sustainability

Article Environmental Assessment of Magnesium Oxychloride Cement Samples: A Case Study in Europe

Gediminas Kastiukas 1, Shaoqin Ruan 1,2,* , Cise Unluer 2, Shuang Liang 1 and Xiangming Zhou 1,* 1 Department of Civil and Environmental Engineering, Brunel University London, Uxbridge, Middlesex UB8 3PH, UK; [email protected] (G.K.); [email protected] (S.L.) 2 School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore; [email protected] * Correspondence: [email protected] (S.R.); [email protected] (X.Z.); Tel.: +44-77-4178-8269 (S.R.); +44-18-9526-6670 (X.Z.)


Received: 20 November 2019; Accepted: 4 December 2019; Published: 6 December 2019


Abstract: This study is the first in the literature to systematically assess the environmental impacts of magnesium oxychloride cement (MOC) samples, which are regarded as a more eco-friendly construction material than Portland cement. The environmental impacts of MOC samples prepared with various molar ratios of MgO/MgCl 6H O and sources of reactive magnesia were obtained via 2· 2 a life cycle assessment (LCA) approach (from cradle to grave), and the obtained outcomes were further compared with the counterparts associated with the preparation of Portland cement (PC) samples. Meanwhile, a sensitivity analysis in terms of shipping reactive magnesia from China to Europe was performed. Results indicated that the preparation of MOC samples with higher molar ratios led to more severe overall environmental impacts and greater CO2 sequestration potentials due to the difference of energies required for the production of MgO and MgCl 6H O as well as 2· 2 their various CO2 binding capacities, whereas in terms of CO2 intensities, the molar ratios in MOC samples should be carefully selected depending on the strength requirements of the applications. Furthermore, various allocation procedures and MgO production processes will greatly influence the final outcomes, and allocation by mass is more recommended. Meanwhile, the environmental impacts associated with the transportation of reactive magnesia from China to Europe can be ignored. Finally, it can be concluded that MOC concrete is no longer a type of ‘low-carbon’ binder in comparison with PC concrete in terms of CO2 emissions, and in view of the single scores and mixing triangles for weighing, MOC concrete can only be identified as a type of more sustainable binder than PC concrete when the main component MgO in MOC samples is obtained through the dry process route rather than the wet process route.

Keywords: magnesium oxychloride cement (MOC); production of reactive magnesia; environmental impacts; CO2 emissions; allocations

1. Introduction Magnesium oxychloride (MOC) cement, which was invented not long after Portland cement (PC), is categorized as a nonhydraulic binder that forms from the reaction between magnesia (MgO) and a magnesium chloride (MgCl2) solution [1,2]. In MOC samples, a MgO–MgCl2–H2O ternary system is formed along with phase-5 (5Mg(OH) MgCl 8H O) and phase-3 (3Mg(OH) MgCl 8H O) crystals. 2· 2· 2 2· 2· 2 These crystals are stable at temperatures <100 ◦C, which contributes to the rapid hardening and the very high early strength, reaching 120 MPa to 140 MPa depending on the formulations [3]. MOC

Sustainability 2019, 11, 6957; doi:10.3390/su11246957 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 6957 2 of 21 is regarded as a sustainable binder [4], and has received great research interest due to its superior properties such as fire and abrasion resistance [5], making it suitable for industrial applications in fire protection, flooring and grinding wheels [6–8]. Reactive MgO, which is the key component in MOC, has been widely investigated in a series of studies in the area of construction [9–14], and can be obtained through the following two main approaches: (i) calcination of magnesia-based minerals (e.g., magnesite, dolomite or serpentine); and (ii) synthesis from brine/seawater [15]. The majority of MgO on the market is obtained via the calcination of natural earth minerals such as magnesite (MgCO3) with a calcination temperature of 700–1000 ◦C. However, the acquired MgO presents low purities and inferior reactivities since impurities are unavoidable in the raw materials [15]. Therefore, in order to obtain MgO with higher purities and reactivities, MgO may be acquired via the chemical synthesis from seawater or magnesium-bearing brine sources, accounting for about 14% of global MgO production annually [16], and the synthetic MgO obtained from seawater/brine has also been proven to outperform MgO obtained from the calcination of magnesite with respect to the aforementioned properties [17], which was related to the larger specific surface area (SSA) achieved in synthetic MgO [18] in comparison with the MgO from calcination. The procedures concerning the production of reactive MgO in industry from seawater/brine involve the following stages: (i) the acquisition of seawater/brine; (ii) the production of a strong alkali base to be used in the precipitation of Mg(OH)2, and (iii) the calcination of Mg(OH)2 [19]. As a result, the overall input (i.e., raw materials and energies) and output (i.e., emissions) during the production of MgO was mainly related to the production of the base and the calcination of Mg(OH)2 [20]. The other key component in MOC, magnesium chloride hexahydrate (MgCl 6H O), is a colourless 2· 2 crystalline material that is present in nature as bischofite, and contains over 70 other elements owing to the presence of impurities [21]. MgCl 6H O crystals can be prepared by dissolving magnesium-bearing 2· 2 materials such as magnesium oxide, hydroxide, or carbonate in an aqueous hydrochloric acid (HCl) solution [22]. Therefore, as magnesite is present as a type of natural mineral, the main energy consumptions and emissions associated with the production of MgCl 6H O are derived from the 2· 2 preparation of the HCl solution. It is reported that the production of MOC resulted in the lowest material usage and environmental burden in contrast to those associated with the preparation of lime-based and magnesium phosphate cement (MPC)-based binders in biocomposites with hemp [23]. Meanwhile, a series of studies [24,25] indicated that construction materials were capable of re-absorbing CO2 during their usage phases, which offsets the carbon footprint associated with their production. As expected, magnesium-bearing materials also present CO2 sequestration potential [25–29], and it was reported that MOC could also sequestrate CO2 after its utilization, and around 20-40% of the emissions associated with the energy consumptions and the decomposition of raw materials may be re-sequestrated due to the carbonation over a period of 15 years [30]. There is also a large number of studies focusing on the energy consumption concerning the production of reactive MgO from the calcination of magnesite or its synthesis from seawater/reject brine. For instance, it was indicated that the major challenge in the production of MgO from seawater was its higher energy demand than that consumed by the calcination of magnesite (17 GJ versus 5.9 GJ per tonne of MgO) [19]. However, the production process can be optimized and the energy demands of MgO production can be greatly reduced by using waste brine [20,31]. Although there are numerous studies investigating the mechanical performance and microstructure of MOC [29,32,33], and MOC is regarded as a type of eco-friendly binder [3,4,7,34], investigations focusing on the environmental impacts of MOC preparation in a life cycle assessment are scarce. Meanwhile, previous studies ignored the value of by-products during the precipitation of Mg(OH)2 such as NaCl; however, the allocation of environmental burdens (i.e., to the by-products) is also of great importance since sometimes a production process may produce more than one end product, and the environmental burdens have to be divided between these two final products in a way that reflects the underlying relationships between them, e.g., allocation by mass or by economic value [35]. Sustainability 2019, 11, 6957 3 of 21

