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Wollastonite Carbonation in Water-bearing Supercritical CO2: Effects of Particle Size Yujia Min, Qingyun Li, Marco Voltolini, Timothy J Kneafsey, and Young-Shin Jun Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04475 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 7, 2017
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Environmental Science & Technology
Wollastonite Carbonation in Water-bearing Supercritical CO2: Effects of Particle Size
Yujia Min,† Qingyun Li,† Marco Voltolini,‡ Timothy Kneafsey,‡ Young-Shin Jun*† †
Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, MO 63130
‡
Earth and Environmental Science, Lawrence Berkeley National Laboratory, CA 94720
E-mail:
[email protected] Phone: (314) 935-4539 Fax: (314) 935-7211 http://encl.engineering.wustl.edu/ Submitted: August 2017
Environmental Science & Technology
*To Whom Correspondence Should be Addressed
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ABSTRACT
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The performance of geologic CO2 sequestration (GCS) can be affected by CO2 mineralization and
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changes in the permeability of geologic formations resulting from interactions between water-
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bearing supercritical CO2 (scCO2) and silicates in reservoir rocks. However, without understanding
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size effects, the findings in previous studies using nanometer or micrometer size particles cannot
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be applied to the bulk rock in field sites. In this study, we report the effects of particle sizes on the
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carbonation of wollastonite (CaSiO3) at 60 oC and 100 bar in water-bearing scCO2. After
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normalization by the surface area, the thickness of the reacted wollastonite layer on the surfaces
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was independent of particle sizes. After 20 hours, the reaction was not controlled by the kinetics
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of surface reactions, but by the diffusion of water-bearing scCO2 across the product layer on
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wollastonite surfaces. Among the products of reaction, amorphous silica, rather than calcite,
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covered the wollastonite surface and acted as a diffusion barrier to water-bearing scCO2. The
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product layer was not highly porous, with 10 times smaller specific surface area than the altered
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amorphous silica formed at the wollastonite surface in aqueous solution. These findings can help
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us evaluate the impacts of mineral carbonation in water-bearing scCO2.
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INTRODUCTION
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To mitigate the adverse impacts of global climate change, anthropogenic CO2 can be
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captured from point sources and sequestered in subsurface environments, such as deep saline
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aquifers or depleted oil and gas reservoirs.1 The safety and efficiency of CO2 trapping in these
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storage sites are related to interactions among supercritical CO2 (scCO2), water, and rock.2 For
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example, the carbonation of silicates can trap CO2 in mineral phases and cause changes in the
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wettability and porosity of reservoir rocks, which can in turn alter their permeability. Furthermore,
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a change of permeability in geologic formations can influence the transport of CO2 and potentially
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affect CO2 leakage. For similar reasons, scCO2–water–rock interactions can be important in other
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subsurface CO2 injection processes, including scCO2-enhanced oil recovery and scCO2-energized
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hydraulic fracturing.3-7
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While most previous studies on scCO2–water–rock chemical interactions focused on
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mineral dissolution and precipitation in the presence of bulk aqueous phase,2 recent studies have
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revealed the following importance of interactions between scCO2 and minerals in the absence of
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aqueous solutions.8-10 First, because injected scCO2 will displace brine, most of the contact areas
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between scCO2 and caprocks and a certain portion of the contact areas between scCO2 and
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formation rocks, are in the absence of bulk aqueous solution.11 Second, scCO2 has higher
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diffusivity, lower viscosity, and thus lower capillary entry pressure than aqueous phase fluid (i.e.,
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CO2-dissolved brine).12 Therefore, scCO2 can enter smaller pore throats which brine cannot enter.
