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Characterization of light and heavy magnesium carbonates using thermal analysis
Abstract
Upon heating, hydrated magnesium carbonates (HMCs) undergo a continuous sequence of decomposition reactions. This study aims to investigate the thermal decomposition of various commercially produced HMCs classified as light and heavy, highlight their differences, and provide an insight into their compositions in accordance with the results obtained from thermal analysis and microstructure studies. An understanding of the chemical compositions and microstructures, and a better knowledge of the reactions that take place during the decomposition of HMCs was achieved through the use of SEM, XRD, and TG/DTA. The quantification of their CO2 contents was provided by TG and dissolving the samples in HCl acid. Results show that variations exist within the microstructure and decomposition patterns of the two groups of HMCs, which do not exactly fit into the fixed stoichiometry of the known HMCs in the MgO-CO2-H2O system. The occurrence of an exothermic DTA peak was only observed for the heavy HMCs, which was attributed to their high CO2 contents and the relatively delayed decomposition pattern.
... Magnesium carbonates exist in different structures with different thermodynamic stability and different degrees of hydration. In general, all magnesium carbonates are of interest in CCS since they have stable and long-lasting forms [3], the most attractive one for CCS is MgCO 3 , which has a 1:1 molar ratio of magnesium to CO 2 , but the hydrated forms are favored during precipitation [4]. This makes the study of the precipitation of magnesium carbonates an important field of study. ...
... Different MgCO 3 structures were obtained, including hydromagnesite (Mg 5 (CO 3 ...
... , and nesquehonite (MgCO 3 ·3H 2 O) according to XRD analysis. The density of these hydrated magnesium carbonates depends on their structure, hydromagnesite usually is 2-2.5 times denser than dypingite [3], therefore, it would present less specific surface area. ...
In this work, we present the results of two synthesis approaches for mesoporous magnesium carbonates, that result in mineralization of carbon dioxide, producing carbonate materials without the use of cosolvents, which makes them more environmentally friendly. In one of our synthesis methods, we found that we could obtain nonequilibrium crystal structures, with acicular crystals branching bidirectionally from a denser core. Both Raman spectroscopy and X-ray diffraction showed these crystals to be a mixture of sulfate and hydrated carbonates. We attribute the nonequilibrium morphology to coprecipitation of two salts and short synthesis time (25 min). Other aqueous synthesis conditions produced mixtures of carbonates with different morphologies, which changed depending on drying temperature (40 or 100 °C). In addition to aqueous solution, we used supercritical carbon dioxide for synthesis, producing a hydrated magnesium carbonate, with a nesquehonite structure, according to X-ray diffraction. This second material has smaller pores (1.01 nm) and high surface area. Due to their high surface area, these materials could be used for adsorbents and capillary transport, in addition to their potential use for carbon capture and sequestration.
... X-Ray Diffraction (XRD) was performed on a Bruker D8 Advance diffractometer (Bruker, Germany) using Cu-Kα radiation (40 kV, 40 mA), with a scanning rate of 0.02 • 2θ/step from 5 • -90 • 2θ. The Relative Intensity Ratio (RIR) technique [18,25] was employed for quantitative phase analysis. A fluorite internal standard material (20 wt% CaF 2 ) was used. ...
... A fluorite internal standard material (20 wt% CaF 2 ) was used. The weight fraction of MgO and Mg(OH) 2 were calculated as y = kx, where × is the RIR of the analyzed phase, calculated by dividing the integrated intensity of the phase with the strongest line by that of fluorite [18,25], and y is the weight fraction of MgO or Mg(OH) 2 . The values of k are 0.2886 for MgO and 0.3651 for Mg(OH) 2 [26]. ...
To further increase the sustainability of reactive magnesia cement (RMC) concrete while not compromising their overall strength, this study used the carbonation technique to pretreat recycled aggregates during the preparation of RMC-based samples, and their strength, pore structures and carbonation degrees were assessed by the compression test, XRD, TG-DTG, FTIR and X-CT. Greater carbonation degrees are considered to contribute to the strength improvement due to the formation of interconnected and well-developed carbonates and reduced porosity in RMC-based samples. However, this study shows that lower w/c ratios, which greatly refined their pore structures, favor the improved mechanical performance of RMC-based samples, albeit their carbonation degrees are lower. In addition, the much weaker correlation between the carbonation degree and strength of RMC-based samples could also be explained by the formation of different types of hydrated magnesium carbonates (HMCs), where samples containing nesquehonite (∼needle-like HMCs), albeit their carbonation degrees could be lower, reveal better strength than samples with hydromagnesite (∼rosset-like HMCs). Therefore, using pretreated recycled aggregates in RMC-based samples delayed their inside CO2 diffusion when they were cured in CO2, which slowed down their strength development, but when cured in air, RMC-based samples containing pretreated recycled aggregates reveal better strength owing to their more compacted pore structures. This study could also prove that CO2 diffusion among RMC-based samples could be through the pores within ITZs instead of capillary pores resulting from water.
... Identification of carbonates by micromorphology is a complex problem because natural carbonates often form solid masses and pseudomorphs can easily transform in aqueous solutions, besides that there is a strong compositional effect on phases morphology (Ming and Franklin 1985;Altay et al. 2007;Li et al. 2008;Munemoto and Fukushi 2008;Unluer and Al-Tabbaa 2014;Singh et al. 2016). ...
... Lansfordite (Lan) forms an orthorhombic prismatic crystals with pseudohexagonal base shape (Fig. 6d). Lansfordite loses water when exposed to air (Ming and Franklin 1985;Unluer and Al-Tabbaa, 2014), which leads to its transformation to nesquehonite (Nes; Fig. 3b) and the formation of cracks (Fig. 6d). ...
The chemical and mineral composition and formation conditions of dense grey-yellow colour crusts on the surface of monuments and bowls of decorative fountains in Peterhof (Russia) were studied using a wide range of methods (XRD, SEM, EMPA, Raman spectroscopy). The crusts consisted of calcium carbonates (monohydrocalcite, aragonite, calcite), magnesium carbonates (lansfordite, nesquehonite), and Sr-, Pb-rich carbonates (aragonite–cerussite–strontianite solid solution; up to 29 wt% PbO and 43 wt% SrO). Carbonates formed on all types of materials (gold, polyester resin, marble, granite) and do not interact with it. The cause of the formation of carbonate crusts on decorative fountains is the high content of carbonate ions in water. The variety of mineral phases is associated with fluctuations in the pH and Mg/Ca ratio. Recommendations are given on conservation measures that inhibit the formation of carbonate layers that threaten the preservation of the cultural heritage of this UNESCO World Heritage Site.
... Such properties are not exhibited by sorbitol, which, under measurement conditions, causes the phenomenon of hormesis, i.e., beneficial effects at low concentrations [23]. The second test compound in this toxicity category III is heavy magnesium carbonate, consisting of MgCO 3 with 40-50% Mg(OH) 2 [24]. It is approved for use in pharmaceutical formulations (tablets, capsules) in amounts up to 45% by weight [25]. ...
The ecotoxicological impact of pharmaceuticals has received considerable attention, primarily focusing on active pharmaceutical ingredients (APIs) while largely neglecting the potential hazards posed by pharmaceutical excipients. Therefore, we analyzed the ecotoxicity of 16 commonly used pharmaceutical excipients, as well as 26 API–excipient and excipient–excipient mixtures utilizing the Microtox® test. In this way, we assessed the potential risks that pharmaceutical excipients, generally considered safe, might pose to the aquatic environment. We investigated both their individual ecotoxicity and their interactions with tablet ingredients using concentration addition (CA) and independent action (IA) models to shed light on the often-overlooked ecotoxicological consequences of these substances. The CA model gave a more accurate prediction of toxicity and should be recommended for modeling the toxicity of combinations of drugs with different effects. A challenge when studying the ecotoxicological impact of some pharmaceutical excipients is their poor water solubility, which hinders the use of standard aquatic ecotoxicity testing techniques. Therefore, we used a modification of the Microtox® Basic Solid Phase protocol developed for poorly soluble substances. The results obtained suggest the high toxicity of some excipients, i.e., SLS and meglumine, and confirm the occurrence of interactions between APIs and excipients. Through this research, we hope to foster a better understanding of the ecological impact of pharmaceutical excipients, prompting the development of risk assessment strategies within the pharmaceutical industry.
... 3 ) content and low curing temperature equal to or below 20°C [10,31]; (3) yet the lower CO 2À 3 content was caused by low UUR as described above; (4) the higher temperature was caused by the combined action of the ambient temperature of 30°C and the exothermic reactions from RMC hydration and bio-carbonation. As important components of HMCs, D and H could be easily identified by their unique rosette-like crystal morphologies in Fig. 10e, f [25,37,44,45]. Due to the denser structure, HMCs also contributed to higher strength increment as a mechanical skeleton in the bio-carbonized samples [38,45]. ...
Bio-carbonation of reactive magnesia cement (RMC) method was proposed for dredged sludge stabilization. A series of RMC bio-carbonation experiments, micro-penetration and direct shear tests were conducted to investigate the feasibility and stabilization performance of the method. The effects of curing agent components, bacteria concentration, urea content and curing age on the mechanical behavior of the stabilized dredged sludge samples were analyzed. Experimental and testing results showed that the proposed method was effective for the stabilization of the dredged sludge with 40% of high initial water content. The maximum penetration resistance of the bio-carbonized sample with 10% of RMC content and 2 M of urea content was up to 39.2 N after 48 h of curing, which was 130.7 times that of the raw dredged sludge sample and 1.7 times that of the Portland cement stabilized dredged sludge sample. Every curing agent component was essential to the effective RMC bio-carbonation process and stabilization effect. Moreover, the stabilization performance was influenced by bacteria concentration and urea content. The optimal formula in this study was the high bacteria concentration (OD600 = 10.89) and medium urea content (2 M), while too high urea content such as above 2 M adversely affected the stabilization effect. The microstructure characteristics analysis revealed that the combined effect of cementation, filling and skeleton support from the mixture of brucite (hydration product of RMC) and HMCs (bio-carbonation products of RMC) was the primary contribution to the improvement of the physico-mechanical properties of the dredged sludge. This study is expected to provide informative insights into the application of new bio-mediated technologies for the efficient disposal of dredged sludge.
... MgO-extensive cements have been gaining popularity for this reason. The extensive hydration of MgO to Mg(OH) 2 , which results in a 117 percent molar solid mass transfer, is used in these studies [41][42][43]. One study was conducted to investigate the mechanical and morphological properties of carbonized corn stalk used to reinforce polyester composites in the manufacture of environmentally friendly composites [32]. ...
