applied sciences

Article Uptake by MOC-Based Materials

OndˇrejJankovský 1, Michal Lojka 1, Anna-Marie Lauermannová 1, Filip Antonˇcík 1, Milena Pavlíková 2, Zbyšek Pavlík 2 and David Sedmidubský 1,* 1 Department of Inorganic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology, Technická 5, 166 28 Prague 6, Czech Republic; [email protected] (O.J.); [email protected] (M.L.); [email protected] (A.-M.L.); fi[email protected] (F.A.) 2 Department of Materials Engineering and Chemistry, Faculty of Civil Engineering, Czech Technical University in Prague, Thákurova 7, 166 29 Prague 6, Czech Republic; [email protected] (M.P.); [email protected] (Z.P.) * Correspondence: [email protected]; Tel.: +420-2-2044-4122

 Received: 6 March 2020; Accepted: 24 March 2020; Published: 26 March 2020 

Featured Application: The highest potential of oxychloride (MOC) is its capability to be used as a component of low-energy building composite materials while acting as a CO2 sink. The results of this contribution also show that MOC can be used as a in advanced building materials that have particular properties, and therefore specific application potentials. Formation and hardening of this material are rather fast, so the material can be used in quick repairs as well as a protection layer. This property is also beneficial for use in prefabrication, due to the possibility of unmolding after a shorter time compared to (PC) materials, so the whole production process can be considered more effective. Somewhat significant importance should be given to its ability to capture CO2, which not only makes it more eco-friendly, but also improves its mechanical properties.

Abstract: In this work, carbon dioxide uptake by cement (MOC) based materials is described. Both thermodynamically stable magnesium oxychloride phases with stoichiometry 3Mg(OH) MgCl 8H O (Phase 3) and 5Mg(OH) MgCl 8H O (Phase 5) were prepared. 2· 2· 2 2· 2· 2 X-ray diffraction (XRD) measurements were performed to confirm the purity of the studied phases after 7, 50, 100, 150, 200, and 250 days. Due to carbonation, chlorartinite was formed on the surface of the examined samples. The Rietveld analysis was performed to calculate the phase composition and evaluate the kinetics of carbonation. The SEM micrographs of the sample surfaces were compared with those of the bulk to prove XRD results. Both MOC phases exhibited fast mineral carbonation and high maximum theoretical values of CO2 uptake capacity. The materials based on MOC cement can thus find use in applications where a higher concentration of CO2 in the environment is expected (e.g., in flooring systems and wall panels), where they can partially mitigate the harmful effects of CO2 on indoor air quality and contribute to the sustainability of the construction industry by means of reducing the carbon footprints of alternative building materials and reducing CO2 concentrations in the environment overall.

Keywords: magnesium oxychloride cement; MOC phases; carbonation; CO2 uptake; carbon footprint

1. Introduction Magnesium oxychloride cement (MOC), also known as , was discovered by French engineer in 1867 [1]. Since then, it has often been compared to Portland cement (PC), which was discovered just a few years earlier. It is formed using a mixture of with an aqueous solution of in a certain ratio. The ratios in the system MgO-MgCl2-H2O

Appl. Sci. 2020, 10, 2254; doi:10.3390/app10072254 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 2254 2 of 11

are significant for each phase of MOC: Phase 2 (2Mg(OH) MgCl 4H O, MOC 2-1-4) and Phase 9 2· 2· 2 (9Mg(OH)Appl. Sci. 2020MgCl, 10, x FOR5H PEERO, REVIEW MOC 9-1-5), which are stable at elevated temperatures (~100 C); and2 of Phase11 2· 2· 2 ◦ 3 (3Mg(OH)2 MgCl2 8H2O, MOC 3-1-8) and Phase 5 (5Mg(OH)2 MgCl2 8H2O, MOC 5-1-8), which are 46 3 (3Mg(OH)·2∙MgCl2·∙8H2O, MOC 3-1-8) and Phase 5 (5Mg(OH)2∙MgCl· 2∙8H· 2O, MOC 5-1-8), which are stable at ambient temperature until they react with CO2 or H2O. [2–4] The crystal structures of Phase 3 47 stable at ambient temperature until they react with CO2 or H2O. [2–4] The crystal structures of Phase 3 and 5 formed by double and triple ribbons of edges shared MgO6 octahedra with water molecules 48 and 5 formed by double and triple ribbons of edges shared MgO6 octahedra with water molecules and and chloride anions in the interstitial region are shown in Figure1. The drawings were produced by 49 chloride anions in the interstitial region are shown in Figure 1. The drawings were produced by VESTA VESTA Software [5]. 50 Software [5]. 51 52 53 54 55 56 57 58 59 60 61 62 63

64 FigureFigure 1. 1. CrystalCrystal structures of Phase 3 3 (left) (left) and and Phase Phase 5 5(right). (right). Mg Mg atoms atoms are are located located in inthe the centers centers of of 65 distorteddistorted octahedra octahedra formedformed by oxygen atoms. Cl Cl at atoms—green;oms – green; O O atoms atoms—blue. – blue. Green-blue Green-blue spheres spheres in in 66 thethe Phase Phase 55 structures structures correspondcorrespond to a mixed site site occupied occupied by by chloride chloride anions anions and and water water molecules molecules 67 ((hydrogen atoms atoms areare notnot shown).