The goal of this study was first to assess the environmental impacts of MOC samples prepared with various molar ratios of MgO/MgCl 6H O during its production, reflected by various impact categories 2· 2 and key parameters. Meanwhile, the CO2 intensities based on the mechanical performance and CO2 sequestration potential of MOC samples were also discussed, and the sustainability of MOC samples3 prepared by various sources of reactive MgO was also investigated. Furthermore, the allocation of environmental impacts burdens associated related to MOCwith the sample transporta preparationtion of wasreactive performed magnesia at thefrom same China time, to andEurope the environmentalwere also assessed; impacts meanwhile, associated the with results the transportationwere compared of reactivewith those magnesia related fromto the China production to Europe of werePortland also cement assessed; (PC) meanwhile, samples. the results were compared with those related to the production of Portland cement (PC) samples. 2. Materials and Methodology 2. Materials and Methodology 2.1. Materials, Mix Design and Sample Preparation 2.1. Materials, Mix Design and Sample Preparation The Mg(OH)2 and MgCl2∙6H2O with a purity of 98% used in this study were purchased from The Mg(OH) and MgCl 6H O with a purity of 98% used in this study were purchased from Magnesia GmbH (Lüneburg,2 Germany).2· 2 The MgO was acquired from the calcination of Mg(OH)2 using Magnesiaa furnace operating GmbH (Lüneburg, at a calcination Germany). temperature The MgOof 650 was°C and acquired a ramp from rate of the 20 calcination °C/min for a of duration Mg(OH) of2 using1 h. The a MgCl furnace2∙6H operating2O was placed at a into calcination deionised temperature water and mixed of 650 using◦C and a magnetic a ramp stirrer rate of until 20 ◦theC/min crystals for a duration of 1 h. The MgCl 6H O was placed into deionised water and mixed using a magnetic were fully dissolved. The MgO2· obtained2 from the calcination of Mg(OH)2 was used as the main binder stirrerfor the untilpreparation the crystals MOC were samples. fully dissolved.In this study, The 1.0 MgO kg obtainedof paste was from prepared the calcination for each of sample Mg(OH) using2 was a usedHobart as thePlanetary main bindermixer for(Peterborough, the preparation UK) MOC by combining samples. In the this MgCl study,2∙6H 1.02O kg solution of paste and was the prepared MgO for each sample using a Hobart Planetary mixer (Peterborough, UK) by combining the MgCl 6H O powders for 5 min. The molar ratios of MgO/MgCl2 were fixed to 3, 5 and 9 separately during2· MOC2 solutionsample preparation, and the MgO with powders the workability for 5 min. of Thethe past molare being ratios maintained of MgO/MgCl by adju2 weresting fixed the water to 3, 5to and solid 9 separatelyratios correspondingly, during MOC as sample shown preparation,in Table 1. As with the MgO the workability used in this of study the pastewas calcinated being maintained within a lab by adjustingand the production the water toefficiency solid ratios was correspondingly,quite low, in orde asr shownto make in full Table use1 .of As the the MgO MgO obtained, used in thisinstead study of wasusing calcinated a 40 × 40 within× 160 mm a lab mould, and the which production is suggested efficiency by EN was 196-1, quite a mould low, in with order a toreduced make fullsize use(20 of× 20 the × MgO obtained, instead of using a 40 40 160 mm mould, which is suggested by EN 196-1, a mould 160 mm) was adopted to save materials× (Figure× 1). After the completion of the mixing, the paste was with a reduced size (20 20 160 mm) was adopted to save materials (Figure1). After the completion moulded into three prismatic× × samples, compacted by a vibrating table. After casting, the samples were ofcured the in mixing, air for theseven paste days was at mouldedlaboratory into room three temperature prismatic with samples, a relative compacted humidity by of a 60 vibrating ± 5% prior table. to Aftermechanical casting, testing. the samples were cured in air for seven days at laboratory room temperature with a relative humidity of 60 5% prior to mechanical testing. ± Table 1. The quantity of each component used to produce 1 kg of MOC sample. Table 1. The quantity of each component used to produce 1 kg of MOC sample. MgO/MgCl2*6H2O w/s Ratio Water (g) MgO (g) MgCl2*6H2O (g) w/s Ratio Water (g) MgO/MgClMolar2*6H Ratio2OMolar Ratio MgO (g) MgCl2*6H2O (g) 0.220.22 181.9181.9 3 305.1 305.1 513.0 513.0 0.180.18 151.2151.2 5 422.5 422.5 426.3 426.3 0.34 253.1 9 475.3 269.1 0.34 253.1 9 475.3 269.1

Figure 1. Size of the mould used in this study. Figure 1. Size of the mould used in this study.

2.2. Methodology

2.2.1. Compressive and Flexural Strength The strength values were obtained through the bending and compression tests, and the values were used during the calculations of CO2 intensities, with the details shown in Section 3.1.2. The flexural strength was obtained by the three-point bending test, with the broken samples being subjected to the compression test to obtain the unconfined compressive strength. Both two tests were conducted using a 50 kN universal testing machine (Instron 5960, High Wycombe, UK) at a loading rate of 12.7 mm/min. It was reported that there were some conversion coefficients between these two sizes (This study: 20 × 20 ×

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2.2. Methodology

2.2.1. Compressive and Flexural Strength The strength values were obtained through the bending and compression tests, and the values were used during the calculations of CO2 intensities, with the details shown in Section 3.1.2. The flexural strength was obtained by the three-point bending test, with the broken samples being subjected to the compression test to obtain the unconfined compressive strength. Both two tests were conducted using a 50 kN universal testing machine (Instron 5960, High Wycombe, UK) at a loading rate of 12.7 mm/min. It was reported that there were some conversion coefficients between these two sizes (This study: 20 20 160 mm vs. 40 40 160 mm) depending on the proportions of the cement binder or the × × × × shape of samples [36]; however, the strength values in this study were still utilized directly without any normalizations as they were mainly involved in the CO2 intensity calculations, which will not change the final outcomes or the conclusions.

2.2.2. Environmental Assessment The environmental impacts of MOC paste prepared with three different molar ratios of MgO/MgCl 6H O and two sources of reactive MgO were obtained via a life cycle assessment 2· 2 (LCA) according to [35], based on a case study in Europe. MOC samples have been widely used in rapid repair due to their relatively fast strength development [37]; therefore, it is very common to use MOC paste in the construction field. However, in practice, aggregates and supplementary cementitious materials (SCMs) are also widely introduced in PC sample preparation; therefore, for a fair comparison between MOC and PC samples in terms of the environmental impacts, a certain amount of aggregates (aggregate content/solids content ratio = 2.5) and pulverized fly ash (i.e., 30% of total solid content) was also assumed to be incorporated in MOC paste, and the environmental impacts of MOC and PC concrete were compared. A w/s ratio of 0.34 was used in both MOC and PC concrete as 0.18 and 0.22 were not practical for PC concrete preparation. As mentioned before, there are two sources with respect to the acquisition of MgO: (i) calcination of magnesite (the dry process route); and (ii) synthesis from seawater (the wet process route). During the wet process route, there were no inputs or outputs associated with the production of seawater since it is a natural material; meanwhile, it is also abundant globally, leading to almost zero emissions related to the acquisition of seawater, which is also in line with a previous study [31]. The first step involved in the synthesis of Mg(OH)2 from seawater is the incorporation of the alkali bases [31]. Secondly, the obtained Mg(OH)2 was then heated to 650 ◦C in a furnace for 1 h, whereas during the dry process, the magnesite was calcined at a temperature of 800 ◦C, undergoing the chemical decomposition shown in Equation (1):

MgCO MgO + CO . (1) 3 → 2 The MgCl 6H O in this study was synthesized via the reaction between the hydrochloric (HCl) 2· 2 and magnesite (MgCO3)[22], which is shown in Equation (2):

MgCO + 2HCl + 5H O MgCl 6H O + CO . (2) 3 2 → 2· 2 2 Inventories: The inventories of three types of alkali used in this study, i.e., NaOH, CaO and NH OH, • 4 which are the most frequently used bases in terms of Mg(OH)2 precipitation in industry [20]. In practice, CaO is more commonly used as an alkali base during the precipitation of Mg(OH)2 due to its cheaper price and larger availability than Ca(OH)2; therefore, CaO was selected in this study instead of Ca(OH)2. Meanwhile, the hydrochloric (HCl) and background data were obtained from the Ecoinvent database developed by the Swiss Centre for Life Cycle Inventories [38] and the ETH-ESU database, provided by Eidgenössische Technische Hochschule [39]. The electricity consumption during the calcination of Mg(OH)2 at the temperature of 650 ◦C for 1 h was recorded using a plug-in electric energy power meter (Floureon, Shenzhen, China). The inventories of Sustainability 2019, 11, 6957 5 of 21

MgO production from the calcination of magnesite were acquired from previous studies [40,41] (Table2). The inventories of PC were obtained from a number of references [ 42–46], the details of which can be found in [40] (Table3). When comparing MOC and PC concrete, the inventories of aggregates and pulverized fly ash were from Ecoinvent database and [47] (Table4). The quantity of each component used to produce 1 kg of MOC and PC concrete is shown in Table5. Goal, functional unit and scope: This goal of this study was to perform a comparative assessment • of the environmental burdens posed by the preparation of MOC paste prepared using three different molar ratios of MgO/MgCl 6H O, where NaOH was used as the alkali base during 2· 2 the synthesis of MgO, then a comparative environmental assessment of MOC paste, in which different alkali bases were used including CaO, NaOH and NH4OH. Finally, the environmental impacts of MOC concrete prepared with two sources of reactive MgO from the wet (i.e., synthesis from seawater) and dry process routes (i.e., calcination of magnesite) were analysed, and then the results were compared with the environmental burdens related to the preparation of PC concrete with the same w/c ratio (i.e., 0.34) afterwards; meanwhile, the transportation of reactive MgO from China to Europe was also taken into account. Therefore, the system boundary used in this study, incorporating the production and utilization phases of MOC and PC samples, is shown in Figure3. The functional unit of assessment used was 1.0 kg of MOC paste prepared according to the mix compositions listed in Table1; however, the final part of this study concerns a comparative environmental assessment of 1.0 kg of MOC concrete (molar ratio: 9) and PC concrete with the same w/c ratio (i.e., 0.34). Impact assessment: In this LCA study, Eco-indicator 99 was employed as it facilitates the selection • of a series of environmental indicators to acquire the environmental loads associated with the production of various binders quantitatively, reflected by several categories. Several key categories such as climate change, affected by CO2 emissions, were also incorporated in this method, and these categories provided a comprehensive profile of the ecological impacts of MOC and PC sample preparation. Meanwhile, these categories also gave a clear representation of the impacts related to human health, ecosystem quality and resources [27,48,49]. Furthermore, the sum of various scores associated with a series of environmental impacts into a single score facilitates a useful comparison of various products and processes, and the weighting and interpretation procedures of environmental burdens used in Eco-indicator 99 [50] are also shown in Figure4. Meanwhile, a mixing triangle was employed to tackle the weighting issues for the two products/processes, in which the indifference curves were calculated by the determination of the points where they cross the boundaries of the triangle [51]. Furthermore, the global warming potential (GWP) of MOC and PC samples was also investigated by IPCC 2007 within a timeframe of 100 years. Description of allocation: Allocation is of great importance, as sometimes a production process • may produce more than one end-product. In this study, during the synthesis of Mg(OH)2 from seawater, which is the main product, some by-products also formed simultaneously, such as NaCl. Therefore, the environmental burdens have to be divided between these two final products in a way that reflects the underlying relationships between them, e.g., allocation by mass or by economic value [35]. Figure2 shows the illustration of allocation in this study. The mass allocation coe fficient CM and economical allocation coefficient CE can be calculated as the mass and economical value ratio between main product and by-product, respectively, shown in Equations (3)–(4):

