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

Journal of Cleaner Production xxx (xxxx) xxx

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Accelerated carbonation of reactive magnesium oxide cement (RMC)based composite with supercritical carbon dioxide (scCO2) Rotana Hay, Kemal Celik* Division of Engineering, New York University Abu Dhabi, Abu Dhabi, P.O. Box 129188, United Arab Emirates

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 June 2019 Received in revised form 6 October 2019 Accepted 11 November 2019 Available online xxx

Reactive magnesium oxide cement (RMC) has the potential to become a sustainable alternative to ordinary Portland cement (OPC). In this study, an approach utilizing supercritical CO2 (scCO2) was investigated to accelerate carbonation of an RMC-based composite and to overcome its long carbonation process under the natural environment. It was found that scCO2 led to an extremely rapid strength gain of the composite, with a mature strength level achievable within a period of hours. CO2 sequestration factors were also increased by three folds as compared to samples cured under a 20% CO2 concentration environment for 28 days. It was also revealed that the carbonation phases under scCO2 were dominated by nesquehonite followed by hydromagnesite and some other intermediate hydrated magnesium carbonates (HMCs). More uniform carbonation within the matrix was also attained under the scCO2 condition. Despite the promising outcomes, technical and cost challenges would need to be resolved before a possible scale-up. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Baoshan Huang Keywords: Reactive-MgO cement Carbonation Supercritical CO2 CO2 sequestration Hydrated magnesium carbonates

1. Introduction Concrete is the most widely used construction material, yet its ubiquitous presence also poses serious environmental challenges and concerns. Ordinary Portland cement (OPC), which is the primary binding agent in concrete, requires intense energy for production, giving rise to an average CO2 emission of 900 kg/tonne (Celik et al., 2014, 2015; Mahasenan et al., 2003). As the annual global production of cement reached 4.2 billion tonnes in 2016 (van Oss, 2017), the industry was estimated to release approximately 3.0 billion tonnes of CO2, accounting for 6e7% of the total anthropogenic CO2 emissions on a global scale (Celik et al., 2019; Hay and Ostertag, 2018; Olivier et al., 2017). The research community has been on an active search for more sustainable construction materials to meet the burgeoning need and to ensure the long-term sustainability of the construction sector. One of the many promising alternatives to cement is reactive magnesium oxide (MgO) cement (RMC), which can be produced from calcination of magnesite (MgCO3) at a relatively low calcination temperature (700e1000
C) to create light-burned MgO of higher porosity, lower crystallinity, larger surface area and higher reactivity as compared to other grades of MgO (Eubank, 1951; Unluer and Al-

* Corresponding author. E-mail address: [email protected] (K. Celik).

Tabbaa, 2013, 2015). In addition, RMC-based concrete gains strength through hydration and carbonation (Dung et al., 2019; Dung and Unluer, 2017b), which allows for permanent sequestration of carbon dioxide (CO2) into its carbonation phases. Its possible full recyclability (Sonat et al., 2017) also reinforces its potential as a sustainable alternative to OPC. Recently, RMC has been formulated for the production of concrete blocks and boards (Unluer and AlTabbaa, 2014; Wang et al., 2016) and used to produce strainhardening composites (Ruan et al., 2018; Wu et al., 2018) and self-healing materials through microbial-induced carbonate precipitation (Ruan et al., 2019). Nevertheless, magnesite deposits are not widely available and are mainly found in China and North Korea (Shand, 2006; Unluer and Al-Tabbaa, 2014). Comprehensive life cycle assessments of the composite for structural applications showed that the decomposition of magnesite is more CO2 intensive than that of calcium carbonate with a CO2 intensity of 1.1 tonnes (Shen et al., 2016), giving a total CO2 emission of 1.7 tonnes per tonne of MgO when fuel combustion is accounted for (Hassan, 2014; Ruan and Unluer, 2016). The geographical limitation of magnesite reserves required for large-scale production of RMC and the higher life cycle carbon footprint with respect to OPC could be combated with the use of concentrated reject brine as raw feeds for its synthesis (Dong et al., 2017, 2018; Walling and Provis, 2016). In this process, the precipitation of brucite (Mg(OH)2) can be achieved with alkali sources such as Ca(OH)2, NaOH or NH4OH (Baird et al., 1988; Dong et al.,

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Please cite this article as: Hay, R., Celik, K., Accelerated carbonation of reactive magnesium oxide cement (RMC)-based composite with supercritical carbon dioxide (scCO2), Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119282

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2017, 2018; Turek and Gnot, 1995), and the precipitated Mg(OH)2 can be further filtered and calcined to form reactive MgO. MgO produced through the reject brine route could potentially offset the 1.1-tonne CO2 emission from the chemical decomposition in comparison to the magnesite route. Also, the process would effectively offset the CO2 released from the decomposition of CaCO3 in OPC production, considering a similar combustion energy level (in the range of 5.60e6.98 GJ/tonne) for the calcination of both OPC and RMC (Dong et al., 2018; Ruan and Unluer, 2016). Another challenge of RMC is related to its extremely slow carbonation processunder natural conditions. Many research works have been dedicated to finding techniques and solutions to accelerate both the hydration and carbonation reactions of the RMC-based composites. The classical ball milling method was used to modify the morphology and microstructure of reactive MgO, generally leading to its enhanced hydration reaction rate (Khalil and Celik, 2019). Acid-based hydration agents such as hydrochloric acid (HCl) and magnesium acetate ((CH3COO)2Mg), and dispersion agents such as sodium hexametaphosphate (NaHMP) were used to increase hydration of MgO to form Mg(OH)2 (Dung and Unluer, 2016, 2017a). High temperature curing at 60
C was also found to have a significant positive effect on the hydration of MgO and the subsequent carbonation and strength gain of RMC-based concrete (Dung and Unluer, 2017a). Considering mixture formulation, high-purity hydrated magnesium carbonates (HMCs) were used as a partial replacement of RMC to increase the exposed surface area of MgO and thus its hydration rate, and also to provide nucleation sites for carbonate formation (Unluer and Al-Tabbaa, 2013). Due to a significant strength enhancement, such inclusion was also postulated to give rise to the formation of an intermediate amorphous phase as a result of a reaction between the added HMCs, Mg(OH)2 and water (Kuenzel et al., 2018). The combined use of hydromagnesite seeds and sodium bicarbonates (NaHCO3 ) to increase dissolved CO2 concentration in the initial solution created a synergistic effect, resulting in an interconnection of carbonate networks and a 142% increase in compressive strengths (Dung and Unluer, 2019). Also, curing under high CO2 concentration up to 99.9% and high humidity levels was adopted to induce the formation of Mg-based carbonates (Liska and Al-Tabbaa, 2008; Mo and Panesar, 2013; Vandeperre and Al-Tabbaa, 2007). Another comprehensive investigation into curing conditions showed that CO2 concentration as low as 5% was sufficient for an almost complete carbonation of MgO and strength development and that 78% relative humidity (RH) provided an optimal condition for the formation of the carbonates (Unluer and Al-Tabbaa, 2014). A simple approach of adjusting mixture formulations of RMCbased concrete was found to enhance carbonation and strength development of the composite, with lower water contents leading to faster CO2 diffusion into the matrix (Ruan and Unluer, 2017). The above-discussed carbonation techniques generally enable strength development of RMC-based composites on a similar time scale to that of OPC-based concrete. Their implementation for commercial production of building elements poses challenges due to the limited availability of large carbonation facilities and generally a requirement to combust propane to elevate CO2 concentration (Liska and Al-Tabbaa, 2008; Liska et al., 2012). The problem could be overcome with specialized chambers where pressure and concentration of CO2 (captured from industrial processes) could be increased to further accelerate the carbonation process, thus shortening carbonation time, increasing yield while maintaining the compactness and versatility of the system. Actually, pressurized CO2 up to 0.2 MPa was implemented to carbonate

