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Hexavalent chromium (Cr6+) is a toxic carcinogenic pollutant that might be released by the mining and processing of ultramafic rocks and nickel laterites and which requires permanent removal from the contaminated biosphere. Ultramafic material can also serve as a feedstock for the sequestration of CO2 resulting from the growth of new minerals, raising the intriguing proposition of integrated sequestration of both pollutants, CO2 and chromium, into magnesium carbonates. Such a synergistic process downstream of ore recovery and mineral processing could be an elegant proposition for more sustainable utilisation and management of the Earth's resources. We have therefore carried out an experimental and microanalytical study to investigate potentially suitable carbonate minerals. Uptake of chromium in carbonate phases was determined, followed by identification of the crystalline phases and characterisation of the local structural environment around the incorporated chromium centres. The results suggest that neither nesquehonite nor hydromagnesite have the structural capacity to incorporate Cr6+ or Cr3+ significantly at room temperature. We therefore propose that further research into this technology should focus on laboratory assessments of other phases, such as layered double hyroxides, that have a natural structural capacity to uptake both chromium and CO2. [ABSTRACT FROM AUTHOR]

A novel process is reported that produces amorphous silica and nesquehonite (MgCO3·3H2O) from the magnesium silicate mineral olivine ((Mg, Fe)2·SiO4). The amorphous silica forms a supplementary cementitious material for use in concrete. The formation of nesquehonite sequesters carbon making the overall process carbon negative. Nesquehonite can also be used to form low-carbon construction products such as bricks, blocks and boards. This article reports on key process optimization studies. The potential for amorphous precipitated silica derived from olivine to produce carbon-negative concrete is discussed.

The result of a complex mineralogical study of the first discovery of the rare hydrated magnesium carbonate minerals of Nesquehonite and Dypingite in the Obnazhennaya kimberlite pipe (of the Yakutian kimberlite province) is presented. The methods of X-ray phase analysis, electronic microscopy, and Raman spectroscopy have established that the main minerals of the samples found in the form of white crust on a small area of rock outcrop of kimberlite breccia are hydrated carbonates: Nesquehonite is MgCO3•3H2O, Dypingite is Mg5(CO3)4(OH)2•5H2O. The formation of Dypingite over Nesquehonite was shown using Raman imaging for the first time. Nesquehonite is represented as aggregates consisting of chaotically oriented prismatic crystals or kidney-shaped formations. Dypingite in the examined samples appears less frequently as rose-shaped aggregates formed from lamellar crystals. It is assumed that the formation of rare carbonates of the Obnazhennaya kimberlite pipe is mainly the result of the weathering of silicates, formation of mineralized solutions, and their subsequent crystallization, including the capture of CO2 from the air. [ABSTRACT FROM AUTHOR]

Reactive magnesia cement is considered an eco-efficient binder due to its low synthesis temperature and CO2 absorption properties. However, the hydration of pure MgO–H2O mixtures cannot produce strong Mg(OH)2 pastes. In this study, nesquehonite (Nes, MgCO3·3H2O) was added to the MgO–H2O system to improve its strength properties, and its hydration products and pore structure were analyzed. The experimental results showed that the hydration product changed from small plate-like Mg(OH)2 crystals to interlaced sheet-like crystals after the addition of a small amount of Nes. The porosity increased from 36.3% to 64.6%, and the total pore surface area increased from 4.6 to 118.5 m2/g. At the same time, most of the pores decreased in size from the micron scale to the nanometer scale, which indicated that Nes had a positive effect on improving the pore structure and enhancing the compressive strength. Combined with an X-ray diffractometer (XRD), a Fourier transform infrared spectrometer (FTIR), and a simultaneous thermal analyzer (TG/DSC), the hydration product of the sample after Nes addition could be described as xMgCO3·Mg(OH)2·yH2O. When Nes was added at 7.87 and 14.35 wt%, the x-values in the chemical formula of the hydration products were 0.025 and 0.048, respectively. These small x-values resulted in lattice and property parameters of the hydration products that were similar to those of Mg(OH)2. [ABSTRACT FROM AUTHOR]

Carbonate minerals are major contributors to carbon sequestration in geological deposits; however, their nature and behavior remain unclear. Amorphous magnesium carbonate (AMC) is formed as a precursor to crystalline magnesium carbonates and as a product of thermal decomposition of nesquehonite (NSQ). In this study, the AMCs formed during the crystallization and decomposition of NSQ were investigated using X-ray diffraction (XRD) and atomic pair distribution function (PDF) methods. An AMC with a hydromagnesite-like structure (AMC-I) was formed immediately after mixing MgCl2 and Na2CO3 solutions. After 5 min of stirring, no change was observed in the XRD pattern; however, the PDF pattern changed. This suggests that the medium-range ordered structure of AMC-I transformed into an intermediate structure (AMC-II) between AMC-I and NSQ. After 10 min of stirring, the AMC-II crystallized into NSQ. In the case of Rb2CO3, the AMC-II structure was formed immediately after the mixing of solutions and was stable for three days. AMC-II in the Rb2CO3 solution appeared to be in equilibrium with energetic local minima, indicating the existence of polyamorphism in AMC. When Cs2CO3 solution was used, the first precipitate had an AMC-I structure. By stirring for 5 min, the AMC-I was transformed to AMC-II, and after 10 min of stirring, a few quantities crystallized into NSQ. After three days, NSQ dissolved and transformed back into AMC-I. Thus, it is inferred that the crystallization of NSQ is significantly influenced by alkali cations in aqueous solutions. The AMC formed during the thermal decomposition also possesses the AMC-I structure. [ABSTRACT FROM AUTHOR]

Nesquehonite is a magnesium carbonate mineral relevant to carbon sequestration envisioned for carbon capture and storage of CO2. Its chemical formula remains controversial today, assigned as either a hydrated magnesium carbonate [MgCO3 ⋅ 3H2O], or a hydroxy‐ hydrated‐ magnesium bicarbonate [Mg(HCO3)OH ⋅ 2H2O]. The resolution of this controversy is central to understanding this material's thermodynamic, phase, and chemical behavior. In an NMR crystallography study, using rotational‐echo double‐resonance 13C{1H} (REDOR), 13C−1H distances are determined with precision, and the combination of 13C static NMR lineshapes and density functional theory (DFT) calculations are used to model different H atomic coordinates. [MgCO3 ⋅ 3H2O] is found to be accurate, and evidence from neutron powder diffraction bolsters these assignments. Refined H positions can help understand how H‐bonding stabilizes this structure against dehydration to MgCO3. More broadly, these results illustrate the power of NMR crystallography as a technique for resolving questions where X‐ray diffraction is inconclusive. [ABSTRACT FROM AUTHOR]

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.

