Carbon dioxide sequestration in magnesium-based binders

Carbon dioxide sequestration in magnesium-based binders

7

Cise Unluer Nanyang Technological University, Singapore, Singapore

7.1

Introduction

The use of reactive magnesium oxide (MgO) in carbonated concrete formulations can enable the permanent sequestration of CO2, which also leads to strength gain (Pearce, 2002). MgO can be obtained by the calcination of magnesium carbonate minerals or extracted from seawater or brine sources (Shand, 2006; Dong et al., 2017, 2018; Ruan and Unluer, 2016; Ferrini et al., 2009; Mignardi et al., 2011). It is classified as “reactive”, “hard-burned,” or “dead-burned” depending on the calcination temperature used during its production. The main applications of MgO within the construction industry include magnesium oxychloride (MOC) cements used in fiber-reinforced fire-resistant boards (Urwongse and Sorrell, 1980; Sorrel and Armstrong, 1976), magnesium oxysulfate (MOS) cements used in lightweight insulation panels (Beaudoin and Feldman, 1977; Beaudoin and Ramachandran, 1978), magnesium phosphate (MAP) cements used in fire protection and rapid repairs (Mestres and Ginebra, 2011; Ribeiro and Morelli, 2009; Viani and Gualtieri, 2014), and MgO-based expansive additives (MEAs) used to compensate concrete shrinkage in dam construction (Gao et al., 2008, 2013; Mo et al., 2010, 2014; Zheng et al., 1991). In addition to these applications, current research initiatives focus on the development of reactive MgO cement (reactive magnesia cement [RMC]) as a binder that gains strength by carbonation (Vandeperre et al., 2008a,b, 2007; Liska and Al-Tabbaa, 2009; Unluer and Al-Tabbaa, 2015, 2014b, 2013; Sonat and Unluer, 2017; Sonat et al., 2017a,b; Ruan and Unluer, 2017a,b,c; Ruan et al., 2017; Pu and Unluer, 2016; Dung and Unluer, 2016, 2017b,c; Mo and Panesar, 2013, 2012, 2014; Liska, 2009; Unluer, 2012; Li, 2013; Vlasopoulos, 2012; Rheinheimer et al., 2017; Panesar and Mo, 2013; De Silva et al., 2009; Liska et al., 2012a,b; Vandeperre and Al-Tabbaa, 2007). This is further supported by the use of MgO derived from magnesium silicates, which has been used in combination with hydrated magnesium carbonates (HMCs) to form a new low-carbon cement system (Flatt et al., 2012). MgO has been used in masonry construction as a mortar component and stabilizer for soil bricks for many centuries. One well-known application of MgO is the Great Wall of China, constructed in the seventh century BC. The amount of MgO in the Great Wall is estimated to be sufficient to meet the global demands of both drywall and plywood at the current level of consumption for over 800 years (Concreate. Magnesium Oxide Germany: Concreate Deutschland, 2013). MgO was also used in the Terracotta Army, manufactured during the third century BC Carbon Dioxide Sequestration in Cementitious Construction Materials https://doi.org/10.1016/B978-0-08-102444-7.00007-1 Copyright © 2018 Elsevier Ltd. All rights reserved.

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Carbon Dioxide Sequestration in Cementitious Construction Materials

(Concreate. Magnesium Oxide Germany: Concreate Deutschland, 2013). MAP cements were used as mortar by the Romans in the construction of the Stupas in India (Mago BP. History, 2014). More recently, the delayed expansion of MgO has been exploited to compensate for the thermal shrinkage of concrete dams in China since the 1970s (Mo et al., 2010). MEA was first used during the construction of the Baishan dam in China, where around 5% MgO was added to Portland cement (PC) to prevent crack formation despite the generation of high-temperature gradients (Du, 2005). MEA has since been used in several Chinese dams (Gao et al., 2008, 2013; Mo et al., 2010; Mo et al., 2014) to speed up construction by casting concrete continuously with reduced dependence on contraction joints, decrease costs and energy associated with expensive cooling measures, and avoid thermal cracks in mass concrete (Mo et al., 2014). Fig. 7.1 provides a summary of the timeline associated with the use of MgO in cements as reported by different commercial and research institutions. The addition of RMC to PC mixes was first suggested by John Harrison, who obtained a patent for “Reactive magnesium oxide cements” in 2001, referring to the use of RMC produced at low temperatures (Harrison, 2001). Following research has investigated the role of RMC as a partial or complete replacement of PC in pastes, mortars, and concrete mixes (Vandeperre et al., 2008a,b, 2007; Liska and Al-Tabbaa, 2009; Unluer and Al-Tabbaa, 2015, 2014b, 2013; Sonat and Unluer, 2017; Sonat et al., 2017a,b; Ruan and Unluer, 2017a,b,c; Ruan et al., 2017; Pu and Unluer, 2016; Dung and Unluer, 2016, 2017b,c; Mo and Panesar, 2013, 2012, 2014; Liska, 2009; Unluer, 2012; Li, 2013; Vlasopoulos, 2012; Rheinheimer et al., 2017; Panesar and Mo, 2013; De Silva et al., 2009; Liska et al., 2012a,b; Vandeperre and Al-Tabbaa, 2007). The production and characterization of different sources of MgO and the performance of samples containing RMC as a binder have also been studied extensively

1940

2001

2007

Magnesium phosphate cements discovered as refractory materials for use in casting dental alloys.

Use of reactive MgO in different binder compositions was proposed to have mechanical and sustainability advantages.

Production of MgO from silicates and development of binders containing MgO, pozzolans and HMCs was suggested by imperial college.

1860s

1970s

2004

2010s

Magnesium oxychloride (sorel) and oxysulfate cements developed and used in industrial flooring, fire protection, and insulating panels.

MgO started to be used as an expansive additive to compensate shrinkage of PC in dam constructions in China.

Extensive research starts in different academic institutions, resulting in several prominent publications on the carbonation of MgO cements.

Research on MgO cements continues in several universities and research institutions on a global scale.

Figure 7.1 Timeline of the use of MgO in the cement context.

Carbon dioxide sequestration in magnesium-based binders

131

(Dong et al., 2017, 2018; Ruan and Unluer, 2016; Hassan, 2014; Pacheco-Torgal et al., 2013; Jin, 2013). The production of RMC from magnesium silicates led to several patents on the development of a binder based on RMC combined with HMCs (Vlasopoulos and Cheeseman, 2009) and silicon oxidee and/or aluminum oxideecontaining materials (Devaraj et al., 2012) and the processing of cements and mixes containing RMC (Vlasopoulos, 2012; Vlasopoulos and Julian, 2012). The use of RMC in concrete applications was reported to enhance carbonation, mechanical properties, and durability. When compared to PC, the main advantages of RMC include its (1) ability to sequester CO2 and gain strength by the formation of HMCs, (2) high-durability performance in extreme and aggressive environments, (3) lower sensitivity to impurities, enabling the use of industrial by-products, and (4) potential to be fully recycled when RMC is used alone as a binder. On the other hand, major issues include unfamiliarity and insufficient track record compared to the high validation and market confidence in PC. The relatively limited availability of raw materials (i.e., magnesite) for producing RMC and the location of existing production facilities, the majority of which are in China, can also lead to increased environmental impact. In addition to its use as a cementitious binder, MgO is used in several other applications. Low-grade MgO is reported to stabilize heavy metals in highly contaminated soils and industrial waste-cement systems (Liu et al., 2007b; Garcia et al., 2004). The use of MgO has been studied in waste immobilization (Garcia et al., 2004; Iyengar, 2008; Ono and Wada, 2007, 2006; Zhang et al., 2011), soil stabilization (Yi et al., 2014a; Jegandan et al., 2010), water treatment (Rotting et al., 2008; Cortina et al., 2003), ground improvement (Al-Tabbaa et al., 2011, 2012), nuclear waste containment (Xiong and Lord, 2008), biodegradation of organic contaminants (Al-Tabbaa et al., 2007; Kogbara et al., 2010), and alkali activation of groundgranulated blast-furnace slag (GGBS) (Jin et al., 2014; Yi et al., 2014b; Haha et al., 2011). Other applications involving MgO include its use in the pharmaceuticals, semiconductor and agricultural industries, and more specifically in the production of wood pulp (Hull et al., 1951), nanowires (Yadong et al., 2002), as a catalyst support (Baird and Lunsford, 1972; Che et al., 1972), and as a plant nutrient (Mengel and Kirkby, 2001). Alternatively, the hydration product of MgO, magnesium hydroxide, is used in flue gas desulfurization, wastewater treatment, and as a flame retardant.

