Reactive Magnesia Research Articles

Solidification of waste excavation clay using reactive magnesia, quicklime, sodium carbonate and early-age oven curing

Kaolin production requires large amounts of excavation waste to be removed, with each tonne of kaolin recovered typically producing up to 9 tonnes of waste. This study introduced quicklime (i.e., CaO), reactive magnesia (i.e., MgO) and sodium carbonate, together with an early-age oven curing regime as a means of re-using kaolin excavation waste to produce more sustainable cementitious materials. The strength development of solidified clay samples was measured and later interpreted by pH tests, porosity measurement, Fourier-Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy-Energy-dispersive X-ray spectroscopy (SEM-EDX). Besides, the environmental impacts with respect to the production of additives for clay solidification were also calculated. The results indicated that the incorporation of CaO and MgO effectively solidified the clay with acceptable compressive strength (CaO group: 12.2 MPa; MgO group: 20.3 MPa after 28 days). Meanwhile, given the strength development of samples investigated in this study, the morphology of hydration phases could be possibly more important than their contents and sample porosity. Furthermore, with the introduction of 3 days of oven curing, MgO took advantage over CaO in clay solidification, which was attributed to the formation of fibrous crystals (i.e., nesquehonite) and fewer micro-cracks. Finally, from a perspective of ‘greenness’ and sustainability, MgO is a more favorable additive than CaO and PC in soil solidification.

Rapid boron removal from wastewater using low-crystalline magnesium oxide

Abstract Boron has applications in industries such as the glass and semiconductor industries. However, boron is toxic and has a significant effect on agriculture, and wastewater contaminated with boron should be treated before being discharged. Magnesium oxide (MgO) can remove boron but has not been used much in practical wastewater treatment systems because it requires between several hours and several days to immobilize boron. We have discovered a method to rapidly remove boron from wastewater by using highly reactive MgO. The MgO, in the form of low-crystalline magnesium oxide (LC-MgO), was obtained from basic magnesium carbonate (BMC) by low-temperature calcination at 500–700 °C, while normal MgO is usually formed at over 1000 °C. Solid analyses showed that LC-MgO had lower crystallinity and a higher specific surface area than normal MgO, both of which helped LC-MgO immobilize boron faster than normal MgO. Studies of the reaction kinetics showed that boron removal by MgO was a three-step process; (i) magnesium ions from MgO first dissolve into the aqueous phase, (ii) the dissolved magnesium ions form complexes with borate ions in the wastewater, (iii) these complexes precipitate out of solution as hydroxides when they react with free borate ions. The kinetic model also revealed that the higher surface area of LC-MgO enhanced the rate of dissolution and precipitation of magnesium ions, which increased the rate of boron removal.

Influence of Early Oven Curing on the Alkali Activated Binders with Reactive Magnesia Replacement

Magnesia is widely used an expansive agent in construction materials due to its delayed hydration, which will compensate for the thermal shrinkage. This study investigates the influence of magnesia and early oven curing on the mechanical performance and microstructures of alkali-activated natural clay samples under different curing regimes after 28 days. Microstructural analysis and pH measurements were conducted to assess the strength of the samples. Results indicate that the strength was greatly improved (in comparison with the control samples) by the incorporation of magnesia due to the formation of nesquehonite. Meanwhile, oven curing is an effective method in the fast strength development in alkali-activated natural clay. The results also demonstrated the possibility of CO2 curing in the strength development of the magnesia-modified alkali-activated natural clay.
 Keywords: magnesia, natural clay, oven curing, compressive strength, microstructure

Efficacy of green alternatives and carbon dioxide curing in reactive magnesia cement-bonded particleboards