Microalgae cultivation is considered an attractive negative emission technology (NET) due to its ability to remove carbon dioxide (CO2) from the atmosphere as well as from flue gases. Moreover, some microalgae can uptake dissolved carbon in the form of bicarbonate (HCO3-) and grow well by removing nutrients from various wastewater effluents without competing with freshwater resources. Conventional carbon mineralization is another NET where carbon is fixed into carbonate minerals such as calcite (CaCO3), magnesite (MgCO3), nesquehonite (MgCO3-3H2O), and nahcolite (NaHCO3), which can be reused in various applications such as construction and cosmetics, and in this case, to supply carbon to microalgae. Previous initial laboratory studies demonstrated the possibility of using metastable carbonates as a source of carbon for biomass growth of the freshwater microalga Scenedesmus obliquus. In this study, we present results from an experimental and modeling work where 100 L open raceway reactors were used to grow an algae polyculture using industrial nitrogen-rich wastewater. In addition, carbonates such as MgCO3-3H2O and sodium carbonate (Na2CO3) were added to the ponds to primarily buffer the pH and maintain it within the acceptable algae physiological values and to provide extra carbon to the cultures. Cultures using BG-11 growth medium and without addition of carbonates were run as controls. Continuous online monitoring of pH, temperature, and dissolved oxygen was performed while nutrients content and dry weight of algae culture were measured offline daily. A mathematical model coupled with geochemistry was implemented to describe the overall process of algae growth, carbonate dissolution, and solution composition. Biomass harvested from the raceway reactors was concentrated and oil and fatty acid methyl esters (FAME) content were determined. Results showed that biomass and oil content obtained were comparable to controls, indicating a successful metabolic-based pH control by the addition of carbonates in the ammonium-based media. Furthermore, FAME analysis revealed different profiles depending on the source of carbon used. Overall, this study suggests that integrating both types of NET strategies can contribute to the reduction of carbon emissions while producing biomass that can be further processed to generate a variety of bioproducts.

• Mg and C isotope fractionation between hydrous Mg-carbonate minerals measured. • If dypingite forms from aqueous Mg2+, it would be preferentially enriched in 26Mg. • Isotopic composition of Mg-carbonate overwitten during mineral phase transformations. • Applications for using Mg and C isotopes as tracers of carbon capture and storage. The fractionation of carbon and magnesium isotopes is a potentially useful tracer of natural weathering in ultramafic catchments and engineered CO 2 storage. To evaluate the use of carbon and magnesium isotopes as tracers of ultramafic weathering and CO 2 storage, we assessed the carbon and magnesium isotope fractionation between hydrous Mg-carbonate minerals and fluid during a mineral phase transformation from nesquehonite [MgCO 3 ·3H 2 O] to dypingite [Mg 5 (CO 3) 4 (OH) 2 · ∼ 5–8H 2 O], two common products of ultramafic rock weathering. Batch reactor experiments containing nesquehonite were conducted at 5 °C, 25 °C, and 35 °C and the evolution of mineralogical composition, fluid composition, and isotopic composition were tracked over time. At 5 °C, the solid remained nesquehonite throughout the experiments, and isotopic equilibrium did not appear to be achieved between the solid and the fluid phase for either carbon or magnesium. At 25 °C, and 35 °C a transformation from nesquehonite to dypingite occurred by dissolution and re-precipitation, which resulted in extensive exchange of Mg and C between solid and fluid. The phase transformation caused the initial C and Mg isotopic composition of the solid phase to be overwritten. The extensive isotopic exchange during the phase transformation suggests C and Mg isotopes likely obtained approximate isotopic equilibrium between dypingite and fluid. For dypingite, the Δ13C dyp-DIC was 4.74 ± 0.12‰ (VPDB) and 4.47 ± 0.17‰ (VPDB) at 25 and 35 °C, respectively. The Δ meas 26 M g dyp - f l u i d between solid and the bulk fluid was −0.76‰, and −0.98 ± 0.08‰ for the 25 and 35 °C experiments, respectively. There was no clear impact of temperature on Mg or C isotope fractionation. The calculated Δ calc 26 M g dyp - M g 2 + between dypingite and the Mg2+ aquo species rather than bulk aqueous Mg values were positive. This indicates that if dypingite is formed by the incorporation of the free Mg2+ ion in the solid, the solid is preferentially enriched in the isotope of higher mass (26Mg). This is opposite to anhydrous Mg-bearing carbonate minerals, which tend to be depleted in 26Mg relative to the forming fluid. These data will help improve interpretation of carbon and magnesium isotope compositions measured in natural and engineered ultramafic weathering environments, and may help to trace the fate of anthropogenic CO 2 during engineered CO 2 storage efforts. [ABSTRACT FROM AUTHOR]

Nesquehonite synthesized by mixing MgCl2·6H2O and Na2CO3 at T 25°C, was thermally treated at various temperatures (170°C, 200°C, 295°C, 350°C, and 390°C), based on thermal analysis study of raw nesquehonite. The mineral phases formed at each temperature were studied by means of Raman microspectroscopy, Fourier transform infrared spectroscopy, and X‐ray diffraction. After thermal treatment of raw nesquehonite above 295°C, its dominant Raman peak at 1,100 cm−1, which corresponds to the v1 CO32− antisymmetric stretching vibrations, exhibits broadening and upshift of up to 10 cm−1, whereas the Raman peaks at 707 cm−1, at 770 cm−1, at 1,425 cm−1, and at 1,710 cm−1 disappear, and new Raman peaks arise at 1,282 cm−1 and at 1,386 cm−1. The latter have been attributed to the decomposition of HCO3−, in accordance with the Fourier transform infrared spectroscopy study. According to the differential thermal analysis study, water of crystallization is lost in two steps until 295°C; therefore, the phase studied above this temperature corresponds to Mg (HCO3)(OH). The Raman peak at 3,550 cm−1 assigned to the existence of OH−, exhibits weakening after thermal treatment. Our results corroborate a Mg (HCO3)OH·2H2O nesquehonite formula. [ABSTRACT FROM AUTHOR]