7.2 7.2.1

Production and properties of MgO Production

Magnesium is found in minerals such as dolomite (CaMg(CO3)2) and magnesite (MgCO3) and is the third most abundant element in seawater, with an average concentration of 1300 ppm (Shand, 2006). MgO is mainly obtained by two methods: 1. calcination of magnesia-based minerals (dry-route) 2. extraction from brine or seawater (wet-route)

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Carbon Dioxide Sequestration in Cementitious Construction Materials

Calcination of MgCO3 to form MgO occurs at temperatures between 550 and 800
C and is an endothermic reaction, as indicated in Eq. (7.1). The lower calcination temperatures used in the production of RMC when compared to PC (i.e., <800 vs. 1450
C) can allow the use of alternative fuels with relatively low heating values. MgCO3 / MgO þ CO2

(7.1)

The calcination of magnesite follows a similar procedure as limestone, during which the applied heat is transferred from the magnesite surface to the micropores between the particle surface and reaction interface. This causes the dissociation of MgCO3 into MgO and CO2, which migrates from the interface to the particle surface through the porous calcined layer. The use of lower temperatures causes lattice strain and porosity in MgO particles, thereby maintaining its reactivity. The microstructure of the MgO layer around the reaction interphase, along with the diffusion of CO2 to particle surface, is the rate-determining step during this process. Around 2.08 kg of pure magnesite is needed for the production of 1 kg of MgO. The kinetics of this reaction are influenced by the calcination temperature used for the decomposition of MgCO3, the duration of calcination to enable the complete decomposition of MgCO3 into MgO, and the removal of CO2 from the kiln to increase the decomposition rate. Sintering occurs during calcination, reducing the porosity of the formed product. Calcination conditions such as temperature and residence time are key parameters that influence the specific surface area (SSA) and reactivity of the final MgO (Shand, 2006; Thomas et al., 2014). Increasing the calcination temperature and residence time decreases the reactivity of MgO. This is accompanied by a reduction in its SSA and an increase in particle size (Mo et al., 2010; Eubank, 1951). Other than the dry-route, MgO can be also obtained from seawater or concentrated brines by adding a base, such as lime (CaO) or dolime (CaO$MgO), which raises the pH of the solution to the precipitation point of Mg(OH)2 (w10.5). However, the use of these Ca-bearing bases can also result in the precipitation of Ca-based compounds and thus reduce the purity and content of Mg-based precipitates. Furthermore, the Ca-bearing bases can react with the sulfate present in the solution to form gypsum (CaSO4$2H2O), which may necessitate the pretreatment of the solution through the addition of CaCl2 to remove the sulfate in seawater/brine. Alternatively, other bases, such as sodium hydroxide (NaOH) or ammonia (NH4OH), have also been proposed for this purpose, leading to the production of MgO samples with relatively high purity levels (Dong et al., 2017, 2018). Following its precipitation, Mg(OH)2 is then heated to produce MgO, as shown in Eqs. (7.2)e(7.4). This product generally contains MgO in the range of 55%e98%. The common impurities in MgO include CaO, SiO2, Fe2O3, and Al2O3, whose concentrations depend on the starting material and the base utilized during the precipitation of Mg(OH)2. For the production of higher grade MgO, the source and pretreatment conditions must be carefully monitored. The purity level of synthetic MgO is usually higher than MgO obtained via the dry-route, whose quality can vary from one location to another. These higher purity levels are accompanied with smaller particle and crystallite sizes, higher surface areas and reactivity, and a whiter color (Shand, 2006).

Carbon dioxide sequestration in magnesium-based binders

133

CaO þ H2O þ MgCl2 / Mg(OH)2 þ CaCl2

(7.2)

CaO$MgO þ 2H2O þ MgCl2 / 2Mg(OH)2 þ CaCl2

(7.3)

Mg(OH)2 / MgO þ H2O

(7.4)

It is estimated that about 14 million tonnes/year of MgO is produced world-wide. For comparison, the production of PC exceeds 4 billion tonnes/year (USGS, 2014). The 3.7 billion tonnes of magnesite reserves in China account for w29% of the world’s total reserves (Li et al., 2015). Although magnesite resources are widely available, 75% of the global magnesite production is from China, North Korea, Slovakia, Turkey, Russia, Australia, and India (Shand, 2006), as shown in Fig. 7.2. The total production of MgO from magnesite is w8.5 million tonnes/year (USGS, 2013), and the leading producers are China, Russia, and Turkey, who are responsible for 49%, 12%, and 6% of total MgO production, respectively (Vandeperre et al., 2008b). Calcination of magnesite usually takes place in shaft or tunnel kilns. Low-quality control measures practiced in China result in inconsistent calcination conditions (temperature and residence time), which can cause large temperature variations at different locations within the kiln, producing MgO with heterogeneous reactivity levels. China has the largest magnesite reserves in the world and is the largest producer, consumer, and exporter of MgO. Export taxes imposed on MgO products by the Chinese government in 2008 increased costs, incentivizing competing countries to increase local MgO production capacities rather than relying on low-cost imports from China (IHS, 2014).

North and South America:

Europe and Russia:

US (13.6 Mt)

Russia (622 Mt) Slovakia (289 Mt) Turkey (145 Mt) Greece (27 Mt) Spain (27 Mt) Austria (18 Mt)

Brazil (59 Mt)

Asia and Australia : China (3.7 Bt) North Korea (680 Mt) Australia (109 Mt) India (50 Mt)

Figure 7.2 Global distribution of magnesite reserves. Bt, billion tonnes; Mt, million tonnes. Figures obtained from Shand, M.A., 2006. The Chemistry and Technology of Magnesia. Wiley-Interscience, Findlay, Ohio and Li, J., Zhang, Y., Shao, S., Zhang S., 2015. Comparative life cycle assessment of conventional and new fused magnesia production. Journal of Cleaner Production 91, 170e179.

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Carbon Dioxide Sequestration in Cementitious Construction Materials

MgO can be categorized according to the temperature under which MgCO3 is calcined, as detailed in Table 7.1. Light-burned (reactive or caustic) MgO is calcined at 700e1000
C and has a very high SSA and reactivity. Along with its use in the pharmaceutical, paper, and agriculture industries, the properties and applications of lightburned MgO have been reported in several studies (Jin and Al-Tabbaa, 2014; Chau and Li, 2008; Fukumoto et al., 2011; Jensen et al., 1986; Rotting et al., 2006; Birchal, 2001), including its use in cementitious materials (Vandeperre et al., 2008a,b, 2007; Liska and Al-Tabbaa, 2009; Unluer and Al-Tabbaa, 2015, 2014b, 2013; Sonat and Unluer, 2017; Sonat et al., 2017a,b; Ruan and Unluer, 2017a,b,c; Ruan et al., 2017; Pu and Unluer, 2016; Dung and Unluer, 2016, 2017b,c; Mo and Panesar, 2013, 2012, 2014; Liska, 2009; Unluer, 2012; Li, 2013; Vlasopoulos, 2012; Rheinheimer et al., 2017; Panesar and Mo, 2013; De Silva et al., 2009; Liska et al., 2012a,b; Vandeperre and Al-Tabbaa, 2007). Hard-burned MgO is calcined at 1000e1400
C and has a relatively low SSA and reactivity. It is typically used in applications that exploit the expansion properties of MgO (Gao et al., 2008; Mo et al., 2014). Deadburned MgO (periclase) is calcined at 1400e2000
C, has a low SSA, and is unreactive (Wagh, 2004). It is mainly used in the refractories industry in the formation of linings and bricks (Silva et al., 2011; Ghanbari Ahari et al., 2002). Fused MgO, which possesses low hydration energy, high thermal shock resistance, and a compact structure, is used in monolithic refractories and electrical insulating (Li et al., 2015; Canterford, 1985). Slightly differing from this classification, the European Commission (Schorcht et al., 2013) defines caustic calcined, dead-burned, and fused MgO to be produced at 600e1300, 1600e2200, and >2800
C, respectively. MgO can be present in PC because of the magnesium carbonate that is found within the raw materials used to produce PC clinker. The production of PC at 1450
C results in the formation of dead-burned MgO, which hydrates slowly under ambient conditions, forming Mg(OH)2. This is associated with a volume expansion, which may be retarded when MgO grains are encapsulated in the concrete matrix. The formation of Mg(OH)2 after concrete has hardened can cause structural damage due to delayed expansion. In the 19th century, bridges and viaducts in France and a town hall in Germany constructed with PC containing 16%e30% MgO from the dolomitic limestone used in the production of PC suffered from delayed hydration due to the expansion of the periclase (Mehta, 1978). This led to limitations on the MgO content in PC to 5% in the UK (BS EN 197e1:2000, 2007) and China (GB 175e2007/XG 1-2009, 2009) and 6% in the USA (ASTM C150M-12, 2012) to prevent unsoundness at late ages caused by the delayed expansion of periclase.

