# Fabrication of Iron-Magnesite under Mild Conditions through Indirect Carbonation of Peridotite and Its Utilization as a Partial Substitute for Cement in Concrete

German Montes-Hernandez, Jérôme Weiss, Sarah Clavier, Benoît Cordonnier, Paul Tafforeau, Olivier Plé, François Renard

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German Montes-Hernandez, Jérôme Weiss, Sarah Clavier, Benoît Cordonnier, Paul Tafforeau, et al. Fabrication of Iron-Magnesite under Mild Conditions through Indirect Carbonation of Peridotite and Its Utilization as a Partial Substitute for Cement in Concrete. Industrial and engineering chemistry research, 2025, 64 (27), pp.13770-13783. 10.1021/acs.iecr.5c00744. hal-05134211

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Fabrication of iron-magnesite under mild conditions through
indirect carbonation of peridotite and its utilization as partial
substitute for cement in concrete

German Montes-Hernandez<sup>a*</sup>, Jérôme Weiss<sup>a</sup>, Sarah Clavier<sup>a</sup>, Benoît Cordonnier<sup>b</sup>, Paul Tafforeau<sup>b</sup>, Olivier Plé<sup>c</sup>, François Renard<sup>a,d</sup>

<sup>a</sup> ISTerre, Univ. Grenoble Alpes, Grenoble INP, Univ. Savoie Mont Blanc, CNRS, IRD, Univ.
Gustave Eiffel, 38000 Grenoble, France

<sup>b</sup> The European Synchrotron Radiation Facility (ESRF), 38000 Grenoble, France

<sup>c</sup> Université Savoie Mont Blanc, CNRS, LOCIE, 73000 Chambéry, France

<sup>d</sup> The Njord Centre, Department of Geosciences and Physics, University of Oslo, 0316 Oslo,
Norway

*Corresponding author: Dr. German Montes-Hernandez

E-mail address: german.montes-hernandez@univ-grenoble-alpes.fr
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The precipitation of magnesite and dolomite from aqueous media is a challenging kinetically
because several hydrated or hydroxylated Mg-carbonates phases can retard or inhibit the
formation of these “enigmatic” minerals under so-called mild conditions (T≤90°C and
atmospheric pressure). Here, we demonstrate that Fe-magnesite can be synthetized at 60°C in
only two days via indirect carbonation of peridotite, following an amorphous-to-nesquehonite-to-
magnesite transformation route. In this reactive configuration, the magnesite production-time is
significantly reduced from 2 days to only 6 hours when temperature is increased at 90°C. The
discovery of these mild conditions for magnesite production is a promising result because
currently these temperatures can be reached by free-carbon energies. We also assess the
incorporation of fabricated magnesite-rich material at 90°C as a partial cement substitute in
concrete manufacturing. Preliminary results show that substituting 15wt.% of cement by
magnesite-rich material could significantly reduce the CO₂ footprint with minimal impact on the
concrete's mechanical-textural properties, including Young's modulus, strength and macro-
porosity. In the practice, this circular carbon approach could provide a dual advantage—reduced
clinker production and permanent CO₂ storage—and presents a significant opportunity to lower
the carbon footprint of cement production.

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40 **Keywords:** Magnesite production; Carbon dioxide utilization; Circular flow material process;

41 Concrete manufacturing; Mechanical properties

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# 1. INTRODUCTION

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Approximately 38±3 Gt of CO₂ from fossil fuel consumption and industrial activities are emitted annually, representing an increase of about 67% since the 1990's. While natural cycles such as photosynthesis and chemisorption in soils and oceans help mitigate this production, a significant surplus remains, contributing to global warming, ocean acidification and major disruptions of ecosystems and climate (1). Concrete is the most widely manufactured material, with production reaching 20 Gt/year. Portland cement accounts for 36% of the 7.7 Gt of CO₂ emissions released in 2010 by construction activities (2) and approximately 8% of total anthropogenic CO₂ emissions (3). Decarbonizing the cement industry is crucial for reducing greenhouse gas emissions, as its CO₂ emissions primarily arise from energy consumption (~1/3) and the calcination of limestone (~2/3) during clinker fabrication (2-6). While the first source can be significantly reduced with low-carbon energies, direct emissions of CO₂ from calcination present more profound challenges. In this context, CO₂ capture from industrial sources (e.g., cement production, metallurgy, and power plants) and its transformation into reusable products (e.g., ethanol, syngas, etc.) and/or non-energetic materials (e.g., magnesite, dolomite, calcite, siderite, and other stable anhydrous carbonates) has been proposed as a means to stabilize CO₂ concentration in the atmosphere (1, 7-10). These measures help maintain a low carbon balance while supporting the transition to carbon-free energy sources (11).

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Ex-situ mineral trapping, the process of indirectly carbonating raw peridotite, serpentinites, basalts or mining wastes to produce readily usable stable and anhydrous Mg-carbonates (e.g., magnesite, iron-magnesite, dolomite, and ankerite), is a promising strategy for significantly reducing industrial CO₂ emissions and aligns with international carbon capture,
79 utilization, and storage programs (1, 7-15). Notably, raw materials/rocks and/or mining wastes 80 containing high concentrations of Mg and other divalent cations such as Fe and Ca, essential for 81 producing stable carbonates, are abundant worldwide (13). However, optimizing process 82 conditions to produce magnesite-rich carbonates under mild conditions and validating their 83 specific applications remain significant challenges.

84 Magnesite (MgCO₃), dolomite (MgCa(CO₃)₂) and magnesite-rich solid solutions (Mg₁-xMxCO₃, 85 where M indicates Fe²⁺ and Ca²⁺) are the most stable-anhydrous carbonate minerals under typical 86 Earth surface conditions, with the highest resistance to leaching and weathering (8). For example, 87 the dissolution rates of magnesite and dolomite are 100-1000 times lower than that of calcite 88 (CaCO₃) across a wide range of conditions, from ambient temperature to 150°C and pH levels 89 ranging from 1 to 14. Thus, these engineered anhydrous Mg carbonates are considered highly 90 relevant minerals for the permanent storage of anthropogenic carbon dioxide. Moreover, both 91 minerals are ready for use as profitable construction materials, filler mineral additives, and flame 92 retardants (8, 16). The precipitation kinetics of magnesite and dolomite (ordered double 93 carbonates) have been widely studied since their abiotic precipitation at ambient temperature 94 (~25°C) is virtually impossible within feasible experimental time scales (8, 16-18). This contrast 95 strongly with anhydrous calcium carbonates (calcite, aragonite and vaterite) than can be easily 96 formed at room temperature under abiotic conditions (17). The strong solvation shell of 97 magnesium ions in aqueous media plays an essential role in this limitation (18 and ref. therein). 98 However, the sole effect of magnesium hydration may not be the only factor inhibiting magnesite 99 and/or dolomite formation. For dolomite for example, recent studies claim that a strong-energetic 100 crystallization barrier prevent the formation and/or regular Ca-Ma alternation in a long-range of
crystal structure at ambient conditions. The fluid chemistry seems also have an important
influence on the magnesite and dolomite formation under mild conditions (e.g., 16-18). In this
context, studies have revealed that carbonate speciation close to pKa2 (pH~10.3), high ionic
concentration (<1M), and the presence of other competitive divalent cations and counterions
accelerate the formation of magnesite and proto-dolomite under ambient and moderate
temperatures (≤90°C) (18-19). Other studies have reported magnesite precipitation under mild
conditions, but in general, catalytic agents (biotic and abiotic) are used, and several days or
weeks are required (20-22).