This study demonstrated a low-carbon and value-added approach for upcycling waste wood formwork into reactive magnesia cement-bonded particleboards, in which green alternatives and carbon dioxide curing were employed to improve mechanical properties and reduce carbon footprint. Spectroscopic and thermogravimetric analyses showed that the replacement of pulverised fly ash and incinerated sewage sludge ash promoted the generation of magnesium silicate hydrates gel via pozzolanic-like reaction. The incorporation of mussel shell densified the microstructure as illustrated from scanning electron microscopy. Therefore, these modified particleboards presented satisfying flexural strength (>9 MPa) and thickness swelling (<2%). However, the spontaneous hydration of blast furnace slag manifested a competition against magnesia cement hydration and resulted in strength reduction. Furthermore, carbon dioxide curing promoted the transformation from magnesium hydroxide to additional magnesium carbonate and hydrated magnesium carbonates, which significantly enhanced early strength. However, the carbonation process consumed magnesium hydroxide, unavoidably suppressing the pozzolanic reaction. By contrast, blast furnace slag-incorporated samples showed synergetic hydration and carbonation. In summary, pulverised fly ash materials are promising substitutes in air curing condition, whereas blast furnace slag is suitable for accelerated carbonation of magnesia cement-based particleboards.

3D printable magnesium oxide concrete: towards sustainable modern architecture

Reactive magnesium oxide cement (RMC) is gaining increasing attention as a sustainable construction material due to its significantly low carbon footprint during the production as well as the operational phase compared to the conventional Portland cement. Whereas several studies have demonstrated the potential of RMC as a suitable and environment-friendly construction material, this study reports that RMC can be shaped into complex structures via three-dimensional (3D) printing technology. By adding suitable additives and only 3 wt. % of caustic magnesium oxide to the commercially available RMC, appropriate rheology and buildability were achieved that enabled smooth 3D printing of complex structures with precise shape retention. Moreover, the 3D printed RMC exhibited higher densification and nearly twofold the compressive strength as compared to its cast counterpart. Therefore, this work demonstrates the potential of RMC as a 3D printable construction material for sustainable and modern architecture.

Hydraulic conductivity characteristics of carbonated reactive magnesia-treated silt

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.

Hydraulic conductivity and self-healing performance of Engineered Cementitious Composites exposed to Acid Mine Drainage.

Engineered Cementitious Composite (ECC) is proposed as a promising vertical cutoff wall material to contain acid mine drainage (AMD). The study presents comprehensive investigations of hydraulic conductivity of ECC permeated with AMD and self-healing of ECC subjected to wet-dry cycles. The effectiveness of incorporating reactive magnesia (MgO) into ECC for self-healing enhancement is also investigated. The chemical species formed in ECC and MgO-ECC specimens after exposure to AMD are investigated via SEM, FTIR, XRD and TGA analyses. The results show hydraulic conductivity of un-cracked and cracked ECC and MgO-ECC specimens pre-strained up to 1.32% is below commonly accepted limits of 10-8m/s when permeated with AMD. The self-healing capacity of ECC specimens subjected to wet-dry cycles using both tap water and AMD as immersing liquids is improved by MgO addition. MgO addition is also beneficial for reducing hydraulic conductivity of un-cracked and cracked ECC specimens permeated with AMD. MgO addition results formation of new self-healing products including hydromagnesite and brucite when exposed to tap water, and hydrotalcite-like phase (Ht) when exposed to AMD.

Influence of accelerated hydration and carbonation on the performance of reactive magnesium oxide concrete

The low dissolution of magnesium oxide (MgO) limits the performance of carbonated reactive magnesium oxide cement (RMC)-based formulations. The initial acceleration of hydration (IAH) of RMC samples involving the use of sodium bicarbonate (NaHCO3) (SBC) was enabled by way of the adjustment of curing conditions. The influence of IAH and SBC on hydration kinetics was evaluated by isothermal calorimetry, hydration degree and pH measurements. Mechanical performance results were supported by X-ray diffraction, thermogravimetric analysis and scanning electron microscopy, which identified the amount and morphology of the final phases. The use of IAH increased Mg2+ concentration and enhanced hydration and carbonation at early ages. The introduction of SBC increased CO32− concentration by providing higher initial pH and additional HCO3− that stimulated carbonation at later ages. RMC samples involving IAH and SBC outperformed others, resulting in ∼130% higher 28 d compressive strengths than the control. This increased performance was associated with improvements in the morphology and amount of carbonates, which were assessed by thermal analysis.