Concrete made with Portland cement is by far the most heavily used construction material 9 in the world today. Its success stems from the fact that it is relatively inexpensive, yet highly versa- 10 tile and functional, and is made from widely available raw materials. However, in many environ- 11 ments concrete structures gradually deteriorate over time. Premature deterioration of concrete is a 12 major worldwide problem. Moreover, cement production is energy intensive and releases much 13 CO2, and this is compounded by its ever-increasing demand, particularly in developing countries. 14 As such, there is an urgent need to develop more durable concretes, to reduce its environmental 15 impact and improve sustainability. For avoiding such environmental problems, researchers are 16 always searching for lightweight structural materials that show high performance during both pro- 17 cessing and application. Among the various candidates, Magnesia (MgO) seems to be the foremost 18 promising to attain this target. This paper presents a comprehensive review for the characteristics 19 and developments of MgO-based composites and its application in cementitious materials and en- 20 ergy efficient buildings. This paper starts with the characterization of MgO in terms of environmen- 21 tal production processes, calcination temperatures, reactivity, and micro-physical properties. Rela- 22 tionships between different MgO composites and energy efficient building design were established. 23 Then, the influence of MgO incorporation on the properties of cementitious materials and indoor 24 environmental quality were summarized. Finally, the future research directions on this were dis- 25 cussed. 26
Keywords:
... Other works showed that the calcination process of the raw dolomitic limestone and the slaking process (atmospheric environment, excess of water, CO 2 presence …) yielded dolomitic limes with different behavior, which gave rise to the formation of different magnesium carbonates after hardening (hydromagnesite, nesquehonite, amorphous carbonates,…) [132]. Whereas in ancient mortars the presence of dolomite was evident together with poorly crystalline phases of other magnesium carbonates, the formation of phases on repair mortars obtained from calcination of dolomitic limestone matches a different pattern [133][134][135][136][137]. The control of burning and slaking processes seems to be of paramount importance for dolomitic lime mortars [35]. ...
The main objective of RILEM TC LHS-277 “Specifications for testing and evaluation of lime-based repair materials for historic Structures” is the revision, adaption and, when necessary proposal, of the test methods to accurately study lime-based binding systems and mixtures, such as mortars and grouts. The empiric use of the lime-based composites and the predominant employ of cement in the field of Civil Engineering have led to the widespread application of test methods developed for cement-based composites to test the former. However, the clear differences in composition and performance between modern cement binders and lime-based materials would advise to explore specific test methods for the latter. To undertake this task the previous knowledge on the mechanisms of setting and hardening of these binders must be revised, arranged and synthesized. Processes such as drying, carbonation, hydration and pozzolanic reaction may occur during the setting and hardening of lime-based mortars and competition between them cannot be underestimated. With the aim of underpinning the revision and proposal of test methods for lime-based systems, this review paper reports a comprehensive study of the mechanisms of setting and hardening of these binders, considering the variability of the composition, which includes pure air lime as well as lime with hydraulic properties, lime-cement and lime-pozzolan systems.
... Unhydrated is available as geological material in its anhydrous carbonate form, magnesite (MgCO 3 ), which was discovered earlier in 1808. It is available into two physical forms: amorphous magnesite and crystalline (Unluer and Al-Tabbaa 2014). Magnesite is considered the most stable carbonate at a different range of temperatures and different partial CO 2 pressures. ...
Environmental global issues affecting global warming, such as carbon dioxide (CO2), have attracted the attention of researchers around the world. This paper reviews and discusses the ground improvement and its contribution to reducing CO2 in the atmosphere. The approach is divided into three parts: the Streamlined Energy and Emissions Assessment Model (SEEAM), the replacement of soil stabilisation materials that lead to the emission of a large amount of CO2 with alternatives and mineral carbonation. A brief discussion about the first two is reviewed in this paper and a detailed discussion about mineral carbonation and its role in enhancing soil strength while absorbing a large amount of CO2. It is emphasised that natural mineral carbonation requires a very long time for a material to reach its full capacity to form CO2; as a result, different acceleration processes can be done from increasing pressure, temperature, the concentration of CO2 and the addition of various additives. In conclusion, it was found that magnesium is more attractive than calcium, and calcium is complicated in terms of strength behaviour. Magnesium has a larger capacity for CO2 sequestration and it has a greater potential to increase soil strength than calcium.
... This trend may be more explored given the obtainment of reactive MgO from industrial byproducts [10]. The great advantage of this type of carbonated cement is the addition of bonded water as part of the Mg carbonates molecules [11,12]. Thus, it is possible to find different reported Hydrated Magnesium Carbonates (HMC) with the general formula xMgCO 3 O] amongst others. ...
This study aims at assessing the potential use of cellulose pulp in MgO-water slurries with potential for precast composites. These systems for composites applications envisage encapsulating CO2 when exposed to fast carbonation. The effect of cellulose fibers was evaluated on samples after the drainage of MgO-cellulose-water slurries. Different cellulose mass fractions were added into MgO-water suspensions – up to 30 wt% – to study the MgO hydration during the first 96 h. Afterward, the carbonation of the hydrated products for 6 and 12 h was evaluated. The addition of cellulose, after hydration, increases the sample porosity, where Mg(OH)2 is the main crystalline phase and only minor traces of unreacted MgO are found. MgO-H2O systems after hydration do not present any binding capacity given the low density and high apparent porosity of the clusters. However, XRD and TG analyses show that exposing samples to a rich CO2 environment promotes the formation of nesquehonite (MgCO3·3H20), which significantly reduces the porosity induced by the cellulose hygroscopy. This reduction in porosity is greater for samples with cellulose fibers because of the greater content of nesquehonite produced in samples with cellulose. Besides, cellulose fibers are covered with nesquehonite nanocrystals after carbonation. By adding cellulose to water-MgO suspensions yields lighter products with promising potential for fiber-cement applications. Moreover, the addition of cellulose contributes to the encapsulation of CO2 in building materials through the Mg(OH)2 carbonation.
... With respect to the sorption capacity, it is important to focus on materials which retain sufficient capacity at elevated temperatures. The steam adsorption capacity of the hydrotalcites, as for the base metal oxides, is however limited [46,47]. ...
Enhancement by steam separation is a promising process intensification for many types of reactions in which water is formed as a byproduct. For this, two main technologies are reactive vapor permeation (membrane technology) and reactive adsorption. Both can achieve significant conversion enhancement of equilibrium limited reactions by in situ removal of the by-product steam, while additionally it may help protecting catalysts
from steam-induced deactivation.
In general, reactive permeation or reactive adsorption would be preferable for distinctly different process conditions and requirements. However, although some advantages of reactive steam separation are readily apparent from a theoretical, thermodynamic point of view, the developments in several research lines make clear that the feasibility of in situ steam removal should be addressed case specifically and not only from a theoretical
point of view. This includes the hydrothermal stability of the membranes and their permselectivity for reactive steam permeation, whereas high-temperature working capacities and heat management are crucial aspects for reactive steam adsorption. Together, these developments can accelerate further discovery, innovation and the
rollout of steam separation enhanced reaction processes.
... Generally, the HMCs begin to dehydrate at~50-100°C, and fully convert to anhydrous magnesium carbonate at 300°C [40]. Hydroxyles, however, can sustain higher temperatures and lose bound water up to 350°C [41]. Fig. 9b shows TGA/DTG results of samples extracted from the healed cracks. ...
Developing self-healing of concrete cracks represents a critical approach for extending material longevity. If demonstrated on concretes with CO 2 stored through carbonation curing, self-healing can further mitigate infrastructure lifecycle emissions. In this study, the self-healing process is investigated on novel Engineered Cementitious Composites (ECC) based on binary blends of reactive MgO cement and siliceous fly ash activated by carbonation. The self-healing was observed under cyclic wetting and drying exposures and was characterized by measurements of resonance frequency, optical microscopy and X-ray computed microtomography (XCT). Thermogravimetric analysis, X-ray powder diffraction and scanning electron microscopy were employed to assist in identifying healing products and mechanisms. It was found that micro-cracks could be autogenously filled in 7 wet-dry cycles, as evidenced by over 95% recovery of resonance frequency and healed crack appearance under optical microscopic observation. Nevertheless, the composite mechanical performance, particularly tensile strength and ductility, did not fully recover with respect to the uncracked specimens. The inadequate post-healing mechanical properties were consistent with XCT observations that indicated low material densities in the cracked area. Based on the chemical and microstructural characterizations, it was determined that nesquehonite crystallites were responsible for the crack filling. Although the nesquehonite filled cracks will be less permeable to invasion of reactive fluids, novel strategies are needed to establish more robust healing capability to completely restore the crack bridging and composite tensile performance.
... With the help of TGA measurements of pure hydromagnesite, see also [67,77], and the MgO/HY 70/30 sample hydrated for 28 days, subsequent TGA mass loss peaks can be simplistically attributed to: ...
The hydration of magnesium oxide in the presence of hydromagnesite (Mg5(CO3)4(OH)2·4H2O) was investigated. The hydration products are a poorly-crystalline form of brucite (Mg(OH)2) and an unknown amorphous or poorly crystalline hydrate, which was evidenced by X-ray diffraction analyses and Raman spectroscopy. By thermogravimetry it was found that in the presence of hydromagnesite more water is bound in hydrate phases than can be explained by the formation of brucite only. The unknown hydrate is hypothesized to lead to cohesive binding in MgO-hydromagnesite blends. Due to thermodynamic predictions artinite (Mg2(CO3)(OH)2·3H2O) should be the stable hydrate in this system. The addition of artinite crystal seeds, however, had no significant effect on hydration kinetics or nature of the reaction products. The hypothesis that the addition of sodium bicarbonate (NaHCO3) could modify or accelerate the hydration reactions due to its high solubility and the supply of additional HCO3− ions was also not supported by the experiments.
... On the basis of these models, the prediction of porosity, carbonation efficiency, pH value and the leaching process of carbonationcured cement paste becomes possible [22,23]. It was reported that CO 2 curing could cause a series of chemical changes in the MgO cement system to form hydrated magnesium carbonates (HMC) such as nesquehonite ( [24][25][26]. These HMC substances can fill small pores and increase the density and mechanical strength of cement pastes [27,28]. ...
This paper aims to investigate the influence of pulverized fuel ash (PFA) and ground granulated blast-furnace slag (GGBS) on compressive strength and water resistance of magnesium oxysulfate (MOS) cement with and without CO2 curing treatment. Hydration products and microstructure of typical samples were also evaluated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential thermal analysis-thermogravimetry (DTA-TG), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) facilities. The results showed that compressive strength of MOS cement was decreased by the addition of PFA or GGBS. The CO2 curing resulted in a negative effect on compressive strength of MOS cement other than Portland cements. The decreased compressive strength of MOS cement induced by CO 2 curing can be alleviated by incorporation of suitable dosages of PFA or GGBS. Both PFA and GGBS behaved an improvement on the water resistance of MOS due to the formation of magnesium silica hydrate gel (M-S-H gel). Therefore, the coupling effect of PFA or GGBS as partial substitution of magnesium oxide (MgO) and CO2 curing treatment provides a potential method to manufacture MOS cement with higher durability and lower environmental impact.