• The needle-like whisker has smooth side faces and irregular hexagonal end faces. • The (1 0 1) surface is the most stable surface and corresponds to the side face. • The states of surfaces are primarily dominated by Mg and O atoms. • A more active surface gets a larger value of transferred electrons after relaxation. By using the first-principles calculations, the structure, energies and electronic properties of four commonly exposed surfaces for the nesquehonite crystal were investigated. The needle-like nesquehonite whisker is well developed with smooth side faces and irregular hexagonal end faces. Surface energy results indicate that the (1 0 1) surface is the most stable surface and corresponds to the side face. The density of dangling bond has a positive relationship with surface energy and the (1 0 1) surface has the least dangling bonds. In terms of relaxed surface energy, the order of relaxed surfaces is (1 0 1) < (2 0 0)-H < (3 0 1) < (2 0 0)-M < (0 0 4). During surface relaxation, the changes in the length of Mg-O bonds and hydrogen bonds contribute to generating a more stable surface with a lower surface energy. The PDOS (partial density of states) of these surfaces are mainly dominated by Mg and O atoms. A small peak value is found in the PDOS of (1 0 1) and (3 0 1) surfaces, which have less exposed Mg-O bonds. Electron transfer causes changes in the length of Mg-O bonds. A more active surface will obtain a larger value of transferred electrons during surface relaxation. [ABSTRACT FROM AUTHOR]

The effect of alcohols (methanol, ethanol, n-propanol, and isopropanol) on the crystallization process of nesquehonite was investigated by combining experiments and molecular dynamics simulation. With the presence of alcohol, the transformation rate from Mg(HCO3)2 to MgCO3 · 3H2O can be accelerated apparently and the solubility of MgCO3 · 3H2O can be reduced significantly. X-ray diffraction (XRD) analysis indicates that Mg(HCO3)2 is unstable and can be transformed to pure MgCO3 · 3H2O easily. Based on the molecular dynamics simulation, we find that alcohol enhances the desolvation of Mg2+ through reducing the water molecules in the first solvation circle of Mg2+. Hence, the solvation energy could be reduced, which is beneficial for the precipitation of Mg2+. The results in this study provide theoretical foundations for the coupled reaction-extraction-alcohol precipitation process to fix CO2 and produce MgCO3 · 3H2O. [ABSTRACT FROM AUTHOR]

• The techno-economics of the production of a carbon-negative construction material has been studied in the context of Qatar. • The product requires CO 2 from power generation, brine from desalination and alkali. • Costs are relatively higher than the conventional product, but there are opportunities for cost-efficiency. In the context of the decarbonisation of the building sector, this work focuses on the process economics of a carbon capture and utilization alternative for the production of a novel construction material based on magnesium carbonate trihydrate, also known as nesquehonite. This material, after a dehydration-rehydration process, can be conformed into moulds with a significant compressive strength in a very similar mechanism to plasterboard manufacturing. This paper discloses the mass and energy balance of the whole process, where the carbon dioxide is captured from flue gas using aqueous alkaline absorption, and magnesium is sourced from desalination brines. The geographical scope of the economic assessment is the Middle East, specifically Qatar, where desalination and power generation are commonly coupled, and where the huge volume of construction materials imports provide an excellent framework for the development of competitive nesquehonite-based products. In fact, although the process economics show high operating costs, the final product would be economically competitive in the Qatari market at a unit cost around USD 410 per tonne, which is achievable at large scale. The sensitivity analysis shows that both alkali source and the process performance are key aspects of process economics. [ABSTRACT FROM AUTHOR]

• Silica and nesquehonite are produced by acid dissolution of olivine. • The silica formed is a viable supplementary cementitious material. • Nesquehonite formation by carbonation makes the process carbon negative. • Global reserve of olivine means this is a viable route to low-carbon concrete. Amorphous silica and hydrated magnesium carbonate (nesquehonite, MgCO 3 ·3H 2 O) have been produced by acid dissolution of olivine ((Mg,Fe) 2 ·SiO 4). The amorphous silica is a viable supplementary cementitious material, and the formation of nesquehonite by carbonation, results in a carbon negative process. Using the amorphous silica as a supplementary cementitious material produces low-carbon cement. The global reserve of olivine makes this a viable route to producing low-carbon concrete. [ABSTRACT FROM AUTHOR]

Synthetic nesquehonite with a Mg(HCO3)OH·2H2O chemical formula is a solid product of CO2 mineralization with cementitious properties. It constitutes an “MHCH” (magnesium hydroxy-carbonate hydrate) phase and, along with dypingite and hydromagnesite, is considered to be a promising permanent and safe solution for CO2 storage with potential utilization as a supplementary material in “green” building materials. In this work, synthetic nesquehonite-based mortars were evaluated in terms of their compressive strengths. Nesquehonite was synthesized by CO2 mineralization under ambient conditions (25 °C and 1 atm). A saturated Mg2+ solution was used at a pH of 9.3. The synthesized nesquehonite was subsequently studied by means of optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Impurity-free nesquehonite formed elongated fibers, often around a centerpiece, creating a rosette-like structure. The synthesized nesquehonite was mixed with reactive magnesia, natural pozzolan, standard aggregate sand and water to create a mortar. The mortar was cast into 5 × 5 × 5 silicone mold and cured in water for 28 days. A compressive strength of up to 22 MPa was achieved. An X-ray diffraction study of the cured mortars revealed the formation of brucite as the main hydration crystalline phase. Carbon dioxide mineralized nesquehonite is a very promising “green” building material with competitive properties that might prove to be an essential part of the circular economy industrial approach.