To overcome these above limitations, we have developed a novel circular-material-flow
laboratory method for the fabrication of liter-scale amounts of iron-magnesite under mild
conditions (T≤90°C and atmospheric pressure), within short time frames of 1 to 48 hours, via
indirect carbonation of peridotites. In this way, the technological readiness level (TRL) of our
process is closed to 2, but, promising basic research was obtained because specific
physicochemical mild conditions were discovered on the magnesite formation at 60°C via direct
transformation of nesquehonite into magnesite. In complement, hydrothermal conditions up to
200°C under saturation pressure were also tested for kinetic comparison.

The primary target product is iron-magnesite, as this anhydrous mineral has multiple applications (e.g., 8). For instance, it can serve as partial substitute for cement during concrete fabrication, significantly reducing CO2 emissions from the cement industry (e.g., 23-25). Additionally, other products of industrial interest can be obtained from this process. For example, nesquehonite (MgCO3·3H2O), a promising green building material (13, 26), can be produced at room temperature when peridotite is leached. Alternatively, siderite (FeCO3) and nesquehonite are
preferentially formed when basalt-rich material is used in the leaching step. In this way, we also assess the use of produced iron-magnesite at 90°C as a partial substitute of cement in concrete fabrication. Herein, Young's modulus, strength and macro-porosity were determined on cylindrical concrete samples in order to assess the impact of magnesite substitution on the concrete performance. Magnesite produced at 90°C was chosen because only 5-6h are required for their synthesis. In the present study, iron-magnesite material refers to magnesite containing up to 10% Fe(II) and minor proportions of adhered iron oxides nanoparticles containing Fe(III).

## 2. MATERIALS AND METHODS

### 2.1 CO₂ capture step

Concentrated NaOH solutions (from 1 to 6M) were used to capture CO₂ as ionic species (CO₃²⁻ and HCO₃⁻) at moderate pressure (PCO₂≤55bar) and ambient temperature (25°C). A specified amount of NaOH and 1 L of water were placed into a reactor cell with 2 L capacity. Then, a defined amount of CO₂ was injected into the reactor at pressures from 10 to 55 bars. The pressure drop related to the CO₂ chemisorption process and the exothermicity (temperature increase during CO₂ chemisorption) were directly monitored in this anisobaric system. Isobaric conditions (20 bars) and sequential CO₂ injections (20+10+10...up to a cumulative total of 100 bars for higher NaOH concentrations) were also tested. In all cases, the maximum reaction duration was 24 hours.

The optimization, reaction mechanisms, and kinetics were determined using time-resolved Raman spectroscopy measurements, as demonstrated in previous studies conducted in our group (8, 17-19). Real-time monitoring enabled better control of the carbonate speciation in the
resulting alkaline solutions ($CO_3^{2-}$-rich solution, $CO_3^{2-}$/HCO$_3^-$ mixed solution, and HCO$_3^-$-rich solution). Moreover, $NaHCO_3$ crystalline-powdered byproduct that have several well-known industrial applications, can also be directly obtained during the carbonation process of NaOH solutions.

## 2.2. Leaching step

Divalent cations (mainly Mg and Fe) in solution were obtained from natural peridotite and basalt-peridotite mixture (collected in France, central massif) through a simple leaching process using acidic (HCl) solutions. The ground samples (millimetric grains ranging from 0.6 to 5mm) were treated with concentrated acidic HCl solutions (up to 10% concentration) at room and moderate temperatures (up to 60°C) for varying durations (up to 10 days) in reactors without agitation (static system), in order to obtain Mg-Fe-rich solutions. Several experiments were conducted to optimize the reaction time, HCl concentration, solution-to-solid weight ratio and temperature. In the present study, raw rocks were used as the source of divalent cations; however, many industrial wastes, such as steel slags, mining waste (except carbonates or sedimentary rocks) and asbestos waste, could also be used and are generally more reactive than raw rocks.

## 2.3. CO$_2$ mineralization or carbonate precipitation step

Solutions obtained from $CO_2$ capture (preferentially alkaline $CO_3^{2-}$/HCO$_3^-$ solution with concentrations close to $pK_{a2}$) and from the leaching step (Mg-Fe-rich acidic solution) were then used to precipitate stable carbonates (preferably anhydrous) by controlled mixing (at given addition rate, 2-12 ml/min) of the two solutions. The main objective was to produce magnesite and Mg-Fe anhydrous carbonates at moderate temperatures (<90°C) preferably, or at higher
temperatures (up to 200°C) within a reduced time. Several of these experiments were monitored
in real-time using Raman spectroscopy, a technique employed to observe the nucleation of
various minerals under mild conditions (8, 17-19). Factors such as the alkaline-solution/acidic-
solution volume ratio, mixing mode, reaction time, and heat-ageing temperature were
investigated. Iron-magnesite (500g) intended for use as a cement substitute in concrete samples
was synthesized at 90°C with a synthesis duration of 24h in all experiments.

All produced carbonates were recovered by simple settling and the residual salt-rich solution was
naturally evaporated in mini-pools (20x20 cm) at ambient temperature to recover the salt (mainly
NaCl).

## 2.4. Ex situ characterization of solutions and precipitates

Density and ionic concentrations were measured for selected solution samples using conventional
methods. Density was determined by weighing the mass of 100 mL of solution with a high-
precision analytical balance. Concentrations of Mg, Fe, Ca, Al, Si, K, and Na were dosed using
ICP-AES. Finally, CO₃²⁻ and HCO₃⁻ concentrations were determined by Raman spectroscopy.

The solid products were recovered by simple settling and washed twice with tap water in order to
remove soluble salts. They were then dried in air at 60°C for 24-48 hours. The dried solid
products were stored in plastic flasks for subsequent characterization of selected samples by Field
Emission Gun Scanning Electron Microscopy (FESEM) and powder X-ray diffraction (XRD).

Powder XRD spectra were acquired using a Siemens D5000 diffractometer in Bragg-Brentano
geometry, equipped with a theta-theta goniometer with a rotating sample holder. Diffraction
patterns were collected using Cu kα₁ (λₖα₁=1.5406 Å) and Cu kα₂ (λₖα₂=1.5444 Å) radiations in
the range $2\theta = 10 - 70^{\circ}$, with a step size of 0.04° and a counting time of six seconds per step. For high-resolution imaging, the solid products were dispersed by ultrasonic treatment in absolute ethanol for five minutes. Two droplets of the suspension were then deposited directly onto an aluminum support and observed without metal coating to avoid any surface perturbation at high magnification. The powders were imaged using a Zeiss Ultra 55 FESEM with a maximum spatial resolution of approximately 1 nm at 15kV.