CM = Mmain product/(Mmain product + Mby-product) (3)

CE = Emain product/(Emain product + Eby-product). (4) 7

Figure 3. Weighting and interpretation of environmental burdens used in Eco-indicator 99 [50].

 Description of allocation: Allocation is of great importance, as sometimes a production process may produce more than one end-product. In this study, during the synthesis of Mg(OH)2 from seawater, which is the main product, some by-products also formed simultaneously, such as NaCl. Therefore, the environmental burdens have to be divided between these two final products in a way that reflects the underlying relationships between them, e.g., allocation by mass or by economic value [35]. Figure 4 shows the illustration of allocation in this study. The mass allocation coefficient CM and economical allocation coefficient CE can be calculated as the mass and economical value ratio between main product and by-product, respectively, shown in Equations (3)–(4):

CM = Mmain product/(Mmain product+ Mby-product) (3)

Sustainability 2019, 11, 6957 CE = Emain product/(Emain product+ Eby-product). 6 of (4) 21

6

 Goal, functional unit and scope: This goal of this study was to perform a comparative assessment of the environmental burdens posed by the preparation of MOC paste prepared using three different molar ratios of MgO/MgCl2∙6H2O, where NaOH was used as the alkali base during the synthesis of MgO, then a comparative environmental assessment of MOC paste, in which different alkali bases were used including CaO, NaOH and NH4OH. Finally, the environmental impacts of MOC concrete prepared with two sources of reactive MgO from the wet (i.e., synthesis from seawater) and dry process routes (i.e., calcination of magnesite) were analysed, and then the results were compared with the environmental burdens related to the preparation of PC concrete with the same w/c ratio (i.e., 0.34) afterwards; meanwhile, the transportation of reactive MgO from China to Europe was also taken into account. Therefore, the system boundary used in this study, incorporating the production and utilization phases of MOC and PC samples, is shown in Figure 2. The functional unit of assessment used was 1.0 kg of MOC paste prepared according to the mix compositions listed in Table Figure 2. Illustration of allocation procedures in this study. 1; however, the finalFigure part 4. of Illustration this study ofconcerns allocation a comparative procedures inenvironmental this study. assessment of 1.0 kg of MOC concrete (molar ratio: 9) and PC concrete with the same w/c ratio (i.e., 0.34). In view of this, the selection of an allocation procedure will greatly influence the outcomes of a study, which has proven to be one of the most controversial issues in LCA studies [52]. However, it is also suggested that the influence of different allocation procedures on the results should be investigated when performing LCA [35].

3. Results

3.1. Environmental Assessment of MOC Paste Prepared with Various Molar Ratios of MgO/MgCl2∙6H2O

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FigureFigure 3. System 2. System boundary boundary used used for for the the LCA LCA ofof MOCMOC samples prepared prepared with with various various molar molar ratios ratios of of MgO/MgO/MgClMgCl 6H2∙6HO2 andO and sources sources ofof reactive MgO MgO and and PC PCsamples. samples. 2· 2  Impact assessment: In this LCA study, Eco-indicator 99 was employed as it facilitates the selection 7 of a series of environmental indicators to acquire the environmental loads associated with the production of various binders quantitatively, reflected by several categories. Several key categories such as climate change, affected by CO2 emissions, were also incorporated in this method, and these categories provided a comprehensive profile of the ecological impacts of MOC and PC sample preparation. Meanwhile, these categories also gave a clear representation of the impacts related to human health, ecosystem quality and resources [27,48,49]. Furthermore, the sum of various scores associated with a series of environmental impacts into a single score facilitates a useful comparison of various products and processes, and the weighting and interpretation procedures of environmental burdens used in Eco-indicator 99 [50] are also shown in Figure 3. Meanwhile, a mixing triangle was employed to tackle the weighting issues for the two products/processes, in which the indifference curves were calculated by the determination of the points where they cross the boundaries of the triangle [51]. Furthermore, the global FigureFigurewarming 4. Weighting 3. Weighting potential and and (GWP) interpretation interpretation of MOC ofandof environmentalenvironmental PC samples was burdens burdens also investigatedused used in inEco-indicator Eco-indicator by IPCC 99 2007 [5 990]. within [50]. a timeframe of 100 years.  Description of allocation: Allocation is of great importance, as sometimes a production process may Sustainabilityproduce 2019more, 11 than, x; doi: one FOR end-product. PEER REVIEW In this study, during the synthesis www.mdpi.com/journal of Mg(OH)2 from/sustainability seawater, which is the main product, some by-products also formed simultaneously, such as NaCl. Therefore, the environmental burdens have to be divided between these two final products in a way that reflects the underlying relationships between them, e.g., allocation by mass or by economic value [35]. Figure 4 shows the illustration of allocation in this study. The mass allocation coefficient CM and economical allocation coefficient CE can be calculated as the mass and economical value ratio between main product and by-product, respectively, shown in Equations (3)–(4):

CM = Mmain product/(Mmain product+ Mby-product) (3)

CE = Emain product/(Emain product+ Eby-product). (4)

Figure 4. Illustration of allocation procedures in this study.

In view of this, the selection of an allocation procedure will greatly influence the outcomes of a study, which has proven to be one of the most controversial issues in LCA studies [52]. However, it is also suggested that the influence of different allocation procedures on the results should be investigated when performing LCA [35].

3. Results

3.1. Environmental Assessment of MOC Paste Prepared with Various Molar Ratios of MgO/MgCl2∙6H2O

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Table 2. Inventory for the production of 1 tonne of MgO [40,41].

Input/Emissions Amount Raw Materials Magnesite (tonne) 2.17 Water (tonne) 0.01 Energy Coal (tonne) 0.27 Electricity (kWh) 8.67 Diesel (tonne) 0.54 Emissions

CO2 (kg) 1096.54 SO2 (kg) 4.21 CO (kg) 0.02 NOx (kg) 0.01 Water vapour (m3) 0.01 Magnesite dust (kg) 0.99

Table 3. Inventory for the production of 1 tonne of Portland cement in Europe [42–45,47].

Raw Materials Limestone (tonne) 1.32 Clay (tonne) 0.32 Sand (tonne) 0.07 Iron ore (tonne) 0.01 Gypsum (tonne) 0.05 Water (tonne) 0.53 Energy Coal (tonne) 5.53E-09 Fuel oil (tonne) 1.53E-09 Electricity (kWh) 71.92 Petroleum coke (tonne) 0.1 Diesel (tonne) 9E-7 Emissions

CO2 (kg) 832.61 SO2 (kg) 0.63 CO (kg) 1.96 NOx (kg) 1.79 Cement kiln dust (kg) 0.87 Particulate matter (kg) 0.03

Table 4. Inventory for the production of 1 tonne of PFA in Europe [47].

Input/Emissions PFA Water (m3) 0.01 Electricity (kwh) 6.8 Natural gas (MJ) 290.0 Fuel oil (kg) 1.0 5 SO2 (kg) 9.1 10− × 2 NOx (kg) 1.8 10 × − Particulate matter (kg) 3.2 10 2 × − CO (kg) 9.1 10 3 × − Sustainability 2019, 11, 6957 8 of 21

Table 5. The quantity of each component used to produce 1 kg of PC and MOC (molar ratio: 9) concrete.

Sample w/s Ratio Water (g) PC (g) MgO (g) MgCl2*6H2O (g) PFA (g) Aggregates (g) MOC concrete 0.34 80.4 - 151.2 85.3 101.3 591.2 PC concrete 0.34 79.3 234.2 - - 100.5 585.5

In view of this, the selection of an allocation procedure will greatly influence the outcomes of a study, which has proven to be one of the most controversial issues in LCA studies [52]. However, it is also suggested that the influence of different allocation procedures on the results should be investigated when performing LCA [35].