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Third, recent studies reported that water dissolved in scCO2 has higher reactivity on mineral
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surfaces than that of water in the aqueous phase.8, 12, 13 Based on these considerations, the reactions
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of minerals with water-bearing scCO2 are at least equally, if not more, important than reactions of
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mineral with aqueous solutions.12, 14
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Our current understanding of water-bearing scCO2–mineral interactions focuses on the
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carbonation of various Ca-, Mg-, or Fe-bearing minerals, including wollastonite,15-18 forsterite,11,
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14, 19-22
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water saturation percentage of CO2,14, 18, 19, 21, 25 temperature,17, 18, 20, 23 and organic ligands,22 on
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carbonation reactions, we need a better understanding of the dependency of water-bearing scCO2–
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silicates reactions on particle sizes. There are several knowledge gaps: First, previous studies
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mostly used micrometer or even nanometer size particles,14-23 a far cry from the bulk rock in field
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sites. Without understanding the effects of particle sizes, we cannot know whether the outcomes
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obtained with small particles are applicable to bulk rock in field sites. Second, findings from
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different studies are not directly comparable due to the different particle sizes used. Thus, results
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from previous studies on water-bearing scCO2–silicate reactions cannot be combined. Third,
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investigating the effects of particle sizes can reveal the mechanisms of mineral carbonation
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reactions. For example, a recent study of metal oxide carbonation (i.e., CaO) in dry CO2 found that
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the reacted fraction is constant for different particle sizes and concluded that the reaction extent of
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CaO carbonation is determined by the porosity of CaO particles.26 Similarly, investigating the
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effects of particle sizes on silicate carbonation in water-bearing scCO2 can inform us about whether
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the reaction is controlled by the porosity of silicates or other limiting factors, such as the kinetics
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of silicate dissolution in water films. However, to the best of the authors’ knowledge, there have
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been no studies on the effects of particle sizes on water-bearing scCO2–silicate interactions.
fayalite,23 dolomite,24 and phlogopite.8 While these studies investigated the effects of the
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The goal of this study is, therefore, to understand the effects of particle size on the
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carbonation of wollastonite in water-bearing scCO2. We chose wollastonite because it has been
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frequently used as a representative of silicates in previous studies on silicate reaction in scCO2 and
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in aqueous solutions.17, 18, 27, 28 We hypothesize that the reaction is controlled by the kinetics of
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wollastonite hydrolysis. If anisotropic reactivity of different crystal faces is not significant, the
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reaction extent of different size particles should be similar after normalization to surface area. To
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test this working hypothesis, particles with five different size ranges were reacted at 60 oC and 100
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bar, conditions closely relevant to GCS. The information provided can contribute to better
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understanding the water-bearing scCO2–silicate reactions and scaling up findings in laboratory
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studies using small size mineral particles to the larger scale bulk rock formations.
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EXPERIMENTAL METHODS
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Minerals
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Natural wollastonite particles with five different size ranges were purchased from NYCO
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Company (Willsboro, NY). By using X-ray powder diffraction (XRPD, Bruker D8), its structure
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was identified to be wollastonite-1A, the most common polymorph in the natural environment.29
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The size distribution and shape for each size range were determined by laser diffraction and image
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analysis (Microtrac Inc.) performed by NYCO Company. Most particles were in cylindrical shape
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(Figure S1). The spherical equivalent volumetric mean diameters were 3.8, 5.2, 11.1, 17.8, and
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82.0 µm, and the respective typical aspect ratios were 3:1, 3:1, 3:1, 4:1, and 15:1. The detailed size
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distribution is available in the Supporting Information (Figure S1). These size ranges were chosen
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because they can provide detectable and significantly different reaction extents. Using N2 as
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adsorbate and 11 points on the isotherm in the BET method (AX1C-MP-LP, Quantachrome
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Instruments), the specific surface areas were determined to be 4.46±0.01, 3.61±0.01, 1.95±0.01,
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1.45±0.01, and 0.54±0.01 m2/g, respectively, for 3.8, 5.2, 11.1, 17.8, and 82.0 µm particles. The
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error ranges are based on duplicate measurements. X-ray fluorescence (Table S1) showed that the
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Ca/Si ratio was 0.959. Thermogravimetric analysis (TGA, Q5000IR, TA Instruments) showed
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0.45 % mass loss between 150–780 oC. As shown by the XRD patterns in Figure S5, the unreacted
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sample contains aragonite, which is estimated to be 1 wt%, based on 0.45% mass loss in TGA. No
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further treatment was performed after the wollastonite were obtained from the vendor.