68 MOCMOC phases phases and and MOC-based MOC-based composites composites have have been been intensively intensively studied studied in recent in years.recent Synthesis,years. 69 structure,Synthesis, and structure, the thermal and the stability thermal of magnesiumstability of magnesium oxychlorides oxychlorides in particular in were particular studied were in detailstudied [6, 7]. 70 MOCin detail is a [6,7]. non-hydraulic MOC is a non-hydraulic versatile binder versatile with binder specific with properties, specific properties, which make which it superior make it tosuperior PC. One 71 ofto the PC. most One significantof the most propertiessignificant isproperties its specific is it density,s specific which density, is considerablywhich is considerably lower and lower possesses and 72 materiallypossesses materially high compressive high compressive and flexural and flexural strength. strength. Other Other superior superior properties properties are are its its elevated elevated fire 73 resistance,fire resistance, resistance resistance to abrasion, to abrasion, and and low low thermal thermal conductivity. conductivity. All of of these these properties properties develop develop in in 74 quitequite a a short short curing curing time,time, soso the material is is suitable suitable for for quick quick repairs. repairs. Further, Further, MOC MOC is isnot not affected affected by by 75 oils,oils, grease, grease, or or paint paint [[8–11],8–11], andand it is less alkaline (pH~10) (pH~10) than than PC, PC, making making it itappropriate appropriate for for use use with with 76 glassglass fibers fibers andand preventingpreventing the aging process as as well well as as with with other other fillers fillers and and aggregates aggregates such such as astire tire 77 rubber,rubber, synthetic synthetic resin, resin, and and wood wood particles particles [[12].12]. The advantageadvantage of MOC MOC also also lies lies in in the the fact fact that that it it has has a 78 higha high bonding bonding capacity capacity with with di differentfferent types types ofof filersfilers including secondary secondary raw raw materials materials coming coming from from 79 industrial processes. industrial processes. 80 The versatility and functional performance of MOC-based materials have spurred considerable The versatility and functional performance of MOC-based materials have spurred considerable 81 research into product design, development, and characterization [13–15]. Properties of MOC cement- research into product design, development, and characterization [13–15]. Properties of MOC 82 based materials make them applicable in many ways—primarily as a flooring material, but also in fire cement-based materials make them applicable in many ways—primarily as a flooring material, 83 protective systems, grinding wheels, decoration, wall insulation, and others [16,17]. 84 but alsoAt inthe fire same protective time, these systems, materials grinding have also wheels, some decoration, disadvantages, wall especially insulation, when and they others come [16 ,into17]. 85 contactAt the with same water time, [18,19]. these MOC materials has a have low alsowater some resistance disadvantages, due to magnesium especially chloride when they leaching, come intoa 86 contactcorrosive with compound water [18 that,19 ].causes MOC seve hasre a damage low water when resistance it comes duein contact to magnesium with metals chloride [20–22]. leaching, The rest a 87 corrosiveof the material compound is then that left causesin the form severe of hydrated damage when it (Mg(OH) comes in2 contact) as the binding with metals phase. [20 This–22 ].process The rest 88 ofcan the be material prevented is then by leftimproving in the form the ofstability hydrated of MOC brucite in (Mg(OH) humid conditions,2) as the binding which phase.can be Thisachieved process 89 cannaturally be prevented by letting by the improving surface of the Sorel stability cement of react MOC with in humid CO2 from conditions, the air. The which reaction can be provides achieved 90 naturallymagnesium by lettingchlorocarbonates the surface to ofthe Sorel surface cement of the react original with material. CO2 from Another the air. method The reaction of improving provides 91 magnesiumMOC is to use chlorocarbonates a very small amount to the of surfaceadditives of such the as original phosphoric material. acid or Another soluble methodphosphates of improving of alkali 92 MOCmetals, is alkaline to use a earth very metals, small amountaluminum, of additivesammonia, suchor iron. as This phosphoric step causes acid the or formation soluble of insoluble of 93 alkaliphosphate metals, complexes alkaline earththat enhance metals, the aluminum, waterproofness , of Sorel or iron. cement This [23,24]. step causesThe addition the formation of silica of 94 fume or is also a suitable way of improving MOC-based materials [25]. 95 The main environmental advantage of MOC is its ability to store CO2—one of the main 96 greenhouse gasses (GHGs)—in the form of carbonates. MOC thus acts as a CO2 sink [26]. This process Appl. Sci. 2020, 10, 2254 3 of 11 insoluble complexes that enhance the waterproofness of Sorel cement [23,24]. The addition of silica fume or fly ash is also a suitable way of improving MOC-based materials [25]. The main environmental advantage of MOC is its ability to store CO2—one of the main greenhouse gasses (GHGs)—in the form of carbonates. MOC thus acts as a CO2 sink [26]. This process is called carbon mineralization, and due to the increasing amount of GHG in the atmosphere, it has recently been intensively studied [27–32]. Carbonation of MOC Phases is beneficial for the environment as well as for MOC itself, making the material harder and tougher as a result of its denser microstructure [33–36]. The significance of these issues has been documented by monitoring air pollution [37–39]. Using carbonation curing of cement-based materials during the preparation of building materials has attracted wide attention [40–43]. Similarly, CO2 uptake within the carbonation of MOC phases can be beneficially used both from an environmental point of view, and in improving the performances of MOC-based materials. In this study, the uptake of CO2 by MOC Phases 3 and 5 is examined along with the long-term kinetics of this process. The morphology of both materials was analyzed using scanning electron microscopy, and the chemical composition of the precipitated MOC phases was determined using energy dispersive spectroscopy. The kinetics of the carbonation of formed stable MOC phases has not previously been reported in the literature in regards to the assumed high CO2 uptake capacity by carbonated minerals and its use in the passive control of indoor air quality. This could partially reduce the concentration of CO2 in the polluted environments, and thus slow down global warming-related effects.