## 2.5. Materials and concrete specimen preparation

Except for the substitution of 15wt.% Portland cement by magnesite or hydromagnesite, we followed the same procedure for materials and concrete specimen preparation as detailed in other studies (27-28). The cement used in all mixes was CEM I 52.5 N type Portland cement, meeting the NF EN 197-1 standard (29). Natural sand and gravel were used as fine and coarse aggregates, respectively. The maximum aggregate size was 3.15 mm for sand and 16 mm for gravel, see reference (27) for the full grading curves. Ordinary tap water was used for mixing and curing. The mass ratio of water to cement+substitute (column W/(C+S) in Table 1) was kept constant at 0.58 for all samples. The concrete without cement substitution can be considered standard concrete (30).

We prepared 14 concrete samples of each type: standard, cement substituted with 15% magnesite, cement substituted with 15% hydromagnesite, resulting in a total of 42 specimens. Each concrete specimen was a cylinder with a height $h=140$ mm and a diameter $\phi=70$ mm. All samples for each material were prepared from a single batch. Casting was performed according to the French standard for normal-weight concrete (30). Following the French norm NF EN 12390-2 (31), the mixtures were poured into cylindrical cardboard molds and compacted using an external 10
208 vibrating table to achieve consolidation. After casting, the specimens were cured in their
209 cardboard molds in air for 5 days, then submerged in water for 7 days, still within the molds.
210 Afterwards, they were demolded, and cured into water for an additional 21 days. Finally, they
211 were air-dried for at least one day before mechanical measurements were conducted. All
212 mechanical tests were performed after a minimum of 28 days, in accordance with the NF EN
213 12390-3 standard (32).

214 2.6. *Texture and porosity*

215 The visual appearance of three representative samples is shown in Figure 1. Aside from a slightly
216 different color, no significant differences were detected. The density of each sample was
217 measured by weighing, and the average values are reported in Table 1. The magnesite concrete
218 was slightly less dense than the standard concrete, while the hydromagnesite concrete was even
219 less dense. However, the density difference from the standard concrete was less than 2%. To
220 compare the texture and the macropore content of the three materials, a representative sample of
221 each material was scanned using a DeskTom 3D X-ray Computed Tomography (CT) scanner.
222 The macropore size distributions are discussed in the main text.

223 Synchrotron scans were conducted at beamline BM18 of the European Synchrotron, using an
224 energy of 164 keV and a propagation distance of 6 meters for phase contrast optimization. The
225 beam was prefiltered with a 3.75mm thick molybdenum plate. Helical scans were performed at a
226 10 µm pixel resolution using an Iris 15 camera and a LuAg2000 scintillator, with an exposure
227 time of 40 ms and an accumulation of three exposures (totaling 120 ms). The scans were laterally
228 offset by 2000 pixels to expand the field of view. A total of 15,000 projections per rotation were
229 acquired, yielding 124,051 projections overall.
230 **2.7. Measurement of elastic properties of concrete samples**

The elastic Young's modulus Y (MPa) and the characteristic compressive strength f<sub>ck</sub> (MPa) are considered fundamental material parameters in the design of concrete structures (e.g., (33)). The characteristic compressive strength, f<sub>ck</sub>, which we discuss in the main text, is used for calculating structures according to the requirements of ultimate limit states, while the elastic Young's modulus, Y, is used to calculate elastic deformations and to design sections of structural concrete elements according to the serviceability limit state (34). It is generally assumed that Y and f<sub>ck</sub> are positively correlated, and several different fully empirical (non-linear) expressions have been proposed to estimate Y from f<sub>ck</sub> (see, e.g., (35-36) for reviews). However, these expressions may only provide reasonable estimates for specific concrete mixes. Different methods, such as the tangent (35) or secant (34) modulus, have been proposed to directly estimate the so-called static Young's modulus Y<sub>s</sub> from the slope of the stress-strain curve in the elastic deformation regime obtained during a uniaxial compression test. However, these methods face challenges in unambiguously defining this elastic regime and accounting for possible non-linearities at the onset of loading, which are related to geometrical imperfections of the samples or to the elastic stiffness of the loading frame.

246 For these reasons, and in accordance with the principles of ultrasonic testing described in ASTM
247 C597-02 (37), we employed a non-destructive method, fully detailed elsewhere (36), based on the
248 measurement of the elastic P-wave velocity Vp, to determine the dynamic modulus Yd. By placing
249 two piezoelectric transducers—one as the “transmitter” and one as the “receiver”—at both ends
250 of the cylindrical samples, we calculated Vp from the travel time Δt of the elastic P-wave through
251 a sample of height h, Vp = h/Δt. Note that in these ultrasonic measurements, the arrivals of
compressive (P) and shear (S) waves are generally indistinguishable. As a result, this method does not allow independent estimation of two elastic constants, such as the dynamic Young's modulus $Y_d$ and the Poisson's ratio $\nu$. However, for normal-weight concrete, data in the literature show no significant dependence of $\nu$ on the concrete mix or preparation, with reported values ranging from 0.20 to 0.25 (35, 38). Consequently, we assumed a material-independent Poisson's ratio of $\nu = 0.22$, and the dynamic modulus can be obtained from the following equation (Equation 1):

$$Y_d = V_p^2 \left[ \frac{\rho(1+\nu)(1-2\nu)}{1-\nu} \right] \qquad (1)$$

### Compressive and characteristic strength of concrete samples

Following the standard (33), the uniaxial compressive strength of each sample was obtained by applying a constant stress rate of 0.5 MPa/s until failure (see (27) for additional details). The corresponding average compressive strength $\sigma_{max}$ and its standard deviation $\delta$ were then calculated for the three sets of 14 samples, for each material, see Table 2. The characteristic compressive strength, $f_{ck}$, used to classify concretes into strength classes for structural design, can be calculated from equation (2):

$$f_{ck} = \sigma_{max} - \lambda \cdot \delta, \qquad (2)$$

where $\lambda=1.645$ for the European standard (39), and $\lambda=1.34$ for the US standard (33). In the European regulation, this means that the characteristic strength is defined as the strength value below which only 5% of the compression tests are expected to fall, while in the US regulation, a larger tolerance of 10% is allowed, assuming a normal distribution of strengths in both cases.
Note that our sample sets (14 samples for each material) were smaller than the recommended size in these standards (e.g., 30 samples for (33)).