7.2.2

Properties

MgO exhibits different properties depending on the source and calcination conditions used during its production. The main parameters used to assess the characteristics of MgO are its reactivity, SSA, crystallite size, crystal structure, density, agglomeration ratio (i.e., the ratio of the primary particle size to the crystallite size), pore structure, total porosity, and morphology (Li, 2013; Jin and Al-Tabbaa, 2014; Chau and Li, 2008; Liska et al., 2006), as shown in Fig. 7.3. SSA can be used as an indicator of

Different grades, properties, and applications of MgO Calcination Temperature (8C)

Reactivity

SSA (m2/g)

Crystallinity

Applications

Light-burned (reactive, caustic calcined)

700e1000

Highest

>20

Lowest

Catalysts, rubber and paper production, sulfur dioxide removal, fertilizer and animal feed supplements, absorbent for inorganic/organic contaminants, neutralizing agent, filtration medium, agriculture, cattle feed, environmental control, cement binders

Hard-burned

1000e1400

Lower

1e20

Higher

MAP, MOC, and MOS cements; MEA in concrete applications (dam constructions in China)

Dead-burned (periclase)

1400e2000

Even lower

<1

Even higher

Refractory applications, MAP cements

Fused

>2800

Lowest

0.01e0.1

Highest

Refractory and electrical insulating markets

Grade

Carbon dioxide sequestration in magnesium-based binders

Table 7.1

135

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Carbon Dioxide Sequestration in Cementitious Construction Materials

Factors influencing properties of MgO

Properties of MgO

Performance of MgO

Properties of parent material

Reactivity

Setting time

Specific surface area

Expansion

1. Composition

Particle size

Hydration

2. Impurities

Agglomeration ratio

Carbonation

Crystal structure

Strength

Calcination conditions

Pore structure

Chemical composition

1. Temperature

Porosity

2. Residence time

Morphology

Microstructural development

3. Heating rate

Figure 7.3 Factors influencing the properties and performance of MgO.

reactivity, which also depends on the characteristics of the MgO source and the system in which it is used. The properties of MgO produced from the calcination of magnesite or other Mg-bearing minerals (dry-route) are primarily determined by the calcination conditions and properties of the precursor. For MgO obtained from brine or seawater (wet-route), preparation methods and precursor properties determine the final properties. These factors influence the hydration and carbonation kinetics of MgO and its interaction with other mix components. In both cases, the processing conditions and properties of raw materials control the performance of MgO and define the potential benefits and limitations of different types of MgO. Accordingly, a carefully controlled production process can enable the production of MgO with desirable properties by customizing specific production parameters in line with the intended end use. The effect of calcination conditions on the production of MgO from various precursors, such as magnesium carbonate and magnesium hydroxide, under different calcination durations and temperatures ranging between 300 and 2000
C has been reported (Mo et al., 2010; Aramendia et al., 1996; Birchal et al., 2000; Liu et al., 2007a; Alvarado et al., 2000; Aramendía et al., 2003; Aphane et al., 2009). At low calcination temperatures, evaporation of gases causes high porosity, surface area, and reactivity. Alternatively, sintering occurs at higher temperatures (i.e., >900
C) and longer residence times, resulting in increased grain size and lower SSA and pore size. Birchal et al. (2000) identified calcination temperature to have a greater influence on SSA than residence time. Reduction in SSA was observed at calcination temperatures above 900
C. Samples with the highest reactivities achieved the highest hydration degree of w70%, which stabilized at w20% for less-reactive samples. Another study (Aphane et al., 2009) reported that the degree of MgO hydration was greater in magnesium acetate solution than in water because of the presence of extra Mg2þ.

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137

The effect of precursors on MgO production was investigated by Aramendia et al. (1996) and Alvarado et al. (2000), who showed that MgO samples with different properties were obtained from different precursors, such as brucite (Mg(OH)2), magnesium carbonate hydroxide (Mg5(OH)2(CO3)4), magnesium sulfate (MgSO4$7H2O), magnesium nitrate (MgNO3$6H2O), magnesium acetate (Mg(CH3CO2)$4H2O), and dolomite (MgCO3$CaCO3). The results highlighted that MgO with higher SSA and basicity was obtained from Mg(OH)2 when compared with Mg5(OH)2(CO3)4. SSA of MgO decreased in the following order: sulphate > nitrate > acetate > dolomite. Other than the calcination conditions and precursor properties, the preparation method can also influence the properties of MgO samples, as reported in Aramendía et al.(2003). In a more recent study (Jin and Al-Tabbaa, 2014), characterization of MgO obtained from 14 different sources, including magnesite, seawater, and chemical precipitation, led to the definition of three categories for MgO reactivity, which was determined according to the acid reactivity, SSA, and agglomeration ratio. Differences between the calcined and synthesized MgO samples included (1) chemical composition: synthetic MgO had a higher Mg content and less impurities than calcined samples; (2) SSA: higher for synthetic MgO, resulting in a higher reactivity than calcined samples; (3) agglomeration ratio: w1 for synthetic MgO (i.e., primary particles were composed of single crystals) and between 1.4 and 7.2 for calcined MgO (i.e., larger primary particles composed of more than one crystallite); and (4) pH: lower for synthetic MgO, typically in the range between w10.1 and 10.8, whereas the pH of calcined samples was between w10.8 and 12.2 at 90 days because of their higher CaO contents. A linear relationship between agglomeration ratio and reactivity was observed. An increase in reactivity and hydration rate was found with increasing SSA, until a limit was reached for the degree of hydration at w80%, due to the incomplete hydration inside MgO particles. In line with the trends observed in its primary properties such as SSA and reactivity, higher calcination temperatures and longer residence times decrease the inner pore volume and hence the hydration activity of MgO. The type of the precursor and kinetics of the hydration reaction directly influence the quality and the properties of the end product (Fruhwirth et al., 1985; Filippou et al., 1999; Smithson and Bakhshi, 1969). The hydration mechanism of MgO (as shown in Eq. (7.5)) follows these steps: (1) adsorption of water at the surface and its simultaneous diffusion inside porous MgO particles; (2) dissolution of oxide within these particles, changing porosity with time; and (3) creation of supersaturation, nucleation, and growth of Mg(OH)2 at the surface of MgO (Rocha et al., 2004). During this process, a layer of Mg(OH)2 forms on the surface of MgO, imposing an additional resistance that limits the further continuation of the hydration process. MgO þ H2O / Mg(OH)2 (Brucite)

(7.5)

In the case of applications involving RMC as the main binder, this mechanism presents a constraint on the progress of hydration and the subsequent carbonation process, which hinders strength development. This is one of the main challenges faced by

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Carbon Dioxide Sequestration in Cementitious Construction Materials

RMC-based formulations that rely on carbonation for hardening. Therefore, understanding the dissolution and hydration kinetics of MgO plays a critical role in determining the mechanical performance of these binders. As the hydration of MgO takes place after the supersaturation of brucite, followed by its precipitation on MgO surface, the reaction rate is controlled by proton attack or desorption of Mg2þ and OH ions. The rate controlling factor depends on the pH interval and is identified as the Mg2þ concentration, diffusion, and adsorption of OH ions when the pH is <5, w5, and >7, respectively (Fruhwirth et al., 1985). It is likely that the structure of the hydrate phase that initially forms on the surface of MgO can show variations when compared to pure brucite because of its interaction with the underlying oxide, resulting in higher solubility levels. Accordingly, the initial hydrate layer can continue to dissolve and reprecipitate as brucite crystals away from the particle surface, especially in the presence of high humidity or water contents. In such a scenario, the progress of hydration is controlled by the dissolving rate of Mg(OH)2 that forms on the MgO surface (Kuenzel et al., 2018).