3. Results

3.1. Environmental Assessment of MOC Paste Prepared with Various Molar Ratios of MgO/MgCl 6H O 2· 2 The individual impact category for 1.0 kg of MOC prepared with various MgO/MgCl2 ratios is presented in Figure5. With the increase in the molar ratio of MOC samples, the score of every impact category increases linearly, and the single score of MOC-3 is 10.4% lower than that of MOC-5, indicating 8 its higher sustainability. The results indicate that the production of MgCl2 6H2O presented lower · 2 environmentalThe impacts, individual which impact is incategory line withfor 1.0 [23 kg], of by MOC contrast prepared with with that various associated MgO/MgCl with ratios the synthesisis of presented in Figure 5. With the increase in the molar ratio of MOC samples, the score of every impact MgO fromcategory seawater, increases which linearly, could and be the related single score to theof MO diC-3fferent is 10.4% amounts lower than of that HCl of MOC-5, (i.e., 0.36 indicating kg) and NaOH (i.e., 2 kg) usedits higher in thesustainability. production The of results 1.0 kg indicate of MgCl that the6H productionO and Mg(OH) of MgCl2∙,6H respectively.2O presented Withlower regard to 2· 2 2 the preparationenvironmental of all three impacts, types which of is MOC in line paste, with [23], respiratory by contrast organicswith that associated present with the lowestthe synthesis score of due to the low organicMgO particulate from seawater, emissions which could associated be related to with the different MOC paste amounts preparation, of HCl (i.e., 0.36 whereas kg) and NaOH the representation (i.e., 2 kg) used in the production of 1.0 kg of MgCl2∙6H2O and Mg(OH)2, respectively. With regard to the clearly identifiespreparation fossil of all fuels three as types the of largest MOC paste, impact respiratory category organics when present preparing the lowest all score three due types to the of low MOC paste since the productionorganic particulate of NaOH emissions and associated HCl involves with MOC the pa processste preparation, of salt whereas or brine the electrolysis,representation clearly which presents a relativelyidentifies low energy fossil fuels effi asciency, the largest asindicated impact category by thewhen ETU-ESU preparing all database, three types leading of MOC paste to massive since the fossil fuel consumptionproduction during of NaOH their and production HCl involves phases. the process Figure of salt 6or shows brine electrolysis, the environmental which presents a impacts relatively of 1.0 kg low energy efficiency, as indicated by the ETU-ESU database, leading to massive fossil fuel consumption MOC pasteduring preparation their production in terms phases. of Figure different 6 shows damage the environmental categories. impacts As expected, of 1.0 kg MOC the preparationpaste of MOC-3 presentspreparation an in advantage terms of different in terms damage of categories. alleviating As expected, damage the topreparation human of health, MOC-3 presents ecosystem an quality and resourcesadvantage in comparison in terms of withalleviatin theg preparationdamage to human of MOC-5 health, andecosystem MOC-9. quality Meanwhile, and resources the in damage to resources wascomparison more severewith the thanpreparation the other of MOC-5 two and categories, MOC-9. Meanwhile, which is the also damage related to resources to the large was more consumption severe than the other two categories, which is also related to the large consumption of fossil fuels in the of fossil fuelspreparation in the of preparation MOC samples, of as MOC explained samples, before. as explained before.

100 92.5 94.2 90 84.4 80 70 60 50 40 30

Single score (mPt) 20 10 0 MOC-3 MOC-5 MOC-9 Carcinogens Resp. organics Resp. inorganics Climate change Radiation Ozone layer Ecotoxicity Acidification/ Eutrophication Land use Minerals Fossil fuels

Figure 5. Environmental impacts of 1.0 kg MOC prepared by various MgO/MgCl2 ratios (alkali base used Figure 5. Environmental impacts of 1.0 kg MOC prepared by various MgO/MgCl2 ratios (alkali base used duringduring the the synthesis synthesis of MgO: MgO: NaOH) NaOH) in terms in termsof different of diimpactfferent categories). impact categories).

Sustainability 2019, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sustainability

Sustainability 2019, 11, 6957 9 of 21 9

70

58.7 60 57.7 53.4

50

40

28.3 28.9

Score (mPt) 30 25.4

20

10 5.5 6.5 5.7

0 MOC-3 MOC-5 MOC-9

Human Health Ecosystem Quality Resources

Figure 6. Environmental impacts of 1.0 kg MOC preparation (alkali base used during the synthesis of MgO:Figure NaOH) 6. Environmental in terms of impacts different of 1.0 damage kg MOC categories. preparation (alkali base used during the synthesis of MgO: NaOH) in terms of different damage categories. 3.1.1. CO2 Sequestration Potential of MOC Paste Prepared with Various Molar Ratios of MgO/MgCl 6H O 3.1.1. CO2 Sequestration2· 2 Potential of MOC Paste Prepared with Various Molar Ratios of MgO/MgClIn addition2∙6H2O to the formation of some hydration phases such as phase-5 and phase-3 crystals, the presence of chlorartinite (Mg (CO )Cl(OH) 3H O) and hydromagnesite (Mg (CO ) (OH) 4H O) In addition to the formation2 of some3 hydration· 2 phases such as phase-5 and5 phase-33 4 crystals,2· 2the waspresence also of found chlorartinite in in MOC (Mg2(CO samples3)Cl(OH) as∙3H the2O) major and hydromagnesite Mg-carbonates, (Mg indicating5(CO3)4(OH) that2∙4H the2O) hydrated was also magnesium-bearingfound in in MOC samples phases as the are major susceptible Mg-carbonates, to carbonation indicating [53 that]. During the hydrated carbonation, magnesium-bearing an alkaline 2+ 2 environment,phases are susceptible the leaching to carbonation of Mg and [53]. the During presence carb ofonation, CO3 − ions an alkaline in solutions environment, are the prerequisites the leaching for of theMg formation2+ and the presence of magnesium-bearing of CO32- ions in carbonates solutions [are2], allowingthe prerequisites for carbonation for the formation to occur in of the magnesium- interfacial waterbearing layers carbonates on the [2], surfaces allowing of MOC for carbonation samples [30 to]. occur With in respect the interfacia to the sourcesl water oflayers ions on involved the surfaces during of 2 theMOC carbonation, samples [30]. the With CO3 respect− ions to are the derived source froms of ions atmospheric involved during CO2, though the carbonation, the concentration the CO32- isions quite are 2+ lowderived (i.e., from 0.04%). atmospheric Regarding CO the2, though Mg theion concentration sources, there is are quite two low main (i.e., ones: 0.04%). (1) Regarding from the dissolutionthe Mg2+ ion ofsources, phase-3 there or phase-5are two main crystals ones: or (1) (2) from from the the dissolut dissolutionion of ofphase-3 brucite or inphase-5 the interfacial crystals or water (2) from layers. the Thedissolution dissolution of brucite of phase-3 in the orinterf phase-5acial water crystals layers. and bruciteThe dissolution also provides of phase-3 a high-pH or phase-5 environment crystals and as wellbrucite as Clalso− ions.provides The a chemical high-pH reactionsenvironment in terms as well of as the Cl carbonation- ions. The chemical of MOC reactions samples in are terms shown of the in Equationscarbonation (5)–(10): of MOC samples are shown in Equations (5)–(10):

2 2 2 →2+ 2+ - - 2 5Mg(OH)5Mg(OH)2 MgCl2 8H∙MgCl2O ∙8H6MgO 6Mg+ 10OH+10OH− ++2Cl2Cl−+9H+ 9HO2 O(5) (5) · · → 2+2+ - - 3Mg(OH)3Mg(OH)MgCl2∙MgCl8H2∙8HO 2O→4Mg4Mg ++6OH6OH+2Cl− + 2Cl+9H−2O+ 9H O (6) (6) 2· 2· 2 → 2 2+ Mg(OH) Mg2+ + 2OH- − (7) Mg(OH)2 2→→Mg +2OH (7) + 2 CO2 + H2O H2CO3 2H + CO3 − (8) CO2+H2O↔↔H2CO3↔↔2H++CO32- (8) 2+ 2 2Mg + CO − + OH− + Cl− + 3H O Mg (CO )Cl(OH) 3H O (9) 3 2 → 2 3 · 2 2+ 2- - - 2Mg2+ + CO32 +OH +Cl +3H2O→ Mg2(CO3)Cl(OH)∙3H2O (9) 5Mg + 4CO − + 2OH− + 3H O Mg (CO ) (OH) 4H O. (10) 3 2 → 5 3 4 2 · 2 Various stages with5Mg respect2++4CO to32- the+2OH carbonation-+3H2O→ Mg of MOC5(CO3) samples4(OH)2∙4H are2O. shown in Figure7. It was(10) reportedVarious that hydromagnesitestages with respect formed to the after carbonation the formation of MOC of chlorartinite samples are [30 ].shown In theory, in Figure the completion 7. It was reported that hydromagnesite formed after the formation of chlorartinite [30]. In theory, the completion Sustainability 2019, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sustainability