A comparative assessment of chemical stabilisers including waste materials, for the treatment of swelling-shrinking soils

This paper assesses comparatively the performance of a number of innovative soil stabilisers for the treatment of a highly swelling-shrinking soil, against that of commercial calcium lime. The production of lime, a most common soil stabiliser, involves high energy consumption, carbon dioxide emissions and the depletion of natural raw materials. Alternatives are actively sought, in particular industrial wastes and by-product materials or lower energy demand cements e.g. reactive magnesia (MgO) cements. In this paper calcium lime, reactive magnesia, industrial wastes and mixes of these with lime are comparatively assessed, based on a number of conventional measures of the propensity of a soil to swell, i.e. plasticity characteristics and swelling characteristics (swelling strains, swelling pressures, swelling indices). Furthermore, as expansive soils are typically in an unsaturated state hence sensitive to both changes in water content and suction, filter paper testing was performed to provide additional evidence of the effect of the treatments on the swelling/shrinking soil. According to the main findings, for the treatment of swelling shrinking soils, binders coming from the paper recycling industry show most promise as alternatives to lime; reactive magnesia cement had a smaller effect than calcium based stabilisers in improving the swelling-shrinking of the soil, yet it also suppressed swelling and shrinkage considerably; it thus shows potential for use as an alternative to common soil stabilisers (Portland cement and calcium lime) to alleviate the environmental impact of the latter.

A Review of Carbonated Reactive MgO-stabilized Soil

The application of new reactive magnesium oxide (MgO) binder in ground improvement has become a research hotspot. This paper summarized the latest research about the mechanical property potential of carbonated reactive MgO-stabilized Soil, described the electrical characteristics and permeability characteristics of solidified soil research results, analyzed the durability and corrosion resistance of solidified soil research, introduced the exploration of new curing agent engineering application measures. In view of the existing studies, further research about the relationship between the indicators of carbonation and unconfined compressive strength (UCS) were suggested, comprehensively study the corrosion resistance of the solidified soil, systematically study the reaction mechanism of fly ash with reactive MgO, and improve the field test of reactive MgO-carbonized mixing piles have been suggested.

Feasibility of Stabilized Zn and Pb Contaminated Soils as Roadway Subgrade Materials

The authors have developed a new binder, KMP, which is made from oxalic acid‐activated phosphate rock, monopotassium phosphate (KH2PO4), and reactive magnesia (MgO). This study explores the acid neutralization capacity, strength characteristics, water‐soaking durability, resilient modulus, and pore size distribution of KMP stabilized soils with individual Zn, Pb, or coexisting Zn and Pb contaminants. For comparison purpose, Portland cement (PC) is also tested. The results show that KMP stabilized soils have a higher acid buffering capacity than PC stabilized soils, regardless of the soil contamination conditions. The water stability coefficient and resilient modulus of the KMP stabilized soils are found to be higher than PC stabilized soils. The reasons for the differences in these properties between KMP and PC stabilized soils are interpreted based on the stability and dissolubility of the main hydration products of the KMP and PC stabilized soils, the soil pore distribution, and concentration of Mg or Ca leached from the KMP and PC stabilized soils obtained from the acid neutralization capacity tests. Overall, this study demonstrates that the KMP is effective in stabilizing soils that are contaminated with Zn or Pb alone and mixed Zn and Pb contaminants, and the KMP stabilized soils are better suited as roadway subgrade material.