7.3

Carbonated reactive magnesia cement systems

The applications involving the use of MgO within the construction industry can be categorized under three main groups: commercial scale, pilot plant scale, and experimental trials. The main commercial applications of MgO include MOC, MOS, and MAP cements, as well as MEA. These are followed by magnesium silicate cements, which have been investigated at a pilot plant scale. These relatively mature applications are not discussed in detail as they are outside the scope of this chapter, in which main focus is on the advances in the development of carbonated RMC formulations.

7.3.1

Background

Carbonated RMC systems evolved from the need to stabilize the amount of greenhouse gases released into the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Until now, various methods have been proposed in the cement industry for cutting down the amount of CO2 emitted through the production of PC, which is held responsible for 5%e10% of global anthropogenic CO2 emissions (Malhotra, 2000; IPCC, 2004; USGS, 2011). Carbon management initiatives that emphasize the reduction of environmental impact and further deployment of existing and new technologies have focused on the following: (1) reduction of energy consumption by increasing the efficiency of energy conversion and utilization, (2) switching to less carbon-intensive fuels and increasing the use of renewable energy sources that emit little or no CO2, and (3) capturing and sequestering CO2 in the form of stable products. Although the first two of these approaches are generally considered to provide incremental improvements, carbon sequestration technologies have the potential to achieve the lowest emissions (Reichle et al., 1999).

Carbon dioxide sequestration in magnesium-based binders

139

The absorption and storage of CO2 in the form of stable solid carbonates is considered as one of the most promising solutions to alleviate the effects of increasing anthropogenic CO2 concentrations in the atmosphere. A natural form of this process, which involves the dissolution of primary phases and their precipitation as carbonate phases, can take place within Ca- or Mg-based cements. Ideally, the CO2 that is emitted during the calcination of limestone (i.e., CaCO3 / CaO þ CO2) or magnesite (MgCO3 / MgO þ CO2) can react with the hydration products within each system, resulting in the formation of calcium/magnesium carbonates. However, the slow rate of this process under ambient conditions and the risk of carbonation-induced corrosion in concrete samples containing steel reinforcement are considered as the two main drawbacks of carbonation curing. Nearly 15e20 years ago, with the use of carbonation, the work on carbonated RMC-based systems initiated curing in the strength development of porous masonry blocks containing RMC and PC without any reinforcement. Initial findings revealed the rapid strength development of these blocks under accelerated CO2 concentrations, whereas their performance under ambient curing conditions was relatively limited. Although the inclusion of PC provided initial strength by hydration, RMC contributed to strength gain mainly by carbonation. This led to the identification of key parameters that controlled the performance of RMC as a binder, such as its production conditions and hydration and carbonation mechanisms under different conditions, which are explained in detail in Section 7.3.2.

7.3.2

Strength and microstructural development

The reaction mechanisms observed in carbonated RMC systems can be explained by the basic thermodynamic cycle shown in Fig. 7.4. The initial step involves the calcination of MgCO3 to obtain MgO (i.e., the main reactive phase within RMC), which then hydrates and forms Mg(OH)2 (brucite). The hydration of MgO in cement-based mixes reduces pore volume because of the volume expansion associated with the formation of Mg(OH)2. Unlike PC-based systems, the hydrate phase, brucite, has a porous structure and does not make a significant contribution to strength development (Liska, 2009; Unluer, 2012; Li, 2013). However, its subsequent carbonation leads to the formation of a range of hydroxy carbonates, such as nesquehonite (MgCO3$3H2O), artinite (Mg2(CO3)(OH)2$3H2O), hydromagnesite (Mg5(CO3)4(OH)2$4H2O), and dypingite (Mg5(CO3)4(OH)2$5H2O). These HMCs consist of different ratios of MgO, H2O, and CO2 and can be defined with the general formula xMgCO3$yMg(OH)2$zH2O. Their details are listed in Table 7.2, where they are grouped according to the number of Mg ions they contain. Micrographs of typical HMCs frequently observed in the MgOeCO2eH2O system are also shown in Fig. 7.5 (Ferrini et al., 2009; Giammar et al., 2005; Power et al., 2007; Teir et al., 2007a). Carbonation is a critical process for RMC systems as they harden and gain strength via the formation of HMCs. Earlier studies on carbonated Mg(OH)2 mortars indicated strength gain via the formation of hydromagnesite under CO2 curing at atmospheric pressure (Dheilly et al., 1999), whereas the use of CO2 at a pressure of 2 MPa led to the formation of nesquehonite in blocks containing Mg(OH)2 (De Silva et al., 2009).

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Carbon Dioxide Sequestration in Cementitious Construction Materials

4. Thermal decomposition (recycling) MgCO3·3H2O → MgO + CO2+ H2O

3. Carbonation Mg(OH)2 + CO2 + 2H2O → MgCO3·3H2O

1.Calcination (production) MgCO3 → MgO + CO2

2. Hydration MgO + H2O → Mg(OH)2

Figure 7.4 Calcination, hydration, carbonation, and decomposition reactions within RMC systems.

Phases forming in the MgOeCO2eH2O system (Liska, 2009)

Table 7.2

Group

I

II

III

IV

No. of MgO moles

1

2

5

7

No. of H2O moles

No. of CO2 moles

Compound

Chemical formula

1

e

Brucite

Mg(OH)2

e

1

Magnesite

MgCO3

2

1

Barringtonite

MgCO3$2H2O

3

1

Nesquehonite

MgCO3$3H2O

5

1

Lansfordite

MgCO3$5H2O

1.5

1

Pokrovskite

Mg2(CO3)(OH)2$0.5H2O

4

1

Artinite

Mg2(CO3)(OH)2$3H2O

5

4

Hydromagnesite

Mg5(CO3)4(OH)2$4H2O

6

4

Dypingite

Mg5(CO3)4(OH)2$5H2O

6e7

4

Giorgiosite

Mg5(CO3)4(OH)2$56H2O

25

5

Shelkovite

Mg7(CO3)5(OH)4$24H2O

Carbon dioxide sequestration in magnesium-based binders

(a)

141

(b)

50 µm

(c)

(d)

1 µm

2 µm

Figure 7.5 Microscopy images of some of the most common magnesium carbonates (a) magnesite (Giammar et al., 2005), (b) nesquehonite (Ferrini et al., 2009), (c) dypingite (Power et al., 2007) and (d) hydromagnesite (Teir et al., 2009).