A sample of MgO was successfully synthesized using thermal decomposition of hydromagnesite and compared to commercial material. The characterization of materials using XRD, SEM, BET and BJH methods showed that the thermal decomposition way led to rectangular mesoporous microsheets with high specific surface area of 100 m2 g−1. This porous magnesium oxide has been shown to be a potential candidate for CO2 capture at low temperatures range (30 and 50 °C), low pressures of CO2 ( P CO 2 = 600 mbar ) and in the presence of water vapor ( P H 2 O = 15 mbar ). In these conditions, our results show that 11% of MgO was converted to hydrated magnesium carbonate MgCO3·3H2O after 8 h of carbonation in a thermobalance and reached 54% after 24 h of carbonation using tube furnace. After carbonation, hydration reaction pore size and surface area have noticeably changed. [ABSTRACT FROM AUTHOR]

Hydromagnesite with different morphologies has been synthesized using self-made nesquehonite whiskers as raw materials. The synthesized samples have been characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that porous rod-like hydromagnesite are generated at 328~353K and in the pH value of 9.30+0.2, while irregular flower-like and flat layered ones are synthesized in the pH values of 10.0+0.05 and 11.0+0.05, respectively. The yield of hydromagnesite improved linearly with the increase of the temperatures and solution pH values. Porous rodlike hydromagneiste crystals with good crystalline and uniform morphology are obtained when the pyrolysis time is over 60 min. Furthermore, the apparent activation energy of phase transformation is calculated to be 3.4080 kJ/mol. According to the results, the experimental data can be well described by the kinetic model, suggesting that the phase transfer rate is dependent on the temperature. [ABSTRACT FROM AUTHOR]

Mineral carbonation is an important technical option for the effective reduction of CO2 emissions. Natural magnesium‐containing minerals, such as serpentine and blast furnace slag (BFS), have recently been used for CO2 storage with an indirect carbonation route using ammonium sulfate. In this study, the effects of the feeding mode and process parameters on the magnesium conversion, product phase, and morphology during the aqueous carbonation of MgSO4 with ammonium carbonate solution were investigated in detail. The results showed that the carbonation ratio with a parallel feed was higher than the forward and reverse feed by about 3–5% with a limited reaction time, and the product size was more uniform. The phase and morphology of the products were affected significantly by the temperature. The highest carbonation ratio appeared at 40°C because only 75% of magnesium was carbonated if hydromagnesite was produced. When the mole ratio of (NH4)2CO3 to MgSO4 was 2:1 and the concentration of magnesium sulfate was higher than 0.4 mol·L−1, the carbonated products contain ammonium magnesium carbonate, and the ammonia should be recovered by selective thermal decomposition. When the mole ratio decreased to 1.5:1, only nesquehonite appears with high crystallinity and uniform size. The optimized conditions were therefore selected as 40°C, a mole ratio of 1.5:1, and magnesium sulfate concentration of 0.7 mol·L−1. Under these conditions the carbonation ratio reached 88%. Additionally, the optimal initial pH of MgSO4 solution was 9.5 (the product was nesquehonite at low pH) while the hydromagnesite will be produced at higher pH (pH more than 10). © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Highlights • The paper focuses on a single CO 2 product with cementitious properties. • The interference of aqueous calcium is studied through thermodynamics. • A sequential precipitation is suggested for aqueous calcium. • The values of precipitation pH are studied for three design options. Abstract Carbon capture and utilisation processes, CCU, are those processes aiming to produce usable products from CO 2 -based materials. Although there are a large number of CCU alternatives, only a few are able to propose routes for the massive production of marketable CO 2 based products with a negative or neutral carbon footprint in the long term. In this work, a conceptual design of a process aimed to produce a cementitious construction material, based on magnesium carbonate trihydrate, also known as nesquehonite, is discussed and experimentally tested. Desalination brines are proposed as the source of aqueous magnesium. However, these brines contain a non-negligible amount of calcium, which interferes in the development of cementitious properties from nesquehonite. In order to avoid this interference, a multi-stage precipitation is proposed, where calcium carbonate precipitates in a first stage, and pure nesquehonite can be obtained in a second stage. A thermodynamic model, based on empirical observations from the precipitation system, is proposed and the technical feasibility of three process alternatives are evaluated against the outcomes from the model. The preferred alternative, from the technical and economic point of view, is to conduct a full absorption of CO 2 as aqueous carbonate, which is then split in two streams for both precipitation stages. The advantage of this alternative is the applicability to different types of brines, the probably faster and more economic absorption stage, and the easier control of the reaction conditions, as pH and temperature. [ABSTRACT FROM AUTHOR]

Magnesium carbonate nesquehonite crystals were synthesized by mixing of the salt manufacture wastewater (bittern) and Na2CO3 with molar ratio 1:1 at room temperature. The effect of pH range (8-10) and initial magnesium concentration range (10,000-40,000 ppm) were investigated. Crystal morphology was observed using scanning electron microscopy and energy dispersive X-ray spectroscopy confirmed with X-ray diffraction. Large changes in morphology were observed in the increasing of initial concentration, which strongly depend on the pH. With an increase of pH, magnesium carbonate nesquehonite crystals change to hydromagnesite. However, reaction crystallization with initial magnesium concentration 40,000 ppm is found do not produce magnesium carbonate hydrated. Crystallization of magnesium carbonate nesquehonite is favored in the pH 8-9 with initial concentration in 10,000-20,000 ppm. [ABSTRACT FROM AUTHOR]

An in-depth scientific survey revealed the deterioration mechanisms affecting the 'Santa Maria della Stella' church in Saluzzo, Italy, where various salt crystallization processes are strongly damaging the building materials and artworks. Rainwater seepage permeates the vault and interior, causing: (1) epsomite growth as interstitial columnar crystals (resulting in pictorial coating detachment) or superficial, powdery efflorescence; (2) formation of nesquehonite/hydromagnesite crusts on wall paintings; and (3) nitratine growth causing pigment staining and detachment. These processes involve selective Mg2+ mobilization from magnesian-lime mortars and bacterial-induced formation of nitrates from guano, with consequent precipitation of degrading salts. The study confirms how characterization of all deterioration agents is fundamental to planning a viable cultural heritage conservation and restoration programme. [ABSTRACT FROM AUTHOR]