... Some of the most common HMC phases observed in RMC-based concrete formulations are nesquehonite (MgCO 3 ·3H 2 O), hydromagnesite (4MgCO 3 ·Mg(OH) 2 ·4H 2 O), dypingite (4MgCO 3 ·Mg(OH) 2 ·5H 2 O) and artinite (MgCO 3 ·Mg(OH) 2 ·3H 2 O). The hardened dense carbonate network provides binding strength, whose extent depends on the degree of carbonation and the morphology of the formed phases [5][6][7]. ...
Reactive magnesia-based cement (RMC) is an emerging group of alternative binder to Portland cement. Recently, the first fiber-reinforced RMC-based strain-hardening composites (SHC) have been developed by the authors. The current work investigated the feasibility of the PC-free RMC-based SHC formulations to engage autogenous healing. Results showed that crack sealing and significant mechanical recovery can be realized through proper environmental conditioning. The presence of water is necessary to engage autogenous healing and elevated CO2 concentration leads to the formation of HMCs that can seal larger crack. However, ample supply of CO2 results in fast sealing of crack on the near surface region, which blocks the pathway for further carbonation and healing of interior region of cracks. Microstructure analysis reveals that the healing products are hydrated magnesium carbonates (HMCs) and different conditioning regimes lead to different types of HMCs as the healing products.
... that reduces the overall pore volume and (2) microstructural evolution as the morphology and the binding strength of the carbonate crystals contribute to the network structure. The use of accelerated carbonation (i.e., 5%e20% CO 2 concentration) in mixes containing 4%e10% RMC as the only cementitious component along with 90%e96% aggregates has been reported to result in compressive strengths 2e3 times higher than corresponding PC mixes, mainly because of the formation of HMCs (Liska, 2009;Unluer, 2012;Unluer and Al-Tabbaa, 2014a), as shown in Eqs. (7.6)e(7.9). ...
... .1 Síntesis por Vía HúmedaEn la Tabla 5 se muestran los resultados de los productos de MgCO3 obtenidos por la síntesis de vía húmeda. Las estructuras de carbonato de magnesio existen en distintos grados de hidratación, tales como la hidromagnesita (MgCO3 tetrahidratado), dypingita (MgCO3 pentahidratado) y nesquehonita (MgCO3 dihidratado), las cuales pueden presentarse tanto en alta densidad (baja porosidad) como en baja densidad (alta porosidad)[49]. La Tabla 5, muestra de manera condensada los resultados de los experimentos de síntesis por vía húmeda; se obtuvieron estructuras de MgCO3 tanto de alta densidad como de baja densidad. Las estructuras de alta densidad fueron producidas en los experimentos que utilizaron metanol como cosolvente, mientras que en los experimentos acuosos se generaron morfologías de baja densidad. ...
... The milling of as-received dolomite, together with acetic acid (AA), in ethanol-water media yielded newly formed [22], in our case a range of acetate hydrates is formed, of different stoichiometries and structures. Figure 5 shows the XRD pattern of the sample D5 (pattern a), together with the standard reflections for the phases CAH (ICDD No: 19-199, blue bars) and CMAH (ICDD No: 49-1106, orange bars). ...
The main goal of this research was to improve a carbonate-type foaming agent for the production of Al foams. Various systematic treatments, i.e. mechanical, thermal and chemical, were applied to naturally occurring dolomite, in order to affect its thermal decomposition. Structural modifications after the treatments as well as after the thermal decomposition were monitored by X-ray diffraction, while thermal gravimetry and differential scanning calorimetry coupled with mass spectrometry were employed to monitor the processes during the heating experiments. The as-received dolomite, without any pre-treatments, decomposes at a relatively high temperature, which prevents its wider application as a foaming agent. However, by using various treatments the decomposition of the dolomite could be shifted towards lower temperatures, making it suitable for the production of aluminium foams.
... One of these initiatives focuses on the development of alternative binders with lower CO 2 emissions and energy consumption. These endeavors include the sequestration of CO 2 within carbonated binders of various compositions (Chen et al. 2016;Morandeau et al. 2015;Shao et al. 2014;Unluer and Al-Tabbaa 2014a;Vandeperre and Al-Tabbaa 2007). Among these carbonated systems, reactive MgO-based binders present a promising alternative due to the lower temperatures used during their production (700-1,000°C versus 1,450°C for PC), ability to gain strength while sequestering CO 2 in the form of stable carbonates, compatibility with large quantities of industrial byproducts and wastes, and potential to be fully recycled when MgO is used as the sole binder (Liska and Al-Tabbaa 2009;Unluer and Al-Tabbaa 2013). ...
This study investigates the performance and microstructural development of reactive MgO and calcined dolomite-based concrete samples subjected to carbonation curing for up to 28 days. The performance of each sample is assessed via compressive strength testing, which is linked with the hydration and carbonation mechanisms studied via isothermal calorimetry, X-ray diffraction (XRD), thermogravi-metric analysis with differential scanning calorimetry (TGA/DSC), and scanning electron microscopy with energy dispersive X-ray spec-troscopy (SEM/EDX). Strength gain depends on the carbonation degree and morphology of carbonate phases such as hydromagnesite and nesquehonite, whose formation is controlled by the binder composition and initial porosity of each sample. Higher contents of MgO enable early strength gain, whereas the presence of undecomposed carbonate phases in dolomite facilitates the continuation of the hydration and carbonation reactions by providing additional nucleation sites and contributing to the formation of a dense carbonate network. Continuous strength gain is achieved by the extensive formation of a carbonate network. Simultaneous use of MgO and dolomite enables 28-day strengths as high as 57 MPa, which are up to 60% higher than that of the control samples containing only MgO or dolomite.
... Strength development of MgO binders subjected to carbonation is related to: (i) the increase in density as carbonation is an expansive process that reduces the overall pore volume (i.e. the formation of HMCs causes a significant expansion and increases the solid volume by a factor of 1.8-3.1) [13,14]; and (ii) microstructure evolution as the morphology and the binding strength of the carbonate crystals contribute to the network structure. ...
Strength development of reactive MgO cement-based concrete is limited by the low hydration and carbonation of MgO. This study aims to improve the hydration and mechanical performance of carbonated MgO mixes with the introduction of various hydration agents (HAs) at different concentrations. Influence of these HAs on the hydration and carbonation mechanisms under ambient and accelerated curing conditions was evaluated through isothermal calorimetry, TG, XRD and FTIR analyses. Introduction of HAs enabled extensive carbonation and strength development reaching up to ∼60 MPa at 28 days, which was 107% and 53% higher than the corresponding MgO and PC-based control mixes, respectively.
Electric Arc Furnace slag (EAF slag) reuse is currently limited by its inconsistent chemical composition and volume instability. However, the alkaline composition suggests the possibility to use this material for carbon capture and storage. This study investigated the CO2 uptake of EAF slag using a direct aqueous carbonation technique. The process was implemented at room temperature and ambient pressure, with minimized energy consumption. The CO2-reactive phases were identified through X-ray diffraction analysis. Different CO2 quantification techniques were employed: thermogravimetric analysis, acid digestion and thermal decomposition. The replicability of experiments and quantification techniques was assessed through analysis of variance and pairwise comparisons. The average CO2 uptake and coefficient of variation resulted respectively 7.9% and 9.0%, with a carbonation degree of about 34%, proving that this simple mineralization process can be promising even in mild conditions.
Magnesium oxychloride cement (MOC), an alternative to ordinary Portland cement (OPC), has attracted increasing research interest for its excellent mechanical properties and its green and sustainable attributes. The poor water resistance of MOC limited its usage mainly to indoor applications; nevertheless, recent advances in water-resistant MOC have expanded the material’s potential applications from indoor to outdoor. This review aims to showcase recent advances in MOC, including water-resistant MOC and ductile fiber-reinforced MOC (FRMOC), exploring their potential applications including in sustainable construction for future generations. The mechanism under different curing procedures such as normal and CO2 curing and the effect of different inorganic and organic additives on the water resistance of MOC composites are discussed. In particular, the review highlights the recent developments in achieving over 100% strength retention under water at 28 days as well as advancements in FRMOC, where tensile strength has surpassed 10 MPa with a remarkable strain capacity ranging from 4–8%. This paper also sheds light on the potential applications of MOC as a fire-resistant coating material, green-wood-MOC composite building material, and in reducing solid waste industrial byproduct accumulations. Finally, this study suggests future research directions to enhance the practical application of MOC.
Bio-carbonization of reactive magnesia (r-MgO) is a new technology for sandy soil solidification. In this study, two sets of tests were conducted to investigate the influence of r-MgO contents on the bio-solidification effects of sandy soils, with the analysis of the unconfined compressive strength (UCS), permeability coefficient, sonic time value, and precipitation content. The relationship between r-MgO contents and solidification effects with a single treatment cycle was studied in the first sand solidification test. Then, the second sand solidification test was further conducted until their permeability coefficient reached about 10−6 cm/s to determine the maximum treatment cycle under various r-MgO contents. The results showed that the UCS, permeation resistance, and carbonate precipitation content were positively related to the r-MgO content if the solidification treatment was applied only once, while the sonic time value showed an opposite trend. Moreover, the maximum treatment cycle obtained under various r-MgO contents varied greatly. A high dosage of r-MgO could clearly reduce the maximum number of treatment cycles of the sand column, especially the r-MgO content larger than 15%. Decreased treatment cycle reduced carbonate precipitations in the sand column and decreased the UCS by over 40%. There was a close relationship between UCS and average carbonate precipitation contents for the bio-carbonated sand columns with the only one treatment cycles. However, the UCS of sand columns with multiple treatment cycles varied greatly within a similar average precipitation content. The results of this study lay a solid foundation for applying bio-carbonization of r-MgO in sandy soil solidification.
The carbonation technology, which is promising in carbon capture and storage, is an emerging soil treatment method adopted in geotechnical engineering and geo-environmental engineering to enhance mechanical properties or mitigate environmental risks. The advantage of utilising CO2 in solving engineering problems shows a large potential for the technology to be widely used to alleviate the greenhouse emission problems; Nevertheless, the changes in soil characteristics, including physical, chemical, mechanical, deformation, and durability characteristics, caused by carbonation are complicated, which brings challenges to possible engineering applications. This paper reviews the recent development of carbonation technology in geotechnical engineering and provides a systematic analysis of the effect of carbonation on soil properties, from the macro behaviours to the micro mechanism. The “double-edged sword” effect of carbonation on the mechanical properties of soils is proposed, together with the micro-scale mechanisms illustrated. A qualitative theoretical model for the prediction of the mechanical strength of carbonated soils is established. The prospective future and important insights of the carbonation technology to be utilized in geotechnical engineering can be concluded from the review, although some potential technical issues, including possible carbonation crack, heavy metal leaching, durability issues, and lack of construction techniques, require further study.