cement kiln dust and other wastes and their reactivity was shown to depend on their total calcium content and mineralogy (Gunning et al., 2010). CO2 gas pressured up to 1 MPa was successfully used to accelerate carbonation and strength development of Portland cement-fly ash-MgO mortars (Mo et al., 2016). A parametric study on gaseous CO2 under various temperature and pressure operating conditions were shown to precipitate different forms of magne€nchen et al., 2008). RMC sium carbonate from MgCl2 solution (Ha and other binder systems were used to form particleboards and paving blocks from wood waste and dredged sediment, respectively, and pressurized CO2 exposure after conventional moist curing was shown to enhance the mechanical properties and carbonation sequestration of the composites (Wang et al., 2016, 2017, 2018). It is hence postulated that an even more promising technique to accelerate carbonation is the use of supercritical CO2 (scCO2). Any solvent in a supercritical state possesses a transport property intermediate between a gas and a liquid and hence exhibits a much higher diffusivity, lower viscosity and lower surface tension than typical liquids (Hyatt, 1984; Kurnik et al., 1981). For CO2, the supercritical condition can be achieved at a temperature greater than 31
C and a pressure higher than 7.3 MPa (Hyatt, 1984). The non-toxic and unique solvent properties of scCO2 have led to its widespread utility in both research and commercial applications in the synthesis of polymeric materials (DeSimone et al., 1994), polymerization medium or plasticization of polymer melt phase (Kendall et al., 1999) and innocuous solvent in catalytic and synthetic processes (Leitner, 2002). Effective carbonation of powder portlandite (Ca(OH)2) with a conversion rate of 80e98% was achieved upon exposure to scCO2 within a period of hours (Gu et al., 2006; Vance et al., 2015). scCO2 was also shown to accelerate transformation of C3S/C2S, CeSeH and Ca(OH)2 in cement paste to lez et al., 2006, 2008; Short et al., calcium carbonates (García-Gonza 2001) and was used to enhance mechanical performance and durability of glass-fiber reinforced cement matrix (Purnell et al., 2001). As a proposed solution to geologically sequester CO2, scCO2 was used to transform forsterite (Mg2SiO4) into nesquehonite (MgCO3.3H2O) and magnesite (MgCO3) at a pressure of 9 MPa and a temperature of 50
C (Schaef et al., 2012). A co-precipitation of magnesite and nesquehonite was also reported from the carbonation of synthetic forsterite under moderate pressure levels and temperatures between 35 and 50
C (Felmy et al., 2012; Giammar et al., 2005). The proven ability of scCO2 to carbonate forsterite and portlandite also indicates its potential to carbonate brucite (Mg(OH)2). scCO2 at 50 and 75
C was shown to effectively convert brucite to nesquehonite and magnesite, respectively, within a few hours when trace amounts of water were present in the system (Schaef et al., 2011). Based on the observation, it is hypothesized that RMC-based composites with brucite as the main hydration product and inherent free water could also be carbonated at a fast rate with scCO2 to form carbonation products of hydrated magnesium carbonates (HMCs). The products would provide the binding ability for the composites for rapid strength development. Hence, the objective of this study is to demonstrate the capability of scCO2 condition to further accelerate the carbonation of hydrated RMC-based composites and to investigate the associated mechanical and chemical evolution. Compressive strength, chemical, physical and morphological analyses were performed on samples under scCO2, and a comparison was made against samples uncarbonated and carbonated under 20% CO2, 80% relative humidity (RH) and 30
C. Carbonation sequestration factors defined as the weight ratio between the sequestered CO2 and raw material were also quantified based on thermal gravimetric analysis (TGA) data. The outcomes of the research would lead to a new technique to accelerate carbonation, strength development and enhancement of carbon

Please cite this article as: Hay, R., Celik, K., Accelerated carbonation of reactive magnesium oxide cement (RMC)-based composite with supercritical carbon dioxide (scCO2), Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119282

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sequestration capability of RMC-based composites. 2. Materials and methods 2.1. Materials Light-burnt reactive magnesium oxide cement (RMC) was used as the main binder. Ordinary Portland cement (OPC) Type I was also used for strength comparison. Their normalized composition of major oxide compounds as analyzed by X-ray fluorescence (XRF) is summarized in Table 1, and their particle size distributions are given in Fig. 1. Both binders exhibited remarkably similar size distributions, with the OPC having a mean particle size of 17.9 mm as compared to 21.2 mm for RMC. Magnesium acetate ((CH3COO)2 Mg) at 0.1 M was used as the hydration agent (HA) in the RMC-based mixtures. The effectiveness of magnesium acetate to hydrate RMC was studied by other researchers (Dung and Unluer, 2017a). In preparation of mortar samples, standard sand conforming to ASTM C778 (American Society for Testing and Materials (ASTM), 2017) was adopted as the fine aggregate. Its size grading is given in Table 2.

Fig. 1. Particle size distribution of OPC and RMC.

Table 2 Size grading of standard sand.

2.2. Mixture design, sample preparation and carbonation conditions Both mortar and paste samples were prepared for mechanical and chemical testing, respectively. A water-to-binder ratio of 0.55 was maintained for both mixtures to obtain a workable fresh mix without a requirement for chemical admixture addition. In mortar preparation, a sand-to-binder ratio of 2.75 as recommended by ASTM C109 (American Society for Testing and Materials (ASTM), 2016) was adopted. The reference OPC samples (designated as PC) and RMC samples (designated as MG) were prepared and moistcured at 22 ± 2
C for a total period of 28 days. It should be noted that after 3 days of moist curing, when most of the hydration was believed to be completed, the carbonation program was initiated on two sets of RMC samples. One set of them was exposed to an accelerated carbonation condition under 20% CO2, 80% RH and 30
C (designated as MGC). The second set of the samples was exposed to supercritical CO2 conditions or scCO2 (designated as MGS) achievable with an equipment (by Supercritical Fluid Technologies Inc.) composed of a reactor, pump and temperature control unit. The temperature of the reactor chamber was maintained at 35 ± 2
C, the minimum temperature range required to create the supercritical condition while liquid CO2 was supplied and pressurized to 9.5 MPa through the pump unit. It is noted that due to the high diffusivity of scCO2 and high initial interconnected porosity of the matrix, the pressure was considered hydrostatic and hence would not induce a differential pressure to cause damage to our samples despite their low early strengths. The samples were exposed to this condition for 1, 2 or 4 h. The set-up of the scCO2 experiment is depicted in Fig. 2. Due to the size of the reactor chamber, cylindrical samples of size Ø25 mm  25 mm were prepared for both the compression and chemical testing. Table 3 summarizes the sample program and corresponding curing conditions implemented in the study.

Square mesh size (mm)

Percentage passing (%) Actual grading

ASTM C778 requirement

1.18 0.6 0.425 0.3 0.15

100 97 69 26 1

100 96e100 60e75 16e30 0e4

2.3. Compression testing An MTS universal testing machine with a load capacity of 100 kN was used for the compression testing, and a loading rate of 1.2 mm/ min was adopted, as recommended by ASTM C469 (American Society for Testing and Materials (ASTM), 2010). The mortar samples were tested at 1, 3, 7, 14 and 28 days after casting for PC and MG samples, and after exposure to the accelerated carbonation condition for MGC samples. The MGS samples were tested after 1, 2 and 4 h upon exposure to the scCO2 condition. 2.4. Phase identification and quantification At the designated compression ages, the corresponding paste samples were sliced and crushed to separate the exterior (5 mm) and interior (10 mm) regions. Hydration termination of the sample fragments was achieved by solvent exchange for 24 h, and vacuum drying was subsequently performed for at least 72 h before the samples were ground with agate pestle and mortar to sizes of less than 63 mm which was checked by sieve No. 230. The ground samples were used for X-ray diffraction (XRD) analysis with PANalytical Empyrean attached with a Cu Ka radiation source and operated under 30 mA, 40 kV. The samples were scanned under programmable gonio mode with a 2q angle between the source and

Table 1 Oxide composition of OPC and RMC.