2. Materials and Methods The following chemicals were used for the syntheses: MgCl 6H O(>99%, Penta, Czech Republic), 2· 2 MgO (>8%, Penta, Czech Republic), and deionized water (16.8 MΩ). The ratio of 5 MgO to MgCl 6H O was used for the synthesis of a stoichiometric phase of MOC 2· 2 with a composition 5Mg(OH) MgCl 8H O (termed as Phase 5, also known as Mg (OH) Cl 4H O). To 2· 2· 2 3 5 · 2 prepare the first sample, 30.6 g of MgCl 6H O was dissolved in 19.0 g of deionized water. In the next 2· 2 step, 30.4 g of magnesium oxide was added, and the suspension was intensively stirred for 10 min. Equation (1) summarizes the formation of Phase 5:

5 MgO + MgCl 6H O + 3 H O 5Mg(OH) MgCl 8H O (1) 2· 2 2 · 2· 2· 2 Similarly, the stoichiometric Phase 3 (3Mg(OH) MgCl 8H O or Mg (OH) Cl 4H O) was prepared 2· 2· 2 2 3 · 2 by mixing MgO, MgCl 6H O and H O in the molar ratio 3:1:5. To prepare the second sample, 39.1 g 2· 2 2 of MgCl 6H O was dissolved in 17.3 g of deionized water. In the next step, 23.2 g of magnesium 2· 2 oxide was added, and the suspension was intensively stirred for 5 min. The formation of magnesium oxychloride cement MOC 3-1-8 is summarized in the following Equation (2):

3 MgO + MgCl 6H O + 5 H O 3Mg(OH) MgCl 8H O. (2) 2· 2 2 → 2· 2· 2 Suspensions were used to prepare small samples in XRD holders (Figure2). Samples were stored indoor at ambient temperature, with an excess of humidity and carbon dioxide. Figure2 shows the photography of prepared samples in XRD holders. For the study of the surface morphology, scanning electron microscopy (SEM) was used (Tescan MAIA 3). Elemental composition and mapping were characterized by energy dispersive spectroscopy (EDS) using X-Max150 analyzer equipped with a 20-mm2 SDD detector (Oxford instruments) and AZtecEnergy software. The sample was put on a carbon conductive tape in order to ensure the conductivity of the experiments. For both SEM and SEM-EDS analyses, the electron beam was set to 10 kV with a 10-mm work distance. X-ray powder diffraction (XRD) was carried out by a Bruker D2 Phaser, a powder diffractometer with Bragg-Brentano geometry, applying CuKα radiation (λ = 0.15418 nm, U = 30 kV, I = 10 mA) with Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 11