The effects of carbonation on the strength and microstructural evolution of MgO-activated reactive SiO2 (M-A-R-S) mortars under different conditions have been investigated. M-A-R-S mortars with a binder-to-silica sand ratio of 50% were prepared by light-calcined MgO and silica fume (SF) under accelerated carbonation at 20% CO2 condition for different periods. Carbonation significantly increases the compressive strength of M-A-R-S mortars. The specimens cured for 28 days in air and placed to accelerated carbonation for ∼14 days achieved the highest strength of ∼83 MPa and the lowest porosity of 13.5%. The increased strength and density resulted from the formation of the hydrated magnesium carbonate (i.e. nesquehonite (MgCO3·3H2O) with a needle/column-like morphology. However, it was also found that continuous carbonation for longer periods (e.g. 28 days), associated with excessive carbonated phase(s) formed, caused micro-cracking and a reduction in compressive strength.
Dypingite is one of the most common phases of crystalline magnesium carbonate hydrates, which are considered potential targets for CO2 mineral sequestration. Ex-situ high-temperature X-ray diffraction and ex-situ high-temperature synchrotron X-ray scattering experiments with pair distribution function analysis were used to explore the thermal transformation process of dypingite. Dypingite transformed to hydromagnesite until 100°C, but its local structure remained unchanged. At the temperature range from 250°C to 350°C, where the amorphous phase was predominant, the second and third coordination spheres were continuously modified and crystallized to MgO at 400°C. Although the local structure of dypingite was as that of hydromagnesite, dypingite exhibited a different thermal transformation pathway from hydromagnesite.
Актуальность исследования обусловлена необходимостью комплексного использования магнезиального сырья, в том числе отходов добычи. При добыче магнезиального сырья как огнеупорного и стратегического материала на некоторых месторождения сопутствующей является гидромагнезитовая порода, которая не находит применения в классических магнезиальных технология. В то же время она обладает характеристиками, позволяющими применять ее для получения важных продуктов: антипиренов для различных материалов, поризующий компонент в огнестойких покрытиях, исходный компонент для получения водостойких магнезиальных вяжущих. Цель: определить возможность и условия применения гидромагнезиатовой породы Халиловского месторождения в качестве исходного сырья при получении водного раствора бикарбоната магния – жидкости затворения водостойкого магнезиального вяжущего. Объекты: гидромагнезитовая порода, сопутствующая магнезиту скрытокристаллической структуры Халиловского месторождения, Оренбургская область. Изучаемая порода состоит из гидрокарбонатных минералов: гидромагнезита, дипингита, несквигонита, а также примеси клинохризотила. Гидрокарбонатный состав не позволяет применять ее для формованных обжиговых магнезиальных изделий. Методы: метод термической активации гидромагнезитовой породы, позволяющий получить высокореакционную дефектную структуру; получение раствора бикарбоната магния искусственной карбонизацией суспензии активированного гидромагнезитового материала; термические методы исследования – дифференциальная сканирующая калориметрия, термогравиметрия; рентгенофазовый анализ; титрометрический метод определения концентрации бикарбонат-ионов. Результаты. Установлена возможность использования гидромагнезитовой породы для получения водного раствора бикарбоната магния с концентрацией по бикарбонат-иону до 3,8 г/л; установлена эффективность термической обработки гидромагнезитов в диапазоне температур 300–375 °С, которая позволяет получить высокодефектный продукт ; термическая активация гидромагнезитов повышает эффективность перехода бикарбонат-ионов и катионов магния в раствор в присутствие СО2 при низком давлении процесса карбонизации 0,2 МПа; полученный при низком давлении газа СО2 водный раствор бикарбоната магния с высокой концентрацией бикарбонат-ионов позволит получить гидравлические магнезиальные вяжущие композиции высокой водостойкости.
The present work reports on foaming of magnesium alloys and composites using MgCO3 as the blowing agent. Foaming was done via the molten metal route by direct addition of MgCO3 in molten Mg. The alloys and composites required for foaming were prepared by varying the concentration of aluminum (10 to 30 wt pct) and calcium (0 and 2 wt pct) in Mg. SiC of 10-lm size and about 10 to 20 vol pct was added as reinforcement particles in the composite. The liquidus temperature of the alloys and composites, the decomposition behavior of MgCO3, and the intrinsic oxides that formed in the melt have a significant effect on the structure of the foams. Mg alloys and composites with 30 wt pct Al showed better foaming behavior with higher expansion, lower density, good cell structure, and uniform cell size distribution due to the smaller difference between their liquidus temperature and the decomposition temperature of MgCO3. The addition of 2 wt pct Ca showed a significant effect on foaming, and the MgO and MgAl2O4 (spinel) particles formed in situ in the molten Mg during foaming acted as the stabilizing agents.
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.
This study investigated hydrothermal fixation of CO2 in serpentine or magnesium hydroxide as one of the compounds containing Mg at an initial CO2 pressure of 7.1 MPa. The carbonation of magnesium hydroxide gave primarily magnesite and the yield increased with increasing initial pH and increasing temperature, reaching 97.9 % at 473 K. Carbonation of serpentine also gave magnesite, and the yield initially increased and then slightly decreased with increasing temperature. Both low and high initial pH enhanced serpentine carbonation at 573 K and the yield of magnesite reached 12.7 % at an initial pH of 1.0. Estimation of chemical species indicated that both the dissolution of serpentine via the action of H⁺ and the formation of magnesite from MgHCO3⁺ and Mg²⁺ via the release of H⁺ were important reactions. Hydrothermal serpentine carbonation is a promising CO2 fixation method for naturally abundant mineral rocks without expensive additives.
We investigated crystal structure and the local structure changes during the thermal decomposition of hydromagnesite by using in situ high-temperature XRD and ex situ high-temperature X-ray total scattering measurements. Hydromagnesite displayed anisotropic thermal expansion up to 220 °C. The a and c lattice parameters exhibited an increase trend with temperature, whereas the b lattice parameter and β angle did not show a regular trend with temperature. The relative expansion between 25 and 220 °C followed the c/c0 > a/a0b/b0. At 260 °C, the a, b, and c lattice parameters significantly decreased. Above 280 °C, hydromagnesite underwent a structural collapse with dehydration and dehydroxylation reactions, but was never accompanied by nucleation and growth of crystal phases up to 425 °C. During the thermal decomposition from hydromagnesite to periclase, the Mg atoms maintained the octahedral coordination environments in the structure.
Carbonation of reactive magnesia (MgO) has recently received increasing attention in the area of soil stabilization and ground improvement. However, as a critical parameter in terms of long-term seepage behavior in the geotechnical analysis, the hydraulic conductivity of carbonated reactive MgO-stabilized silt has not been fully studied. In this context, the effect of water-MgO ratio (ratio of initial water content to MgO content, w0/c) and carbonation time on hydraulic conductivity (or permeability) characteristics was systematically investigated. Serial microstructural tests including mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) analyses were applied to elucidate the intrinsic mechanisms. The obtained results indicate that as the initial water-MgO ratio decreases, the void ratio gradually decreases and the reduction of hydraulic conductivity becomes less prominent because of the little presence of flow paths. The hydraulic conductivity of carbonated MgO-admixed silt similar to that of PC-treated silt is mainly governed by the porosity, and its correlation with void ratio is proposed in the article. The variations of permeability with void ratio are consistent with those of the cumulative pore volume from MIP results in general, and the medium pores (3–30 μm) are substantiated to be the primary contributor in controlling the permeability. SEM and DSC analyses reveal that the cementation of soil particles and filling of hydrated magnesium carbonates marginally reduce the voids and permeability. The reasons for changes of permeability behaviors have been confirmed by the pore-size distribution and microstructure characteristics.
The interaction between CO2 and a commercial CuO/ZnO/Al2O3 (CZA) catalyst has been investigated by employing temperature-programmed desorption (TPD), FTIR spectroscopy and equilibrium adsorption-desorption experiments. TPD runs were carried out after exposure of the reduced or oxidized catalyst to a CO2-containing gaseous stream at temperatures of 25, 200 and 300 °C. Equilibrium CO2 adsorption-desorption experiments were performed up to 2 MPa at 25, 50, 100, 200 and 300 °C. In addition, the formation of carbonate species following adsorption at room temperature and at high temperatures (200, 300 °C) has been investigated via in situ FTIR experiments in a DRIFTS cell. CO2 adsorption revealed extensive surface heterogeneity as indicated by: i) the shape of TPD profiles, ii) the fact that no standard model (Langmuir, Freundlich and others) could provide a good fit with random distribution of residuals and physical meaning of estimated parameter values and iii) the non-linear Clausius-Clapeyron plots at all coverages. CO2 adsorption at high temperatures takes place through creation of new adsorption states on both oxidized and reduced catalysts corresponding to polydentate carbonates at the expense of other surface carbonates. The presence of hysteresis between adsorption-desorption isotherm branches indicates that a fraction of CO2 adsorbs irreversibly on oxidized CZA.
This paper reveals the effect of calcium and magnesium ions in carbonation experiments carried out to regenerate sodium hydroxide from a biogas upgrading unit. This novel study arises as an alternative to standard physical process whose elevated energy consumption imposes economic restrictions. Previous works employed alkaline waste to turn them into value added product. Nevertheless, no attractive economical results were obtained due to the low regeneration efficiencies. Our hypothesis is that both calcium and magnesium waste composition percentages have an impact in the result, hence this work propose an isolated study aiming to determine the of each one in the global performance. To this end, the operational parameters (reaction time, reaction temperature and molar ratio) were tuned as well as physicochemical properties of the final solid samples were analyzed by several techniques. The results indicate that calcium is much more prone than magnesium to reach high efficiencies in aqueous carbonation experiments. Additionally, higher quality products were achieved with calcium. The results of this study suppose an important step for understanding the aqueous carbonation through waste in the path to achieve a more sustainable city and society.