Sustainability 2019, 11, 6957 10 of 21 10 10 ofof the the precipitation precipitation of of chlorartinite chlorartinite would would consume consume all all the the Cl Cl- ionsions and and a proportion a proportion of the of theMg Mg2+ ions2+ ionsin the in of the precipitation of chlorartinite would consume all the Cl- ions− and a proportion of the Mg2+ ions in the interfacialthe interfacial layer, layer, leaving leaving the remaining the remaining Mg2+ Mg ions2+ ionsin the in layer the layer to be to involved be involved in the in thelater later formation formation of interfacial layer, leaving the remaining Mg2+ ions in the layer to be involved in the later formation of hydromagnesite.of hydromagnesite. The Thecalc calculatedulated results results of CO of2 CO-eq 2sequestration-eq sequestration potential potential of 1.0 of kg 1.0 MOC kg MOC preparation preparation are hydromagnesite. The calculated results of CO2-eq sequestration potential of 1.0 kg MOC preparation are shownare shown in Figure in Figure 8. The8. The figure figure shows shows that that MOC MOC samples samples with with a agreater greater molar molar ratio ratio present present a a greater shown in Figure 8. The figure shows that MOC samples with a greater molar ratio present a greater sequestrationsequestration potential potential of ofCO CO2. For2. For instance, instance, the theCO2 CO absorption2 absorption capacity capacity of MOC-9 of MOC-9 is 57.7% is 57.7% higher higher than sequestration potential of CO2. For instance, the CO2 absorption capacity of MOC-9 is 57.7% higher than thatthan of that MOC-3, of MOC-3, which whichcould be could ascribed be ascribed to the different to the di sequestrationfferent sequestration capacities capacities of CO2 shown of CO by2 shown MgO that of MOC-3, which could be ascribed to the different sequestration capacities of CO2 shown by MgO (i.e.,by MgO1.1 kg (i.e., CO2 1.1/kg kgMgO) CO2 and/kg MgO)MgCl2∙ and6H2O MgCl (i.e.,2 0.226H 2kgO (i.e.,CO2/kg 0.22 MgCl kg CO2∙6H2/kg2O), MgCl leading2 6H to2 O),a higher leading CO to2 (i.e., 1.1 kg CO2/kg MgO) and MgCl2∙6H2O (i.e.,· 0.22 kg CO2/kg MgCl2∙6H2O), leading· to a higher CO2 absorptiona higher CO capacity2 absorption in MOC capacity samples inwith MOC greater samples molar withratios greater of MgO/MgCl molar ratios2∙6H2O. of The MgO CO/MgCl2 re-absorbed2 6H2O. absorption capacity in MOC samples with greater molar ratios of MgO/MgCl2∙6H2O. The CO2 re-absorbed· byThe MOC CO 2samplesre-absorbed offset bythe MOC CO2 emissions samples o associatedffset the CO with2 emissions their preparation. associated with their preparation. by MOC samples offset the CO2 emissions associated with their preparation.

FigureFigure 7. 7.Illustration Illustration of various stages stages with with respec respectt to to the the carbonation carbonation of ofMOC MOC samples samples [30]. [30 ]. Figure 7. Illustration of various stages with respect to the carbonation of MOC samples [30].

400 400 353.4 353.4 350 334.9 350 334.9

300 300

250 250 224.0 224.0 200 200

150 150

100 100 -eq sequestration potential (grams) potential -eq sequestration 2 -eq sequestration potential (grams) potential -eq sequestration 2 50 CO 50 CO 0 0 MOC-3 MOC-5 MOC-9 MOC-3 MOC-5 MOC-9

FigureFigure 8. 8. COCO2-eq2-eq sequestration sequestration potential potential of 1.0 of 1.0kg kgMOC MOC sample sample preparation preparation (alkali (alkali base base used used during during the Figure 8. CO2-eq sequestration potential of 1.0 kg MOC sample preparation (alkali base used during the synthesisthe synthesis of MgO: of MgO: NaOH). NaOH). synthesis of MgO: NaOH).

3.1.2.3.1.2. CO CO2 Intensities2 Intensities of ofMOC MOC Paste Paste Prepared Prepared with with Various Various Molar Molar Ratios Ratios of MgO/MgCl of MgO/MgCl2∙6H22O6H 2O 3.1.2. CO2 Intensities of MOC Paste Prepared with Various Molar Ratios of MgO/MgCl2∙6H2·O TheThe use use of of a unit a unit of volume of volume or ma orss mass as a functional as a functional unit of unit sample of sampleformulations formulations is often not is oftenpractical not The use of a unit of volume or mass as a functional unit of sample formulations is often not practical orpractical comprehensive or comprehensive enough enoughsince the since mechanical the mechanical performances performances of sample of sample formulations formulations such such as as or comprehensive enough since the mechanical performances of sample formulations such as compressivecompressive and and flexural flexural strength strength are are a amore more accurate accurate reflection reflection of ofthe the major major function function of ofsamples samples in ina compressive and flexural strength are a more accurate reflection of the major function of samples in a majoritya majority of ofconstructions constructions instead instead of ofvolume volume or or ma massss [54]. [54]. Therefore, Therefore, the the eco-efficiency eco-efficiency indicator indicator is is majority of constructions instead of volume or mass [54]. Therefore, the eco-efficiency indicator is proposedproposed as as it itcan can be be expressed expressed with with respect respect to the to theenvironmental environmental burden burden caused caused by delivering by delivering one unit one proposed as it can be expressed with respect to the environmental burden caused by delivering one unit ofunit functional of functional performance performance (i.e., comp (i.e.,ressive compressive and flexural and flexuralstrength) strength) in MOC insamples MOC in samples this study. in this The study. use of functional performance (i.e., compressive and flexural strength) in MOC samples in this study. The use of a performance indicator as a functional unit facilitates the elimination of the distinction between a of a performance indicator as a functional unit facilitates the elimination of the distinction between a Sustainability 2019, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sustainability Sustainability 2019, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sustainability

Sustainability 2019, 11, 6957 11 of 21

The use of a performance indicator as a functional unit facilitates the elimination of the distinction between a material scale (e.g., kg or m3), and a structural scale, allowing for a fair comparison between the efficiency of mixtures with different performances, which facilitates the search for an optimal molar ratio of MgO/MgCl 6H O in turn. As a result, the use of functional performance should be 2· 2 adopted instead of a functional unit (e.g., m3) or mass (e.g., tonne) in the environmental assessment [54]. Under this study, the calculation of CO2-eq intensities was proposed to investigate the global warming potential (GWP) in Equation (11), enabling a more comprehensive comparison of eco-efficiency of MOC samples prepared with various molar ratios of MgO/MgCl 6H O: 2· 2

Ci = Cd/f, (11) where Cd is the total CO2-eq emissions (kg) and f is the seven-day compressive or flexural strength (MPa) of each MOC sample. As mentioned before, the strength development of MOC samples is very fast [37], as seen in Table6, so seven-day strength of MOC paste is su fficient for its usage in practice, meeting the strength requirements of most applications.

Table 6. The quantity of each component used to produce 1 kg of MOC sample (details and the standard deviations can be found in our previous study [55]).