OPC Type I Reactive MgO

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

Na2O

K2O

TiO2

P2O5

19.30 1.20

4.22 0.42

3.31 0.71

65.60 1.31

1.67 96.00

4.26 0.18

0.23 e

0.89 0.04

0.26 0.01

0.22 0.02

Please cite this article as: Hay, R., Celik, K., Accelerated carbonation of reactive magnesium oxide cement (RMC)-based composite with supercritical carbon dioxide (scCO2), Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119282

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Fig. 2. Supercritical CO2 (scCO2) test set-up: (a) reactor equipped with pressure gage, (b) pressure control unit, (c) temperature control unit, and (d) tube to liquid CO2 source (arrow).

Table 3 Sample program and corresponding curing conditions of paste and mortar samples. Sample annotation

Binder

Curing conditions

PC MG MGC MGS

OPC RMC RMC RMC

Moist-curing at 22 ± 2
C Moist-curing at 22 ± 2
C 20% CO2 under 80% RH and 30
C scCO2 under 9.5 MPa and 35 ± 2
C

the detector (PIXel3D) varied from 5 to 70
. A fixed divergence slit of 1/2
was used during the X-ray data acquisition to minimize strayed X-rays and noise signals. A step of 0.013
and a scan step time of 350 s were implemented. Based on the XRD signals, phase quantification was conducted for MG at 3 days (MG-3D), MGC at 14 days (MGC-14D), and MGS at 4 h (MGS-4H). In the process, Rietveld refinements were performed using X’Pert HighScore Plus software from PANalytical with parametric refinement for the background, the scale factors, the peak shape parameters of W, V and U, the specimen displacement, the preferred orientation, asymmetry, and the unit cell. The crystallographic information files utilized for artinite, brucite, hydromagnesite, nesquehonite, magnesite, and periclase were ICSD 1320, 79031, 920, 91710, 10264 and 159367, respectively. Raman spectra of the exterior surfaces of the paste samples were also recorded using a confocal Raman microscope (alpha300 RA from WITec GmbH, Ulm, Germany) equipped with a 488 nm laser and a lens-based spectrometer with a CCD camera. The detector was tuned to a grating of 1800 l/mm for high resolution scanning from 50 to 1250 cm1. An area of 170 mm  170 mm was scanned under Confocal Raman mode with 170 points per line and 170 lines per image under an integration (trace) time of 0.2 s/point. Thermal gravimetric analysis (TGA) of the ground samples was performed with SQ600 TG/DTA series under nitrogen gas at a flow rate of 100 ml/min. The temperature was regulated from 25 to
1000
C with a ramping rate of 10 C/min. Approximately 50 mg of individual samples was used for each run. Decarbonation of HMCs was assumed to occur between 350 and 600
C (Frost et al., 2008; Jauffret et al., 2015; Sawada et al., 1978a), from which the amount of sequestered CO2 (MCO2) was calculated based on deconvolution and area integration of the differential thermal weight loss curves. The

deconvolution was necessary due to peak overlapping with the decomposition of Mg(OH)2, dehydration and dehydroxylation of HMCs. The Gaussian peak fitting method was implemented for the deconvolution in Origin Lab software. The CO2 sequestration ratio (SCO2) was subsequently calculated based on Eq. (1), where MCO2 is the content of CO2 sequestered (i.e., the weight loss due to the decarbonation) and MRMC is the amount of residual RMC at 1000
C. Considering the high number of abbreviations and annotations used in this study, Table 4 summarizes all their meanings for an easy reference to the readers. SCO2 ¼ MCO2/MRMC

(1)

2.5. Nitrogen (N2) gas adsorption N2 gas adsorption was performed for the interior regions of the samples at 28 days (under moist curing and accelerated carbonation) and 4 h (under scCO2) to investigate their porosity properties. The vacuum-dried samples were further crushed to particle sizes between 1 and 3 mm and degassed under a pressure of 0.1 mmHg and a temperature of 40
C for 24 h, a method adopted ndez, 2008; Mantellato et al., by other researchers (Costoya Ferna 2016) to preserve the composition and to remove surface moisture and organic contaminants. The adsorption and desorption characteristics were analyzed with a 3Flex surface analyzer by Micrometrics, USA at 196
C, the liquid temperature of N2, and with applied pressure (P) varied between 1.3  109 and 1 with respect to the saturation pressure of N2 (Po). From the isotherm curves, specific surface area (SSA) was estimated from the linear region of the adsorption-desorption curves, and pore size distribution based on nonlocal density functional theory (NLDFT) was derived. 2.6. Microstructural analysis The fragmented paste samples and fractured mortar samples were mounted on aluminum stubs using double-sided adhesive carbon tape and the assemblies were coated with gold to prevent charging and subjected to scanning electron microscopy (SEM)

Please cite this article as: Hay, R., Celik, K., Accelerated carbonation of reactive magnesium oxide cement (RMC)-based composite with supercritical carbon dioxide (scCO2), Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119282

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Table 4 List of abbreviations and annotations. Abbreviation/annotation

Full form

HMCs MG MGC MGS OPC RMC SSA MCO2 MCO2, effective MCO2, intrinsic MCO2, total MRMC MTotal scCO2 SCO2, effective SCO2, intrinsic SCO2, total

Hydrated magnesium carbonates Samples under the moist-curing condition Samples exposed to accelerated carbonation Samples exposed to supercritical CO2 (scCO2) Ordinary Portland cement Reactive MgO cement Specific surface area Typical CO2 content Effective CO2 content absorbed during carbonation Intrinsic CO2 content inherent in raw material Total CO2 content Weight of reactive MgO cement Total sample weight used in thermal gravimetric analysis Supercritical CO2 Effective sequestration factor based on effective CO2 content absorbed during carbonation Intrinsic sequestration factor based on intrinsic CO2 content Total sequestration factor based on total CO2 content

with Quanta FEG 450 to investigate the morphology of hydration and carbonation products. Images were captured under secondary electron mode (SE) with an accelerating voltage of 15 kV and a spot current of 2.5 nA.

3. Results 3.1. Compression test results The compressive strength development for PC, MG, MGC and MGS mortar samples is given in Fig. 3. The carbonation time for MGS (Fig. 3 (b)) was expressed in hours. As expected, the compressive strengths of PC samples increased with curing time due to its continuing hydration. Interestingly, RMC samples under moist curing exhibited a strength improvement of up to 3 days, after which the prolonged moist curing led to a gradual reduction in strength. The phenomenon could be attributed to the continuing hydration of magnesia (MgO) to form Mg(OH)2 which has a tendency to imbibe moisture (Frost and Kloprogge, 1999) and the hy€nchen et al., 2008) present in the pore drated nature of Mg2þ (Ha solution. After 7 days of moist curing, expansion and cracking were visually seen in the paste samples, and it is inferred that a certain extent of cracking would also occur in the mortar counterpart as aggravated by a restraint induced by the aggregate. A significant strength enhancement was observed when the samples were exposed to the accelerated carbonation condition. After 1 day of carbonation, the strength of MGC increased to 13.7 MPa from 8.3 MPa of MG at 3 days, representing a 65.7% improvement. At all ages, MGC exhibited a superior mechanical performance when compared with the PC reference samples. At 3 days, the strength of MGC reaches 41.9 MPa, a threefold increase as compared to that of PC at the same age. The strength values of MGC are consistent with findings by other researchers (Dung and Unluer, 2017b) when magnesium acetate at 0.1 M was used as the hydration agent. However, the strength development of MGC stabilized after 3 days of carbonation. It is postulated that the formation of HMCs led to pore refinement and their growth or precipitation on the uncarbonated brucite crystals (Mo et al., 2016), resulting in restricted CO2 diffusion and carbonation of the remaining brucite despite the longer carbonation periods. Remarkably rapid strength development of the samples exposed to scCO2 was revealed in Fig. 3 (b). Within 1 h of scCO2 exposure, the strength of MGS suddenly increased to 21.0 MPa, representing a 153% increase over MG at 3 days. Upon 4 h of the exposure, its