In this work a novel strategy for bio-methane production and magnesium chloride waste valorization is addressed. The proposed process is a potential alternative path to the already existing biogas upgrading technologies by carbon dioxide mineralization into valuable magnesium carbonate. The main parameters affecting the precipitation efficiency (reaction time, reaction temperature, and molar ratio reactant/precipitator) are studied, leading to promising results which spark further investigation in this innovative route. Additionally the purity and the morphology of the obtained solid product was accurately analysed through different physicochemical characterization techniques such as Raman, X-Ray diffraction and Scanning electron microscope. The characterisation study reveals a mixture of Nesqueonite and Dypingite carbonate phases obtained in the process being the later the dominant phase in the resulting precipitate. Overall, the results discussed herein confirmed the technical feasibility of this innovative strategy for synergizing carbon dioxide mineralization and renewable energy production.
Pure nanocrystalline akermanite powder was synthesized using calcium carbonate, magnesium carbonate, and silicate as the base materials. Base powders were mechanically activated and then heat treated at various temperatures. The produced powders were evaluated by simultaneous thermal analysis (STA), x-ray diffraction (XRD), and scanning electron microscopy (SEM) characterization techniques. The results showed that pure nanocrystalline akermanite was synthesized from powder mechanically activated for 50 h with subsequent annealing at 900 °C for 1 h. Nanostructured akermanite had a crystalline size of 67 nm. The formation mechanism of akermanite was scrutinized. It was found that akermanite forms through the formation of several transition compounds such as enstatite, wollastonite, larnite, and merwinite.
In this study, mechanical properties of rock flour mortar were investigated through experimental studies. The tests results were employed to assess compressive strength, water absorption and specific gravity of rock flour mortar in the presence of reactive MgO. The results showed that the use of rock flour-to-cement ratio of 1.5 with 2.5% magnesium oxide had the maximum compressive strength and adding more MgO to mortar could not play an effective role in strength property. Water absorption also increased with increasing amounts of magnesium oxide for almost all of the samples. In this study, the lowest water absorption was 11.16% for the sample containing 2% magnesium oxide and rock flour-to-cement ratio of 2.5. The specific gravity of the samples also increased with an increase in the amount of magnesium oxide which varied between 16.2 and 2.4 g/cm³. The field emission scanning electron microscope and energy-dispersive X-ray spectroscopy tests were also used to verify the results. In addition, backpropagation neural network was used for better parameter estimation. This network showed better precision for predicting the 28th day compressive strength with 0.936 regression.
Amorphous intermediates play a crucial role during the crystallization of alkaline earth carbonates. We synthesized amorphous carbonates of magnesium, calcium, strontium, and barium from me-thanolic solution. The local environment of water and the strength of hydrogen bonding in these hydrated modifications were probed with Fourier transform infrared (FTIR) spectroscopy, 1 H nuclear magnetic resonance (1 H NMR) spectroscopy, and heteronuclear correlation (HETCOR) experiments. Temperature dependent spin-lattice (T1) relaxation experiments provided information about the water motion in the amorphous materials. The Pearson hardness of the respective divalent metal cation predominantly determines the strength of the internal hydrogen bonding network. Amorphous magnesium carbonate deviates from the remaining carbonates, as it contains additional hydroxide ions, which act as strong hydrogen bond acceptors. Amorphous calcium carbonate (ACC) exhibits the weakest hydrogen bonds of all alkaline earth carbonates. Our study provides a coherent picture of the hydrogen bonding situation in theses transient species and thereby contributes to a deeper understanding of the crystallization process of carbonates.
The effect of the heat treatment regime on the hydraulic activity of magnesian materials was determined from the heat release during interaction with water. Hydration processes proceed efficiently, hardening structures are formed after calcination of the initial magnesian materials in the range of 500 – 800°C. Therefore, in the production of hydraulically active magnesium oxide used to make the magnesian binder, it is necessary to bake magnesian materials regardless of their nature at low or moderate temperatures in the range of 500 – 800°C. The change in the structure and properties of magnesian materials during heat treatment has a great influence on the processes of sintering refractory materials.
This study focuses on the development of a new strain-hardening composite (SHC) involving carbonated reactive MgO cement (RMC) and fly ash (FA) as the main binder. Rheological properties of the developed composites were investigated by varying FA and water contents to achieve desirable fiber dispersion. A suitable mix design, in which polyvinyl alcohol (PVA) fibers were introduced to provide tensile ductility, was determined. The effect of key parameters such as w/b ratio and curing age on the mechanical properties of carbonated RMC-SHC was evaluated. Adequate binder content and w/b ratio was necessary for desirable fiber dispersion. Lower water contents and longer curing ages contributed to the strength development of RMC-SHC by improving the fiber-matrix interface bond and enhancing the formation of a dense carbonate network.
A formula for an environmentally friendly and safe fire-proof paint was developed based on a potassium silicate aqueous solution with addition of a magnesium-containing fire retardant. The predominant use in paint of magnesium-containing fire retardant such as hydromagnesite compared to brucite and magnesite was demonstrated. It was established that a paint containing 5.5 wt% of hydromagnesite provides the first group of fire resistance for a wooden surface. The high fire resistance of the paint with this formula is proven by the high intumescence ratio (150%) and low weight losses (8.4%) after tests in a ceramic tube. When the paint is heated, a gradual loss of weight is detected, which is related to the characteristics of the hydromagnesite structure and helps to obtain an effectively foamed protective layer that prevents the diffusion of combustible gases and flame.
Reactive magnesium oxide (RMO) and circulating fluidised bed combustion (CFB) slags were used to prepare magnesium silicate cements using sodium hexametaphosphate (NaHMP) as a water reducer. The effects of curing condition and the initial levels of RMO and silica fume (SF) were studied for up to 90 d. The mechanical properties of the prepared pastes were evaluated through measurements of compressive strength. Mercury intrusion porosimetry (MIP) was employed to investigate the pore structure. X-ray diffraction, thermogravimetric analysis, mercury penetration analysis and scanning electron microscopy (SEM) were carried out to investigate the reaction products and final products. The results indicated that the final products were mainly magnesium silicate hydrate, hydrotalcite phases and hydromagnesite. Steam curing yielded higher strength, increased reaction products and closure of the macropores. SEM analysis showed that the product after steam curing exhibited abundant nanolattice structures (length ≤100 nm). MIP analysis showed that steam curing led to lower porosity and fewer macropores. The specimen prepared with 64 wt% CFB slag, 16 wt% SF and 20 wt% RMO exhibited the highest compressive strength (105 MPa).
Magnesium silicide (Mg2Si) is as known a thermoelectric material with low environmental load and high power factor around 400 °C. It should be performed to solve the problems. i.e., its low oxidation resistance and high chemical activity. In the current decades, oxidation resistance of Mg2Si has been researched because it is essential to thermoelectric power generation. The oxidation factor of Mg2Si still remains unexplained. In this paper, forming process of oxidation resistant layer of Mg2Si powder is examined by thermal analysis and X-ray diffraction. It was revealed that Mg2Si induced to compound with elements of the atmosphere. The oxidation process of Mg2Si was related to humidity without oxide, since oxidation layer absorbs humidity and forms a magnesium hydroxide (Mg(OH)2) layer. The Mg(OH)2 layer was decomposed at 400 °C, which is the temperature required for practical use of thermoelectric generation. It is found that a humidity resistant layer should be formed on the surface of Mg2Si.
Accelerated carbonation of reactive magnesia (MgO)-treated soil is an innovative and sustainable method for improving ground through absorbing CO2. The strength and microstructural properties of various types of carbonated soils at varying initial water contents (w0) are explored, and liquid limit (wL), a parameter representing soil type, is highlighted in this paper. The strength properties of carbonated soils are investigated by unconfined compression tests. To explain the carbonation mechanism, the microstructural properties are obtained from different experimental tests: X-ray diffraction, scanning electron microscopy, thermogravimetric analysis and mercury intrusion porosimetry. The results indicate that large amounts of CO2 and water are consumed in the carbonation process, which results in the reduction of water content and the formation of carbonation products such as nesquehonite and hydromagnesite/dypingite. The strength of carbonated soils decreases with the increase in wL or water content. A simplified equation is proposed for predicting the strength of carbonated soils when both the wL and w0 are given. Silt is easier to absorb CO2 for carbonation than silty clay and clay. Moreover, the pore volume and the quantity of carbonation products decrease with increase in wL, and the content of CO2 absorbed decreases with increase in wL or w0.
The reinforcement technology of carbonation based on reactive magnesia (MgO) and carbon dioxide (CO2) is a low-carbon and high-efficiency foundation treatment method. This paper investigates the compaction, mechanical and microstructural characteristics of carbonated reactive MgO-stabilized silt with varying MgO-soil ratios, carbonation time and water-soil ratios. The results indicate that the maximum dry density of uncarbonated reactive MgO-stabilized silt increases while the optimum moisture content reduces compared to the parent soil. The unconfined compressive strength of reactive MgO-stabilized soil was found to have increased after CO2 carbonation for several hours. With increasing MgO-soil ratio and carbonation time, the failure mode changes from elasticplastic to brittleness, and the failure strain of carbonated specimens mainly ranges between 0.8% and 1.6% and the ratio of the deformation modulus to unconfined compressive strength is about 30 to 200. The water-soil ratio has a slight influence on the evolution of strength. Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) analyses have indicated that the carbonation products facilitate the strength growth of reactive MgO-stabilized silt. Thermogravimetric analysis (TGA) shows that CO2 uptake increases with increasing carbonation time and achieves the highest under the MgO-soil ratio of 20%, carbonation time of 6 h, and water-soil ratio of 25%. The stabilization mechanism of carbonated reactive MgO-stabilized silt is proposed according to microstructure analyses, providing a deeper understanding of the application of the reactive MgO carbonation technology in the ground reinforcement.
This study assesses the influence of mix design on the hydration and carbonation of reactive MgO cement (RMC)-based concrete formulations by varying the water and cement contents. Samples were subjected to accelerated carbonation under 10% CO2 for up to 28 days and compared with corresponding PC-based samples. Their performance was analyzed by compressive strength, porosity, density, water sorptivity and thermal conductivity measurements. XRD, TGA/DSC and FESEM/SEM analyses were employed to investigate the formation of hydration and carbonation products and microstructural development. RMC samples achieved 28-day strengths of 62 MPa, which was comparable with PC samples. Strength gain of RMC samples was accompanied with a substantial decrease in porosity, determined by the amount and morphology of carbonates. The initial water content was more influential on final performance and thermal conductivity than cement content. Lower water contents led to higher strengths due to lower porosities and faster CO2 diffusion within dry mediums.