## 3. RESULTS AND DISCUSSION

### 3.1. A novel circular flow material method for producing Fe-magnesite

Previous studies have shown that magnesite can be produced more quickly using commercial magnesium sources, such as fine-powdered brucite ($Mg(OH)_2$) or various soluble Mg-salts, including chloride ($MgCl_2 \cdot 6H_2O$), sulphate ($MgSO_4 \cdot 7H_2O$), nitrate ($Mg(NO_3)_2 \cdot 6H_2O$) and acetate ($Mg(CH_3COO)_2 \cdot 4H_2O$) (8, 17-19). In all cases, a temperature of at least 90°C was required to produce magnesite within a reasonable time frame (1-5 days). High carbonate alkalinity, close to the $pK_{a2}$ of the ionic carbonate speciation, was also a key factor for magnesite production (17-19). Carbon dioxide pressure does not appear to be a critical parameter in magnesite formation in the investigated systems, except when brucite mineral $Mg(OH)_2$ is used as magnesium source (8).

Here, we present a novel circular material flow process that consists of three fundamental steps: (1) $CO_2$ capture in concentrated NaOH solutions, resulting in an alkaline solution (solution S1: $HCO_3^/-CO_3^-$) and $NaHCO_3$ crystals as byproduct (if NaOH concentration exceeds 4M). (2) Leaching of Mg-Fe-rich peridotite in concentrated HCl solution (up to 10%), producing an acidic solution enriched in Mg and Fe divalent ions (solution S2), amorphous silica (as a byproduct) and residual leached material. (3) Mineralization of $CO_2$ (carbonate precipitation), by mixing the solutions S1 and S2. In this precipitation step several carbonate minerals mixed with iron oxides can be produced by a simple heat-ageing step from room T to 200°C. Figure 1 illustrates this circular material flow process, including also the Fe-magnesite utilization as partial substitute of
cement in concrete. The more relevant fundamental results are described below and all steps are commented in the following sub-sections.

As discussed below, one notable result is the fabrication of iron-magnesite at 60°C in just 2-4 days. Using real-time Raman spectroscopy, we monitored the direct transformation of nesquehonite into iron-magnesite (Figure 2, top plot on the left). Conversely, a hydromagnesite transient phase (e.g., 8) was also observed at 60°C under the investigated conditions. This is the most significant result because magnesite formation at low T (≤60) has been reported, but additives and/or catalysts (in vitro and/or in living systems) have systematically used (20-22). Moreover, several days or weeks are generally required and the proofs provided for magnesite crystals formation at room temperature are not convincing (20-22). At higher temperatures, hydromagnesite transient phase was systematically detected at 90, 150 and 200°C (Figure 2). As expected, magnesite production time significantly decreased at higher temperatures, with only 90 minutes required at 150°C.

In practice, this method utilizes standard and abundant reactants (Mg silicates, water, NaOH, HCl, and CO₂). Additionally, all byproducts (NaHCO₃ crystals, amorphous silica nanoparticles, NaCl-rich material, and unreacted silicate grains) and products (nesquehonite, hydromagnesite, magnesite, and carbonate-iron oxide composites) have well-established industrial applications. Furthermore, the permanent storage of industrially captured CO₂ as highly stable iron-magnesite is promising, as this mineral can replace up to 15 wt.% fresh cement in concrete fabrication without significantly affecting concrete's porosity or mechanical properties. Currently, the Technology Readiness Level (TRL) for our process is close to TRL 2. In effect, maturating technological studies on the optimization concerning the magnesite fabrication and its use and its extent application to other Mg sources are still required to reach an expected TRL 4. In parallel to
maturating studies, an overall life cycle assessment (LCA) and a techno-economic assessment
(TEA) on the magnesite production and its use in concrete manufacturing will be required in a
near future. In the present study, basic technological research is mainly reported for each step of
process, i.e., that LCA and TEA studies are not relevant.

## 3.2. CO₂ capture step

CO₂ capture from industrial sources does not present a significant technological or
scientific barrier; however existing methods and technologies, including the recovery of high
purity CO₂ and its liquefaction, remain expensive (40-43). Therefore, improving and optimizing
CO₂ capture units still pose important challenges. In this study, we propose the capture of CO₂ by
chemisorption in concentrated NaOH solutions. While this chemical approach involves well-
understood reactions, NaOH solution have not, to our knowledge, been widely suggested for
capturing carbon dioxide from industrial sources. This may be because CO₂ is primarily captured
in ionic form (as rich-carbonate CO₃²⁻/HCO₃⁻ solution) and, under specific precipitating
conditions, also in crystalline form as NaHCO₃. Conversely, NaOH micro-drop solutions have
been suggested to remove/capture CO₂ from atmospheric air (44-46).

The advantage of using NaOH solutions for capturing industrial CO₂ is that the resulting ionic
carbonate solutions can be easily manipulated, stored in conventional tanks, and used to form
stable carbonates, as proposed in this study. Additionally, NaOH solutions are strongly selective
for CO₂; the accompanying gases from combustion processes (e.g., NOx, SOx, N₂...) have little
or no influence on the CO₂ chemisorption reactions. In the present study, we conducted several
CO₂ capture experiments using concentrated NaOH solutions under different conditions, varying
mainly the CO₂ injection mode (anisobaric and isobaric), initial gas pressure (from 5 to 55 bar),
and NaOH concentration (1-6M). In all cases, the ionic carbonate concentration and speciation
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can be determined by using Raman spectroscopy in real time. For example, time-resolved Raman spectroscopy was then employed to monitor the reaction mechanism and kinetics (Figure 3). Our results indicate that the injected CO₂ is rapidly chemisorbed as carbonate ions (main Raman signature at 1065 cm⁻¹) according to the following reaction (Reaction 1):

$$344 \quad CO_{2(aq)} + 2OH^{-} \rightarrow CO_{3}^{2-} + H_{2}O \quad (\text{chemisorption}). \\ 345 \quad (1)$$

For example, a rapid chemisorption phase reaches its maximum (spectral equilibration) within the first 70 minutes (Figure 3). Afterwards, the chemisorption process involves additional concurrent reactions, during which some of the carbonate ions are partially protonated, leading to the formation of bicarbonate ions (Raman peak at 1015 cm⁻¹). The direct transformation of aqueous carbon dioxide is also expected, introducing protons in the system. This second chemisorption regime can be quickly followed by the precipitation of NaHCO₃ crystals (Raman peak at 1045 cm⁻¹), depending on the initial concentration of the NaOH solution ([NaOH]>4M). All these reaction events monitored in real time by Raman spectroscopy, can be summarized by the following reactions (Reactions 2 to 4):

$$355 \quad CO_{2(aq)} + OH^{-} \rightarrow HCO_{3}^{-} \quad (\text{chemisorption}), \\ 356 \quad (2)$$

$$357 \quad CO_{3}^{2-} + H^{+} \rightarrow HCO_{3}^{-} \quad (\text{protonation}), \\ 358 \quad (3)$$

$$359 \quad HCO_{3}^{-} + Na^{+} \rightarrow NaHCO_{3} \quad (\text{crystals precipitation}). \\ 360 \quad (4)$$

In summary, this CO₂ capture method enables the production of an alkaline carbonate solution
where carbonate/bicarbonate ratio and NaHCO₃ formation can be controlled by adjusting the NaOH concentration and the carbon dioxide injection pressure. Additionally, this system does not require high-pressure equipment because all above reactions take place from 5 bar of carbon dioxide pressure and all injected CO₂ is completely chemisorbed. Herein, the monitoring in real time by Raman spectroscopy provide clearly the reaction mechanism and kinetics during carbon dioxide capture process by chemisorption reactions. However, this capture step should be tested using more realistic hot gas stream containing carbon dioxide.

