious materials is investigated. The mechanical strength and durability behaviour of cement pastes, mortars and concrete mixes made with MgO are discussed. The studied properties of MgO–cement mixes include compressive strength, flexural strength, tensile strength, modulus of elasticity, water absorption, porosity, carbonation, chloride ion penetration, shrinkage, expansion, and hydration degree. In addition, microscopic analyses of MgO-cement mixes are also assessed. Summarizing the results of different studies, it is concluded that MgO incorporation in cementitious materials generally decreases the mechanical strength and shrinkage, and increases the porosity, expansion, carbonation and chloride ion migration. However, it should be emphasized that the properties of the specific MgO used (mainly the calcination temperature, the reactivity and the surface area) have a significant influence on the characteristics of the cementitious materials produced.