The reaction of magnesium minerals such as brucite with CO2 is important in the sequestration of CO2. The study of the thermal stability of hydromagnesite and diagenetically related compounds is of fundamental importance to this sequestration. The understanding of the thermal stability of magnesium carbonates and the relative metastability of hydrous carbonates including hydromagnesite, artinite, nesquehonite, barringtonite and lansfordite is extremely important to the sequestration process for the removal of atmospheric CO2. This work makes a comparison of the dynamic and controlled rate thermal analysis of hydromagnesite and nesquehonite. The dynamic thermal analysis of synthetic hydromagnesite proves that dehydration takes place in two steps at 135 and 184°C, dehydroxylation at 412°C and decarbonation at 474°C. Controlled rate thermal analysis shows the first dehydration step is isothermal and the second quasi-isothermal at 108 and 145°C, respectively. In the CRTA experiment both water and carbon dioxide are evolved in an isothermal decomposition at 376°C.
CRTA technology offers better resolution and a more detailed interpretation of the decomposition processes of magnesium carbonates such as nesquehonite via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition processes of non-isothermal nature reveal partial nesquehonite structure.
This study provides experimental evidence for biologically induced precipitation of magnesium carbonates, specifically dypingite (Mg5(CO3)4(OH)2.5H2O), by cyanobacteria from an alkaline wetland near Atlin, British Columbia. This wetland is part of a larger hydromagnesite (Mg5(CO3)4(OH)2.4H2O) playa. Abiotic and biotic processes for magnesium carbonate precipitation in this environment are compared.
Field observations show that evaporation of wetland water produces carbonate films of nesquehonite (MgCO3.3H2O) on the water surface and crusts on exposed surfaces. In contrast, benthic microbial mats possessing filamentous cyanobacteria (Lyngbya sp.) contain platy dypingite (Mg5(CO3)4(OH)2.5H2O) and aragonite. Bulk carbonates in the benthic mats (delta13C avg. = 6.7%, delta 18O avg. = 17.2%) were isotopically distinguishable from abiotically formed nesquehonite (delta13C avg. = 9.3%, delta 18O avg. = 24.9%). Field and laboratory experiments, which emulated natural conditions, were conducted to provide insight into the processes for magnesium carbonate precipitation in this environment. Field microcosm experiments included an abiotic control and two microbial systems, one containing ambient wetland water and one amended with nutrients to simulate eutrophic conditions. The abiotic control developed an extensive crust of nesquehonite on its bottom surface during which [Mg2+] decreased by 16.7% relative to the starting concentration. In the microbial systems, precipitation occurred within the mats and was not simply due to the capturing of mineral grains settling out of the water column. Magnesium concentrations decreased by 22.2% and 38.7% in the microbial systems, respectively. Laboratory experiments using natural waters from the Atlin site produced rosettes and flakey globular aggregates of dypingite precipitated in association with filamentous cyanobacteria dominated biofilms cultured from the site, whereas the abiotic control again precipitated nesquehonite.
Microbial mats in the Atlin wetland create ideal conditions for biologically induced precipitation of dypingite and have presumably played a significant role in the development of this natural Mg-carbonate playa. This biogeochemical process represents an important link between the biosphere and the inorganic carbon pool.
Direct mineral carbonation was investigated as a process to convert gaseous CO2 into a geologically stable final form. The process utilizes a slurry of water, with bicarbonate and salt additions, mixed with a mineral reactant, such as olivine (Mg2SiO4) or serpentine [Mg3Si2O5(OH)4]. Carbon dioxide is dissolved into this slurry, resulting in dissolution of the mineral and precipitation of magnesium carbonate (MgCO3). Optimum results were achieved using heat-pretreated serpentine feed material and a high partial pressure of CO2 (PCO2). Specific conditions include 155°C, PCO2 = 185 atm and 15% solids. Under these conditions, a 78% conversion of the silicate to the carbonate was achieved in 30 minutes. Process mineralogy was utilized to investigate the appropriate feed characteristics, identify the process products and interpret the mineral dissolution and carbonate precipitation reaction paths.
Dypingite was found in materials obtained during the production of high-purity MgO from crude magnesite by the calcination/CO2-leaching process. The snow-white, finely crystalline dypingite contains MgO 41.0%. XRD data reveal a long period structure with a strong peak at 31.0 A; the strongest lines are 10.4(100), 31.0(40) , 15.62(40), 5.86(30), 6.34(20) A. DTA, TGA and IR curves are given; the DTA curve shows 10 endothermic reactions and one sharp exothermic reaction at 495oC.-R.A.H.
A comprehensive low-temperature thermodynamic model for the
geochemically important Na 2CO 3-MgCO
3-CaCO 3-H 2O system is presented. The
model is based on calorimetrically determined Δ fH°
298 values, S° 298 values and C°
p( T) functions taken from the literature as well as on
μ° 298 values of solids derived in this work from
solubility measurements obtained in our laboratories or by others. When
these thermodynamic quantities were combined with temperature-dependent
Pitzer parameters taken from the literature, solubilities calculated for
a wide range of conditions agree well with experimental data. The
results for several subsystems were summarized by depicting the
respective phase diagrams. For the MgO-CO 2-H 2O
subsystem, it was found that the commonly believed stability relations
must be revised, i.e., in the temperature range covered, nesquehonite
never becomes more stable than hydromagnesite at pCO 2
≤ 1 atm. Although the recommended set of thermodynamic data on
sparingly soluble solids was derived from experimental results on mainly
NaClO 4 systems, it can be incorporated in databanks
containing additional Pitzer parameters for modeling more complex fresh-
or seawater systems.
In view of loss prevention and hazard control, traditional engineers use adsorbents to adsorb volatile organic compounds (VOCs) in the semiconductor, photonics, and petrochemical industries. To save funds and promote green energy application, industries usually apply a zeolite processing desorption step under high temperature in the zeolite rotor-wheel system. Many thermal runaway accidents and flame incidents have occurred in the desorption step. Zeolite has been used to adsorb VOCs and applied in the processing desorption step in a reactor without considering oxygen concentration situation, which could easily lead to a flame followed by thermal explosion. Nitrogen is a critically important purge gas regarding passive action for avoiding an accident. Home-made zeolite was investigated for the best manufacturing ratio, which was 20. Brunauer–Emmett–Teller of zeolite (Si/Al = 20) was analyzed to be 400 m2 g−1, which is easy for adsorbing pollutants. According to our previous studies, home-made zeolite has prominent adsorption capacities on VOCs. Zeolite rotor-wheel system was developed to desorb the pollutants of interest. Zeolite was applied to analyze the thermal stability, runaway reaction under various oxygen concentrations, reuse rates, etc. Zeolite is a thermally stable material under room temperature to 650 °C. An endothermic reaction (30–100 °C) of home-made zeolite was analyzed by differential scanning calorimetry and thermogravimetric analyzer. Clearly, water has a significant effect on deteriorating for the zeolite adsorption. Home-made zeolite is a suitable adsorbent and catalyst in the petrochemical and environmental industries. As far as pollution control and loss prevention are concerned, versatility in the analysis of recycled adsorbents is required and is useful for various industrial applications.
The degradability and durability for polymer–nanocomposites under various environmental conditions are from the essential fields of research. This study was carried out to examine the thermal stability of polystyrene loaded by carbon (C) nanoparticles up to 20 wt% content. The thermal degradation of PS/C nanocomposites were studied by thermogravimetry analysis and differential scanning calorimetry (DSC) under non-isothermal condition and inert gas atmosphere at constant heating rate 10 °C min−1. The variation of degradation characteristic temperatures as a function of C content has been a non-monotonic behavior. The obtained results suggested that the C nanoparticles act as a promoter slowing down the degradation and providing a protective barrier to the nanocomposite, except 5 wt% C content. The latter exception was confirmed by DSC curve through the emergence of a small endothermic peak before the fundamental endothermic, melting, one.
Nesquehonite, synthesized in the laboratory, can be readily altered to hydromagnesite via an intermediate phase which is morphologically similar to hydromagnesite. This intermediate phase exhibits an X-ray pattern similar to that of the newly discovered mineral dypingite. The transformation is accompanied by a large loss of water, with resulting increase in magnesium. Chemical analysis indicates that dypingite occurs in a range of phases from slightly altered nesquehonite to hydromagnesite. The alteration of nesquehonite at 52°C is exceedingly rapid, explaining why hydromagnesite is the more common hydrate in nature. This formation of a new mineral via the aqueous phase also occurs in the solid state at temperatures in excess of 100°C, as indicated by differential thermal analysis and thermal gravimetric analysis.
The dramatic increase in atmospheric carbon dioxide since the Industrial Revolution has caused concerns about global warming. Fossil-fuel-fired power plants contribute approximately one third of the total human-caused emissions of carbon dioxide. Increased efficiency of these power plants will have a large impact on carbon dioxide emissions, but additional measures will be needed to slow or stop the projected increase in the concentration of atmospheric carbon dioxide. By accelerating the naturally occurring carbonation of magnesium silicate minerals it is possible to sequester carbon dioxide in the geologically stable mineral magnesite (MgCO3). The carbonation of two classes of magnesium silicate minerals, olivine (Mg2SiO4) and serpentine (Mg3Si2O5(OH)4), was investigated in an aqueous process. The slow natural geologic process that converts both of these minerals to magnesite can be accelerated by increasing the surface area, increasing the activity of carbon dioxide in the solution, introducing imperfections into the crystal lattice by high-energy attrition grinding, and in the case of serpentine, by thermally activating the mineral by removing the chemically bound water. The effect of temperature is complex because it affects both the solubility of carbon dioxide and the rate of mineral dissolution in opposing fashions. Thus an optimum temperature for carbonation of olivine is approximately 185 degrees C and 155 degrees C for serpentine. This paper will elucidate the interaction of these variables and use kinetic studies to propose a process for the sequestration of the carbon dioxide.
It has been suggested that the highly hydrated character of the Mg 2+ ion in aqueous solution is responsible for the often encountered difficulty of precipitating stable, anhydrous phases of magnesium carbonate and calcium-magnesium carbonate. In an effort to investigate this, a study of magnesite crystallization kinetics was undertaken, utilizing the reaction of hydromagnesite plus CO 2 to yield magnesite at 126°C. The reactions were characterized by prolonged initial quiescent periods prior to the onset of detectable crystallization. The length of the initial period was found to vary with Mg concentration, pCO 2 and ionic strength. Contrary to classical kinetics, the reaction studied was inhibited by increased Mg concentration. Ionic strength and pCO 2 acted as positive catalysts.