### 3.3. Acid leaching step

A Mg-rich solution is produced by acid leaching of Mg-rich minerals, such as olivine from peridotite. The density and ionic concentration of the recovered solutions for a given magnesium source are strongly dependent on the temperature and duration of leaching in the investigated non-stirred experiments. For peridotite material (millimetric grains; 0.5 < size grain < 5 mm), optimal Mg concentrations were achieved when the acidic leaching was conducted at 60°C for 5-6 days. A hyperbolic empirical model, $\Delta P=(a.t/(b+t))$, describes the behavior of the increase of ionic concentration, $\Delta P=P_i-P_0$, with leaching time t (Figure 4). This simple measurement/analysis provides an overall ionic enrichment of solution as a function of leaching time until a saturation state at a fixed solid/liquid ratio and at a given acid concentration. However, similar experiments and calculations would be necessary if parameters such as grain size, mineral composition, acid concentration, solid/solution ratio, and temperature are varied. The high ionic concentrations in leaching solutions after 5 days of reaction were also determined by ICP-AES. Mg was present at 22 g/L and Fe at 4.7 g/L when peridotite millimetric grains were used. Conversely, a more complex ionic solution was obtained using basalt-rich material. In this case, only 7.6 g/L of Mg was leached, but the solution also contained other ions such as Al (5.2
g/L), Fe (5.12 g/L), Na (3.6 g/L), Ca (1.4 g/L), and K (0.64 g/L). Si concentration for both cases is slight ([Si]<110ppm) because amorphous silica is mainly formed during leaching process. Therefore, iron-magnesite is preferentially fabricated when peridotite grains are used in the leaching step. However, other minerals and mineral composites with potential industrial value can be produced when peridotite, basalt and/or serpentinite grains are used, such as layered double hydroxides, magnetite-magnesite, and/or hematite-magnesite composites (Figure SI-1), whether specific temperature and ionic concentration are controlled. In practice, all leaching experiments using peridotite, basalt and serpentinite produced amorphous silica. This byproduct material has several industrial applications, including in microelectronics, chromatography, semiconductors, and as mineral additive or filler. Additionally, the recovered residual leached material could be reused in subsequent leaching cycles. At an industrial scale, millimetric leached-peridotite grains could also serve as a sand substitute (due to their similar grain sizes) in concrete fabrication and other construction materials. Obviously, all these circular uses possibilities remain to be assessed in a near future, but, conceptually all generated solids and solutions can be used and/or reused as illustrated in Figure 1.

In this first study, peridotite was chosen due to its abundant natural availability and high Mg content. However, serpentinite and basalt materials are also suitable candidates for obtaining Mg-Fe-rich solutions to produce stable anhydrous carbonates and/or mineral composites. Additionally, several other sources containing divalent cations (Mg, Fe and Ca), excepting carbonates, can be used, such as mine wastes, steel slags, asbestos wastes, paper mill wastes, gypsum (raw or waste), and industrial brines (7, 13, 47-48). This means that additional exploratory and maturation studies using different divalent-cations sources are required to advance on this circular flow material process. In the present study, we focus exclusively on the
408 formation of iron-magnesite from peridotite because magnesite is an enigmatic challenging mineral as described in the introduction.
409

410 **3.4. Iron-magnesite fabrication by indirect carbonation of peridotite**

411 In our process, the production of iron-magnesite is highly dependent on the CO₂ capture step, as it requires the fabrication of an alkaline solution with a controlled carbonate/bicarbonate ratio and concentrations. The leaching step is also crucial because magnesite amount and the iron content in the structure and as iron oxide adhered particles are directly related to this step.
412
413
414 Fabricating magnesite involves then the controlled mixing of the leaching solution (x volume of S1) into alkaline solution (2x volume of S2) at a flow rate from 2 to 12 ml/min and under ambient conditions (room T and without redox control). This approach helps prevent CO₂ degassing (e.g., HCO₃⁻ + H⁺ = CO₂ + H₂O) until complete neutralization occurs (see Figure SI-2 and SI-3).
415
416
417
418
Following this, a maturation or a heat-ageing step, under mild conditions (60 or 90°C and atmospheric pressure), is then required to produce iron-magnesite in 2-4 days at 60°C or in 5-10h at 90°C as determined by time-resolved Raman spectroscopy (see Figure 2). As expected, higher temperature reduces strongly the time for magnesite production (Figure 2). Energetic requirements and the possibility to use free-carbon energies remains to be examined and refined in an overall context via a carbon footprint analysis, but this was not relevant for this reported basic research, focused mainly on the kinetics and reaction mechanism. For example, the time-resolved Raman spectroscopy measurements (Figure 5) show that an amorphous magnesium and/or ferrous carbonate phase initially forms before the nucleation of nesquehonite at room temperature. A gelling aspect during solutions mixing step at room T until a solid-liquid separation process agrees with this assumption. Although amorphous or poorly crystalized iron oxyhydroxides are also expected in minor proportion, but they were not detected by Raman 419
420
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430
20
spectroscopy. Moreover, the red-brown color and FESEM imagery of the recovered
nesquehonite-rich material supports this assumption (Figure 6 and Figure SI-2). The relevant
chemical precipitation reactions determined from time-resolved Raman spectroscopy and ex-situ
solid characterization, can be simplified for Mg chemistry as follows (Reactions 5 and 6):

$$
\begin{align*}
& 435 \quad Mg^{2+} + CO_3^{2-} + (x + 3)H_2O \rightarrow MgCO_3 \cdot (x + 3)H_2O \quad (\text{AMC formation}), \\
& 436 \quad (5)
\end{align*}
$$

$$
\begin{align*}
& 437 \quad MgCO_3 \cdot (x + 3)H_2O \rightarrow MgCO_3 \cdot 3H_2O + xH_2O && (\text{nesquehonite formation}). \\
& 438 \quad (6)
\end{align*}
$$

439 Ferrous iron ions can also incorporate into the carbonate phases and/or partially oxidize to form
440 ferric iron phases, since the ionic mixtures were not isolated from the atmosphere and the redox
441 conditions were not systematically controlled in the closed reactors. Consequently, only
442 nesquehonite mixed with minor amounts of iron phases were obtained at room temperature
443 (Figure 6, FESEM image on the left). The production of magnesite requires higher temperature
444 (T≥60°C), as shown in Figure 2. Interestingly, time-resolved Raman spectroscopy measurements
445 have revealed two different reaction pathways for magnesite formation.