Sample Flexural Strength (MPa) Compressive Strength (MPa) MOC-3 38.6 71.8 MOC-5 19.6 93 MOC-9 14.2 111

According to Equation (11), when the binders emit similar CO2 emissions, a mixture with greater strength would show lower CO2 intensity, and improving the mechanical properties of samples would definitely lead to the higher eco-efficiency of samples (i.e., a lower CO2-eq intensity) in this study, leading to the production of a ‘green binder’ [56]. The CO2-eq intensities for the preparation of 1.0 kg MOC paste with different molar ratios of MgO/MgCl 6H O are presented in Figure9. Our previous 2· 2 study [55] indicated that with the increase of the MgO/MgCl2 molar ratio, the flexural strength of MOC samples reduced due to the formation of a larger number of microcracks on the surface of MOC samples. Meanwhile, a larger MgO/MgCl2 molar ratio in MOC samples led to greater compressive strength, which was associated with a higher amount of phase-5 crystal (5Mg(OH) MgCl 8H O) formation, 2· 2· 2 namely the main phase contributing to mechanical strength. Therefore, the representation indicated that the molar ratios of MOC samples should be reduced in terms of eco-efficiency (i.e., CO2-eq intensity) considering the flexural strength of MOC paste, whereas in terms of environmental efficiency, which takes compressive strength into account, as expected, the indicator of MOC-9 presents an advantage over the rest of the MOC samples, and a higher molar ratio should be adopted due to the lower CO2-eq intensity. Therefore, in view of the CO2-eq intensities, the molar ratios should be selected carefully depending on the strength requirement of the applications. Sustainability 2019, 11, 6957 12 of 21 12

0.16 0.03 ) -1 ) MPa -1

0.14 ∙ 0.025 MPa ∙ 0.12

0.02 0.1

0.08 0.015

0.06 0.01

0.04

0.005

-eq intensity (flexual strength) (kg strength) (flexual intensity -eq 0.02 2 -eq intensity (compressive strength) (kg (compressive strength) -eq intensity 2 CO 0 0 CO MOC-3 MOC-5 MOC-9

CO₂-eq intensity (flexual strength) CO₂-eq intensity (compressive strength)

Figure 9. CO2-eq intensity of 1.0 kg MOC preparation (alkali base used during the synthesis of FigureMgO: 9. NaOH). CO2-eq intensity of 1.0 kg MOC preparation (alkali base used during the synthesis of MgO: NaOH). 3.2. Environmental Assessment of MOC Paste Prepared with MgO Synthesized by Various Alkali Bases

3.2. EnvironmentalThe type of Assessment alkali base of used MOC during Paste Prepared the precipitation with MgO ofSynthesized Mg(OH) 2byfrom Various seawater Alkali Bases greatly affects the energy inputs [20]; however, previous studies [18,19] failed to provide the outputs associated The type of alkali base used during the precipitation of Mg(OH)2 from seawater greatly affects the energywith the inputs production [20]; however, process previous of synthetic studies MgO. [18,19] Moreover, failed to provide during the the outputs precipitation associated of Mg(OH)with the2 (i.e., main product) from seawater, several by-products are also produced simultaneously, so the production process of synthetic MgO. Moreover, during the precipitation of Mg(OH)2 (i.e., main product) fromenvironmental seawater, several burdens by-products have to be are divided also produced between simultaneously, them in a way so that the indicates environmental the underlying burdens haverelationships to be divided between between them them [35 ].in a way that indicates the underlying relationships between them [35]. AsAs indicated indicated in in Section Section 2.2.2, 2.2.2 ,three three types types of of comm commonlyonly used used alkali alkali bases bases were were chosen, chosen, i.e., i.e., CaO, CaO, NaOH and NH OH, and they generated the by-products of NaCl, CaCl and NH Cl, respectively, as NaOH and NH4OH,4 and they generated the by-products of NaCl, CaCl2 and2 NH4Cl,4 respectively, as a resulta result of ofthis this process, process, as as illustrated illustrated in in Equation Equationss (12)–(15). (12)–(15). Furthermore, Furthermore, two two types types of of widely widely used used allocationallocation procedures, procedures, namely namely allo allocationcation by by mass mass or or by by economic economic value, value, were were applied applied during during this this productionproduction process process to toshow show the the different different scenarios scenarios in this in study. this study. In this In part, this the part, mix the composition mix composition of MOC- of 9MOC-9 was selected: was selected: 2NaOH + MgCl2 Mg(OH)2 + NaCl (12) 2NaOH+MgCl→2 →Mg(OH)2+NaCl (12)

CaO + H O + MgCl Mg(OH) + CaCl (13) 2 2 → 2 2 CaO+H2O+MgCl2→Mg(OH)2+CaCl2 (13) NH OH + MgCl Mg(OH) + NH Cl (14) 4 2 → 2 4 NH4OH+Mg(OH) MgCl2→Mg(OH)MgO +2+NHH O.4Cl (14) (15) 2 → 2 Figure 10 shows the (a) single scores and (b) CO2-eq emissions of 1.0 kg of MOC-9 prepared with Mg(OH)2→MgO+H2O. (15) MgO synthesized by the use of alkali bases including CaO, NaOH and NH4OH. The use of CaO as the 2 alkaliFigure base 10 presented shows the an (a) advantage single scores over and the (b) use CO of NaOH-eq emissions and NH of 41.0OH kg during of MOC-9 theprecipitation prepared with of 4 MgOMg(OH) synthesized2 from seawater by the use in of terms alkali of bases the overall including environmental CaO, NaOH impacts, and NH reflectedOH. The byuse the of singleCaO as score, the 4 alkaliwhereas base the presented adoption an of NHadvant4OHage posed over thethe largest use of burdens NaOH toand the NH environment.OH during Thisthe isprecipitation interpreted of by 2 Mg(OH)the fact that from the seawater production in terms of NH of4 OHthe hasoverall a higher environmental energy demand impacts, (i.e., reflected pressure by (i.e., the 200single atm)) score, and whereas the adoption of NH4OH posed the largest burdens to the environment. This is interpreted by the fact that the production of NH4OH has a higher energy demand (i.e., pressure (i.e., 200 atm)) and involves Sustainability 2019, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sustainability

Sustainability 2019, 11, 6957 13 of 21 involves the use of some catalysts (e.g., iron), leading to more severe overall environmental impacts in 13 contrast with the production of CaO, which is obtained from the calcination of limestone (i.e., a type of naturalthe mineral)use of some at acatalysts temperature (e.g., ir ofon), around leading 850 to more◦C, shown severe overall in Equation environmental (16), thus impacts causing in lesscontrast damage to the environment.with the production In terms of CaO, of CO which2-eq is emissions, obtained from the the use calcination of CaO led of limestone to a slightly (i.e., highera type of global natural warming mineral) at a temperature of around 850 °C, shown in Equation (16), thus causing less damage to the potential than NaOH, which could be ascribed to the large CO2 emissions during the calcination of environment. In terms of CO2-eq emissions, the use of CaO led to a slightly higher global warming limestone (CaCO3) (Equation (16)), leading to a larger GWP related to sample preparation: potential than NaOH, which could be ascribed to the large CO2 emissions during the calcination of limestone (CaCO3) (Equation (16)), leadingCaCO to a largerCaO GWP+ relatedCO . to sample preparation: (16) 3 → 2 CaCO3→CaO+CO2. (16)

300

250

200

150

100 Single score (mPt)

50

0 MOC-9 (alkali base: MOC-9 (alkali base: MOC-9 (alkali base: CaO) NaOH) NH₄OH)

No allocation Allocation by mass Allocation by economical value

(a)

3.5

3

2.5

2

1.5

1 -eq (kg) emission 2 0.5 CO 0 MOC-9 (alkali base: MOC-9 (alkali base: MOC-9 (alkali base: CaO) NaOH) NH₄OH) No allocation Allocation by mass Allocation by economical value

(b)

FigureFigure 10. 10.Single Single score score ( a(a)) andand CO CO2-eq2-eq emission emission (b) of (b )1.0 of kg 1.0 MOC-9 kg MOC-9 prepared prepared with MgO with synthesized MgO synthesized by by variousvarious alkali bases. bases. The The results results were wereallocated allocated by mass by and mass economical and economical value allocation. value allocation.

TableTable7 shows 7 shows the the percentages percentages of of allocation allocation by mass by mass and economic and economic value for value Mg(OH) for2, Mg(OH)CaCl2, NaCl2, CaCl2, NaCland and NH NH4Cl.4 AsCl. can As be can seen be from seen the from table, the percentages table, percentages of allocation ofby allocationmass were much by mass lower were compared much lower comparedwith allocation with allocation by economic by value economic in terms valueof the ma inin terms product, of and the the main percenta product,ges of andallocation the percentagesby mass of allocationare quite by close, mass ranging are quite from close, 33.1% ranging to 35.1%. from Howeve 33.1%r, a to large 35.1%. variance However, concerning a large the percentages variance concerning of the percentagesSustainability 2019 of, 11 allocation, x; doi: FOR PEER by economic REVIEW value (i.e., 56.7–66.7%) www. is observed.mdpi.com/journal As a/sustainability result, the single scores and CO2-eq emissions using allocation by mass were much smaller in comparison with those involving the use of allocation by economic value. Meanwhile, the use of CaO as the alkali base still 14 allocation by economic value (i.e., 56.7–66.7%) is observed. As a result, the single scores and CO2-eq emissionsSustainability using2019, 11allocation, 6957 by mass were much smaller in comparison with those involving the use14 of of 21 allocation by economic value. Meanwhile, the use of CaO as the alkali base still outperformed the use of NaOH and NH4OH during the precipitation of Mg(OH)2 irrespective of the allocation methods, as shown inoutperformed Figure 9. the use of NaOH and NH4OH during the precipitation of Mg(OH)2 irrespective of the allocation methods, as shown in Figure9.