Chemically pure lansfordite (MgCO 3.5H 2O) and nesquehonite (MgCO 3.3H 2O) were synthesized from CO 2-saturated Mg(HCO 3) 2 solutions. Crystalline and chemical properties of the synthesized minerals were determined and results were compared with the properties reported for geological specimen minerals. Crystalline data determined for synthesized lansfordite are: monoclinic; a 1.249, b 0.762, c 0.743 nm, beta 1.785 + or - 0.002o, Z = 4. Nesquehonite is monoclinic (pseudo-orthorhombic); a 1.199, b 0.769, c 0.537 nm, beta 1.573 + or - 0.012o, Z = 4. Lansfordite is unstable when exposed to the atmosphere at T >10oC. Large lansfordite crystals (>150 mu m) were observed with SEM to undergo dehydration above 10oC. XRD data and SEM observations indicate that, upon dehydration, large lansfordite crystals convert to nesquehonite but remain pseudomorphous after lansfordite. Small lansfordite crystals (<20 mu m) appeared to dissolve upon an increase in T >10oC when viewed with SEM. Crystal forms of nesquehonite, lansfordite and dehydration states of lansfordite are shown in SEM micrographs. (Authors' abstract)-C.N.
Dypingite, Mgs(COa)4(OH)2-SH20 , was found in materials obtained during the production of high purity magnesia from crude magnesite by the calcination/ CO2-1eaching process. The snow-white, finely crystalline dypingite contained 41.0 ~o MgO; the theoretical value is 41.5 ~. The X-ray powder data revealed a long period structure with a strong peak at 31.0 A. The five strongest reflections (/~) and their estimated intensities (I/Io) were 10.4 (100), 31.0 (40), 15.62 (40), 5.86 (30), and 6.34 (20). The DTA curve showed ten endothermic reactions with maxima at 43, 50, 77, 86, 105, 127, 180, 260, 420, and 514~ and an exothermic reaction at 495 ~ As part of a detailed examination of the produc- tion of high purity magnesia from crude magnesite by the calcination/CO2-1eaching process (Canter- ford et al., 1981, 1983), the phases that precipitate when the clarified magnesium bicarbonate solution is heated and/or aerated are being studied. The chemical composition of the precipitate that forms is affected markedly by the temperature of the solution. For a carbon dioxide partial pressure of 1 atmosphere, the approximate transformation temperatures are reported as follows (Langmuir, 1965). IO~ 55~
No.: 99
A comprehensive low-temperature thermodynamic model for the geochemically important Na2CO3−MgCO3−CaCO3−H2O system is presented. The model is based on calorimetrically determined ΔfH°298 values, S°298 values and C°p(T) functions taken from the literature as well as on μ°298 values of solids derived in this work from solubility measurements obtained in our laboratories or by others. When these thermodynamic quantities were combined with temperature-dependent Pitzer parameters taken from the literature, solubilities calculated for a wide range of conditions agree well with experimental data. The results for several subsystems were summarized by depicting the respective phase diagrams. For the MgO−CO2−H2O subsystem, it was found that the commonly believed stability relations must be revised, i.e., in the temperature range covered, nesquehonite never becomes more stable than hydromagnesite at pCO2 ≤ 1 atm. Although the recommended set of thermodynamic data on sparingly soluble solids was derived from experimental results on mainly NaClO4 systems, it can be incorporated in databanks containing additional Pitzer parameters for modeling more complex fresh- or seawater systems.
The reaction path in the MgO–CO2–H2O system at ambient temperatures and atmospheric CO2 partial pressure(s), especially in high-ionic-strength brines, is of both geological interest and practical significance. Its practical importance lies mainly in the field of nuclear waste isolation. In the USA, industrial-grade MgO, consisting mainly of the mineral periclase, is the only engineered barrier certified by the Environmental Protection Agency (EPA) for emplacement in the Waste Isolation Pilot Plant (WIPP) for defense-related transuranic waste. The German Asse repository will employ a Mg(OH)2-based engineered barrier consisting mainly of the mineral brucite. Therefore, the reaction of periclase or brucite with carbonated brines with high-ionic-strength is an important process likely to occur in nuclear waste repositories in salt formations where bulk MgO or Mg(OH)2 will be employed as an engineered barrier. The reaction path in the system MgO–CO2–H2O in solutions with a wide range of ionic strengths was investigated experimentally in this study. The experimental results at ambient laboratory temperature and ambient laboratory atmospheric CO2 partial pressure demonstrate that hydromagnesite (5424) (Mg5(CO3)4(OH)2 · 4H2O) forms during the carbonation of brucite in a series of solutions with different ionic strengths. In Na–Mg–Cl-dominated brines such as Generic Weep Brine (GWB), a synthetic WIPP Salado Formation brine, Mg chloride hydroxide hydrate (Mg3(OH)5Cl · 4H2O) also forms in addition to hydromagnesite (5424).
The thermal decomposition of hydromagnesite, 4MgCO3 - Mg(OH)2 -4H2O, was studied under two typical experimental conditions: the condition in which an exothermic phenomenon at ~ 520 °C was absent (PCO2 = 0.00 atm) and the one in which it was present (Pco2 = 0.50 atm).The specimen formed an amorphous phase including a lower carbonate intermediate after dehydration was completed at ~300°C. At PCO2 = 0.00 atm, decarbonation proceeded and MgO was formed at ~ 500°C. At PCO2 = 0.50 atm, crystallization of MgCO3, the evolution of heat and a rapid evolution of carbon dioxide took place coincidentally at ~ 520°C. The MgCO3 was decomposed at higher temperatures.A model was proposed to explain the exothermic phenomenon; the crystallization of MgCO3 was assumed to cause the rapid evolution of heat which decomposed the specimen and generated the rapid gas evolution when the specimen powder was packed tightly.
Isothermal and non-isothermal decomposition of hydromagnesite 4 MgCO3 · Mg(OH)2 · 4 H2O was studied thermogravimetrically. Decarbonation was strongly influenced by the partial pressure of carbon dioxide. Decarbonation in an argon atmosphere proceeded via an amorphous lower carbonate to MgO. Decarbonation in a carbon dioxide atmosphere was interrupted at ∼460–480°C. This interruption was explained by the formation of a metastable intermediate and the subsequent crystallization of MgCO3, both from the amorphous lower carbonate. This explanation was supported by DTA and power X-ray diffraction analysis of the quenched specimens.
A study has been made of the thermal decomposition of MgCO3 . 3H2O (nesquehonite) and MgCO3 . (NH4) 2CO3 . 4H2O using a variety of experimental techniques, including thermogravimetric analysis, differential thermal analysis and optical microscopy. The solids remaining at various stages during decomposition were characterized by measure-ment of their X-ray powder diagrams, surface areas, and particle densities. Nesquehonite decomposes first to crystalline MgCO3. H2O, then to an amorphous magnesium carbonate of essentially zero surface area and finally to high area "active" magnesia (∼ 250 m2/g). Magnesium ammonium carbonate loses (NH4)2CO3 and most of its water below 100°C to give a second amorphous magnesium carbonate differing from that derived from nesquehonite in having a high surface area (∼ 400 m 2/g) and smaller CO2 content. Both of the amorphous carbonates have the pseudomorphic form of the parent crystal, and both recrystallize abruptly to magnesite on heating to 510°C in an atmosphere of CO2. The rehydration of these two amorphous carbonates is investigated. A tentative picture of their disordered relic structure is advanced. In the presence of steam, MgCO3 . 3H2O undergoes hydrothermal decomposition at 100°C to hydromagnesite. The pyrolysis of hydromagnesite is studied briefly. Some properties of active magnesia are discussed in an addendum.
Ten ancient mortars from the Church of Santa Marı́a de Zamarce (Navarra, Spain) have been studied by means of chemical analysis, X-ray diffraction and simultaneous thermogravimetric and differential thermal analysis (TGA–DTA). All the studied samples have shown to be dolomitic mortars. Sixteen simulated standards of typically present compounds in dolomitic mortars were prepared and studied. Particularly the occurrence of hydrated magnesium carbonate hydroxide (hydromagnesite (HY), Mg5(CO3)4(OH)2·4H2O) as result of the setting of dolomitic mortars is discussed. No HY has been checked in mortars from Santa Marı́a de Zamarce. However, TGA–DTA studies have clearly shown the thermal decomposition of HY in the prepared mixtures, specially the exothermic peak at 500°C. The experimental conditions in TGA–DTA, with a high CO2 pressure around the sample, seem to play an important role in the occurrence of this phenomenon. Suitable experimental conditions for thermal studies of these materials have been indicated.
The thermal analysis of basic magnesium carbonate has been investigated by thermogravimetry (TGA), derivative thermogravimetry (DTGA) and differential thermal analysis (DTA). The end product was magnesium oxide formed from the decomposition of magnesium carbonate. The composition of the samples studied varied, but the general formula that represented them was xMgCO3·yMg(OH)2·zH2O. In the present study, the thermal analysis of such magnesium carbonates in nitrogen, carbon dioxide and air was carried out at different heating rates and the origin of the exothermic peak was studied. Generally, the decomposition peaks were endothermic, but a persistent exothermic peak was noted, always accompanied by a very sharp drop in the TG curve, and the weight loss in this region may be significant. The exothermic DTA peak was found to be strongly influenced by the rate at which the samples were heated, sample size and atmospheric conditions.
Thermal analysis (TG, DTG, DTA) of a number of basic magnesium carbonate samples, prepared by precipitation using different magnesium salts (magnesium nitrate, sulphate, chloride, and acetate) and precipitating agents (viz. sodium carbonate and bicarbonate and potassium carbonate and bicarbonate) at different concentrations of magnesium salt (0.005–2.0 M), temperatures (0–100°C) and pH9-11 using different modes of mixing in the precipitation and also by ageing the precipitate for different periods (0.5–96 h) has been carried out in the temperature range 30–600°C. The thermal decomposition and surface area of the resulting MgO are found to be strongly influenced by the preparation conditions of basic magnesium carbonate.
The processes, which influence the decomposition of 4MgCO3 ·-Mg(OH)2 · 4H2O, could be determined by systematical variation of the analytical parameters. The original crystal structure exists in a wide temperature range during the decomposition. The formation of magnesite is a secondary reaction of the gaseous phase with the reaction product.H2O and CO2, are released simultaneously in different proportions during the decomposition to 500°C. Stoichiometric intermediates were not found. The original crystal structure collapsed, when the last H2O escapes. The ratio of MgCO3: MgO can be influenced by partial pressure of CO2 in a wide range.ZusammenfassungDurch systematische Variation der Analysenparameter konnten für den komplexen Zersetzungsverlauf von 4MgCO3·Mg(OH)2·4H2O die reaktionsbestimmenden Prozesse erfasst werden. Die Ausgangsstruktur bleibt in einem relativ weiten Temperaturbereich während des Zersetzungsvorganges haltbar. Eine Magnesitbildung erfolgt unter Normaldruck durch eine Sekundärreaktion der Gasphase mit dem Reaktionsprodukt.Bei der Zersetzung wird bis ca. 500°C in unterschiedlichem Verhältnis gleichzeitig H2O und CO2 freigesetzt. Definierte Zwischenverbindungen liessen sich bei der dynamischen Zersetzung nicht nachweisen. Endpunkt der Zersetzung ist der Zusammenbruch des Ausgangsgitters unter Abgabe des letzten Wassergehaltes. Das Verhältnis von MgCO3:MgO kann durch den CO2 Partialdruck in relativ weiten Grenzen beeinfiusst werden.