strength increased to 46.3 MPa, slightly higher than the level achievable under 3e7 days of the accelerated carbonation. The results confirm that the scCO2 carbonation condition could be used to accelerate carbonation further and to achieve a mature strength development of RMC-based composites within a period of hours rather than days as under the accelerated carbonation at an elevated CO2 concentration. It is also observed that prolonged exposure to scCO2 leads to a moderate strength increase from 34.8 MPa in MGS-2H to 46.3 MPa in MGS MGS-4H, attributable to the ability of scCO2 to diffuse through the pore system and to further carbonate the remaining brucite within the matrix.

3.2. Phase identification 3.2.1. X-ray diffraction Fig. 4 presents the XRD patterns of the interior regions of RMC paste samples under the three different curing regimes. Peak assignment was performed and was based on Powder Diffraction Files (PDFs) obtained from open-source databases. They are unhydrated periclase (MgO; PDF #00-043-1022), uncarbonated brucite (Mg(OH)2; PDF #00-007-0239), artinite (Mg2CO3(OH)2$3H2O; PDF #00-006-0484), hydromagnesite (4MgCO3$Mg(OH)2$4H2O; PDF #00-025-0513), nesquehonite (MgCO3$3H2O PDF #00-020-0669) and magnesite (MgCO3; PDF #96-900-0097). It is observed in Fig. 4 (a) that the peaks associated with periclase became less intense as the curing progressed. This was due to its hydration to form brucite whose peaks became correspondingly more intense up to 3e7 days of curing. Beyond this age, the peaks of brucite did not significantly change, indicating a near-complete transformation of RMC to brucite within this short period of hydration under the aid of the hydration agent. Noticeable peaks at 26.8
2q for hydromagnesite, 29.4
2q for nesquehonite, 32.6
2q for magnesite and 32.7
2q for artinite existed in all samples, regardless of the conditions or duration of carbonation. The observation implies an inherent presence of the different forms of magnesium carbonates in the raw RMC. It is observed in Fig. 4 (b) that exposure to the accelerated carbonation led to a remarkable reduction in the peak intensities of brucite between 1 and 3 days due to its partial carbonation to form HMCs. As compared to MG samples, minor peaks at 9.6
2q for hydromagnesite, and 23.1
and 35.9
2q for nesquehonite became more apparent due to carbonation. Nevertheless, longer CO2 exposure did not significantly change the peak intensities, possibly due to restricted carbonation, as discussed earlier. Also, a small

Please cite this article as: Hay, R., Celik, K., Accelerated carbonation of reactive magnesium oxide cement (RMC)-based composite with supercritical carbon dioxide (scCO2), Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119282

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Fig. 3. Compressive strength development for: (a) OPC (PC), RMC under moist curing (MG), RMC under accelerated carbonation (MGC), and (b) RMC under scCO2 (MGS).

difference in peak intensities associated with the carbonation phases between MG and MGC samples indicates a relatively low degree of carbonation under the accelerated carbonation regime for the interior regions. In contrast, more apparent peaks corresponding to nesquehonite at 13.7
, 23.1
and 35.9
2q were observed under the exposure to scCO2 as shown in Fig. 4 (c), confirming that the scCO2 carbonation regime provided a conducive environment for the formation of this type of carbonation product. Despite a reduction in the peak intensity of brucite between 1 and 2 h of scCO2 (at 50.9
2q), longer exposure of 4 h only slightly reduced the peak intensity and this indicates a slow-down in carbonation rate. The quantification results for unhydrated MgO (periclase), uncarbonation brucite and main carbonate phases within MG-3D, MGC-14D and MGS-4H are provided in Table 5. At 3 days, only 12.0% of periclase remained in the matrix, and this confirmed the fast hydration reaction under the aid of the hydration agent to form brucite whose level correspondingly reached 83.1%. Consistent with the XRD spectra, various forms of carbonates, including artinite, hydromagnesite, nesquehonite and magnesite were inherently present in MG-3D. Under the accelerated carbonation, the content of hydromagnesite increased to 1.8% in MGC-14D from

Fig. 4. XRD patterns for interior regions of: (a) MG, (b) MGC, and (c) MGS samples.

Please cite this article as: Hay, R., Celik, K., Accelerated carbonation of reactive magnesium oxide cement (RMC)-based composite with supercritical carbon dioxide (scCO2), Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119282

R. Hay, K. Celik / Journal of Cleaner Production xxx (xxxx) xxx Table 5 Phase contents of MG, MGC, and MGS at selected curation durations based on XRDRietveld analysis.

Periclase Brucite Artinite Hydromagnesite Nesquehonite Magnesite Total Goodness of fit

MG-3D

MGC-14D

MGS-4H

12.0 83.1 0.8 0.2 0.6 3.3 100 10.5

12.3 80.9 0.8 1.8 1.2 3.0 100 7.0

17.3 73.3 2.0 0.4 4.1 2.9 100 3.7

0.2% in MG-3D. It is also revealed that both hydromagnesite and nesquehonite co-existed in MGC-14D, although hydromagnesite was the dominant phase. Conversely, scCO2 led to more formation of nesquehonite. In MGS-4H, the content of nesquehonite was found to be approximately 10 times as compared to the level of hydromagnesite. Artinite was also observed to be one of the main HMCs phases in MGS-4H. Interestingly, the content of the remaining periclase was relatively high in MGS-4H as compared to MG-3D. This could be attributed to a slightly earlier time when the samples were exposed to the scCO2 condition. 3.2.2. Raman spectroscopy results To complement the XRD results and to identify the various forms of HMCs, Raman spectroscopy on the external surfaces of MG at 28 days of curing, MGC at 28 days, and MGS at 1, 2 and 4 h of carbonation was performed, and the spectra are presented in Fig. 5. The external regions were used due to their higher abundance of carbonation products as compared to those found in the interior regions. Normalization was performed with respect to the maximum intensity between Raman shifts of 50 and 1250 cm1. According to Raman spectra obtained from RRUFF database for brucite (R040077), the peaks at 277, 442, 726 and 1086 cm1 are attributed to OH vibration modes in brucite. The free CO2 3 ion in HMCs exhibits four normal vibrational modes: a symmetric stretching vibration (n1) with a strong Raman peak at around 1100 cm1, an out-of-plane bending mode (n2) with a peak at 800 cm1, a doubly degenerate asymmetric stretching mode (n3)