The accelerated carbonation or RMC-PC mixes with and without GGBS under elevated CO2 concentrations of 99.9% was shown to result in the formation of nesquehonite as well as CaCO3 (calcite and aragonite) (Mo and Panesar, 2013, 2012; Panesar and Mo, 2013). Although the CO2 concentration used in this study was extremely high, one of the main findings was the influence of Mg on calcite formation, which led to the formation of magnesian calcite, along with nesquehonite. These findings differed from those of Vandeperre et al. (2008a) and Vandeperre & Al-Tabbaa (2007), where the interaction of Mg with calcite was not observed in samples carbonated under lower CO2 concentrations. Other studies have reported the formation of magnesium silicate hydrate (M-S-H) and hydrotalcite (Mg6Al2(CO3)(OH)16$4(H2O)) in the presence of pulverized fly ash (PFA) or GGBS (Vandeperre et al., 2008a; Yi et al., 2014a,b). Strength development of RMC formulations subjected to carbonation is related to (1) the reduction in porosity as the carbonation reaction is an expansive process (i.e., the formation of HMCs increases the solid volume by a factor of 1.8e3.1) that reduces the overall pore volume and (2) microstructural evolution as the morphology and the binding strength of the carbonate crystals contribute to the network structure. The use of accelerated carbonation (i.e., 5%e20% CO2 concentration) in mixes containing 4%e10% RMC as the only cementitious component along with 90%e96% aggregates has been reported to result in compressive strengths 2e3 times higher than

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Carbon Dioxide Sequestration in Cementitious Construction Materials

corresponding PC mixes, mainly because of the formation of HMCs (Liska, 2009; Unluer, 2012; Unluer and Al-Tabbaa, 2014a), as shown in Eqs. (7.6)e(7.9). Mg(OH)2 þ CO2 þ 2H2O / MgCO3$3H2O (Nesquehonite)

(7.6)

2Mg(OH)2 þ CO2 þ 2H2O / Mg2CO3(OH)2$3H2O (Artinite)

(7.7)

5Mg(OH)2 þ 4CO2 / Mg5(CO3)4(OH)2$4H2O (Hydromagnesite)

(7.8)

5Mg(OH)2 þ 4CO2 þ H2O / Mg5(CO3)4(OH)2$5H2O (Dypingite)

(7.9)

The morphology of carbonates, in addition to their amount, determines the final performance of RMC samples. HMCs have microstructures with varying morphologies and add microstructural strength to RMC formulations because of their strong, fibrous, acicular, or otherwise elongated nature via the interlocking effects occurring between the crystals. Fibrous and needlelike crystal growths are more beneficial than rounded or tabular crystals because of the 3D structures formed. Nesquehonite is prismatic and forms starlike clusters, whereas dypingite and hydromagnesite are composed of rounded rosettelike crystals. The elongated morphology of nesquehonite reduces porosity and increases stiffness, raising the solid volume by a factor of 2.3 upon its conversion from brucite. The compact and interlocked networklike structure established by interconnected and well-developed crystals controls the overall performance (De Silva et al., 2006). This is further supported by the bulk “microaggregate” formed through localized carbonation, which also contributes to the creation of a dense matrix compared to PC pastes (Mo and Panesar, 2012). Optimized use of RMC in concrete mixes requires an understanding of the influence of parameters such as mix composition (i.e., RMC content, water/binder ratio, and aggregate type and packing), curing conditions (i.e., CO2 concentration, relative humidity, temperature, and wet-dry cycling), and the use of additives and admixtures on the final performance. Although none of these parameters individually determine the extent of carbonation, their optimum combination can enable complete carbonation and associated strength gain. As the CO3 group is the fundamental building block of carbonate minerals, the availability of CO2 is one of the main factors that play a key role in the formation of strength providing HMCs. In this respect, along with the properties of the binder and aggregate components, the rate of carbonation depends on the overall porosity and the connectivity of the pores, cross-section of the member, and the environmental conditions (Pu and Unluer, 2016; Unluer and Al-Tabbaa, 2014b, 2011, 2012; Unluer, 2014). One of the main applications of carbonated RMC mixes studied in detail is porous masonry blocks. Key advantages of these blocks include their ability to carbonate and sequester CO2, lack of need for in-situ treatment as carbonation can take place during fabrication, and high potential for commercialization, supported by a large volume of research (Vandeperre et al., 2008a,b, 2007, 2006a,b; Liska and Al-Tabbaa, 2009; Unluer and Al-Tabbaa, 2015, 2014b, 2013, 2011; Sonat and Unluer, 2017; Sonat et al., 2017a,b; Ruan and Unluer, 2017a,b,c; Ruan et al., 2017; Pu and Unluer, 2016;

Carbon dioxide sequestration in magnesium-based binders

143

Dung and Unluer, 2016, 2017b,c; Mo and Panesar, 2013, 2012, 2014; Liska, 2009; Unluer, 2012; Li, 2013; Vlasopoulos, 2012; Rheinheimer et al., 2017; Panesar and Mo, 2013; De Silva et al., 2009; Liska et al., 2012a,b, 2006, 2008; Vandeperre and Al-Tabbaa, 2007; Harrison, 2001; Cwirzen and Habermehl-Cwirzen, 2012; Liska and Al-Tabbaa, 2008, 2012). Several studies have investigated hydration (Vandeperre et al., 2008b; Thomas et al., 2014; Rocha et al., 2004; Kuenzel et al., 2018; Kasselouris et al., 1985; Jin and Al-Tabbaa, 2013; Jin et al., 2013) and microstructure (Vandeperre et al., 2008a; Mo and Panesar, 2012) of RMC and RMC-PC systems, some of which contained PFA and GGBS. Much attention has been given to the high CO2 sequestration potential of RMC and the associated strength gain and microstructural development due to the carbonation of mixes with varying compositions of RMC, PC, and/or PFA and/or GGBS (Unluer and Al-Tabbaa, 2014b, 2013; Mo and Panesar, 2013, 2012; Vandeperre and Al-Tabbaa, 2007; Hovelmann et al., 2012; Jia et al., 2004; Chang and Chen, 2006; Orlando et al., 2012; Fernandez Bertos et al., 2004). Recent studies on RMC blocks include durability tests using hydrochloric acid (HCl) and magnesium sulfate (MgSO4) solutions (Liska, 2009; Liska and Al-Tabbaa, 2012). The effect of mix design in RMC, RMC-PC, and RMC-PC-GGBS systems (Ruan and Unluer, 2017a,b; Panesar and Mo, 2013); curing conditions (Dung and Unluer, 2017b; Unluer and Al-Tabbaa, 2014b); aggregate profile (Unluer and Al-Tabbaa, 2012; Unluer, 2014); and use of additives (Unluer and Al-Tabbaa, 2015, 2013; Dung and Unluer, 2016, 2017a; Ruan and Unluer, 2017c) on the rate and extent of carbonation have also been reported. The benefits of RMC are particularly highlighted in systems where it has the potential to carbonate and contribute to strength gain. Studies on the mechanical performance of 20% RMC/PC and 80% PFA mixes indicated an increase in strength with the cement content (Vandeperre et al., 2007). The strengths of mixes cured under ambient CO2 levels increased with the amount of PC, reaching 24, 7, and 1 MPa for those containing only PC, both PC and RMC in equal amounts, and only RMC, respectively. With the introduction of accelerated carbonation at a CO2 concentration of 20% under a relative humidity (RH) of 80%e90%, the 28-day strength of pastes containing only RMC increased from 1 to 6 MPa, which was still less than the 28-day strength of PC pastes cured under ambient conditions. When the pastes were replaced by concrete mixes incorporating 10% cement, mixes containing only RMC outperformed all other mixes with 28-day strength of 18 MPa under accelerated carbonation. This was because of the increased penetration of CO2 through the porosity provided by the imperfect packing of aggregates. Other studies have looked into the effect of water and cement content (Ruan and Unluer, 2017b; Liska and Al-Tabbaa, 2008) and curing conditions (ambient vs. 20% CO2 concentration) (Liska and Al-Tabbaa, 2009) on the strength development of RMC and PC blocks. These studies have also highlighted the differences in the strength gain mechanism of PC and RMC. Although PC blocks outperformed RMC blocks under ambient conditions (2 vs. 12 MPa at 28 days), RMC blocks achieved a strength of 22 MPa after 14 days of accelerated carbonation due to the formation of HMCs. The decomposition of C-S-H to calcium carbonate and silica gel decreased the strength of PC blocks.