Table 7. Percentages of allocation by mass and economic value used for Mg(OH)2 and CaCl2 NaCl and Table 7. Percentages of allocation by mass and economic value used for Mg(OH) and CaCl NaCl and NH4Cl. Individual mass and market prices are shown to justify the results. 2 2 NH4Cl. Individual mass and market prices are shown to justify the results. Mass Market Main and Allocation by Mass Allocation by Economic Reaction. Main and by ProducedMass Allocation by PriceMarketa Price a Allocation by Reaction by Product (%) Value (%) Product Produced(kg) (kg) Mass (%) (GBP/kg)(GBP /kg) Economic Value (%) Seawater (alkali Mg(OH)2 1 34.3 117 61.5 Seawater (alkali Mg(OH)2 1 34.3 117 61.5 base:base: CaO)CaO) CaClCaCl22 1.91 1.91 65.7 65.7 73.2 73.2 38.5 38.5 SeawaterSeawater (alkali (alkali Mg(OH)Mg(OH)22 1 133.1 33.1 117 117 66.7 66.7 base:base: NaOH) NaOH) NaClNaCl 2.01 2.0166.9 66.9 28.9 28.9 33.3 33.3 SeawaterSeawater (alkali (alkali Mg(OH)Mg(OH)22 1 135.1 35.1 117 11756.7 56.7 base:base: NH NH44OH)OH) NHNH44Cl 1.84 1.8464.9 64.9 48.6 48.6 43.3 43.3 aThea The market market prices prices are are from from Sigma Sigma Aldrich Aldrich and correctcorrect as as of of April April 2019. 2019.

3.3.3.3. Comparative Comparative Environmental Environmental Assessment Assessment of MOC of MOC Concrete Concrete Prepared Prepared with with Different Different MgO MgO Sources Sources and PC and ConcretePC Concrete

3.3.1.3.3.1. Contribution Contribution Analysis Analysis and and CO CO2-eq2-eq Emissions Emissions FigureFigure 11 11 presents presents the the contribution contribution analysis analysis of each of each environm environmentalental impact impact associated associated with 1 with kg MOC 1 kg concreteMOC concrete prepared prepared by (a) MgO by (a) from MgO the from dry theprocess dry processroute or route(b) the or wet (b) process the wet route; process and route; (c) PC and concrete. (c) PC Theconcrete. contributions The contributions of climate ofchange climate (the change orange (the area), orange related area), to related CO2 emissions, to CO2 emissions, vary greatly vary among greatly differentamong diconcretefferent concretesample preparations. sample preparations. The score of The climate score change of climate is 21.2% change of the is overall 21.2% single of the score overall in MOCsingle concrete score in made MOC by concrete MgO from made the by dry MgO process from route the dry(Figure process 11a), route a value (Figure much 11 highera), a valuethan that much of thehigher other than two that groups of the (Figure other 11b,c), two groups which (Figure can be 11ascribedb,c), which to the can decomposition be ascribed toof themagnesite decomposition (Equation of (1))magnesite and its emission (Equation of (1)) a large and itsamount emission of CO of2 a(1.1 large kg amountCO2/kg MgO) of CO 2during(1.1 kg this CO process.2/kg MgO) Meanwhile, during this as theprocess. calcination Meanwhile, temperature as the of calcination PC is very temperature high (i.e., 1450 of °C) PC isand very the high production (i.e., 1450 of alkali◦C) and bases the in production synthetic MgOof alkali is an bases energy-intensive in synthetic process, MgO isthe an scores energy-intensive of fossil fuels in process, MOC concrete the scores prepared of fossil by MgO fuels from in MOC the wetconcrete process prepared route and by PC MgO concrete from ar thee 65.2% wet process (Figure route11b) and and 72.5% PC concrete (Figure 11c) are 65.2%of the total (Figure single 11b) score, and respectively,72.5% (Figure which 11c) of is themuch total higher single than score, MOC respectively, concrete whichwhen the is much MgO higher is obtained than MOC via the concrete dry process when routethe MgO (Figure is obtained 11a). Figure via the 12 dryreveals process the CO route2-eq (Figure emissions 11a). of Figure 1.0 kg 12 MOC reveals concrete the CO prepared2-eq emissions by MgO of obtained1.0 kg MOC from concrete the dry prepared and wet byprocess MgO routes, obtained and from PC theconcrete. dry and As wet expected, process the routes, CO2-eq and of PC 1 kg concrete. MOC concreteAs expected, prepared the CO by2 -eqMgO of 1from kg MOCthe dry concrete and wet prepared process by routes MgO is from 35.0% the and dry 25.0% and wet higher process than routes that emittedis 35.0% by and the 25.0% PC concrete higher than preparation, that emitted indicating by the PCthat, concrete in contrast preparation, with PC indicating concrete, that,MOC in concrete contrast cannotwith PC be concrete, categorized MOC as a concrete ‘low-carbon’ cannot binder. be categorized as a ‘low-carbon’ binder.

Carcinogens Resp. organics Resp. inorganics Climate change Radiation Ozone layer Ecotoxicity Acidification/ Eutrophication

(a) Sustainability 2019, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sustainability Figure 11. Cont.

15 Sustainability 2019, 11, 6957 15 of 21 15

Carcinogens Resp. organics CarcinogensResp. inorganics Resp.Climate organics change Resp.Radiation inorganics ClimateOzone layer change RadiationEcotoxicity OzoneAcidification/ layer Eutrophication

Ecotoxicity Acidification/ Eutrophication

(b) (b)

Carcinogens Resp. organics CarcinogensResp. inorganics Resp.Climate organics change Resp.Radiation inorganics ClimateOzone layer change RadiationEcotoxicity OzoneAcidification/ layer Eutrophication

Ecotoxicity Acidification/ Eutrophication

(c) (c) Figure 11. Contribution analysis of each environmental impact associated with (a) 1 kg MOC concrete Figure 11. Contribution analysis of each environmental impact associated with (a) 1 kg MOC concrete Figureprepared 11. by Contribution MgO from theanalysis dry process of each route environmental (calcination impact of magnesite), associated (b) with1 kg MOC(a) 1 kgconcrete MOC preparedconcrete prepared by MgO from the dry process route (calcination of magnesite), (b) 1 kg MOC concrete prepared preparedby MgO fromby MgO the fromwet processthe dry processroute (synthesized route (calcination from seawater; of magnesite), alkali ( bbase:) 1 kg CaO; MOC allocation: concrete preparedby mass; by MgO from the wet process route (synthesized from seawater; alkali base: CaO; allocation: by mass; byMolar MgO ratio: from 9), theand wet (c) 1process kg PC concrete.route (synthesized from seawater; alkali base: CaO; allocation: by mass; Molar ratio: 9), and (c) 1 kg PC concrete. Molar ratio: 9), and (c) 1 kg PC concrete. 0.35 0.308 0.35 0.285 0.3 0.308 0.3 0.285 0.25 0.228 0.25 0.228 0.2 0.2 0.15 0.15 0.1 0.1 0.05 -eq (kg) emission

2 0.05 0 -eq (kg) emission 2 CO 0 MOC concrete (MgO MOC concrete (MgO PC concrete CO MOCfrom concrete dry process (MgO MOCfrom concrete wet process (MgO PC concrete from dryroute) process from wetroute) process route) route) Figure 12. CO2-eq emissions of 1.0 kg MOC concrete prepared by MgO obtained from the dry process routeFigure (calcination 12. CO2-eq emissions of magnesite) of 1.0 kg and MOC the concrete wet process prepared route by (synthesized MgO obtained from from seawater; the dry process alkali route base: FigureCaO;(calcination allocation: 12. CO of2-eq magnesite) byemissions mass; Molar ofand 1.0 the kg ratio: MOCwet 9), process concrete and 1.0 route prepared kg PC (synthesized concrete. by MgO obtained from seawater; from the alkalidry process base: routeCaO; (calcinationallocation: by of mass; magnesite) Molar ratio:and 9),the and wet 1.0 process kg PC concrete.route (synthesized from seawater; alkali base: CaO; Sustainabilityallocation: 2019 by, 11 mass;, x; doi: Molar FOR ratio:PEER 9),REVIEW and 1.0 kg PC concrete. www.mdpi.com/journal/sustainability Sustainability 2019, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sustainability