The Mg content incorporated in calcite is crucial to the formation of nesquehonite in a system where magnesium calcite and nesquehonite co-precipitate. Because of the importance of nesquehonite on the mechanical properties of paste and mortar containing reactive MgO, the Mg content that can be incorporated in calcite impacts the design of concrete containing reactive MgO. However, the maximum incorporation of Mg in calcite and co-precipitation behaviour of magnesium calcite and nesquehonite in a paste system are different from those in a solution system. Therefore, this study exposes blends of reactive MgO and calcium hydroxide to carbonation environments (50%–95% relative humidity, 75%–99% CO 2 concentration and temperature of 20 ± 3 °C) to investigate the precipitation behaviour of magnesium calcite and nesquehonite and factors influencing the Mg content in calcite in a paste system. The results reveal that the initial Mg:Ca ratio in the original system greatly determines the average Mg content incorporated in calcite, while the influence of curing condition (relative humidity and CO 2 concentration) is relatively negligible. The highest mole% of MgCO 3 in calcite can be ∼30% in the study, and the high stability of this high magnesium calcite is attributed to the co-existence of nesquehonite in the system. Moreover, the thermal decomposition of nesquehonite in this research is further characterized in detail. [ABSTRACT FROM AUTHOR]

Four lithologies of the Dumont nickel project were studied for assessing the carbonation capacity in ambient conditions of waste rocks and mineral processing tailings consisting of dunite, peridotite, gabbro, and volcanic materials. Mineral carbonation of these mine waste and tailing minerals is contemplated as a premium solution to permanently trap atmospheric CO 2 into solid carbonates using a differential batch carbonation cell and carbonate precipitation columns. The different mafic to ultramafic lithologies of the Dumont nickel project were characterized before and after carbonation by means of X-ray diffraction and thermogravimetry analyses, Fourier transform infrared spectroscopy, optical microscopy, and quantitative evaluation of minerals by scanning electron microscopy. It was found that for identical size fractions, carbonation of rock wastes was quite limited as compared to the tailings. Brucite more abundant in dunite and peridotite substrates was found to be the main reactant involved in carbonation as compared to other less-reactive magnesium silicate minerals. Nesquehonite, the prevalent magnesium carbonate species formed in wet ambient carbonation, remained stable despite prolonged exposition in dry ambient air conditions. Finally, some design recommendations were formulated to overcome the dilemma due to separate storage of high-permeability brucite-poor (lowly reactive) waste rock stockpiles, and finely-ground low-permeability brucite-rich (highly reactive) tailings. [ABSTRACT FROM AUTHOR]

In this paper the thermodynamic properties and the chemical reaction kinetics of the reversible reactions where sorption of water in magnesium hydro carbonates are analysed for thermal energy storage (TES). Depending on the conditions mainly nesquehonite, lansfordite and hydromagnesite may be formed from magnesite, all with a certain heat effect. Magnesite and water vapour can form nesquehonite or lansfordite via reaction (R1) and (R2): MgCO3 + 3H2O(g) ↔ MgCO3∙3H2O ΔH = -1.0 MJ/kg MgCO3∙3H2O, T=298K (R1) MgCO3 + 5H2O(g) ↔MgCO3∙5H2O ΔH = -1.41 MJ/kg MgCO3∙5H2O, T=298K (R2) Compared to other chemical sorption compounds, its advantages are low operating temperatures while they can act as a fire retardant. Experimental data is presented on the reactivity of the dehydration at various temperatures. The rate of dehydration of the nesquehonite is sufficient at low temperatures such as 50 °C and the reaction is about 90 % completed after 120 minutes. Magnesite reaches partial re-hydration to about 37% conversion after 24 hours. For better contact between reagents, mixtures with silica gel were used. A too large amount of water vapour, causing condensation of the water, appears to make the reactions irreversible. The temperatures of operating the process are presented as well as which compounds give an optimal energy storage. [ABSTRACT FROM AUTHOR]

X-ray diffraction of solid products combined with attenuated total reflection-Fourier transform infrared spectroscopy analysis reveals that during mineral carbonation of brucite-rich nickel mining tailings, parallel to brucite dissolution, hydrated magnesium carbonates such as nesquehonite are being formed. Wetting/drying cycles of the carbonation products revealed the important impact of temperature (25–70 °C) oscillations on the stability of the primary carbonates. While nesquehonite was observed to transform into an amorphous phase during hot dry episodes, evidences are presented on nesquehonite transformation into hydromagnesite and dypingite on the surface of already carbonated layers during long-term contacts under occasional wetting and drying episodes. Such observations, made at controlled laboratory temperatures, suggest that even under environmental CO 2 partial pressures and over long periods, the hydrated magnesium carbonates act as precursors for the formation of more stable carbonate products. Moreover, it was observed that drying and freeze/thaw cycles were at the origin of a thermomechanical “peel-off” effect which inflicted micro-fractures to the carbonate product layers enabling water and gas to engulf beneath and react with freshly unearthed Mg donor sites. Results of an experimental campaign designed to evaluate the nature and stability of the ambient carbonation products are also discussed. [ABSTRACT FROM AUTHOR]

It is generally agreed that aqueous carbonation of basic oxygen furnace slag (BOFS) is a promising and effective strategy to sequestrate CO 2 and enhance the slag properties for use as a construction material. However, the handling of wastewater produced in the aqueous carbonation route is still a challenge. This work investigates the impact of co-utilizing carbonated BOFS as a partial cement replacement and carbonated filtrate as a mixing solution on the fresh and mechanical properties as well as the hydration and microstructural characteristics of cement paste. The carbonation duration of BOFS via the aqueous route (0, 10, 20, 30, 40, 80, and 120 min) was studied, and it was found that calcite was the dominant CaCO 3 polymorph with different crystal shapes depending on the concentration of Mg ion in the slurry solution. The early-age compressive strength of blended cement pastes improved with the addition of 15% BOFS slurry carbonated for 40 min compared to as-received BOFS since the in-situ produced fine CaCO 3 particles in the former could promote nucleation sites for cement hydration. However, the strength of pastes containing BOFS slurry carbonated for a longer duration was decreased due to the inhibited hydration reaction induced by the nesquehonite (MgCO 3 ∙3H 2 O). At later ages of hydration, the nucleation effect was less obvious and the strength of cement paste was limited by the lower cement content. Moreover, the volume expansion issue caused commonly by f-CaO/MgO in BOFS can be improved through the precipitation of CaCO 3 and nesquehonite. • Reaction duration on aqueous-phase carbonation of BOFS was studied. • Rhombohedral to cauliflower calcite crystals were found. • Carbonated BOFS and filtrate can be co-utilized in cement paste. • The optimal carbonation duration of BOFS was 40 min. • No soundness concern with prolonged carbonation (>30 min) of BOFS slurry. [ABSTRACT FROM AUTHOR]