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Carbon Dioxide Sequestration in Cementitious Construction Materials

In addition to the water and cement content, other factors such as the CO2 concentration (0.04 vs. 5%), type of aggregates (natural vs. lightweight), and level of saturation (natural water content vs. immersed in water or oven dried) influence the carbonation of RMC blocks (Liska et al., 2008). Increased concentration of CO2, use of natural aggregates, and intermediate levels of saturation were reported to enhance carbonation and increase the strength and modulus of elasticity due to the rapid diffusion of CO2 through the unsaturated pore network. While highly concentrated levels of CO2 are needed for rapid strength gain, increasing the CO2 concentration higher than 10% did not improve the progress of carbonation (Unluer and Al-Tabbaa, 2014b). This confirmed that beyond a certain value, CO2 concentration was not the limiting parameter for achieving complete carbonation. Samples cured under 5% CO2 revealed the required strength for masonry blocks after only 1 day. Lower w/c ratios initially produced higher strengths due to the rapid diffusion of CO2, whereas longer curing required extra water and wet/dry cycling for continuous carbonation. Similar to PC samples, an intermediate level of RH resulted in the highest strengths. These findings indicated that the role water plays in the carbonation process also has to be fully comprehended as the presence of water is required for continuous formation of HMCs, whereas excessive water fills the available pores and slows down CO2 penetration (i.e., the diffusion of CO2 is much slower in water than in air [Haynes, 2010]). Therefore, an intermediate saturation level is required to transform brucite into HMCs as water provides the medium for carbonation, meanwhile maintaining the continuous diffusion of CO2 within the pore system. Another parameter that has shown to induce carbonation is the incorporation of gap-grading in the particle size distribution (PSD) of the overall mix. Gap-grading, introduced through aggregates with a coarser and narrower PSD, can increase the degree of carbonation because of the high porosity and continuous pathway provided for the diffusion of CO2 (Unluer and Al-Tabbaa, 2012). Amongst the different aggregate profiles incorporating granular MgO, MgCO3, and limestone, those containing granular MgO demonstrated improved mechanical performance because of the extra MgO content available for carbonation and the higher overall porosity provided by the gap-grading of monosized MgO granules (Unluer, 2014). The optimization of the aggregate PSD could lead to further reductions in the cement content and CO2 concentration to achieve the strength requirement of 7 MPa for masonry units (Institution BS. BS EN 771e773:2011/A1, 1992). Apart from their mechanical performance, another advantage of carbonated RMC formulations is their improved durability when compared to PC samples under aggressive environments. The resistance of these samples was assessed under HCl and MgSO4 solutions for up to 12 months (Liska and Al-Tabbaa, 2012). The detrimental effect of both solutions on PC blocks was attributed to the formation of ettringite as a result of the expansive reaction of portlandite. Alternatively, RMC blocks indicated improved resistance because of the absence of any reaction between brucite and the test solutions. Accelerated CO2 curing enabled the conversion of brucite into various HMCs, thereby retaining most of the strength. Although this study was a good indicator for the durability of RMC samples, further investigation on the performance of these samples under different durability conditions is needed. These studies led to a

Carbon dioxide sequestration in magnesium-based binders

145

comparison between commercial-scale (400  100e150  215 mm) and laboratoryscale (50 mm diameter, 60e75 mm height) RMC blocks (Liska et al., 2012a,b). Some problems related to the sensitivity to aggregate PSD and water content were encountered during the scaled up production of RMC blocks. Strengths of up to 21 MPa were obtained after 2 weeks of curing under 20% CO2, depending on the aggregate profile. Commercial and laboratory samples had similar densities, compressive strengths, and microstructural properties, underlining the feasibility of producing RMC blocks on a large scale. Similar to PC mixes, the incorporation of environmentally friendly building materials such as PFA and GGBS plays a key role in the development of sustainable RMC formulations. As also observed in RMC systems, the benefits of carbonation in the strength gain of PC-RMC and PC-RMC-GGBS mortars have been demonstrated (Panesar and Mo, 2013). Mixes containing 40% GGBS, 0%e40% RMC, and PC as their cement component were cured under ambient and accelerated carbonation (0.04% vs. 99.9% CO2 at 23  2
C and 98% RH) for up to 56 days. Carbonated samples achieved higher strengths than those subjected to ambient curing. The main carbonation product of samples containing only PC was calcite, with magnesium calcite forming in the presence of RMC. The formation of magnesium calcite increased carbonate agglomeration and resulted in high strengths. The presence of GGBS improved strength development through the increased diffusion of CO2 via the increased porosity and the formation of pozzolanic reaction products in uncarbonated zones.

7.3.3

Recent initiatives on the enhancement of hydration and carbonation of reactive magnesia cement mixes

To fully exploit their benefits, it is necessary to recognize certain chemical and physical properties required for the complete carbonation of RMC systems. As the carbonation reaction is initially diffusion controlled, parameters that contribute to the continuous diffusion of CO2 into the solid matrix and the growth of carbonated material must be fully evaluated. With this goal in mind, the previous studies, whose findings were summarized in Section 7.3.2, established the pathway for the strength gain mechanism of RMC formulations. These studies have demonstrated the advantages of using elevated CO2 concentrations in the curing of RMC samples, which resulted in rapid strength gain by increasing the rate of carbonation and associated formation of HMCs. However, the hydration and carbonation capability of the developed binders have not been fully optimized until now. In terms of hydration, the degree of the conversion of MgO to Mg(OH)2 under ambient conditions is limited to 40%e80% (Unluer and Al-Tabbaa, 2014b; Rocha et al., 2004). This also reduces the subsequent formation of carbonation products and presents an inefficient use of RMC as a binder. The low hydration of MgO, the initial limiting reaction in carbonated RMC systems, is mainly attributed to the incomplete dissolution and precipitation processes, shown in Eqs. (7.10)e(7.13) (Dung and Unluer, 2016, 2017a). Other than the slow dissolution of MgO (Fruhwirth et al., 1985; Vermilyea, 1969), the hydration and carbonation

146

Carbon Dioxide Sequestration in Cementitious Construction Materials

products precipitate on the surface of unreacted MgO/Mg(OH)2 particles, preventing any further contact with H2O/CO2. This mechanism slows down the hydration and carbonation reactions by establishing a physical barrier around unreacted phases, which reduces the degree of hydration/carbonation and results in low strength gain and porous microstructures within carbonated RMC samples. MgO-alkaline oxide plays an electron donator role in water: MgOðsÞ þ H2 OðlÞ / MgðOHÞþ ðsurfaceÞ þ OH ðaqÞ

(7.10)

OH anions are adsorbed on the positively charged surface: MgðOHÞþ ðsurfaceÞ þ OH ðaqÞ / MgOHþ $OH ðsurfaceÞ

(7.11)

OH anions are desorbed from the surface, releasing Mg2þ and OH ions into the solution: MgOHþ $OH ðsurfaceÞ / Mg2þ ðaqÞ þ 2OH ðaqÞ

(7.12)

Ion concentration reaches supersaturation, at which the hydroxide starts to precipitate as brucite on the oxide surface: Mg2þ ðaqÞ þ 2OH ðaqÞ / MgðOHÞ2ðsÞ

(7.13)

This limitation in the hydration of MgO was improved through the introduction of various hydration agents (HAs) into the initial mix design (Dung and Unluer, 2016, 2017a). Some of the most effective HAs reported in literature are HCl, magnesium chloride (MgCl2), and magnesium acetate ((CH3COO)2Mg) (Filippou et al., 1999; Matabola et al., 2010). The presence of HCl increases the Hþ concentration, thereby enhancing the solubility of MgO and the precipitation of Mg(OH)2 away from the original particles (Ruan and Unluer, 2017a,b). This alteration of the hydration mechanism provides further space for the continuation of hydration, which increases Mg(OH)2 formation and improves the subsequent carbonation reaction. The hydration of MgO in the presence of MgCl2 or (CH3COO)2Mg takes place in a similar mechanism, as explained in detail in Dung and Unluer (2016, 2017a). The complex magnesiaacetate ions (CH3COOMgþ) migrate away from their original particles to enable the precipitation of Mg(OH)2 within the bulk solution (Filippou et al., 1999). This mechanism increases the space available for the continuous hydration of MgO and enhances the precipitation of brucite via the presence of additional Mg2þ. The increased formation of brucite in the bulk system increases the amount of material available for carbonation. The addition of HA to RMC-based concrete mix formulations was shown to improve the dissolution of MgO and the precipitation of brucite, which in turn increased the rate and degree of hydration (Dung and Unluer, 2016, 2017a). The use of HA (magnesium acetate at 0.05 M) provided hydration degrees as high as