Sustainability 2019, 11, 6957 16 of 21

3.3.2. Single Score and Weighting Process 16 Figure 13a presents a comparison of the single score for 1.0 kg of MOC concrete prepared by MgO3.3.2. obtained Single fromScore theand dryWeighting process Process route (i.e., calcination of magnesite) and the wet process route (i.e., synthesized from seawater; allocation by mass; alkali base: CaO), and 1.0 kg PC concrete. The figure Figure 13a presents a comparison of the single score for 1.0 kg of MOC concrete prepared by MgO showsobtained that MOC from concrete the dry preparedprocess route by MgO (i.e., obtainedcalcination from of magnesite) the dry process and the route wet process outperformed route (i.e., the PC samplesynthesized in terms from of the seawater; overall allocation environmental by mass; alkali impacts, base: reflectedCaO), and by1.0 kg a 45.2% PC concrete. lower The single figure score. shows Also, it is interestingthat MOC concrete to find prepared that the by preparation MgO obtained of PCfrom concrete the dry process is more route eco-friendly outperformed than the MOC PC sample concrete preparedin terms by of MgO the overall obtained environmental from the impacts, wet process reflected route. by a Meanwhile,45.2% lower single the score. preparation Also, it is of interesting PC concrete led toto more find that severe the preparation damage to of resources PC concrete than is more MOC eco-friendly concrete than prepared MOC concrete by MgO prepared obtained by MgO from the dry processobtained route, from asthe also wet seenprocess in Figureroute. Meanwhile, 13a, which th coulde preparation be ascribed of PC to concre the factte led that to themore preparation severe of PCdamage samples to resources involves than the MOC use of concrete multiple prepared raw materials by MgO obtained including from limestone, the dry process sand, route, clay, as ironalso and seen in Figure 13a, which could be ascribed to the fact that the preparation of PC samples involves the use gypsum, whereas the preparation of MOC concrete only requires the utilization of magnesite in the of multiple raw materials including limestone, sand, clay, iron and gypsum, whereas the preparation of dry process route, as seen in Equations (1) and (2). Furthermore, two mixing triangles were used to MOC concrete only requires the utilization of magnesite in the dry process route, as seen in Equations (1) assessand the (2). overall Furthermore, environmental two mixing impacts triangles of were each used type toof assess concrete the overall on human environmental health, impacts ecosystem of each quality and resources,type of concrete improving on human the health, clarity ecosystem of the quality weighting and resources, process improving via addressing the clarity the of weighting the weighting issues moreprocess efficiently via addressing than the the single weighting score issues method, more thus efficiently facilitating than the better single score decision-making. method, thus facilitating The mixing trianglesbetter are decision-making. shown in Figure The mixing13b, and triangles the figures are shown indicated in Figure the 13(b1,b2), same findingsand the figures as mentioned indicated the before, wheresame only findings MOC concreteas mentioned prepared before, by where MgO only from MO theC concrete dry process prepared route by canMgO be from categorized the dry process as a more sustainableroute can binder be categorized than PC. as Meanwhile, a more sustainable as indicated binder before,than PC. MOC Meanwhile, concrete as presentsindicated before, a rapid MOC strength developmentconcrete presents [37], so, a comparedrapid strength with development the strength [37], ofso, PCcompared concrete with [57 the] afterstreng seventh of PC days, concrete the [57] authors after seven days, the authors believe that MOC concrete revealed higher strength within the same curing believe that MOC concrete revealed higher strength within the same curing age, which led to lower age, which led to lower CO2-eq intensities in comparison with those related to the preparation of PC CO -eq intensities in comparison with those related to the preparation of PC concrete. 2 concrete.

14

12

10

8

6

4 Single score (mPt)

2

0 MOC concrete (MgO from MOC concrete (MgO from PC concrete dry process route) wet process route)

Human Health Ecosystem Quality Resources

(a)

Figure 13. Cont.

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Sustainability 2019, 11, 6957 17 of 21

17

.

(B

(.

s(b)

Figure 13. 13. ComparisonComparison of of (a () asingle) single score score and and (b) (mixingb) mixing triangle triangle of environmental of environmental impacts impacts of 1.0 ofkg 1.0MOC kg concreteMOC concrete prepared prepared by MgO by obtained MgO obtained from the from dry theprocess dryprocess route (calcination route (calcination of magnesite) of magnesite) and the andwet processthe wet route process (synthesized route (synthesized from seawater; from seawater; alkali base: alkali CaO; base: allocation: CaO; allocation: by mass; molar by mass; ratio: molar 9), and ratio: 1.0 kg 9), PCand concrete. 1.0 kg PC concrete.

4. Discussion 4. Discussion The majority of magnesite deposits and magnesia production (i.e., the key component in MOC The majority of magnesite deposits and magnesia production (i.e., the key component in MOC production) are based in China [40], which reduces the convenience of the magnesia requirement production) are based in China [40], which reduces the convenience of the magnesia requirement from from the calcination of magnesite in Europe and keeps the price of magnesia higher than that of the calcination of magnesite in Europe and keeps the price of magnesia higher than that of PC. Meanwhile, PC. Meanwhile, the transportation of magnesia from China to Europe puts further stress on the the transportation of magnesia from China to Europe puts further stress on the environment. In Europe, Austria, Slovakia, Greece and Spain have some magnesite deposits, but their annual output of magnesia

Sustainability 2019, 11, x; doi: FOR PEER REVIEW www.mdpi.com/journal/sustainability

Sustainability 2019, 11, 6957 18 of 21 environment. In Europe, Austria, Slovakia, Greece and Spain have some magnesite deposits, but their annual output of magnesia is far less than the amount of magnesia imported from China. In view18 of this, a sensitivity analysis is necessary since the majority of magnesia is from China, and the travel distances of reactiveis far less magnesia than the amount from Liaoning of magnesia Province imported (which from China. is the In major view provinceof this, a sensitivity for MgO analysis production is in China)necessary to Europe since wasthe majority assumed of magnes to be 7000–9000ia is from China, km, separatelyand the travel by di freighter.stances of reactive From Figure magnesia 14 ,from it is clear Liaoning Province (which is the major province for MgO production in China) to Europe was assumed to that the shipping of reactive magnesia from China to Europe has a negligible influence on the single be 7000‒9000 km, separately by freighter. From Figure 14, it is clear that the shipping of reactive magnesia scores and CO emissions associated with the preparation of MOC concrete when MgO is obtained from China 2to Europe has a negligible influence on the single scores and CO2 emissions associated with fromthe the preparation dry process of MOC route. concrete when MgO is obtained from the dry process route.

12

10

8

6

4

2

0 Single score (mPt) Single MOC concrete MOC concrete MOC concrete MOC concrete PC concrete (MgO from dry (MgO from dry (MgO from dry (MgO from dry process route): process route): process route): process route): base distance: 7000 distance: 8000 distance: 9000 km km km

Human Health Ecosystem Quality Resources

(a)

0.35

0.3

0.25

0.2

0.15

0.1 -eq (kg) emission

2 0.05 CO 0 MOC concrete MOC concrete MOC concrete MOC concrete PC concrete (MgO from dry (MgO from dry (MgO from dry (MgO from dry process route): process route); process route); process route); base distance: 7000 km distance: 8000 km distance: 9000 km

(b)

FigureFigure 14. 14.Sensitivity Sensitivity analysis analysis in terms of of the the influence influence of various of various transportati transportationon distances distances on the (a on) the

(a) singlesingle scores scores andand ((b) CO22-eq-eq emissions emissions associated associated with with MOC MOC concrete concrete from from the dry the and dry wet and process wet process routesroutes (base: (base: transportation transportation is is not not considered considered in the the MOC MOC concrete concrete preparation). preparation).

In addition to magnesite, some alternative magnesium-bearing minerals such as magnesium silicate (talc)Sustainability or dolomite 2019, could 11, x; doi: also FOR be PEER ideal REVIEW precursors for the production of www. magnesia,mdpi.com/journal as they/sustainability are abundant

Sustainability 2019, 11, 6957 19 of 21 worldwide and can be extracted easily, similar to the calcination of magnesite, thereby increasing the supply and reducing the current price of MgO from magnesite in Europe.

5. Conclusions 1. Increasing the molar ratios in MOC paste led to more severe environmental impacts and greater CO sequestration potentials, which are related to the difference between MgO and MgCl 6H2O 2 2· in terms of their energy requirements for production and their CO2 binding capacities. 2. In terms of environmental efficiency, as reflected by CO2 intensities, the molar ratios of MOC paste should be carefully selected depending on the strength requirements of the applications, and allocating the environmental impacts of MgO production by mass is more favourable. 3. The transportation of reactive MgO from China to Europe has a negligible influence on the environmental impacts of MOC concrete preparation when MgO is from the dry process route, and MOC concrete cannot be categorized as a type of ‘low-carbon’ binder, by contrast with PC concrete. Meanwhile, in terms of the overall environmental impacts, MOC concrete can only be recognized as a more sustainable binder than PC concrete when the main component, MgO, is obtained through the dry process route rather than the wet process route in MOC concrete preparation.

Author Contributions: G.K. and S.L. studied the mechanical performance of samples. S.R. and C.U. designed the process and performed the LCA studies. X.Z. is the project leader and revised the final manuscript. Funding: This research was funded by the European Commission Horizon 2020 Research and Innovation Programme through the grant 723825 (i.e., the Green INSTRUCT project). Acknowledgments: The authors would like to acknowledge the lab technician, Neil Macfadyen, for his assistance. Conflicts of Interest: The authors declare no conflict of interest.

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