Acid leaching of ultramafic rocks to produce Mg 2+ - and Ca 2+ -rich solutions for mineral carbonation may inadvertently leach toxic trace metals. This study investigates the capacity of nesquehonite (MgCO 3 ·3H 2 O), a common product of mineral carbonation at Earth’s surface conditions, to sorb Cr, Ni, Mn, Co and Cu from solution. Our results demonstrate that upon precipitation, nesquehonite rapidly sequesters transition metals present in solution at concentrations from 10 to 100 mg/L. Trace metal uptake appears to occur by substitution for Mg 2+ in the nesquehonite crystal structure, and also by incorporation into minor, metal-rich phases, such as Fe-oxyhydroxides. This indicates that first row transition metals will likely be trapped and stored within Mg-carbonate minerals produced in industrial mineral carbonation reactors and in landscapes modified to capture atmospheric CO 2 via enhanced weathering. Thus, it is unlikely that trace metals will pose an environmental risk in the event of accidental release of wastewater. [ABSTRACT FROM AUTHOR]

As magnesium and many of its alloys are a promising class of degradable implant materials, a thorough understanding of their degradation under physiological conditions is a key challenge in the field of biomaterial science. In order to increase the predictive power of in vitro studies, it is necessary to imitate the in vivo conditions, track the decomposition process and identify the products that form during the degradation pathway. In this in vitro study, slices of pure magnesium were exposed to Hank's Balanced Salt Solution (HBSS), Dulbecco's Modified Eagle Medium (DMEM) and simulated body fluid (SBF), respectively, under cell culture conditions, which included CO2 gassing. The series were repeated with supplements of fetal bovine serum (FBS), added to the respective media. Degradation rates, osmolality and pH were found to vary with the choice of medium and supplementation with proteins. In order to identify the crystalline degradation products, the crusts formed on the specimens were investigated via X-ray diffraction (XRD) measurements. As expected, brucite, Mg(OH)2, was found among the degradation products; interestingly, nesquehonite, Mg(HCO3)(OH)·2H2O, was found to be the dominant degradation product in this study. The experimental data are well in accordance with solubility calculations. [ABSTRACT FROM AUTHOR]

The feasibility of synthesis of nesquehonite powders from natural talc was evaluated both through theoretical and experimental approaches. The change of the Gibbs free energy was − 318 kJ/mol with its corresponding equilibrium constants (K) of 5.53 × 10 55 , which indicated that the reaction could proceed spontaneously. Pure phase of nesquehonite was successfully obtained from natural talc at a low temperature and ambient pressure under the action of ammonia. The effects of reaction temperature and ammonia dosage on the precipitation of MgCO 3 ·3H 2 O have been investigated. XRD and SEM results demonstrated that the reaction temperature had a significant impact on the crystal phase of the products, while the ammonia dosage showed no serious effect on the morphology of the products, but had important influence on the length of the crystals. With the optimization of operating conditions (reacted at 60 °C with 8 mL ammonia), the nesquehonite crystals were prepared and could grow up to a length of about 19.31 μm and a width of 0.96 μm. During the crystallization process, the nesquehonite crystals were transformed from magnesium bicarbonate obtained from magnesium, and have a preferential orientation growth in the (002) direction. The growth process of nesquehonite crystals accorded with the solution-liquid–solid mechanism. [ABSTRACT FROM AUTHOR]

Computational chemistry, nesquehonite, NMR spectroscopy, VASP, 13C{1H} REDOR Keywords: computational chemistry; nesquehonite; NMR spectroscopy; VASP; 13C{1H} REDOR EN computational chemistry nesquehonite NMR spectroscopy VASP 13C{1H} REDOR 1 1 1 01/27/23 20230124 NES 230124 B Mg-containing species b can "chemically trap" CO SB 2 sb , and nesquehonite is one such metastable mineral. [Extracted from the article]

Among the phases in the MgO-CO-HO system, nesquehonite, MgCO ·3HO, attracts particular attention because of its potential application to carbon capture. However, its stability and the reported sequence of phases formed during the course of its thermal decomposition differ between authors and the corresponding decomposition mechanisms find various explanations. To improve the knowledge on the thermal decomposition of nesquehonite, new thermal data are presented and X-ray diffraction was used to follow the evolution of the solid products of decomposition. During thermal decomposition, nesquehonite loses its water below 300-350 °C whereas CO is lost above 300-350 °C, but the mechanism of thermal decomposition process is influenced by the choice of experimental conditions. The first loss of water, between ~55 and ~135 °C, results in a partial collapse of the nesquehonite structure and in the formation of a crystalline phase which is referred to as 'phase X' (approximately MgCO·2HO) and this gradually converts into an amorphous phase upon further heating and water loss. The regeneration of nesquehonite upon rehydration of either 'phase X' or the amorphous phase suggests that sufficient structural elements persist throughout the initial stages of decomposition to reconstitute nesquehonite. [ABSTRACT FROM AUTHOR]

Nesquehonite (MgCO 3 ·3H 2 O) has in the past been proposed as a low-cost, long-term mineral host for CO 2 . Here, stability of the phase was investigated under low temperatures (50 °C, 100 °C), moderate water vapour pressures (pH 2 O = 0.02–0.04 atm) and in both open and closed experimental systems. Specifically, this study explores CO 2 storage security in nesquehonite exposed to (1) atmospheric humidity, (2) self-generated humidity, and (3) humidity in simulated flue gases during ex situ carbonation. Both CO 2 and N 2 were used as carrier gases for H 2 O vapour to establish the influence of CO 2 on nesquehonite decomposition under humid conditions. Nesquehonite thermal stability was clearly enhanced under humid conditions for short-term (<20 h) in situ X-ray diffraction and thermogravimetric experiments. Formation of hydrous surface layers may impede structural H 2 O release from nesquehonite; delaying dehydration and preventing subsequent decomposition. Enhanced stability of nesquehonite was also observed under a CO 2 atmosphere. This study presents novel insights into the importance of temperature, pH 2 O and pCO 2 when considering the suitability of nesquehonite as a long-term CO 2 store. Additionally, it establishes basic, previously overlooked conditions that are essential considerations when tailoring disposal, storage and ex situ carbonation to enhance CO 2 stability in metastable Mg-carbonate phases. [ABSTRACT FROM AUTHOR]