Carbon dioxide sequestration in magnesium-based binders

147

90% at the end of 14 days, which were almost twice the hydration degree demonstrated by the control sample without any HA. This increase in the hydration degree led to the higher utilization of MgO, which increased the amount of brucite available for carbonation and resulted in the formation of a denser carbonate network composed of a higher amount of HMCs with larger sizes than the control sample. The improvement in the quantity and morphology of HMCs translated into improved mechanical performance, reaching strengths as high as 60 MPa after 28 days of curing under 10% CO2 concentration, which were 107% higher than the corresponding strength of the control sample. In another study (Dung and Unluer, 2017b), the use of high-temperature precuring, which allowed the hydration of samples under 60
C for 1e2 days before they were subjected to carbonation under 10% CO2 and ambient temperatures for up to 28 days, led to an increase in the hydration of RMC samples by 54% after 1 day (Dung and Unluer, 2017b; Filippou et al., 1999; Matabola et al., 2010). The simultaneous introduction of HA (magnesium acetate at 0.05 M) and 1 day of hightemperature precuring under 60
C also resulted in 40% higher 28-day strength in comparison to the control sample (56 vs. 40 MPa) (Dung and Unluer, 2017b). These improvements in strength were associated with the increased dissolution of MgO and the subsequent formation of brucite available for carbonation. Following these improvements in the hydration process, a subsequent study investigated the simultaneous use of HA (magnesium acetate at 0.05 M) and carbonate seeds (up to 1% of cement content) in enhancing carbonation, and therefore the mechanical performance of carbonated RMC concrete mixes (Dung and Unluer, 2018). A comparison of the unseeded and seeded systems illustrated in Figs. 7.6 and 7.7 reveals the approach adopted with the inclusion of seeds within RMC systems. In the absence of seeds, the formation of brucite and HMCs results in the encapsulation of unreacted MgO grains, thereby preventing them from participating in hydration/ carbonation. Seeds can alleviate this problem by enabling the formation of hydrate/carbonate phases on the seed surfaces, away from the MgO grains, thereby increasing access to unreacted particles. As carbonation proceeds, the nuclei of carbonation products not only form on the surface of the MgO/Mg(OH)2 grains but also around the seeds dispersed within the pore space. This dispersed formation of hydration products into the pore space can enhance the carbonation process, leading to an increase in the amount of carbonate phases that fill in the initially available pore space (Dung and Unluer, 2017c). The notable reduction in porosity associated with carbonation, as well as the strong binding network established by the increased formation of HMCs, were shown to significantly improve the mechanical performance of RMC-based formulations including HAs and seeds. The obtained results (Fig. 7.8) indicate the role of (1) HA in improving the dissolution of MgO and increasing the amount of well-dispersed brucite available for carbonation and (2) of seeds in facilitating the nucleation of brucite away from the original MgO particles, extending the amount of contact surface area between Mg-phases and diffused CO2, and acting as microaggregates that filled the pore space (Dung and Unluer, 2018). To fully harvest the benefits of seeding on the performance of RMC formulations, it is crucial to determine the optimum amount of seeds to be introduced into the mix and facilitate their thorough dispersion during

148

(a)

Carbon Dioxide Sequestration in Cementitious Construction Materials

MgO

Grains

(b)

Mg(OH)2(aq,s)

(c)

HMCs

Figure 7.6 Schematic representation of the hydration and carbonation processes in unseeded pastes, showing (a) MgO grains before hydration, (b) hydration product (Mg(OH)2) nucleates on MgO particle surfaces, and (c) carbonation products (HMCs) nucleate on the outer layer of Mg(OH)2 and limit further carbonation (Dung and Unluer, 2017c).

(a)

MgO

Grains

(b)

Mg(OH)2(aq,s)

(c)

HMCs

Seeds

Figure 7.7 Schematic representation of the hydration and carbonation processes in seeded pastes, showing (a) seeds introduced within the pore space to facilitate nucleation on their surfaces, (b) Mg(OH)2 nucleates both on MgO and seed surfaces to enable a higher degree of carbonation and (c) HMCs form both on Mg(OH)2 and seed surfaces, thereby filling up the pore space and densifying the microstructure (Dung and Unluer, 2017c).

the mixing process. Samples containing up to 1% seeds achieved strengths of 70 MPa, which were 50% higher than the control sample. These improvements translated into an increase in the carbonation degree by up to 96% and the densification of the microstructure via the formation of a strong network composed of large and firmly bonded carbonate (nesquehonite) crystals with improved morphologies (Dung and Unluer, 2017c, 2018). Apart from these improvements in the carbonation of RMC-based concrete samples, a recent study (Kuenzel et al., 2018) also looked into the effects of the presence of hydromagnesite on the hydration of RMC. Although the hydration of RMC by itself led to weak pastes, RMC-hydromagnesite blends revealed pastes with short setting times and higher compressive strengths. This increase in strength in the presence of hydromagnesite was attributed to the potential formation of an amorphous phase,

Carbon dioxide sequestration in magnesium-based binders

149

Compressive strength (MPa)

70 60 50 40 30 Control sample Sample with seeds

20

Sample with magnesium acetate Sample with seeds & magnesium acetate

10 0

7

14 Age (days)

21

28

Figure 7.8 Strength and microstructural development of carbonated RMC samples with the introduction of hydration agent and seeds (Dung and Unluer, 2018).

whose composition could be resembling that of brucite or other HMC phases, whereas thermodynamic calculations suggested the formation of artinite, which was not observed in the analyses performed. Collectively, the findings of these studies have indicated advancements in the reaction mechanisms associated with the use of additives (HAs and seeds) and adjustment of the curing conditions (temperature and CO2 concentration). The potential of carbonated RMC formulations in achieving high strengths based on the degree of hydration/carbonation and the properties of the resulting phases have been demonstrated. The carbonation process was also enhanced by improving the preceding hydration, which is usually the limiting step in RMC mixes. The obtained results highlighted the ability of RMC to hydrate/carbonate and gain strength within a few days, while demonstrating significant mechanical performance when the right conditions were provided. These advancements can improve the cost effectiveness of RMC systems by increasing the CO2 sequestration capability of RMC and facilitating high strength development at early ages, which will enable higher utilization rates of RMC and shorten the required curing period.

7.3.4

Stability under high temperatures

The composition and properties of HMCs forming in carbonated RMC systems depend on the reactivity and impurities present within RMC, along with the conditions they are exposed to. Theoretically, the formation and stability patterns of the most

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Carbon Dioxide Sequestration in Cementitious Construction Materials

commonly observed HMCs follow the transformation pathway shown in Eq. (7.14). An increase in temperature can lead to the transformation of carbonates into those that are less hydrated, whereas variations in the CO2 concentration can also result in the formation of different phases (Xiong and Lord, 2008). Other parameters, such as water activity and pH, also influence the formation of different HMCs. The transformation pathway of HMCs under different curing conditions and its effect on the overall performance of RMC mixes play a significant role in comprehending the stability and determining suitable end products using RMC in the long term. Nesquehonite / Dypingite / Hydromagnesite / Magnesite

(7.14)

Previous studies (Ferrini et al., 2009; Xiong and Lord, 2008; K€onigsberger et al., 1999; Marini, 2007; H€anchen et al., 2008; Fernandez et al., 2000; Hopkinson et al., 2012, 2008; Botha and Strydom, 2001; Ming and Franklin, 1985; Swanson et al., 2014; Jauffret et al., 2015; Hales et al., 2008; Dong et al., 2008; Davies and Bubela, 1973; Robie and Hemingway, 1972; Chaka and Felmy, 2014; Ballirano et al., 2010) have reported the effects of temperature and CO2 concentration on the formation, stability, and transformation of various Mg-carbonates, albeit not in the cement context. The formation of lansfordite and artinite was observed in ambient temperatures and CO2 concentrations (K€ onigsberger et al., 1999; Ming and Franklin, 1985), whereas others (Marini, 2007) reported the formation of hydromagnesite under similar conditions as the formation of artinite was suggested to require less CO2 than the amount present at ambient levels (8e40  104 vs. 0.04%). Apart from ambient conditions, the formation of hydromagnesite was also observed under elevated temperatures (>40
C) (Xiong and Lord, 2008; K€onigsberger et al., 1999; Marini, 2007), preceding the formation of magnesite at around 120
C (H€anchen et al., 2008; Botha and Strydom, 2001). Increasing the CO2 concentration at ambient temperatures is known to lead to the formation of nesquehonite (Xiong and Lord, 2008; K€onigsberger et al., 1999; Davies and Bubela, 1973). Different CO2 levels have been suggested regarding the boundary above which the formation of nesquehonite commences (Xiong and Lord, 2008; K€ onigsberger et al., 1999). Lansfordite was also reported to form under similar conditions but at lower temperatures and CO2 concentrations than nesquehonite (Ming and Franklin, 1985). The stability of nesquehonite, which is governed by not only temperature but also the water activity and CO2 concentration, was reported by several studies (Ming and Franklin, 1985; Jauffret et al., 2015; Hales et al., 2008; Dong et al., 2008; Hopkinson et al., 2008; Davies and Bubela, 1973; Dell and Weller, 1959; Langmuir, 1965; Lanas and Alvarez, 2004). Nesquehonite is stable under natural conditions and is synthesized for different processes including CO2 sequestration, where it was shown that its structure can remain substantially unaffected at up to 100
C (Ferrini et al., 2009; Ballirano et al., 2010). Alternatively, some studies have claimed that nesquehonite is metastable and can decompose into more stable carbonates, such as hydromagnesite, at temperatures above w50
C (Hopkinson et al., 2012; Swanson et al., 2014; Jauffret et al., 2015; Hales et al., 2008; Dong et al., 2008; Hopkinson et al., 2008; Robie and Hemingway, 1972).