7

with a weak peak at 1400 cm1 and another doubly degenerate bending mode (n4) with a peak at around 700 cm1 (Frost, 2011; Nakamoto, 2006). Consistently, the RRUFF database for nesquehonite (R050639), hydromagnesite (R060011), dypingite (R070086) and artinite (R060166) exhibit a dominant peak in the vicinity of 1100 cm1 as associated with the vibration of CO2 3 . It is observed in Fig. 5 that all samples showed spectrum peaks at ~279.1, ~444.1 and ~727.5 cm1, which corresponded to the OH vibration mode in the uncarbonated brucite. The findings confirmed that brucite persisted in the system despite the carbonation exposure. A broad peak with a center at 794.3 cm1 appeared in all samples under carbonation and was attributed to the out-of-plane bending mode (n2) of CO2 3 . Also, peaks in the vicinity of 715.4 cm1 were observed in all the samples and could be due to the doubly degenerate bending mode (n4) of CO2 3 in HMCs and magnesite, which were inherently present in the raw material. In MGC-28D, an asymmetric peak broadening in the vicinity of 1100 cm1 corresponded to CO2 3 of HMCs. A new peak at 1120 cm1 was also revealed and was assigned to CO2 3 vibration of hydromagnesite, as found by other researchers (Frost, 2011). On the other hand, the peak broadening in MGS samples was centered at 1098.4 cm1 and was associated with nesquehonite (Frost and Palmer, 2011; Hales et al., 2008). In line with the XRD results, the finding confirms that hydromagnesite was the dominant carbonation phase under a prolonged accelerated carbonation condition as compared to nesquehonite, which was found to be prevalent under scCO2. Stoichiometry could be used to explain the phenomenon. As observed from Eqs. (2) and (3) (Unluer and Al-Tabbaa, 2014), the molar ratio of Mg(OH)2:CO2 for nesquehonite is 1:1 while that of hydromagnesite is 5:4. The higher concentration of CO2 under scCO2 would hence induce more formation of nesquehonite. A recent study conducted by Dong et al. (2019) also confirmed that higher Mg(OH)2:CO2 molar ratios (>2) achieved by sparging CO2 into Mg(OH)2 slurry or reject brine led to a growth of rosette-like hydromagnesite/dypingite flakes over nesquehonite at high pH values. The higher stability of hydromagnesite (Davies and Bubela, 1973) could also explain its dominant presence for MGC-28D under an extended carbonation duration. Mg(OH)2 þ CO2 þ 3H2O / MgCO3.3H2O (nesquehonite)

(2)

5 Mg(OH)2 þ 4CO2 / 4MgCO3.Mg(OH)2.4H2O (hydromagnesite)(3) Phase distribution was analyzed by applying filtering on the area scan spectrums at 1086, 1100 and 1118 cm1 for brucite, nesquehonite, and hydromagnesite, respectively. A filter width of 10 cm1 was adopted, and phase distribution maps, with intensity count normalized to a scale of 0e100, for MG-28D, MGC-28D and MGS-2H are depicted in Fig. 6. Brighter color intensity is associated with a higher content of the corresponding phase. It is confirmed that brucite is the dominant phase in MG-28D, with only traces or background of nesquehonite and hydromagnesite appearing on its surface. Prolonged exposure to the concentrated CO2 condition induced the formation of HMCs, leading to a reduced intensity of OH from brucite, as seen in the phase mapping of MGC-28D. Although traces of nesquehonite were observed in MGC-28D, the dominant HMCs phase was hydromagnesite, whose growth seemed to be in agglomerated clusters. scCO2 further reduced the relative intensity of brucite, and it is also revealed that the dominant HMCs phase was mainly nesquehonite, consistent with the average Raman spectrum results of the scanned area. Fig. 5. Raman spectra with 1800 g/mm detector setting for MG at 28 days, MGC at 28 days and MGS at 1, 2 and 4 h..

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Fig. 6. Phase distribution based on Raman spectra for exterior regions of RMC under moist curing for 28 days (MG-28D), RMC under accelerated carbonation for 28 days (MGC-28D), and RMC under scCO2 for 2 h (MGS-2H).

3.3. Phase quantification 3.3.1. TGA results of interior regions The weight loss and differential weight loss results obtained from the middle or interior 10 mm regions of MG and MGC samples for a curing period of up to 28 days and MGS samples for a curing period of up to 4 h are plotted in Fig. 7. Early weight losses in MG samples up to approximately 200
C were attributed to dehydration from the matrix. The decomposition of Mg(OH)2 in all samples started at around 300
C, as observed by the change in the slope of the differential weight loss curves, consistent with the value given by other researchers (Hollingbery and Hull, 2010). As the curing duration increased, the endothermic peaks with centers at 390
C became more intense for MG samples due to the progress of hydration and more formation of Mg(OH)2. In line with this, the prolonged hydration periods also increased the ultimate weight losses of MG samples. Two endothermic peaks were also observed at 560 and 650
C and were attributed to the decomposition of the intrinsic MgCO3 and other forms of carbonates. The decomposition temperatures of the carbonation phases were consistent with findings by other researchers (Frost et al., 2008; Hollingbery and Hull, 2010; Jauffret et al., 2015; Sawada et al., 1978b). Under carbonation, it is revealed that the weight losses up to 200
C increased for MGC and MGS samples due to the higher content of water within the HMCs and the subsequent dehydration upon heating. The endothermic peaks due to dehydroxylation of Mg(OH)2, either in its own isolated form or as part of HMCs,

increased to 405
C for MGC-28D of carbonation and 410
C for MGS-4H. The shift could be attributed to a delayed decomposition of Mg(OH)2 as a result of the physical barrier effects provided by HMCs growth. It is apparent that as carbonation progressed, shoulder peaks at approximately 455 and 500
C started to appear in both MGC and MGS samples due to decarbonation of the newlyformed HMCs. No remarkable difference in the weight loss and differential weight loss curves was observed for MGC samples between carbonation durations of 3 and 28 days. It is postulated that the carbonation rate of the interior regions dramatically slowed down after 3e7 days of carbonation due to matrix refinement of the exterior regions and a protective barrier created by the newlyformed carbonation products, resulting in a reduced CO2 diffusion and carbonation of the remaining brucite. Also, due to the dehydration, decarbonation and additional dehydroxylation in MGC and MGS samples, their total weight losses were found to be higher than those of MG samples. In phase quantification of MGC and MGS samples, the dehydration peak was assumed to be in the vicinity of 100
C, while dehydroxylation occurred between 300 and 460
C (Frost et al., 2008; Hollingbery and Hull, 2010) with two distinct distributions of peaks at 385 and 410
C. A wide temperature spectrum over a range of 360e950
C has been reported for the decomposition of HMCs (Frost et al., 2008; Hollingbery and Hull, 2010; Jauffret et al., 2015; Sawada et al., 1978b), which also overlaps the decomposition temperature of brucite. In the deconvolution of the weight loss curves to quantify the carbonation content, peak centers

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Fig. 7. Thermal gravimetric analysis results for interior regions of: (a) RMC under moist curing (MG), (b) RMC under acceleration carbonation (MGC), and (b) RMC under scCO2 carbonation (MGS).