Fig. 1. Schematic diagram of the pore construction mechanism of porous nesquehonite spheres. [Display omitted] • Porous nesquehonite sphere assembled by nanosheets was synthesized. • The role of dextran in the CO 2 biomimetic mineralization process was explained. • The effect of synthesis conditions on nesquehonite characteristics was investigated. • The formation mechanism of porous spherical nesquehonite was explored. Mineralizing CO 2 is an effective way to reduce the greenhouse effect. Due to the high cost, most CO 2 mineralization projects are basically difficult to be commercialized. Mineralizing CO 2 with magnesium salt is an effective and achievable method. The hydrated magnesium carbonate prepared by this method can be used as a functional material, which has great economic benefits. Inspired by biomineralization, nesquehonite (MgCO 3 ·3H 2 O) was prepared by indirect CO 2 mineralization of MgCl 2 and (NH 4) 2 ·CO 3 under the regulation of biosugars. Porous spherical nesquehonite with complex hierarchical structure was synthesized with the addition of 10.5 % dextran at the pH value of 9.5 ± 0.05. The formation mechanism of nesquehonite with different morphologies was further revealed. Under the regulation of dextran, 3D porous nesquehonite spheres are formed by the epitaxial growth of 2D-2D nanoarrays assembled and arranged by nanosheets. This work will provide references for further preparation of hierarchical nesquehonite by CO 2 biomimetic mineralization, and give new insights into the formation mechanism of porous spherical nesquehonite. [ABSTRACT FROM AUTHOR]

This work evaluates the influence of a CO 2 -saturated curing environment on thin (5 mm thick) boards of magnesium silicate hydrated (M-S-H) cement composites reinforced with cellulose fibers (CF). The first stage of the work consists of the optimization of the curing temperature when samples are subjected to high CO 2 curing conditions. By means of XRD, the optimal carbonation temperature was set at 45 °C, at which a higher peak intensity attributed to nesquehonite (MgCO 3 ·3H 2 O) was reached. At the selected temperature, the carbonation reaction was assessed by TGA. Carbonation conditions promoted the formation not only of hydrated magnesium hydroxicarbonates (HMHC) crystals, but also of other poorly crystallized compounds. As a consequence, carbonation increased the total amount of hydrated phases present in the matrix. Mercury intrusion porosimetry (MIP) tests were performed in order to evaluate a modification of the pore structure after carbonation. In this regard, carbonation reduced total cumulative intrusion by up to 38%. Finally, bending tests revealed that carbonation increases the strength of the composites while preserving their high. This results also into unprecedented toughness values (8.78 kJ/m2) considering only the deflection up to the modulus of rupture (0.17%). Therefore, carbonation is presented as a curing technique able to leverage the potential of CF in fiber reinforced cementitious composites (FRCC). [ABSTRACT FROM AUTHOR]

Nesquehonite, hydromagnesite, and brucite are important precursors for the preparation of high-purity magnesia (MgO) using magnesium resources from salt lake as raw materials. In this paper, TG-DTG and DSC were used to investigate the thermal decomposition behaviors of the three precursors. Decomposition kinetic parameters at each stage were evaluated based on the TG data using the iso-conversional method. Decomposition mechanisms were determined using the master-plots method. The decomposition temperature range, heat absorption, and kinetic parameters of the three phases were then compared. The most probable mechanism of each stage from the perspective of crystal structure was found to be consistent with the calculation results from the master-plots method. Results led to the conclusion that nesquehonite is the most appropriate precursor for the preparation of high-purity MgO. Further studies on precursor selection and calcining condition selection for the preparation of MgO using bischofite will benefit from this research. [ABSTRACT FROM AUTHOR]

The preparation of nesquehonite and calcium carbonate using low-grade magnesite as a starting material, as well as the chemical treatment of copper-bearing heavy metal wastewater by using nesquehonite as a novel precipitating agent was investigated for the first time. The precipitation of copper with nesquehonite was shown to be successful in reducing the level of soluble copper in solution. The precipitation completion being reached within 30 min. The removal efficiency of copper was not affected by the addition of nesquehonite and a removal efficiency up to 99.97% was achieved as excess coefficient of nesquehonite was 0%. At this time, the precipitated amount of copper by nesquehonite was 459 mg/g. The residual copper concentration was 0.1 mg/L, and final pH value was 6.8, which met the Chinese discharge limit (0.5-2.0 mg/L, GB8978-1996) and the USEPA enforceable limit of 0.25 mg/L. The precipitate produced was very rich in copper, which occurred in the form of Cu(OH)2, CuCO3, Cu2(OH)2CO3, Cu3(OH)2(CO3)2, and Cu4(OH)6SO4·H2O. © 2012 American Institute of Chemical Engineers Environ Prog, 32: 543-546, 2013 [ABSTRACT FROM AUTHOR]

Abstract: In this study, nesquehonite MgCO3·3H2O whiskers with long length and high aspect ratio were synthesized through reactive crystallization, in which MgCl2 and Na2CO3 precipitated in the presence of sodium dodecyl sulfate (SDS). The effects of SDS concentration, reactive temperature and reactants concentration were investigated. SEM, XRD and FT-IR were used to character morphology and structure of the products. It was found that the presence of SDS in the mixed aqueous solution was an important parameter for the morphology and size of nesquehonite crystals. The morphology of nesquehonite crystals changed from rod-like to bundle-like with increasing SDS concentration. The average length of nesquehonite whiskers was about 87–180μm while the aspect ratio was 18–45. The longest length reached 250μm and the biggest aspect ratio was about 100. The reactive temperature and reactant concentration also affect the size even the crystal type. It is concluded that SDS was beneficial for oriented growth of one-dimensional nesquehonite whiskers based on the formation process of the nesquehonite whiskers. The possible mechanism of SDS in nesquehonite whiskers precipitation was ascribed to physical adsorption of SDS on the individual amorphous nanoparticles or crystallites. The critical point of oriented growth occurred when the SDS concentration was about 4.33mM. [Copyright &y& Elsevier]