Towards a clean environment: The potential application of eco-friendly magnesia-silicate cement in CO2 sequestration

Abstract The key point of this study is the fabrication of magnesia-based cement with promising mechanical properties and high efficiency of CO2-capture. The naturally occurring volcanic ashes (white & red ashes) and reactive magnesium oxide are the main materials used in the synthesis of eco-friendly CO2-capture materials. Volcanic ashes were individually mixed with reactive magnesium oxide at ash to magnesium oxide ratio of 25:75 wt %. The dry blends can react with water to yield hardened materials (at ambient temperature) with compressive strength depends on the type of volcanic ash. A considerable change in the features of the hardened samples was recorded when the fabricated materials exposed to 100% CO2 gas for 28-days. This change is mainly due to CO2-capture by magnesium hydroxide Mg(OH)2 within the fabricated materials, resulting in the formation of Nesquehonite minerals MgCO3.3H2O as proved by X-ray diffraction, thermo-gravimetric, and infra-red instrumental techniques. The thermo-gravimetric analysis demonstrates that, the fabricated sample containing low amorphous red ashes has higher CO2-capture capacity (∼260 kg/ton) compared to that having high amorphous white volcanic ashes (∼220 kg/ton) at 28-days of CO2-exposure. Accordingly, the fabricated magnesia-based cement is not only used as cementitious material with outstanding mechanical properties, but also used as a super CO2-absorbent precursor. This can strongly contribute in the mitigation of global warming potential caused by different industrial activities.

Environmental Assessment of Magnesium Oxychloride Cement Samples: A Case Study in Europe

This study is the first in the literature to systematically assess the environmental impacts of magnesium oxychloride cement (MOC) samples, which are regarded as a more eco-friendly construction material than Portland cement. The environmental impacts of MOC samples prepared with various molar ratios of MgO/MgCl2∙6H2O and sources of reactive magnesia were obtained via a life cycle assessment (LCA) approach (from cradle to grave), and the obtained outcomes were further compared with the counterparts associated with the preparation of Portland cement (PC) samples. Meanwhile, a sensitivity analysis in terms of shipping reactive magnesia from China to Europe was performed. Results indicated that the preparation of MOC samples with higher molar ratios led to more severe overall environmental impacts and greater CO2 sequestration potentials due to the difference of energies required for the production of MgO and MgCl2∙6H2O as well as their various CO2 binding capacities, whereas in terms of CO2 intensities, the molar ratios in MOC samples should be carefully selected depending on the strength requirements of the applications. Furthermore, various allocation procedures and MgO production processes will greatly influence the final outcomes, and allocation by mass is more recommended. Meanwhile, the environmental impacts associated with the transportation of reactive magnesia from China to Europe can be ignored. Finally, it can be concluded that MOC concrete is no longer a type of ‘low-carbon’ binder in comparison with PC concrete in terms of CO2 emissions, and in view of the single scores and mixing triangles for weighing, MOC concrete can only be identified as a type of more sustainable binder than PC concrete when the main component MgO in MOC samples is obtained through the dry process route rather than the wet process route.

Mechanical and compressibility characteristics of a soft clay stabilized by slag-based mixtures and geopolymers

In the present study, industrial wastes such as Granulated Blast Furnace Slag (GBFS) and Basic Oxygen Furnace Slag (BOFS) activated with calcium oxide (CaO) and medium reactive magnesia (MgO) are used for chemical stabilization of a soft clay. This environmentally friendly approach can eliminate the hazards associated with improper waste disposal and reduce greenhouse gas emissions generated by cement production. To this end, various amounts of additives (ranging from 2.5 to 20%) with the activator:slag ratio of 1:3 are added to kaolinite clay and cured at two temperatures of 20 and 45°C. A series of laboratory tests, including pH, Electrical Conductivity (EC), one-dimensional consolidation, Unconfined Compressive Strength (UCS), are conducted on the stabilized samples. The increasing temperature causes a faster formation of cementitious products and a higher UCS value, as confirmed by the SEM micrographs and XRD analysis, particularly in the case of MgO-BOFS (MB) and CaO-BOFS (CB) samples with the UCS values of 4 and 4.7 MPa after 90 days of curing, respectively. Furthermore, the MB- and CB-stabilized clay samples show a better compressibility characteristic compared to the MgO-GBFS (MG) and CaO-GBFS (CG) blends. To further enhance the activity of the additives and to prepare a slag-clay based geopolymers, two types of alkaline solutions at various Na2SiO3:NaOH ratios are added to the mixtures at the final step. In addition, the energy absorption capacity (Eu) and the secant modulus (E50) of the optimum blends are determined to assess the toughness and stiffness of the samples. The results indicate that the UCS values of the MB and CB samples increase up to 7.41 MPa and 8.44 MPa after 90 days of curing, respectively, when the Na2SiO3:NaOH ratio is 80:20. Generally, the use of slag-clay based geopolymers, particularly BOFS, is very effective to address the problems associated with the soft soil and the optimum mixtures are successful in decreasing settlements and enhancing compressive strength of the soft soil, which can be considered for use as a pavement base material.