9

corresponding to decarbonation were assigned at 455, 500, 560 and 650
C with a margin of ±10
C depending on the actual profiles of data. The weight loss results attributed to dehydration, dehydroxylation, and decarbonation (MCO2, total) along with CO2 sequestration factors for the interior regions of MGC and MGS samples are summarized in Table 6 and Table 7, respectively. MTotal and MCO2, total shown in the tables refer, respectively, to the total TGA feed and the total CO2 content (as a proportion of the sample weight) which comprises the intrinsic CO2 from the raw RMC (MCO2, intrinsic) and CO2 newly sequestered during the carbonation period (MCO2, effective). Their CO2 sequestration factors were respectively annotated as SCO2, total, SCO2, intrinsic, and SCO2, effective. SCO2, intrinsic was estimated from the TGA profile of MG at 1 day, assuming that no significant induced carbonation occurred within this short period of moist curing. It is noted in the tables that MTotal was not summed up to 100 wt%, and a consistent underestimation of approximately 2 wt% could be attributed to the deconvolution and curve fitting errors. It is observed that dehydration generally increased in tandem with the decarbonation level. For MGC samples, dehydration increased from 2.0 wt% to 5.8 wt% while decarbonation increased from 4.0 wt% to 4.2 wt% under 1 and 3 days of carbonation, respectively. This disproportionate increase of dehydration level could be attributed to the presence of surface water, which may not have been removed by the solvent exchange as part of the drying process. Dehydroxylation and decarbonation levels for MGC samples stabilized between 3 and 28 days of carbonation, attributable to a slowdown of hydration and carbonation within the interior regions. The carbonation slowdown was mainly attributed to matrix densification due to the formation of HMCs (Dung et al., 2019; Dung and Unluer, 2017b), resulting in restricted diffusion of CO2 into the matrix system. Also, the precipitation of HMCs on brucite formed a protective layer and further limited the carbonation of the remaining brucite (Mo et al., 2016). The level of effective CO2 sequestered was estimated to be 1.6 wt% for MGC-1D and increased to 2.5 wt% for MGC-3D, after which the sequestration factors stabilized. The trend is consistent with the compressive strength results where no significant strength change occurred after 3 days of carbonation. The ultimate CO2 sequestration factor at 28 days of carbonation was estimated at 2.8 wt%. Due to a fast carbonation and transformation of brucite into HMCs, MGS samples exhibited a more noticeable reduction in dehydroxylation with increasing exposure time to scCO2. The reduction was a result of the short carbonation duration that did not allow enough time for hydration to progress to form more brucite to offset its transformation to HMCs. The ability of scCO2 to carbonate RMC at a fast rate was revealed by a significant increase in the effective sequestration factor. Within just 1 h of carbonation under scCO2, the sequestration factors increased to 3.1 wt%, as compared to just 2.8 wt% at 28 days achieved with the accelerated carbonation regime under 20% CO2. Within 4 h of scCO2, the factor increased to 8.8 wt%, representing a three-fold increase in CO2 sequestration as compared to the level obtained from the accelerated carbonation process. Despite the much larger carbonation content, the strengths of MGS samples were observed to be of similar magnitude to those of MGC samples. This could be attributed to the formation of HMCs products at a very fast rate under scCO2, causing phase incompatibility, microcracking of the matrix and the resulting strength plateau. In general, the results imply that RMC requires only a small carbonation level to achieve a similar or

Please cite this article as: Hay, R., Celik, K., Accelerated carbonation of reactive magnesium oxide cement (RMC)-based composite with supercritical carbon dioxide (scCO2), Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119282

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Table 6 Weight loss results and CO2 sequestration factors for MGC interior regions. Age (day)

Dehydration

Dehydroxylation

Decarbonation (MCO2, total)

MRMC

MTotal

SCO2, total (MCO2, total/MRMC)

SCO2,

5.7 6.6 6.5 6.7 6.9

4.1

1.6 2.5 2.5 2.6 2.8

intrinsic

SCO2, effective (SCO2, total- SCO2,

intrinsic)

(wt% of sample) 1 3 7 14 28

2.0 5.8 6.2 6.2 7.2

23.1 23.9 23.4 23.9 23.3

4.0 4.2 4.2 4.3 4.4

70.2 64.7 64.5 63.9 63.4

99.3 98.6 98.3 98.2 98.3

Table 7 Weight loss results and CO2 sequestration factors for MGS interior regions. Age (hour)

Dehydration

Dehydroxylation

Decarbonation (MCO2, total)

MRMC

MTotal

SCO2, total (MCO2, total/MRMC)

SCO2, intrinsic

SCO2, effective (SCO2, total- SCO2, intrinsic)

64.7 60.2 58.4

98.6 98.0 97.9

7.2 11.8 12.9

4.1

3.1 7.7 8.8

(wt% of sample) 1 2 4

5.9 8.7 10.3

23.4 22.1 21.8

4.6 7.1 7.5

better strength level to that of its OPC counterpart.

Fig. 8. Thermal gravimetric analysis results for exterior regions of: (a) RMC under accelerated carbonation (MGC), and (b) RMC under scCO2 carbonation (MGS).

3.3.2. TGA results of exterior regions and carbonation distribution It was observed that the strength development of the mortar samples and the carbonation contents of the interior regions of the corresponding paste samples exposed to the accelerated carbonation (MGC) stabilized after 3 days. It was also argued that carbonation and the subsequent formation of HMCs led to matrix refinement of the exterior regions and resulted in restricted CO2 diffusion into the matrix. In order to prove this, the exterior 5 mm regions of the paste samples were extracted and subjected to TGA analyses. Their weight loss and differential weight loss curves, together with those of MGS samples, are given in Fig. 8. It is revealed that at 1 day of CO2 exposure, the exterior region of MGC1D exhibited more apparent differential weight losses due to dehydration and decarbonation relative to its internal region. Carbonation content results, as given in Table 8, showed that the effective CO2 sequestration factor of MGC-1D was 2.0 wt% in the exterior region as compared to 1.6 wt% in the interior region. Prolonged exposure to CO2 also led to increased carbonation contents of the exterior regions. The CO2 sequestration factors increased by almost 2 times to 4.0 wt% at 28 days. The finding implies that regions in direct contact with the concentrated CO2 environment continued to be carbonated, leading to more formation of HMCs and increased densification of the exterior matrix, which ultimately impeded further carbonation of the interior regions. The finding confirms that carbonation of RMC is non-uniform and that of strength enhancement could potentially be achieved with techniques to enhance carbonation distribution of the samples, one of which is the scCO2 being investigated in the study. Table 9 provides carbonation quantification results for the exterior regions of the samples under scCO2. It is observed that at 1 h, the exterior region of MGS-1H was more carbonated as compared to its interior region, whose CO2 sequestration factors were found to be 6.6 wt% and 3.1 wt%, respectively. However, at 2 and 4 h, the carbonation contents of both the interior and exterior regions of MGS-2H and MGS-4H converged. This was attributed to an improved permeation of the gas-liquid phase of scCO2 into the matrix through three combined mechanisms: (i) enhanced solvent

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Table 8 Weight loss results and CO2 sequestration factors for MGC exterior regions. Age (day)

Dehydration

Dehydroxylation

Decarbonation (MCO2, total)

MMRC

MTotal

SCO2, total (MCO2, total/MRMC)

SCO2, intrinsic

SCO2, effective (SCO2, total- SCO2, intrinsic)

23.1 24.4 23.4 23.8 22.5

4.0 4.1 4.5 4.8 5.1

66.1 64.8 63.5 62.2 62.5

98.7 98.8 98.4 98.1 97.8

6.1 6.4 7.0 7.7 8.1

4.1

2.0 2.3 3.0 3.6 4.0

(wt% of sample) 1 3 7 14 28

5.5 5.4 7.0 7.3 7.7

Table 9 Weight loss results and CO2 sequestration factors for MGS exterior regions. Age (hour)

Dehydration

Dehydroxylation

Decarbonation (MCO2, total)

MMRC

MTotal

SCO2, total (MCO2, total/MRMC)

SCO2, intrinsic

SCO2, effective (SCO2, total- SCO2, intrinsic)