446 Firstly, a direct transformation of nesquehonite into magnesite at 60°C is illustrated in Figure 2
447 (top plot on the left) and summarized by the following overall reaction (Reaction 7):

$$
\begin{align*}
& 448 \quad MgCO_3 \cdot 3H_2O + xFe^{2+} \rightarrow Mg_{1-x}Fe_xCO_3 + xMg^{2+} + 3H_2O && (\text{Fe-magnesite formation}). \\
& 449 \quad (7)
\end{align*}
$$

Here, the intensity of the bicarbonate ion signature decreases significantly with time, indicating
that the transformation of nesquehonite into magnesite is also accompanied by an additional
carbonation of residual dissolved ions and the incorporation of ferrous iron, as qualitatively
evidenced by a shift in lattice vibration modes from 330 until 323 cm⁻¹ for magnesite at 60°C
21
(330 cm⁻¹ correspond to typical peak for high-purity magnesite crystals). This suggests a partial dissolution-recrystallization pathway. The recovered iron-magnesite was characterized by sub-micrometric rhombohedral crystals mixed with iron oxyhydroxides nanoparticles (Figure 6, FESEM image on the right). X-ray diffraction and EDS chemical analyses (see Figure SI-4 and SI-5) of the magnesite crystals also suggest some structural incorporation of Fe(II) because a shift of reflection peaks was also detected (SI-5), indicating a partial substitution of Mg by Fe(II) in magnesite crystals, as summarized in reaction 7. In summary, the magnesite production at 60°C via a direct transformation of nesquehonite in only 2 days is the most important scientific result because this insight has not been reported in the literature to the best of our knowledge. Some studies have reported magnesite formation at lower temperature in biotic systems and/or in vitro using organic catalysts, but, several days or weeks were required (20-22).

Secondly, the more common transformation of nesquehonite to magnesite involves hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O) as a transient phase, as previously described (8). In this study the transformation from hydromagnesite to magnesite also involves the incorporation of iron into the magnesite structure, as summarized by the following overall reaction (Reaction 8):

$$Mg_5(CO_3)_4(OH)_2 \cdot 4H_2O + HCO_3^- + 5xFe^{2+} \rightarrow 5Mg_{1-x}Fe_xCO_3 + 5xMg^{2+} + 5H_2O + OH^- \quad (Fe-magnesite formation). \quad (8)$$

The dissolution-recrystallization pathway is also observed at higher temperatures. However, when hydromagnesite forms at 60°C (in 25% of the experiments), it slows down the overall nesquehonite-to-magnesite transformation, which lasts for 2-3 days. In fact, hydromagnesite is a more stable phase than nesquehonite as clearly demonstrated in (44). In contrast, at higher temperatures (90, 150 and 200°C), the hydromagnesite transient-phase has a reduced impact on the nesquehonite-to-magnesite transformation. At these higher temperatures, the persistence of
hydromagnesite is limited to several minutes at 150 and 200°C, and several hours at 90°C (Figure 2). In the present study, 90°C seems to be the more promising because it remains a moderate temperature and the formation kinetics of magnesite is less than 24h. Moreover, the magnesite formation mechanism at 90°C is independent of the ionic Mg-Fe concentration (or leaching time) as illustrated in Figure 7. This means that in all cases magnesite is formed via hydro-magnesite transient phase and only nucleation and persistence time (or life time) of transient phases are impacted as summarized in Figure 7 and Table 3.

In summary, magnesite can be produced from the indirect carbonation of peridotite by mixing two ionic solutions and heating the resulting suspensions at mild temperature and atmospheric pressure (60 and 90°C; see Figures 2 and 7). As expected, the production time for iron-magnesite decreases significantly with increasing temperature as illustrated in Figure 2, from several days to just minutes (spectral Raman equilibration in real-time). For instance, producing iron-magnesite at 90 °C from indirect carbonation of peridotite could be a promising technology for pilot and industrial scales, as it requires only 5-6 hours to produce iron-magnesite from ionic solutions (Figure 7). Herein, the amorphous phase and nesquehonite formed at ambient conditions are rapidly transformed into a hydromagnesite transient phase, which has only a slight impact on iron-magnesite formation due to its short lifetimes (Figure 7 and Table 3). In a scientific context, the magnesite formation from direct transformation of nesquehonite at 60°C remains the most important insight as invoked above. Then, these results are significant because high quantities of magnesite can be produced at moderate temperatures in reasonable times—2-4 days at 60°C and only 5-6 hours at 90°C. The fabrication time of magnesite decreases sharply with temperature, requiring only 90 minutes at 150°C and less than 60 minutes at 200°C (Figure 2). It is important to note that the effectiveness of these mild conditions for magnesite production was probably
influenced by the presence of competitive cations in the system, specifically Fe(II) and, to a lesser extent, Ca. The role of divalent cations in magnesite formation kinetics has been demonstrated recently, particularly regarding the influence of calcium during Ca-magnesite formation at 90°C (19).

### 3.5. Utilization of iron-magnesite as a partial substitute of cement

We used iron-magnesite material produced at 90°C from indirect carbonation of peridotite as a 15 wt.% substitute for cement (Figure 1). In the cement industry, this substitution could reduce clinker production and securely store CO₂ within the material, minimizing the risk of re-emission during the concrete's lifespan or when reused as aggregate for concrete recycling (50). A key concern is ensuring that this substitution does not significantly affect the mechanical and textural properties of concrete. To address this, we tested concrete samples with substituted cement with both iron-magnesite synthesized at 90°C from indirect carbonation of peridotite (this study), and hydromagnesite synthesized at 60°C from direct carbonation of brucite following the method described in (8). We prepared cylindrical concrete samples (7 cm in diameter and 14 cm high) with either iron-magnesite or hydromagnesite as a partial substitute for Portland cement. We assessed these materials in terms of internal texture and porosity using X-ray tomography imaging, as well as elastic stiffness (Young's modulus) and compressive strength. The standard concrete for its casting presents a ratio of W/(C+S) equal at 0.58 (see Table 1). This ratio leads a concrete C20/25 i.e. a concrete which have a characteristic strength ($f_{ck}$) equal to 20 MPa after 28 days of curation (26, 50). The compressive strength results for the three types of concrete are summarized in Table 2, using the European standard ($\lambda$=1.645, see Supplementary Information). We observed a slight decrease in strength of 1.3 MPa (5.5%) for magnesite concrete and a more notable decrease for hydromagnesite concrete (4 MPa, or 16.9%). The greater reduction in
strength for hydromagnesite concrete is expected, as the introduction of hydromagnesite
increases the total water content of the cement paste. The results for magnesite concrete are
promising (Figure 8). However, these assessments are preliminary. Strength differences might be
reduced with optimization of the mix (water, sand, gravels) or the preparation process. Further
work could involve exploring the mechanical properties of the cements at scales ranging from the
nanometer-sized particles of calcium-silicate-hydrate (C-S-H) to the size of the magnesite micro-
particles and/or micro-aggregates (see Figure 9) using techniques such as nano- and micro-
indentation (51). Additionally, the rheology of fresh concrete pastes (52) was not analyzed, as we
focused only on the mechanical properties of hardened concrete samples. Lastly, due to the
limited amount of magnesite or hydromagnesite synthetized at the laboratory scale, we tested
only 14 samples of each material — fewer than the recommended 30 samples for accurately
estimating characteristic concrete strength according to regulatory codes (30).