Accelerated carbonation of reactive magnesium oxide cement (RMC)-based composite with supercritical carbon dioxide (scCO2)

Reactive magnesium oxide cement (RMC) has the potential to become a sustainable alternative to ordinary Portland cement (OPC). In this study, an approach utilizing supercritical CO2 (scCO2) was investigated to accelerate carbonation of an RMC-based composite and to overcome its long carbonation process under the natural environment. It was found that scCO2 led to an extremely rapid strength gain of the composite, with a mature strength level achievable within a period of hours. CO2 sequestration factors were also increased by three folds as compared to samples cured under a 20% CO2 concentration environment for 28 days. It was also revealed that the carbonation phases under scCO2 were dominated by nesquehonite followed by hydromagnesite and some other intermediate hydrated magnesium carbonates (HMCs). More uniform carbonation within the matrix was also attained under the scCO2 condition. Despite the promising outcomes, technical and cost challenges would need to be resolved before a possible scale-up.

Performance of reactive magnesia cement formulations containing fly ash and ground granulated blast-furnace slag

This study investigated the performance of carbonated reactive MgO cement (RMC) and sealed MgO-SiO2 (RMS) concrete formulations. Fly ash (FA) and ground granulated blast-furnace slag (GGBS) replaced half of the binder component in each system. The assessment of hydration mechanisms via isothermal calorimetry was supported by performance evaluation involving the measurement of permeable pores and compressive strength. Samples were also studied via x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and field emission scanning electron microscopy (FESEM). The formation of a continuous carbonate network was responsible for the strength development of RMC samples, whereas hydrate phases such as M-S-H were observed in RMS samples. Partial replacement of RMC with GGBS led to similar performances under carbonation. RMS samples revealed comparable strengths to PC-based samples, highlighting the potential of MgO-SiO2 binders to be used in similar applications without requiring any special curing conditions.

Mechanical and durability performance of mortars with fine recycled concrete aggregates and reactive magnesium oxide as partial cement replacement

In this paper, mortar specimens were produced using two types of commercially-available reactive MgO as partial cement replacement (10%, 15% and 20%, by weight) and fine recycled concrete aggregate as siliceous sand substitute (50% and 100%, by volume). The specimens were subjected to thermogravimetric, energy dispersive X-ray, differential thermal and powder X-ray diffraction analyses. The mechanical and durability performance of all specimens was evaluated in terms of their flexural and compressive strength, dynamic modulus of elasticity, water absorption by capillary action, carbonation, and shrinkage. The main results indicate an overall decline in mechanical and durability-related performance with the use of both MgO and fine recycled concrete aggregates, but enhanced shrinkage behaviour was observed in all MgO-containing specimens.

Highly Reactive Magnesia Production: Modeling and Experiment

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.

Effects of Drying-Wetting Cycles on Durability of Carbonated Reactive Magnesia-Admixed Clayey Soil

AbstractThe 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...

Effect of MgO Activity Index on Physicochemical, Electrical and Mechanical Properties of Carbonated MgO-admixed Silt

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.