22.6 22.4 21.5

6.4 6.8 7.5

60.3 60.3 58.5

98.2 98.0 97.7

10.7 11.2 12.7

4.1

6.6 7.1 8.6

(wt% of sample) 1 2 4

8.9 8.6 10.2

characteristics of scCO2, (ii) high pressure level required to achieve the supercritical state of CO2, and (iii) possible formation of microcracking due to multiphase formation and internal restraint. The latter is postulated to play a less dominant role in view that such microcracking was observed in both the MGC and MGS samples (to be shown in SEM results). The results affirm that scCO2 could be utilized to enhance carbonation and to achieve a more uniform carbonation distribution throughout the matrix. 3.4. Porosity measurement with N2 gas adsorption Adsorption-desorption isotherms for the interior regions of MG28D, MGC-28D and MGS-4H are shown in Fig. 9. The vertical axis represents the amount of N2 absorbed per gram of the solid materials (at standard temperature and pressure or STP), while the

horizontal axis denotes the relative pressure (P/Po). Hysteresis loops corresponding to Type IV isotherms (Sing, 1985) are observed for all the samples and are associated with capillary condensation in the mesopores of internal width between 2 and 50 nm (Scrivener et al., 2016). For MG-28D, the closure point was observed at P/ Po ¼ 0.4, corresponding to the N2 boiling point, and the desorption curve followed the adsorption path at lower relative pressures. On the other hand, the desorption curves for MGC-28D and MGS-4H deviated from their respective adsorption curves below the N2 boiling point, and this could be attributed to the instrument’s sensitivity in capturing the small data values. It is evident from the isotherms that the total amounts of N2 adsorbed on MG-28D, MGC28D and MGS-4H were in decreasing order. At P/P0 of 1, the quantities of N2 adsorbed were estimated to be 56.9, 1.5 and 0.9 cm3/g STP for MG-28D, MGC-28D and MGS-4H, respectively,

Fig. 9. Adsorption-desorption curves of interior regions of MG at 28 days (MG-28D), MGC at 28 days (MGC-28D), and MGS at 4 h (MGS-4H).

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Fig. 10. Pore distribution for MG at 28 days (MG-28D), MGC at 28 days (MGC-28D), and MGS at 4 h (MGS-4H).

indicating a significant reduction in capillary porosity of the samples under concentrated CO2 carbonation and scCO2. The results of pore size distribution expressed in cumulative pore volume per unit sample weight (cm3/g) derived from nonlocal density functional theory (NLDFT) are shown in Fig. 10. The pore sizes shown were in the range of 2e100 nm as restricted by the detectable limit of the equipment and could be assigned to capillary pores of the matrix. In line with the discussion on N2 adsorption results, it was revealed that a noticeably larger cumulative pore volume was present present in MG-28D as compared to MGC-28D and MGS-4H. Also, MGS-4H exhibited a smaller cumulative pore volume relative to that of MGC-28D and this confirms the ability of scCO2 to further refine nanopores in such a short carbonation time. Pore size less than 13 nm was not observed in MGS-4H due to carbonation and the resulting pore refinement. Meanwhile, the relatively high initial cumulative pore volumes for MG-28D and MGC-28D possibly indicate the presence of pores with size less than 2 nm in the matrix respective matrices. The specific surface areas (SSA) were also derived from the linear regions of the adsorption isotherms over the P/Po range of 0.05e0.30 based on the Brunauer-Emmett-Teller (BET) theory of monolayer adsorption capacity of porous materials (Sing, 1985) and the SSA values were found to be 23.90, 0.75 and 0.36 m2/g for the three samples, respectively. The findings confirm that the formation of the expansive HMCs reduced the porosity by pore-filling mechanism, leading to multi-scale matrix densification, reduction in sample surface area and ultimately strength enhancement of the composite. The reduction in the SSA of the carbonated samples also implies a reduction in the reactive area due to the formation of HMCs, thus supporting the diffusion control and protective barrier effect in restricting further carbonation. 3.5. Microstructural analysis Fig. 11 shows the morphology of the exterior regions of RMC paste samples under the three different curing regimes at various ages. At 1 day of hydration, disk-like brucite minerals with diameters less than 1 mm were observed in MG-1D. A prolonged hydration duration of 28 days helped to increase the size of the brucite disks, although no noticeable morphology change was observed. The formation of brucite and the resulting matrix densification contributed to the strength gain of MG samples (up to 3 days). An exposure to CO2 induced carbonation and the formation

of HMCs. At 1 day under the accelerated carbonation environment of 20% CO2, a small amount of needle-like nesquehonite was observed on the sample surface of MGC-1D (Fig. 11 (c)). The disklike brucite disappeared from the surface and the matrix became more densified. It is postulated that some of the brucite minerals in the exterior regions were transformed into intermediate carbonation phases of no exact morphology. Examination of MGC-7D (Fig. 11 (d)) revealed the rosette-like hydromagnesite growing radially from clusters of an intermediate phase, which took a resemblance to nesquehonite. At 28 days of carbonation, clusters of rosette-like hydromagnesite were observed on the sample surface (Fig. 11 (e)), and the finding is consistent the observation from Raman analyses. As expected, larger and more apparent features of the rosette-like hydromagnesite were observed for MGC-28D as compared to MGC-7D due to a significant improvement to the morphology of HMCs upon a prolonged exposure to CO2. Intriguingly, the nesquehonite observed at 1 day of carbonation disappeared under longer carbonation periods, and this was attributed to its transformation to hydromagnesite through a transitional phase called proto-hydromagnesite with structural similarity to dypingite (Davies and Bubela, 1973). Consistent with the XRD and TGA results, scCO2 was conducive to the formation of nesquehonite on the sample surfaces. In comparison to MGS-1H under 1-h exposure to sCO2, larger and more elongated nesquehonite was observed for MGS-2H, as shown in Fig. 11 (g). At a pH level of 10e11 which is the level expected in the pore solution of RMC-based composites (Dung and Unluer, 2019), rosette-like hydromagnesite/dypingite flakes were observed on the surface of nesquehonite needles, attributable to a dissolution-recrystallizationself-assembly growth mechanism with nesquehonite serving as a precursor for further nucleation and seeding of hydromagnesite/ dypingite on the surface (Dong et al., 2019). Such flakes were not observed here and could be due to the less hydrated condition of the reactor, which limited the dissolution and recrystallization mechanism. Under 4 h of scCO2, as in Fig. 11 (h), most of the nesquehonite on the surface of MGS-4H seems to be fused together. The morphology change could be attributed to its phase transformation under a prolonged scCO2 exposure, although further studies are needed for its identification. Indeed, other researchers observed the formation of amorphous nesquehonite under scCO2 with the presence of water (Schaef et al., 2011) or higher temperature conditioning of HMCs (Jauffret et al., 2015; Morrison et al., 2016). Fig. 12 shows representative SEM images of the fractured surfaces of MGC-28D, MGS-2H paste samples, and MGC-28D, MGS-4H mortar samples. Again, it was shown that hydromagnesite and nesquehonite were the main HMCs in MGC and MGS samples, respectively. However, the morphology of these carbonation products was not as apparent as in the exterior regions and was only noticeable in voids or at aggregate-paste interfacial (pulledout) regions. This could be due to reduced carbonation through the depth, specifically for the samples under the accelerated carbonation, or space confinement, which restricted the growth and morphology development of the HMCs. The finding indicates that the HMCs within the interior regions of the matrix could be mostly of intermediate or amorphous forms with no clear morphology. The presence of microcracks, possibly due to phase incompatibility, was apparent in both MGC and MGS paste and mortar samples, and this could potentially serve as a strength limiting factor for RMC-based concrete. The implementation of cracking mitigation strategies such as mixture proportioning with minimal binder content and introduction of fibers to provide a crack-arresting mechanism could be beneficial to the strength enhancement of the RMC-based composites. Nevertheless, the cracking extent under different carbonation conditions and its effects on mechanical properties

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Fig. 11. SEM images of external surfaces for: (a) RMC under moist curing at 1 day (MG-1D), (b) RMC under moist curing at 28 days (MG-28D), (c) RMC under accelerated carbonation for 1 day (MGC-1D), (d) RMC under accelerated carbonation for 7 days (MGC-7D), (e) RMC under accelerated carbonation for 28 days (MGC-28D), (f) RMC under scCO2 for 1 h (MGS1H), (g) RMC under scCO2 for 2 h (MGS-2H), and (h) RMC under scCO2 for 4 h (MGS-4H).