One sample of each concrete type was scanned before deformation using a DeskTom 3D
X-ray Computed Tomography (CT) scanner at a resolution of 50µm and after deformation at the
BM18 beamline at the European Synchrotron Radiation Facility with a voxel size of 10 µm using
propagation phase-contrast. The raw X-ray images were segmented to identify pores, but only
those larger than 3 ×3 ×3 voxels were considered meaningful. Thus, these tomographic analyses
capture only macropores and micropores and do not resolve nano-porosities present in the
hardened cement paste (e.g., 26, 50).

From the reconstructed 3D pore structures, no significant differences of texture were
detected. The pore size distributions before deformation, expressed in terms of equivalent
spherical diameters (dₚ) are shown in Figure 8 (plot on the right). These distributions follow a
power-law trend $f(d_p) \sim d_p^{-\alpha_p}$ with $\alpha_p \simeq 3.3$. The only detectable difference is the presence of a
few larger pores ($6 \text{ mm} \le d_p \le 10 \text{ mm}$) in magnesite- and hydromagnesite concrete, which are
absent in standard concrete. Consequently, macroporosity ($p$) is slightly higher in magnesite
concrete ($p = 1.25\%$) and hydromagnesite concrete ($p = 1.69\%$) compared to the standard
concrete ($p = 0.86\%$). These differences are relatively small and may not fully represent all
samples, but are consistent with minor differences in density noted previously. In addition,
macro-porosity seems to be slightly affected after deformation into no-fractured zones as
illustrated from tomographic synchrotron measurements (Figure 10). Herein, macropores and
fractures were reconstructed at higher resolution on BM18 beamline offering a powerful tool to
determine a textural impact in doped concretes.

Eight elastic velocity measurements were performed on each type of concrete sample,
with results reported in Table 3. The dynamic Young's modulus decreased by approximately
9.5% for magnesite concrete and 16% for hydromagnesite concrete, compared to standard
concrete. The observed decrease in magnesite concrete cannot be explained by a simple mixing
law for composite materials, as the Young's modulus of bulk magnesite at room temperature and
atmospheric pressure is 174 GPa, significantly higher than the typical values for consolidated
cement (~10-30 GPa).

This reduction cannot be easily explained by the small differences of (macro) porosities
noted earlier. Classical models relating the Young's modulus $Y$ to porosity $p$, such as $Y(p) =
Y_0 (1 - \frac{p}{p_c})^\alpha$, where $Y_0$ is the Young's modulus of the bulk (pore-free) material and $p_c$ is the
porosity at which the Young's modulus vanishes (53), suggest only a minor decrease in elastic
stiffness. For standard concrete, with $\alpha = 1$ and $p_c \approx 42\%$, this model reasonably fits
experimental data (54). Based on the measured porosities, the model predicts a decrease in elastic
stiffness of less than 1% for magnesite concrete and about 2% for hydromagnesite concrete.
Therefore, the observed elastic softening is likely due to the structure of the hardened cement
containing magnesite or hydromagnesite particles, including the specific structure and
interactions of these particles within the cement matrix. Future analyses, such as nano- and
micro-indentation measurements, will be essential to further explore these structural effects and
their impact on mechanical properties.

## 4. CONCLUSION

The present study introduces a circular flow material method for producing iron-
magnesite via indirect carbonation of peridotite. Conducted on a laboratory scale with liter-
volume capacity, this method efficiently generates iron-magnesite under mild ($T \le 90^\circ C$) and
hydrothermal (up to $200^\circ C$) conditions within a timescale coherent for industrial processes (from
h to days). In practice, the process offers significant potential for high-use applications and
provides a mechanism for permanently storing industrial captured $CO_2$. Our preliminary analysis
suggests that the partial substitution of cement with magnesite particles could notably reduce the
carbon footprint of the cement on concrete industry. Evaluations reveal only marginal differences
in density, porosity, or texture between hardened magnesite concrete and standard concrete.
Additionally, the observed degradation of mechanical properties, such as Young's modulus and
characteristic strength, is minimal. These findings support the viability of iron-magnesite material
as a promising sustainable alternative in concrete production.
In a scientific context, this study reports for the first time than the magnesite “an enigmatic mineral in geology” can be significantly produced at 60°C and atmospheric pressure via a direct transformation of nesquehonite into magnesite; this chemical-mineral transformation was then monitored in real time by Raman spectroscopy. Obviously, these results open new possibilities to investigate on the nucleation of “enigmatic” anhydrous magnesium carbonates such as magnesite, dolomite, huntite and ankerite debated in the last two centuries.

592
593 ASSOCIATED CONTENT

594 Supporting Information

595 The pdf file includes: Figures SI-1 to SI-5 concerning FESEM images, X-ray diffraction patterns,

596 pH titration and some photographs for selected experiments

597

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610

611
612 Acknowledgements

613 The authors acknowledge funding from the French National Centre for Scientific Research
614 (exploratory project PEPS-CNRS 2023 and multi-disciplinary project MITI-CNRS "Ressources
615 et Sobriété" 2024-2025), The Université Grenoble Alpes (BQR-ISTerre 2022), The European
616 Synchrotron Radiation Facility BM18 beamline for inhouse beamtime access, and The European
617 Union Horizon 2020 program (Accelerating CCS Technologies, project FUNMIN No. 294766).

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630
631 **Competing interests:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

633
634 **Data and materials availability:** All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supporting Information.

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648
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<table><caption>**Table 1.** Mixture proportions and density for 1 m³ of concrete (standard, magnesite, and hydromagnesite).</caption><thead><tr><th>Concrete</th><th>Substitution<br>(wt.%)</th><th>Water<br>(W)<br>(kg)</th><th>Cement<br>(C) (kg)</th><th>Substitute<br>(S) (kg)</th><th>Ratio<br>W/(C+S)</th><th>Sand<br>(kg)</th><th>Gravels<br>(kg)</th><th>Density<br>(kg/m³)</th></tr></thead><tbody><tr><td>standard</td><td>0</td><td>195</td><td>335</td><td>-</td><td>0.58</td><td>800</td><td>1065</td><td>2311±25</td></tr><tr><td>magnesite</td><td>15</td><td>195</td><td>284.75</td><td>50.25</td><td>0.58</td><td>800</td><td>1065</td><td>2286±23</td></tr><tr><td>hydromagnesite</td><td>15</td><td>195</td><td>284.75</td><td>50.25</td><td>0.58</td><td>800</td><td>1065</td><td>2269±14</td></tr></tbody></table>
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<table><caption>**Table 2.** Mechanical properties of the three types of concrete produced (N=14 samples of each type): standard concrete, concrete with 15wt.% magnesite substitution, and concrete with 15wt.% hydromagnesite substitution.</caption><thead><tr><th>Concrete</th><th>Y<sub>d</sub> (dynamic modulus), GPa</th><th>σ<sub>max</sub> (average compressive strength), MPa</th><th>f<sub>ck</sub> (characteristic strength), MPa</th></tr></thead><tbody><tr><td>standard (N = 14)</td><td>42.1±2.9</td><td>28.5±2.9</td><td>23.7</td></tr><tr><td>magnesite (N = 14)</td><td>38.1±2.9</td><td>25.5±1.9</td><td>22.4</td></tr><tr><td>hydromagnesite (N = 14)</td><td>35.2±2.1</td><td>21.7±1.2</td><td>19.7</td></tr></tbody></table>

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839 **Table 3.** Summary of kinetic indicators determined from time-resolved Raman spectroscopy
840 measurements for transient phases and magnesite nucleation until spectral equilibration at 90°C.