The application of hydrated Mg-carbonates as CO2 sequestering media is a pressing environmental challenge, which requires a deep knowledge of the phase transitions occurring in the Mg–CO–H2O system as well as the thermal and structural stability of these phases. In this paper we investigate the phase transition of nesquehonite (MgCO3 ·3H2O) to dypingite (Mg5(CO3)4(OH)2 ·5H2O), occurring after an incubation of months and years in solution, at ambient conditions. However, as the kinetics of this process resulted to be slow, the phase transition of dypingite to hydromagnesite (Mg5(CO3)4(OH)2 ·4H2O) was investigated at non-ambient conditions using in situ real-time high-resolution X-ray powder diffraction. Moreover, the thermal behaviour of dypingite and its decomposition have been also investigated with the aim to explore the appropriateness of this carbonate and the products of its decomposition as sinks of anthropogenic carbon dioxide. The results suggest that the dypingite structure remains unaffected up to 438K. At temperature above this threshold, dypingite transforms into hydromagnesite. A further increase in temperature converts the well-ordered hydromagnesite into a “collapsed form” at 528K. The heating of dypingite does not produce a loss of CO2 as the intermediate phases have the same CO2:Mg molar ratio. The final product of the heating is periclase MgO. Its nucleation occurs at temperature ranging from 573 to 663K and it becomes the only phase in the temperature range 633–678K. These results highlighted that dypingite assures a stable storage of CO2 in the conditions that prevail at the Earth's surface. Moreover, the transformation of dypingite in more thermodynamically stable hydromagnesite, occurring without release of CO2, enhances the safety of carbon dioxide disposal in solid form. Furthermore, the observed volume changes during phase transitions, which in turn could affect the porosity and permeability of the geological reservoir, have been evaluated in order to improve the prediction of the safe and permanent storage of CO2 in underground. [ABSTRACT FROM AUTHOR]

Abstract: When magnesium is added to source-separated urine, struvite (MgNH4PO4·6H2O) precipitates and phosphorus can be recovered. Up to now, magnesium salts have been used as the main source of magnesium. Struvite precipitation with these salts works well but is challenging in decentralized reactors, where high automation of the dosage and small reactor sizes are required. In this study, we investigated a novel approach for magnesium dosage: magnesium was electrochemically dissolved from a sacrificial magnesium electrode. We demonstrated that this process is technically simple and economically feasible and thus interesting for decentralized reactors. Linear voltammetry and batch experiments at different anode potentials revealed that the anode potential must be higher than −0.9 V vs. NHE (normal hydrogen electrode) to overcome the strong passivation of the anode. An anode potential of −0.6 V vs. NHE seemed to be suitable for active magnesium dissolution. For 13 subsequent cycles at this potential, we achieved an average phosphate removal rate of 3.7 mg P cm−2 h−1, a current density of 5.5 mA cm−2 and a current efficiency of 118%. Some magnesium carbonate (nesquehonite) accumulated on the anode surface; as a consequence, the current density decreased slightly, but the current efficiency was not affected. The energy consumption for these experiments was 1.7 W h g P−1. A cost comparison showed that sacrificial magnesium electrodes are competitive with easily soluble magnesium salts such as MgCl2 and MgSO4, but are more expensive than dosing with MgO. Energy costs for the electrochemical process were insignificant. Dosing magnesium electrochemically could thus be a worthwhile alternative to dosing magnesium salts. Due to the simple reactor and handling of magnesium, this may well be a particularly interesting approach for decentralized urine treatment. [Copyright &y& Elsevier]

Around the world large quantities of sludge wastes derived from nuclear energy production are currently kept in storage facilities. In the UK, the British government has marked sludge removal as a top priority as these facilities are nearing the end of their operational lifetimes. Therefore chemical understanding of uranium uptake in Mg-rich sludge is critical for successful remediation strategies. Previous studies have explored uranium uptake by the calcium carbonate minerals, calcite and aragonite, under conditions applicable to both natural and anthropogenically perturbed systems. However, studies of the uptake by Mg-rich minerals such as brucite [Mg(OH)2], nesquehonite [MgCO3·3H2O] and hydromagnesite [Mg5(CO3)4(OH)2·4H2O], have not been previously conducted. Such experiments will improve our understanding of the mobility of uranium and other actinides in natural lithologies as well as provide key information applicable to nuclear waste repository strategies involving Mg-rich phases. Experiments with mineral powders were used to determine the partition coefficients (Kd) and coordination of UO22+ during adsorption and co-precipitation with brucite, nesquehonite and hydromagnesite. The Kd values for the selected Mg-rich minerals were comparable or greater than those published for calcium carbonates. Extended X-ray absorption fine structure analysis results showed that the structure of the uranyl-triscarbonato [UO2(CO3)3] species was maintained after surface attachment and that uptake of uranyl ions took place mainly via mineral surface reactions. [ABSTRACT FROM AUTHOR]

Precipitation of carbonate minerals can play important roles in geological carbon sequestration and engineered processes for mineral carbonation. Influences of temperature, solution composition, and the presence of a solid substrate on the nucleation and precipitation of magnesium carbonate minerals were examined in a set of batch experiments. Conditions studied are relevant to full-scale geological carbon sequestration systems. Aqueous phase analysis by inductively coupled plasma mass spectrometry quantified the extent of precipitation. X-ray diffraction analysis was conducted to identify solids. Temperature significantly affected the identity of the solid obtained. At 25°C and 60°C the solids were magnesium carbonate minerals, and at 100°C the solid phase was identified as brucite [Mg(OH)2]. Although magnesite (MgCO3) was predicted to be the most thermodynamically stable magnesium carbonate phase, no magnesite precipitated and instead metastable magnesium carbonate phases formed. Evolution of dissolved concentrations was consistent with precipitation of these metastable phases. Presence of the magnesium silicate forsterite had no measurable effect on the rate or extent of precipitation. Mineralization in geological systems is likely to also be controlled by ionic strength, pressure, and mineralogy of the host formation. [ABSTRACT FROM AUTHOR]