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Fig. 12. SEM images of fractured surfaces for: (a) RMC under accelerated carbonation for 28 days (MGC-28D), and (b) RMC under scCO2 for 2 h (MGS-2H), (c) RMC mortar under accelerated carbonation for 28 days (MGC-28D mortar), and (d) RMC mortar under scCO2 for 4 h (MGS-4H mortar).

would need to be further elucidated.

4. Conclusions Supercritical CO2 (scCO2) was proposed as a process to accelerate the carbonation of reactive magnesium oxide cement (RMC), which is deemed to be one of the sustainable alternatives to ordinary Portand cement (OPC). RMC was allowed to hydrate for 3 days before exposure to the scCO2 condition for a duration of 1, 2 and 4 h. Compressive strength, chemical, and morphological analyses were performed and compared with uncarbonated and carbonated samples under 20% CO2, 80% RH and 30
C (accelerated carbonation). From the results and discussion presented above, the following can be concluded:  RMC-based samples exposed to scCO2 exhibited an extremely rapid strength gain with a mature strength development achievable within hours.  Carbon dioxide (CO2) sequestration factors increased by three folds upon 4 h of exposure to scCO2 as compared to 28 days under the accelerated carbonation of 20% CO2.  The effective CO2 sequestration factor under scCO2 seems to reach a maximum threshold of 8e9% with respect to the RMC mass. The level could be the maximum possible CO2 absorption capacity of the composite.  Although nesquehonite and other intermediate carbonation products were observed in the exterior regions at early ages of









the accelerated carbonation, the more stable hydromagnesite was found to be the dominant hydrated magnesium carbonates (HMCs) upon a prolonged exposure to the 20% CO2 environment. The scCO2 condition, on the other hand, induced more formation of nesquehonite. For the interior regions, an apparent morphology of the HMCs was not easily discernible, and the carbonation products were postulated to be dominated by intermediate or amorphous HMC phases. Uneven distribution of carbonation within the samples was observed with the carbonation rate of the interior regions slowed down upon 3e7 days of the accelerated carbonation or upon 2 h of exposure to scCO2, attributable to the protective barrier induced by the growth of HMCs over brucite and the matrix densification resulting in restricted diffusion of CO2. An effective carbonation level of only approximately 2 wt% was required to provide a significant binding action and strength development of the RMC-based composite. Carbonation and the resulting formation of HMCs led to a refinement of capillary porosity and a reduction in the specific surface area. Despite having higher carbonation sequestration factors, samples under scCO2 did not exhibit strength development in proportion to their carbonation content, and the mechanism behind this would need to be further explored.

The findings demonstrated that scCO2 could be effectively used to further accelerate the carbonation of RMC-based composites to

Please cite this article as: Hay, R., Celik, K., Accelerated carbonation of reactive magnesium oxide cement (RMC)-based composite with supercritical carbon dioxide (scCO2), Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119282

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achieve mature strength development within hours and to enhance its carbonation sequestration factors. It is envisioned that the technique could be scaled up for industrial production of precast concrete elements to cut down on curing and carbonation time and thus to increase the production cycles. However, such a system would be technologically challenging and cost-prohibitive, considering the high pressure the reactor chamber needs to sustain and the requirement for liquid CO2 as solvent and reactant. One plausible variation would be carbonation implementation under pressurized CO2 condition (1e3 MPa) in which an autoclave system widely used in precast plants could be readily adopted. The required CO2 could be of lower quality and could be captured from industrial sources. The study is being investigated by the research team. Declaration of competing interest 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. Acknowledgment The authors wish to express their gratitude and sincere appreciation to New York University Abu Dhabi for grant number ADHPG-ST254 to make this research possible and to Core Technology Platforms (CTPs) experts, specifically Dr. James Weston and Dr. Liang Li for guidance and assistance with some of the experiment. The help by undergraduate student Joel Ntwali Rukazambuga is also much appreciated. References American Society for Testing and Materials (ASTM), 2010. ASTM C469/469M-10: Standard Test Method for Static Modulus of Elasticity and Poisson’s Ratio of Concrete in Compression, 100 Barr Harbor Dr., PO box C-700. ASTM International, West Conshohocken, Pennsylvania. American Society for Testing and Materials (ASTM), 2016. ASTM C109/C109M-16a: Standard Test Method for Compressive Strength of Hydraulic Cement Mortar, 100 Barr Harbor Dr., PO box C-700. ASTM Standards. ASTM International, West Conshohocken, Pennsylvania. American Society for Testing and Materials (ASTM), 2017. ASTM C778-17: Standard Specification for Standard Sand, 100 Barr Harbor Dr., PO box C-700. ASTM International, West Conshohocken, Pennsylvania. Baird, T., Braterman, P.S., Cochrane, H.D., Spoors, G., 1988. Magnesium hydroxide precipitation as studied by gel growth methods. J. Cryst. Growth 91 (4), 610e616. Celik, K., Hay, R., Hargis, C.W., Moon, J., 2019. Effect of volcanic ash pozzolan or limestone replacement on hydration of Portland cement. Constr. Build. Mater. 197, 803e812. Celik, K., Jackson, M.D., Mancio, M., Meral, C., Emwas, A.H., Mehta, P.K., Monteiro, P.J.M., 2014. High-volume natural volcanic pozzolan and limestone powder as partial replacements for portland cement in self-compacting and sustainable concrete. Cement Concr. Compos. 45 (0), 136e147. Celik, K., Meral, C., Petek Gursel, A., Mehta, P.K., Horvath, A., Monteiro, P.J.M., 2015. Mechanical properties, durability, and life-cycle assessment of selfconsolidating concrete mixtures made with blended portland cements containing fly ash and limestone powder. Cement Concr. Compos. 56 (0), 59e72. Costoya Fern andez, M.M., 2008. Effect of Particle Size on the Hydration Kinetics and Microstructural Development of Tricalcium Silicate. EPFL. Davies, P.J., Bubela, B., 1973. The transformation of nesquehonite into hydromagnesite. Chem. Geol. 12 (4), 289e300. DeSimone, J.M., Maury, E.E., Menceloglu, Y.Z., McClain, J.B., Romack, T.J., Combes, J.R., 1994. Dispersion polymerizations in supercritical carbon dioxide. Science 265 (5170), 356e359. Dong, H., Unluer, C., Yang, E.-H., Al-Tabbaa, A., 2017. Synthesis of reactive MgO from reject brine via the addition of NH 4 OH. Hydrometallurgy 169, 165e172. Dong, H., Unluer, C., Yang, E.-H., Al-Tabbaa, A., 2018. Recovery of reactive MgO from reject brine via the addition of NaOH. Desalination 429, 88e95. Dong, H., Unluer, C., Yang, E.-H., Jin, F., Al-Tabbaa, A., 2019. Microstructure and carbon storage capacity of hydrated magnesium carbonates synthesized from different sources and conditions. J. CO2 Util. 34, 353e361. Dung, N.T., Lesimple, A., Hay, R., Celik, K., Unluer, C., 2019. Formation of carbonate phases and their effect on the performance of reactive MgO cement formulations. Cement Concr. Res. 125, 105894.

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Please cite this article as: Hay, R., Celik, K., Accelerated carbonation of reactive magnesium oxide cement (RMC)-based composite with supercritical carbon dioxide (scCO2), Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119282

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Please cite this article as: Hay, R., Celik, K., Accelerated carbonation of reactive magnesium oxide cement (RMC)-based composite with supercritical carbon dioxide (scCO2), Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.119282