841 Ionic solutions were obtained by leaching of peridotite millimetric grains from 1 to 10 days at
842 60°C (ionic concentration was reported in Figure 4).

<table><thead><tr><th rowspan="2">Kinetic indicator (minute)</th><th colspan="8">Leaching Time (day)</th></tr><tr><th>1</th><th>2</th><th>3+</th><th>3*</th><th>4</th><th>5+</th><th>6+</th><th>10+</th></tr></thead><tbody><tr><td>Nesquehonite persistence</td><td>51</td><td>30</td><td>50</td><td>38</td><td>148</td><td>35</td><td>45</td><td>36</td></tr><tr><td>Hydromagnesite nucleation</td><td>21</td><td>25</td><td>37</td><td>30</td><td>38</td><td>28</td><td>32</td><td>34</td></tr><tr><td>Hydromagnesite persistence</td><td>1446</td><td>1040</td><td>1077</td><td>661</td><td>503</td><td>421</td><td>240</td><td>780</td></tr><tr><td>Fe-Magnesite nucleation<sup>++</sup></td><td>566</td><td>260</td><td>177</td><td>121</td><td>148</td><td>121</td><td>100</td><td>120</td></tr><tr><td>Fe-magnesite equilibration</td><td>&gt;1446</td><td>1360</td><td>1317</td><td>841</td><td>923</td><td>576</td><td>375</td><td>1080</td></tr></tbody></table>

843 *: replica; +: spectral behavior reported in figure 7; ++: concern the detection time of lattice
844 mode at 330 cm⁻¹ characteristic for magnesite crystals; Nesquehonite peaking at 1098-1100 cm¹;

845 Hydromagnesite peaking at 1117-1119 cm⁻¹
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<figure><img src="image_1.png" /></figure>

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853 Figure 1. Schematic diagram flow of materials used in three fundamental, interdependent steps for iron-magnesite production, and its use as a partial substitute in concrete fabrication. S1 and S2 indicate the two solutions prepared and mixed to mineralize carbon dioxide and produce an iron-magnesite material that can be used as a partial substitute of cement for the fabrication of concrete.

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<figure><img src="image_2.png" /><figcaption>Figure 2. Fe-Magnesite-rich material production at four maturation temperatures (60 and 90°C (mild conditions) 150 and 200°C (hydrothermal conditions) through indirect carbonation of peridotite. Kinetics and reaction mechanisms were monitored in real-time using Raman</figcaption></figure>

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spectroscopy. The time axis concerns times when magnesite (the more stable phase) was reached a spectral equilibration, i.e. that the peak intensity for magnesite is constant and/or reaches a maximum value.

<figure><img src="image_3.png" /></figure>

Figure 3. Time-lapse Raman spectroscopy monitoring of CO₂ capture by chemisorption in concentrated NaOH solution (4M) under anisobaric conditions. Left: Continuous evolution of the carbonate concentrations of the different species over time. Right: Kinetic behavior of carbonate
ions and sodium bicarbonate precipitation including only spectra at different time points.

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<figure><img src="image_4.png" /></figure>

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896 Figure 4. Increase in ionic concentration (ΔP=Pᵢ-P₀ in g/100 mL) for the leaching solution over time at 60°C and using peridotite grains (0.6mm < size < 3mm). P₀ represents the ionic

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concentration for HCl at 10% and Pᵢ represents the ionic concentration of the leaching solution as a function of time. In all experiments, an amorphous SiO₂ byproduct was also recovered (see also Figure 1).

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<figure><img src="image_5.png" /></figure>

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Figure 5. Left: time-lapse Raman spectroscopy monitoring of nesquehonite formation at room temperature, by mixing alkaline and leaching solutions. Right: Raman spectra after 30 minutes showing the formation of an amorphous carbonate phase prior to the nucleation of nesquehonite.

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<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

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Figure 6. FESEM images showing Nesquehonite and iron-magnesite crystals formed at room temperature and 60°C, respectively. Both Mg-carbonate minerals mixed with iron oxyhydroxide nanoparticles adhered to carbonate crystals (in minor proportions), providing yellow-red-brown coloration of powdered recovered materials. Iron oxyhydroxide nanoparticles were not detected by in-situ Raman spectroscopy or by laboratory X-ray diffraction, but their presence was clearly identified by FESEM imagery and local EDS analyses.
<figure><img src="image_8.png" /><figcaption>Figure 7. Fe-Magnesite-rich material production at 90°C for four different concentrations of ionic solution (solution S2 (Figure 1) as a function of leached time (see also figure 4)), i.e., solutions</figcaption></figure>

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leached for 3, 5, 6 and 10 days respectively. Kinetics and reaction mechanisms were monitored in real-time using Raman spectroscopy (see also kinetic indicators summarized in Table 1). The time axis was constrained at 24h (1440 minutes) and this time is enough to produce magnesite at 90°C (the more stable phase). In all cases, magnesite was reached a spectral equilibration, i.e., that the peak intensity for magnesite is constant and/or reaches a maximum value.

<figure><img src="image_9.png" /><figcaption>Figure 8. Compressive strength (left) and pore sizes distributions (densities of probability), for the three concretes: Standard concrete, Magnesite-concrete, and Hydromagnesite-concrete. On the left figure, each colored dot corresponds to the strength of one individual compression test. The inset shows the visual appearance of virgin concrete samples (before mechanical loading). On the right figure, the pore sizes are expressed in terms of equivalent spherical diameters.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 9. FESEM images at four different magnifications for Fe-magnesite produced at 90°C from indirect carbonation of peridotite. Millimetric grains of peridotite leached 5 days at 60°C</figcaption></figure>

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969 under in a static system (without stirring). Kinetics and reaction mechanism was also reported in
970 Figure 7.

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<figure><img src="image_11.png" /></figure>

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Figure 10. Tomographic analysis of standard, magnesite, and hydro-magnesite concretes. The images labeled 1, 2, 4, and 6 show the samples before testing deformation, while those labeled 3, 5, and 7 display the samples after failure. The top section presents 3D volume rendering with transparency to highlight pores (blue) and cracks (red). The bottom section shows the middle slice of samples 1, 3, 5, and 7, including a zoomed-in view. Porosity is indicated by black regions, and the cement phase is shown in gray.
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<figure><img src="image_12.png" /></figure>