The preparation of a magnesium hydroxy carbonate from magnesium hydroxide and carbon dioxide is described. The procedure involves the formation of a magnesium hydroxide slurry and sparging CO2 gas through it. Various experimental conditions are evaluated in order to obtain the conditions that result in the formation of the magnesium hydroxy carbonate. Slurry pH, slurry temperature, drying temperature, drying time and HCl addition are the conditions that are evaluated. The products that are obtained are identified by XRD and its decomposition characteristics studied with TG/DTA. It is evident from the results obtained that the experimental parameters have a significant influence on the products obtained.
Direct mineral carbonation has been investigated as a process to convert gaseous CO2 into a geologically stable final form. The process utilizes a slurry of water, with bicarbonate and salt additions, mixed with a mineral reactant, such as olivine (Mg2SiO4) or serpentine [Mg3Si2O5(OH)4]. Carbon dioxide is dissolved into this slurry, resulting in dissolution of the mineral and precipitation of magnesium carbonate (MgCO3). Optimum results have been achieved using heat pretreated serpentine feed material and high partial pressure of CO2 (PCO2). Specific conditions include: 155?C; PCO2=185 atm; 15% solids. Under these conditions, 78% conversion of the silicate to the carbonate was achieved in 30 minutes. Process mineralogy has been utilized to characterize the feed and process products, and interpret the mineral dissolution and carbonate precipitation reaction paths.
The occurrence of dolomites with phosphorites in geosynclinal areas of the U. S. S. R. , the wide distribution of dolomitic sediments in the Paleozoic and their absence in the Jurassic and Cretaceous of the Russian platform, and the absence of dolomite formation in modern marine sediments have led to the study of their origin. The possibility of a simultaneous (synchronous) development of phosphorites and dolomites (magnesites) is excluded. Concurrence is explained by subsequent impositions of diagenetic and epigenetic dolomites on previously deposited phosphate. Primary dolomites are negative prospecting indicators for phosphorites; secondary dolomites are not. Analysis of magnesite and dolomite systems at the 20°, 60°, and 150°C isotherms is offered in the experimental part of this report. Fields of crystallization and stability are defined for nesquehonite, magnesite, dolomite, basic magnesium carbonate (artinite and hydromagnesite), and brucite systems.
An exothermic phenomenon and a simultaneous rapid evolution of a small amount of carbon dioxide at -500°C during thermal decomposition of hydromagnesite 4 MgCO3 · Mg(OH)2 · 4 H2O was studied by isothermal DSCTG in a carbon dioxide atmosphere. It was quantitatively confirmed that the exothermic phenomenon was due to crystallization of MgCO3 from the amorphous phase and that the evolution of carbon dioxide was due to decomposition of the MgCO3 by the heat of crystallization ({reversed tilde equals}3.4 kcal mole-1.
Thermal decomposition of basic magnesium carbonates, hydromagnesite 4MgCO3·Mg(OH)2·4H2O and nesquehonite MgCO3·3H2O, was studied under high-pressure carbon dioxide, nitrogen and argon atmospheres (≤ 50 kg cm−2) by high-pressure DTA.The decarbonation was strongly affected by the partial pressure of carbon dioxide. A new decomposition process with a new metastable intermediate was found at high-pressure carbon dioxide atmospheres: hydromagnesite, amorphous dehydrated, amorphous lower carbonate, intermediate, MgCO3, MgO. A model was proposed to explain the decomposition mechanism integratedly throughout the various partial pressures of carbon dioxide; the formation of the intermediate governed the decarbonation rate and the crystallization of MgCO3 from the amorphous lower carbonate at ∼500°C.
Abstract Fossil fuels play a crucial role in satisfying growing world energy demands, but their continued use could cause irreparable harm to the environment. Unless virtually all anthropogenic carbon dioxide is captured, either at the source or subsequently from the air, and disposed of safely and permanently, fossil fuels may have to be phased out over the next few decades. Sequestration of waste carbon dioxide will require methods that can safely store several trillion tons of carbon dioxide. Long-term storage of a gaseous substance is fraught with uncertainty and hazards, but carbonate chemistry offers permanent solutions to the disposal problem. Carbonates can be formed from carbon dioxide and metal oxides in reactions that are thermodynamically favored and exothermic, which result in materials that can be safely and permanently kept out of the active carbon stocks in the environment. Carbonate sequestration methods require the development of an extractive minerals industry that provides the base ions for neutralizing carbonic acid.
The rehydration characteristics of a commercially produced hydromagnesite and two basic magnesium carbonates synthetically
produced from Mg(OH)2, are presented. The products were dehydrated and dehydroxylated at 325°C before rehydration was attempted. DTA and FT-IR
were used to follow the structural changes that occurred during the rehydration processes. The results obtained for the commercially
and synthetically produced hydromagnesite products indicated that the original symmetry of the groups was reclaimed during
rehydration. This was not observed for the synthetically produced unidentified basic magnesium carbonate product. This investigation
provides insight into the rehydration characteristics of a select group of basic magnesium carbonates.
The products of forsterite dissolution and the conditions favorable for magnesite precipitation have been investigated in experiments conducted at temperature and pressure conditions relevant to geologic carbon sequestration in deep saline aquifers. Although forsterite is not a common mineral in deep saline aquifers, the experiments offer insights into the effects of relevant temperatures and PCO2 levels on silicate mineral dissolution and subsequent carbonate precipitation. Mineral suspensions and aqueous solutions were reacted at 30 °C and 95 °C in batch reactors, and at each temperature experiments were conducted with headspaces containing fixed PCO2 values of 1 and 100 bar. Reaction products and progress were determined by elemental analysis of the dissolved phase, geochemical modeling, and analysis of the solid phase using scanning electron microscopy, infrared spectroscopy, and X-ray diffraction. The extent of forsterite dissolution increased with both increasing temperature and PCO2. The release of Mg and Si from forsterite was stoichiometric, but the Si concentration was ultimately controlled by the solubility of amorphous silica. During forsterite dissolution initiated in deionized water, the aqueous solution reached supersaturated conditions with respect to magnesite; however, magnesite precipitation was not observed for reaction times of nearly four weeks. Magnesite precipitation was observed in a series of experiments with initial solution compositions that simulated extensive forsterite dissolution. The precipitation of magnesite appears to be limited by the process of nucleation, and nucleation requires a critical saturation index between 0.25 and 1.14 at 95 °C and 100 bar PCO2. Magnesite precipitation is fastest in the presence of an initial magnesite seed. Although magnesite precipitates do form on the surfaces of forsterite particles, the presence of the forsterite surface does not significantly accelerate magnesite precipitation relative to solid-free systems.
Nesquehonite (MgCO3·3H2O) (N) can be obtained from a dolomitic quicklime paste in a CO2-rich atmosphere. Thermal decomposition of this synthetic nesquehonite has been studied by TG-DTA analysis. It is very similar to the hydromagnesite (HY) thermal decomposition, as it show similarities in the decarbonations at 440 and 550 °C, and an exothermic phenomenon at 510 °C.It has been reported in the literature that some intermediate in N→HY transformation occur, but this process cannot be directly detected during the heating. Samples were heated at 115, 230, 280, 370, 460, 520, 600, 800 and 1000 °C and XRD and FT-IR were used in order to determine the structural changes in nesquehonite and the intermediate phases formed.Results show that nesquehonite transforms at lower temperatures (115 °C) into a stable amorphous magnesium carbonate with chemical composition very close to that of HY. Thermal decomposition of nesquehonite, during a gradual temperature increase, proceeds via the formation of this compound. At higher temperatures (460 °C/short heating times), nesquehonite transforms into HY.The occurrence of an exothermic peak at 510 °C has been also discussed.
With the use of thermogravimetry and dilatometry the maximum temperatures attained at various locations in a concrete construction during a fire can be determined, if samples of the concrete can be obtained within 1 or 2 days of the fire. On peut déterminer les températures maximales atteintes en divers endroits d'une construction en béton au cours d'un incendie en se servant de la thermogravimétrie et de la dilatométrie, si des échantillons de béton peuvent être pré levés un ou deux jours après l'incendie. RES
Pure nesquehonite (MgCO3.3H2O)/Mg(HCO3)(OH).2H2O was synthesised and characterised by a combination of thermo-Raman spectroscopy and thermogravimetry with evolved gas analysis. Thermo-Raman spectroscopy shows an intense band at 1098 cm-1 which shifts to 1105 cm-1 at 450 degrees Celsius, assigned to the ν1 CO32- symmetric stretching mode. Two bands at 1419 and 1509 cm-1 assigned to the ν3 antisymmetric stretching mode shift to 1434 and 1504 cm-1 at 175 degrees Celsius. Two new peaks at 1385 and 1405 cm-1 observed at temperatures higher than 175 degrees Celsius are assigned to the antisymmetric stretching modes of the (HCO3)- units. Throughout all the ThermoRaman spectra a band at 3550 cm-1 attributed to the stretching vibration of OH units. Raman bands at 3124, 3295, and 3423 are assigned to water stretching vibrations. The intensity of these bands is lost by 175 degrees Celsius. The Raman spectra were in harmony with the thermal analysis data. This research has defined the thermal stability of one of the hydrous carbonates namely nesquehonite. ThermoRaman spectroscopy enables the thermal stability of the mineral nesquehonite to be defined and further the changes in the formula of nesquehonite with temperature change can be defined. Indeed Raman spectroscopy enables the formula of nesquehonite to be better defined as Mg(OH)(HCO3).2H2O.
In this paper is reported a novel method to synthesize nesquehonite, MgCO(3) x 3H(2)O, via reaction of a flux of CO(2) with Mg chloride solution at 20+/-2 degrees C. The reaction rate is rapid, with carbonate deposition almost complete in about 10 min. The full characterization of the product of synthesis has been performed to investigate its potential role as a "CO(2)-sequestering medium" and a means of disposing Mg-rich wastewater. We investigated the nesquehonite synthesized using SEM, XRD, FTIR and thermal analysis. The thermodynamic and chemical stability of this low-temperature hydrated carbonate of Mg and its possible transformation products make our method a promising complementary solution to other methods of CO(2) sequestration. Carbonation via magnesium chloride aqueous solutions at standard conditions represents a simple and permanent method of trapping CO(2). It could be applied at point sources of CO(2) emission and could involve rejected brine from desalination plants and other saline aqueous wastes (i.e., "produced water"). The likelihood of using the resulting nesquehonite and the by-products of the process in a large number of applications makes our method an even more attractive solution.
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