97 is called carbon mineralization, and due to the increasing amount of GHG in the atmosphere, it has 98 recently been intensively studied [27–32]. 99 Carbonation of MOC Phases is beneficial for the environment as well as for MOC itself, making 100 the material harder and tougher as a result of its denser microstructure [33–36]. The significance of 101 these issues has been documented by monitoring air pollution [37–39]. Using carbonation curing of 102 cement-based materials during the preparation of building materials has attracted wide attention [40– 103 43]. Similarly, CO2 uptake within the carbonation of MOC phases can be beneficially used both from 104 an environmental point of view, and in improving the performances of MOC-based materials. 105 In this study, the uptake of CO2 by MOC Phases 3 and 5 is examined along with the long-term 106 kinetics of this process. The morphology of both materials was analyzed using scanning electron 107 microscopy, and the chemical composition of the precipitated MOC phases was determined using 108 energy dispersive spectroscopy. The kinetics of the carbonation of formed stable MOC phases has not 109 previously been reported in the literature in regards to the assumed high CO2 uptake capacity by 110 carbonated minerals and its use in the passive control of indoor air quality. This could partially reduce 111 the concentration of CO2 in the polluted environments, and thus slow down global warming-related 112 effects.

113 2. Materials and Methods

114 The following chemicals were used for the syntheses: MgCl2∙6H2O (>99%, Penta, Czech Republic), 115 MgO (> 8%, Penta, Czech Republic), and deionized water (16.8 MΩ). 116 The ratio of 5 MgO to MgCl2∙6H2O was used for the synthesis of a stoichiometric phase of MOC 117 with a composition 5Mg(OH)2∙MgCl2∙8H2O (termed as Phase 5, also known as Mg3(OH)5Cl∙4H2O). To 118 prepare the first sample, 30.6 g of MgCl2∙6H2O was dissolved in 19.0 g of deionized water. In the next 119 step, 30.4 g of magnesium oxide was added, and the suspension was intensively stirred for 10 min. 120 Equation (1) summarizes the formation of Phase 5:

121 5 𝑀𝑔𝑂 + 𝑀𝑔𝐶𝑙 ∙6𝐻𝑂+3 𝐻 𝑂→5𝑀𝑔(𝑂𝐻) ∙𝑀𝑔𝐶𝑙 ∙8𝐻𝑂 (1)

122 Similarly, the stoichiometric Phase 3 (3Mg(OH)2∙MgCl2∙8H2O or Mg2(OH)3Cl∙4H2O) was prepared 123 by mixing MgO, MgCl2∙6H2O and H2O in the molar ratio 3:1:5. To prepare the second sample, 39.1 g Appl. Sci. 2020, 10, 2254 4 of 11 124 of MgCl2∙6H2O was dissolved in 17.3 g of deionized water. In the next step, 23.2 g of magnesium oxide 125 was added, and the suspension was intensively stirred for 5 min. The formation of magnesium 126 oxychloride cement MOC 3-1-8 is summarized in the following Equation (2): a rotation of 5 rpm. The step size was set to of 0.02025◦ (2θ), and the overall data were acquired in the 127 3 𝑀𝑔𝑂 + 𝑀𝑔𝐶𝑙 ∙6𝐻 𝑂+5 𝐻 𝑂→3𝑀𝑔(𝑂𝐻) ∙𝑀𝑔𝐶𝑙 ∙8𝐻 𝑂 range of 5–80◦. Measurements were performed on samples matured for. 7, 28, 50, 100, 150, 200,(2) and 250 days. After 250 days, the particular samples were manually homogenized in an agate mortar and 128 measuredSuspensions in a powder were used form to to prepare obtain information small samples on in the XRD bulk holders phase composition.(Figure 2). Samples Rietveld were analysis stored 129 indoor at ambient temperature, with an excess of humidity and carbon dioxide. Figure 2 shows the Appl.was Sci. performed 2020, 10, x forFOR the PEER quantification REVIEW of the identified phases. 4 of 11 130 photography of prepared samples in XRD holders. 133 For the study of the surface morphology, scanning electron microscopy (SEM) was used (Tescan 134 MAIA 3). Elemental composition and mapping were characterized by energy dispersive spectroscopy 135 (EDS) using X-Max150 analyzer equipped with a 20-mm2 SDD detector (Oxford instruments) and 136 AZtecEnergy software. The sample was put on a carbon conductive tape in order to ensure the 137 conductivity of the experiments. For both SEM and SEM-EDS analyses, the electron beam was set to 138 10 kV with a 10-mm work distance. 139 X-ray powder diffraction (XRD) was carried out by a Bruker D2 Phaser, a powder diffractometer 140 with Bragg-Brentano geometry, applying CuKα radiation (λ = 0.15418 nm, U = 30 kV, I = 10 mA) with 141 a rotation of 5 rpm. The step size was set to of 0.02025° (2θ), and the overall data were acquired in the 142 range of 5–80°. Measurements were performed on samples matured for 7, 28, 50, 100, 150, 200, and 250 143 days. After 250 days, the particular samples were manually homogenized in an agate mortar and 131144 measured in a powder form to obtain information on the bulk phase composition. Rietveld analysis 132145 was performedFigureFigure 2.2. for Photography the quantification of prepared of thesamplessamples identified inin XRDXRD phases. holders:holders: Phase 3 (left) andand PhasePhase 55 (right).(right). 3. Results and Discussion 146 3. Results and Discussion Two MOC phases were prepared and characterized. The phases under study with the compositions 147 Two MOC phases were prepared and characterized. The phases under study with the 3Mg(OH)2 MgCl2 8H2O and 5Mg(OH)2 MgCl2 8H2O are referred to as Phases 3 and 5, respectively. 148 compositions· 3Mg(OH)· 2∙MgCl2∙8H2O and· 5Mg(OH)· 2∙MgCl2∙8H2O are referred to as Phases 3 and 5, The phase purities of both phases were determined using XRD analysis after 7 days (see Figure3). A fast 149 respectively. The phase purities of both phases were determined using XRD analysis after 7 days (see formation of Phase 3 was confirmed, and a single-phase composition was obtained (ICDD 00-007-0412). 150 Figure 3). A fast formation of Phase 3 was confirmed, and a single-phase composition was obtained Similar results were also obtained for Phase 5. The formation of Phase 5 (ICDD 00-007-0420) was 151 (ICDD 00-007-0412). Similar results were also obtained for Phase 5. The formation of Phase 5 (ICDD confirmed with no significant impurities. Figure3 shows the XRD patterns measured after 7 days for 152 00-007-0420) was confirmed with no significant impurities. Figure 3 shows the XRD patterns measured MOC Phases 3 and 5. 153 after 7 days for MOC Phases 3 and 5.