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Carbonation in water-bearing scCO2 at simulated GCS conditions
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Carbonation experiments were conducted in a 300 mL high pressure and high temperature
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reactor (Parr Instruments, Moline, IL) modified from the reactor used in our previous studies.30-34
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A schematic diagram of the reaction system setup is available in the Supporting Information
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(Figure S2). The conditions (60 oC and 100 bar CO2) are similar to typical conditions at GCS sites:
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65 oC and 150 bar in the Frio formation,35 37 oC and 100 bar at the Sleipner site,36 and 63 oC and
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140 bar at the Weyburn field site.37 To investigate carbonation of wollastonite in the absence of
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aqueous phase, wollastonite particles were placed in 10 ml Teflon tubes, while ultra-purified
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deionized water (Barnstead, > 18.2 MΩ·cm) was added outside the tubes. After scCO2 was injected
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into the vessel, water dissolved in the scCO2 and generated water-bearing scCO2. Five PTFE tubes
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were placed in the reactor during each test. Considering the volume taken up by tubes and liners
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in the reactor, the volume of CO2 during the experiment was 201 mL. The tubes were capped, but
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have small holes allowing contact between the wollastonite and water-bearing scCO2. The
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solubility of water in CO2 was predicted using Spycher’s model.38 At 35 and 60 oC, the mole
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fraction of 100% saturated water in 100 bar CO2 is 0.41 and 0.49%, which indicates 234 and 117
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µL water in the 201 mL scCO2. The solubility of water in scCO2 is affected by the salinity of water.
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As a starting point, ultra-purified water was used for simplicity. This result can serve as an
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important underpinning for understanding the effects of salinity on silicate carbonation in the
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future. Each sample contained 0.3 g particles. To check the possible influence of particle stacking
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inside the tubes on the reaction, results were compared with samples using 0.05 g particles.
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Selected samples were analyzed using XRPD. In this study, no attempt of XRPD quantitative
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analysis was made. Instead, the reaction extent was determined by TGA analysis. A previous study
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has shown that the results from XRPD quantitative analysis are comparable with the results from
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TGA analysis.18
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Determination of the reaction extent
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The reacted fraction of wollastonite was determined based on TGA results. After the
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sample was recovered from the reactor, the samples were stored at atmospheric pressure and
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analyzed within 24 hours. The powder sample was briefly mixed using a plastic spatula, and
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approximately 10 mg was used for each TGA analysis. At least duplicate samples were analyzed.
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Samples were heated to 900 oC with a ramp of 20 oC/min under N2 flow (25 ml/min). Wollastonite
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and amorphous silica were stable below 900 oC, while calcium carbonate completely decomposed
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into CaO and CO2.18 The amount of CO2 was measured by the mass loss between 150-780 oC
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during TGA (Figure 1B). Given the amount of CO2, the reacted fraction of the original wollastonite
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was derived using this equation:
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(1) (2)
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where mL is the mass loss during TGA, which is the mass of CO2 from CaCO3 decomposition.
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According to equation (2), the mass loss is equal to the mass of CO2 consumed during reaction
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with water-bearing scCO2, so the mass of wollastonite consumed as reactant can be calculated
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using mL and the molecular weight of CO2 and wollastonite; mo is the mass of the sample reacted
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with water-bearing scCO2, which includes the mass of the original wollastonite (wollastonite
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before reaction with CO2) and the mass of sorbed CO2 on wollastonite (equal to mL); and M is the
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molecular weight of different species. Given the 0.45% mass loss in unreacted wollastonite, the
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detection limit of this method is estimated to be 1%. The reacted fraction of the largest particle
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size used (82 µm) is 7.8%, so the uncertainty of the reaction extent determined using this method
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is no more than 15%.
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Calculation of the reacted thicknesses of wollastonite samples
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Based on the reacted fraction, we calculated the thicknesses of the reacted layer by using a
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shrinking core model. The schematic diagram in Figure 2A shows the geometry. Before the
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reaction, each particle was assumed to be cylindrical in shape, with aspect ratio R and diameter D,
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calculated using equation (3) based on the spherical equivalent diameter (Deq) measured by laser
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diffraction by the NYCO Company. During the reaction, a shell with thickness L was consumed
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as the reactant, and according to the model, the wollastonite particle shrank to a cylindrical shape
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core with diameter D-2L. The volume fraction of the shell is the reacted fraction of the particle.
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By integrating all the size intervals shown in the size distribution (Figure S1), the reacted fraction
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of each size range was calculated. The reacted thickness L was obtained by solving equation (4),
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which equated the calculated reacted fraction and the reacted fraction determined by TGA.
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(3) ∑
(4)
∑
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In Equations 3 and 4, L is the thickness of the reacted layer, F is the reacted fraction, Deq is the
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spherical equivalent diameter of each size range from laser diffraction results provided by NYCO
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Company, D is the cylindrical equivalent diameter of each size range, and R is the aspect ratio. L
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was obtained by solving this equation. For small particle size ranges, in which (R × D – 2 × L)