154 Figure 3. XRD patterns for magnesium oxychloride cement (MOC) Phases 3 and 5 after 7 days. 155 Figure 3. XRD patterns for magnesium oxychloride cement (MOC) Phases 3 and 5 after 7 days. Another XRD measurement was performed after 50 days of samples maturing. In both 156 Another XRD measurement was performed after 50 days of samples maturing. In both samples, samples, chlorartinite was formed on the surface due to carbonation (see Figure4). Phase 3 reacted 157 chlorartinite was formed on the surface due to carbonation (see Figure 4). Phase 3 reacted with carbon with carbon dioxide forming Mg2(CO3)(OH)Cl 2H2O (ICDD 00-061-0391), while Phase 5 formed 158 dioxide forming Mg2(CO3)(OH)Cl∙2H2O (ICDD· 00-061-0391), while Phase 5 formed Mg2(CO3)(OH)Cl 3H2O (ICDD 00-00-0278). A tubular crystal structure of the dihydrate formed 159 Mg2(CO3)(OH)Cl∙3H· 2O (ICDD 00-00-0278). A tubular crystal structure of the dihydrate formed by by MgO6 octahedra interconnected by carbonate anions with chloride anions and water molecules 160 MgO6 octahedra interconnected by carbonate anions with chloride anions and water molecules located 161 inside the channels is depicted in Figure 5. It is apparent from the comparison with the initial structures 162 of Phases 3 and 5 (Figure 1) that the CO2 incorporation into the system required a complete 163 reconstruction of the MgO6 network and, in the case of Phase 5, the formation of another phase (see 164 Equation (4) below). Hence, a relatively slow kinetics of the carbonation process can be anticipated. 165 Figure 4 shows the XRD pattern of MOC Phases 3 and 5 after 50 days of maturing. Figure 5 shows the 166 crystal structure of the chlorartinite phase. Appl. Sci. 2020, 10, 2254 5 of 11

located inside the channels is depicted in Figure5. It is apparent from the comparison with the initial structures of Phases 3 and 5 (Figure1) that the CO 2 incorporation into the system required a complete reconstruction of the MgO6 network and, in the case of Phase 5, the formation of another phase (see Equation (4) below). Hence, a relatively slow kinetics of the carbonation process can be anticipated. Appl.Figure Sci.4 2020 shows, 10, x the FOR XRD PEER pattern REVIEW of MOC Phases 3 and 5 after 50 days of maturing. Figure5 shows5 of the 11 Appl.crystal Sci. 2020 structure, 10, x FOR of thePEER chlorartinite REVIEW phase. 5 of 11

167 167 168 FigureFigure 4. 4. XRDXRD patterns patterns of of MOC MOC Phases Phases 3 3 and and 5 5 after after 50 50 days days. . 168 Figure 4. XRD patterns of MOC Phases 3 and 5 after 50 days .

169 169 170 FigureFigure 5. 5. CrystalCrystal structure structure of of the the chlorartinite chlorartinite phase. Colo Colorr scheme scheme is the same as in FigureFigure1 1;; carbon carbon 170171 Figureatomsatoms located5. located Crystal in in thestructure the centers centers of of ofthe triang triangular chlorartiniteular carbonate carbonate phase. molecules molecules Color scheme are are in in isred. red. the same as in Figure 1; carbon 171 atoms located in the centers of triangular carbonate molecules are in red. 172 XRDXRD measurements measurements with with subsequent subsequent Rietveld Rietveld analysis analysis were performed again after 100, 150, 200,200, 172173 andand 250XRD 250 days days measurements of of maturing. maturing. with In In both bothsubsequent samples, samples, Rietveld the the cont content analysisent of chlorartinite were performed increased again withwith after thethe 100, agingaging 150, time time200, 173174 andofof the the 250 samples. samples. days of However, However,maturing. as asIn can canboth be be samples,seen seen from from the Figure Figure cont 6ent6,, thethe of rateratechlorartinite ofof chlorartinitechlorartinite increased formationformation with the was was aging di differentfferent time 174175 offorfor the Phases Phases samples. 3 3 and and However, 5. 5. In In the the caseas case can of of be Phase Phase seen 3, 3,from MOC MOC Figure almost almost 6, thedisappeared disappeared rate of chlorartinite after 250 days,days, formation whilewhile was chlorartinitechlorartinite different 175176 forrepresentedrepresented Phases 3 andthe the dominant5. dominant In the case phase. phase. of Phase The The carbonation carbonation3, MOC almost of of Phase Phasedisappeared 5 5 was slower after 250 than days, Phase while 3,3, likely likelychlorartinite duedue toto 176177 representedthethe phase phase separation. separation. the dominant After After phase. 150 150 days, days, The the thecarbonation surface surface of of of the Phase sample 5 was contained slower similarsithanmilar Phase amountsamounts 3, likely ofof PhasePhase due to 55 177178 theandand phase chlorartinite. chlorartinite. separation. After After After 250 250 150days, days, days, this this thesample sample surface contained of the sample 46% of contained Phase 5 andand similar 54%54% of ofamounts chlorartinite.chlorartinite. of Phase TheThe 5 178179 andXRDXRD chlorartinite. patterns patterns acquired acquired After 250on on 250-daydays, 250-day this samples samples sample are arecontained shown 46%in Figure of Phase7 7 for for 5 both andboth 54% surfaces surfaces of chlorartinite. and and bulk. bulk. It ItThe is is 179180 XRDobvious patterns from Figureacquired 7 thaton 250-daythe carbonation samples tookare shownplace mainlyin Figure on 7the for surface. both surfaces Although and the bulk. Phase It is3 180181 obvioussample contained from Figure over 7 that95 wt% the chlorartinite,carbonation tookin the pl wholeace mainly volume on thethe contentsurface. of Although chlorartinite the Phasedid not 3 181182 sampleexceed 15contained wt%. Similar over results95 wt% were chlorartinite, obtained infor the Phase whole 5, where volume the the content content of chlorartiniteof chlorartinite in the did bulk not 182183 exceedwas even 15 lower.wt%. Similar Figure results 6 shows were the obtained surface phase for Phase composition 5, where theof Phases content 3 ofand chlorartinite 5 after 7, 50, in 100,the bulk 150, 183184 was200, evenand 250 lower. days Figure of sample 6 shows ageing. the surface Figure phase7 shows composition the phase ofcomposition Phases 3 and of 5the after surface 7, 50, and100, bulk150, 184185 200,(powder) and 250 of Phases days of 3 andsample 5 assessed ageing. for Figure 250-day 7 sh samples.ows the phase composition of the surface and bulk 185 (powder) of Phases 3 and 5 assessed for 250-day samples. Appl. Sci. 2020, 10, 2254 6 of 11 obvious from Figure7 that the carbonation took place mainly on the surface. Although the Phase 3 sample contained over 95 wt% chlorartinite, in the whole volume the content of chlorartinite did not exceed 15 wt%. Similar results were obtained for Phase 5, where the content of chlorartinite in the bulk was even lower. Figure6 shows the surface phase composition of Phases 3 and 5 after 7, 50, 100, 150, 200, and 250 days of sample ageing. Figure7 shows the phase composition of the surface and bulk (powder) of Phases 3 and 5 assessed for 250-day samples. Appl.Appl. Sci. Sci. 2020 2020, 10, 10, x, xFOR FOR PEER PEER REVIEW REVIEW 6 6of of 11 11

186186

187187Figure 6. FigurePhaseFigure 6. composition6. Phase Phase composition composition of the of of the surface the surface surface of of of Phases Phases Phases 3 3 3an andand d5 5after 5 after after 7, 7, 50, 7,50, 100, 50,100, 150, 100, 150, 200, 150,200, and and 200, 250 250 anddays days 250of of days of 188188samples ageing.samplessamples ageing. ageing.

189189 190190Figure 7. FigureFigurePhase 7. 7. Phase composition Phase composition composition of of theof the the surface surface and and and bulk bulk bulk (powde (powde (powder)r)r) of of Phases Phases of 3 3and Phases and 5 5at at 250 250 3 days and days of 5of sampleat sample 250 days of 191 ageing. 191sample ageing.ageing. Appl. Sci. 2020, 10, 2254 7 of 11

SEM micrographs were acquired to study the microstructure of the 250-day MOC samples. The typical needle-like crystalline structure of Phases 3 and 5 were obtained in the bulk at high magnification (see FigureAppl.8). Sci. The 2020 needles, 10, x FOR PEER were REVIEW 1–5 µm long and approximately 0.5 µm wide. On the other7 of 11 hand, the morphology192 ofSEM the micrographs surface was were di acquiredfferent. to Due study to the carbonation microstructure (formation of the 250-day of chlorartinite),MOC samples. The the surface contained193 typical a lower needle-like amount crystalline of needles structure in both of samples.Phases 3 and More 5 were needles obtained (MOC in phase)the bulk were at high detected in Phase194 5 thanmagnification Phase 3, which(see Figure is in 8). good The needles agreement were 1–5 with μm the long XRD and approximately results. The EDS0.5 μm chemical wide. On compositionthe 195 other hand, the morphology of the surface was different. Due to carbonation (formation of was196 also measuredchlorartinite), at the three surface diff erentcontained locations a lower inamount the sample of needles (surface, in both samples. center, More and bottom).needles (MOC High purity of both197 samplesphase) were was detected confirmed in Phase when 5 than only Phase magnesium, 3, which is in oxygen,good agreement , with andthe XRD carbon results. were The detected. The198 EDS dataEDS chemical confirmed composition the highest was carbonalso measured content at th onree the different surface locations of the in tested the sample samples. (surface, The carbon content199 incenter, the center and bottom). and on High the purity bottom of both of thesamples analyzed was confirmed samples when was only significantly magnesium, lower.oxygen, Figure8 200 chlorine, and carbon were detected. The EDS data confirmed the highest carbon content on the surface shows201 theof SEM the tested micrographs samples. The of carbon the surface content in and the bulkcenter (fractureand on the surface)bottom of the of theanalyzed Phases samples 3 and was 5 samples scanned202 aftersignificantly 250 days lower. of Figure materials 8 shows ageing. the SEM Themicrographs EDS data of the measured surface and frombulk (fracture the surface, surface) center,of and bottom203 ofthe the Phases samples 3 and are 5 samples also provided. scanned after 250 days of materials ageing. The EDS data measured from 204 the surface, center, and bottom of the samples are also provided.

205

206Figure 8. FigureSEM micrographs8. SEM micrographs of the of the surface surface and and bulkbulk (fractu (fracturere surface) surface) of samples of samples of Phases of 3 Phases and 5 3 and 5 207scanned afterscanned 250 after days 250 of days ageing. of ageing. The The EDS EDS measurements measurements were were performed performed from fromthe surface, the surface, center, and center, and 208bottom ofbottom the samples. of the samples.

Both MOC phases analyzed in this study exhibited fast formation of chlorartinite as a result of their reaction with carbon dioxide. The carbon footprint of MOC cement-based materials will be thus significantly and quickly reduced. Based on the great sorption activity of Phase 3 towards CO2, Appl. Sci. 2020, 10, 2254 8 of 11

it is estimated that 1 kg of Phase 3 could theoretically incorporate 211 g of CO2, as formulated in Equation (3):

3Mg(OH) MgCl 8H O + 2 CO 2 [Mg (CO )(OH)Cl 2H O] + 6 H O (3) 2· 2· 2 2 → 2 3 · 2 2 Similarly, assuming carbonation of Phase 5 in accordance with Equation (4),

5Mg(OH) MgCl 8H O + 4 CO 2 [Mg (CO )(OH)Cl 3H O] + 2 MgCO + 6 H O (4) 2· 2· 2 2 → 2 3 · 2 3 2 where maximum calculated theoretical CO2 uptake capacity of Phase 5 was 331 g/kg. However, these are theoretical assumptions only, requiring a high concentration of CO2 in the surrounding air, and full-volume carbonation of MOC phases. On the other hand, increased global CO2 emissions pose a threat to the Earth’s atmospheric environments. It was reported that the annual CO2 concentration at the Earth’s surface in 2017 reached 405 ppm [44], which is approximately quadruple that of the early 1960s [45]. Therefore, any attempt at a reduction of the carbon footprint of construction materials is of particular importance from a sustainability point of view. The production of Portland cement, as a material that is competitive to MOC cement, represents 5–8% of global CO2 emissions [46,47]. In this sense, alternative approaches to reducing CO2 emissions associated with the manufacture of the binder phase in concrete and other construction materials are in demand [48]. Seo et al. [44] reported on the potential of CO2 reduction materialized by carbonation curing. He calculated the CO2 uptake capacity of Portland cement during an accelerated carbonation test as 12.3%. The computing was done based on the thermogravimetry (TG) and calculation of the difference between the weight loss at 500–720 ◦C of carbonated and noncarbonated cement paste samples. This was a similar level of CO2 uptake as observed in a study published by Jang and Lee [41]. Nevertheless, this CO2 uptake capacity corresponds with the 5% CO2 concentration within the accelerated carbonation test. The normal level of CO2 concentration in the air is 250–400 ppm [49]. In occupied spaces with good air exchange, the concentration of CO2 varies in the 400–1000 ppm range. This means that the applied CO2 concentration in accelerated carbonation tests was typically even 200 times higher than the normal CO2 concentration in air. The significant reduction of CO2 uptake by Portland cement paste can be therefore be anticipated under standard atmospheric conditions. Moreover, the sequestration of CO2 as a result of the carbonation of Portland cement paste in normal environments is extremely slow [50], contrary to the fast formation of chlorartinite from MOC phases. Based on the data provided and analysis conducted in this study, it can be concluded that MOC cement represents an alternative construction binder for the production of low carbon materials that can partially mitigate the atmospheric concentration of CO2, which is the main constituent of greenhouse gases related to global warming issues [51].

4. Conclusions The issue of global warming has resulted in a climate crisis faced by people all over the world. Annually, billions of metric tons of Portland cement are produced around the world, which represents a serious environmental burden due to the release of huge amounts of CO2 within the decomposition of limestone and burning of coal in cement production plants. As Portland cement and cement-based materials are the most widespread materials in the construction industry, there is a need to develop and discover alternative cement materials with similar or better functional properties than Portland cement, with a lower negative environmental impact. In this work, MOC was studied as an alternative to Portland cement binder. Based on the conducted tests and analyses, the fast formation and hardening of both stable MOC phases was documented, which is an advantageous property and contrary to the significantly slower hardening of Portland cement-based materials. This could be beneficially exploited in repair applications, prefabrication, and the speeding up of construction processes. Specific attention was paid to the ability of MOC to capture CO2 from the environment, making it eco-friendly, but also resulting in a more Appl. Sci. 2020, 10, 2254 9 of 11 dense and mechanically resistant structure than was anticipated. Both MOC phases proved their fast mineral carbonation, whereas the maximum theoretical CO2 uptake capacity was high and significantly greater than that of Portland cement paste. Nevertheless, one must consider the surface and bulk carbonation effects, relative humidity values, and CO2 concentration available for carbonation reaction in this respect. MOC-based composites could find use in applications where a higher concentration of CO2 in the environment is expected (e.g., in areas and construction sites exposed to higher traffic density, industrial air pollution, etc.). Here, they can be applied in industrial flooring and wall panels. In this case, the concentration of the prevailing component of greenhouse gases will be safely reduced by CO2 uptake in newly formed carbonated MOC phases which will be more durable and resistant against mechanical loads. Based on the free carbonation capacity of MOC-based products, they could also serve as the passive moderators of indoor air climates with respect to the mitigation of excessive CO2 concentrations (which can cause serious health problems).

Author Contributions: Conceptualization, D.S., O.J., M.P., and Z.P.; methodology, O.J., M.P., Z.P., and D.S.; investigation, M.L., A.-M.L., F.A., O.J., M.P., and Z.P.; data curation, M.P., O.J., D.S., and Z.P., writing—original draft O.J., M.P., D.S., and Z.P.; supervision, O.J. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the CZECH SCIENCE FOUNDATION, grant number 19-00262S—Reactive magnesia -based composites with selected admixtures and additives. Acknowledgments: The technical support provided by Pavel Košata from the Faculty of Civil Engineering, CTU Prague, is greatly acknowledged. Conflicts of Interest: The authors declare no conflict of interest.

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