Carbon dioxide sequestration in magnesium-based binders

151

It is noteworthy to mention that the stability of nesquehonite, along with other carbonate phases within the MgOeCO2eH2O system depend on the analytic conditions under which they are investigated, as well as the synthesis process of each phase. For this reason, their behavior and the effect of this on the long-term performance of carbonated RMC systems cannot be predicted by solely referring to these earlier studies that were not in the cement context. Although it is known that the performance of carbonated RMC samples is dependent on the formation and stability of HMCs, the changes in the properties of HMCs under different conditions and their effect on the overall sample performance have not been studied in detail until now. The only preliminary study on this was performed by Liska (2009), who assessed the changes in the properties of carbonated RMC blocks subjected to a range of temperatures up to 80
C. The results presented in this study concluded that an increase in the amount of dypingite/hydromagnesite was accompanied by a decrease in nesquehonite under increased temperatures. These changes were attributed to the potential transformation of nesquehonite into dypingite/hydromagnesite. A decrease in the quantity of brucite with increased temperature was also observed, which was due to its carbonation via its reaction with the CO2 released during the decomposition of nesquehonite, as shown in Eq. (7.15). In terms of performance, the compressive strength results measured over 196 days at elevated temperatures indicated high levels of variation. The negative effect of drying on strength development was highlighted, which could explain the high variability observed. Despite the reduction in solid volume, the transformation of nesquehonite into other HMCs still enabled the maintenance of the required strength level for masonry blocks. 5MgCO3$3H2O / Mg5(CO3)4(OH)2$5H2O þ CO2 þ 9H2O

(7.15)

Whereas the contribution of nesquehonite to strength gain has been mentioned by previous studies (Dung and Unluer, 2016, 2017b, 2018; Unluer and Al-Tabbaa, 2014b), other HMCs, such as dypingite, hydromagnesite, and artinite, are also known to provide strength in carbonated RMC systems. The development of strength in RMC samples is dependent not only on the degree of carbonation but also on the morphology and binding strength provided by various HMCs, which enable the formation of an interconnected microstructural network. The coexistence of different HMCs has been demonstrated by previous studies that focused on both laboratory- and commercial-scale RMC blocks (Liska and Al-Tabbaa, 2009; Liska et al., 2012a,b), indicating that strength development is not merely dependent on a single carbonate phase but a combination of various phases that lead to a dense carbonate network. Understanding the long-term properties of HMCs and their influence on mechanical performance plays a significant role in comprehending the stability of the developed formulations and determining suitable end products using RMC as the main binder. In line with this need, further research is currently focusing on investigating the changes in the strength and microstructural development of carbonated RMC samples under different exposure conditions. These studies are investigating the changes in the morphology, volume, and crystal composition of HMCs that form within carbonated RMC samples. These findings will identify the stability and transformation of hydrate

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Carbon Dioxide Sequestration in Cementitious Construction Materials

and carbonate phases of RMC under elevated temperatures in different environments and reveal their effect on the overall performance of carbonated RMC samples, thereby enabling the industry to identify possible uses for RMC in line with its stability under different conditions.

7.3.5

Environmental impact

As in other novel binders that are proposed as a more sustainable alternative to currently used materials, it is crucial to determine the environmental impact of RMC in comparison to existing building materials, such as PC, for a comprehensive evaluation of its suitability within the construction industry. From a sustainability perspective, some of the main advantages of RMC are its lower production temperatures than PC (800 vs. 1450
C), ability to fully carbonate and gain strength accordingly, and recyclability at the end of use, which is discussed further in Section 7.3.6. The main production method of RMC is via the dry-route that involves the calcination of magnesite. Similar to the production of PC from limestone, magnesite is heated to produce RMC, which results in CO2 emissions. Calcination of magnesite to form RMC occurs at temperatures between 550 and 800
C and is an endothermic reaction, with a total energy requirement of w2415 kJ/kg (Shand, 2006). The exact calcination temperature depends on several conditions such as the presence of impurities and the physical properties of the parent material. Higher calcination temperatures may be necessary as these parameters become more prominent. Nevertheless, the lower calcination temperatures used in the production of RMC when compared to PC can enable the use of alternative fuels with low heating values. A recent study (Hassan, 2014) reported that the theoretical energy demand of RMC production via the dry-route (5.9 GJ/tonne of RMC) was larger than the production of PC from calcite (4.5 GJ/tonne of PC). The higher energy demand of RMC production was linked with (1) the higher total energy required for the acquisition of magnesite (0.06 vs. 0.04 GJ/tonne required for limestone), (2) the higher quantity of raw materials needed for RMC production (2.08 vs. 1.52 tonne for PC), and (3) the higher work index value of magnesite than that of limestone (16.3 vs. 12.3 kWh/t). Therefore although the calcination temperature of magnesite is lower, the energy required for the production of RMC is generally higher than PC. Similarly, the total amount of CO2 emitted during this process was reported as 1.7 tonne/tonne (t/t) of RMC (1.1 t/t from the calcination of magnesite and 0.6 t/t from fuel combustion), whereas a corresponding value of 1 t/t (0.67 t/t from the calcination of calcite and 0.33 t/t from fuel combustion) is emitted during PC production. However, several studies have shown that the CO2 emitted during the calcination of magnesite can be fully sequestered during the curing process of RMC-based mixes, which gain strength via carbonation (Liska and Al-Tabbaa, 2009; Unluer and Al-Tabbaa, 2014b, 2013). If sourced sustainably, this can bring down the final net CO2 emissions of RMC to 0.5e0.6 t/t. Another comprehensive life-cycle assessment (LCA) study on MgO production was presented by Li et al. (2015), whose main focus was the production of fused MgO, during which reactive MgO was only mentioned as an intermediate product. Nevertheless, this study provided an inventory with data obtained from actual

Carbon dioxide sequestration in magnesium-based binders

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production plants, which was used in the assessment of the environmental implications of RMC production via the calcination of magnesite in a recent study (Ruan and Unluer, 2016). The comparative LCA of RMC and PC production highlighted the benefits and drawbacks of the production of each binder. The obtained results showed that the production of RMC via the calcination of magnesite outperformed PC production in mitigating a majority of the environment impacts imposed on the ecosystem and resources. Production of RMC was reported to result in the consumption of a larger fossil energy because of the large quantity of coal used, whereas it required the utilization of a single raw material as opposed to the several different raw materials used in PC production. The decomposition of magnesite released a higher amount of CO2 than that of limestone used in PC production, leading to a higher climate change score for RMC in spite of the lower production temperatures. However, when the high carbonation capability of RMC within concrete mixes was considered, the net CO2 emissions of RMC was calculated to be up to 73% (0.71 t/t) lower than those of PC (Ruan and Unluer, 2016). The main disadvantage of RMC production when compared to PC was identified as its effect on human health, which was attributed to the large quantity of coal used in the production of RMC. This problem could be addressed through the replacement of coal with more sustainable fuel sources and the introduction of up-to-date kiln technologies that have already been optimized for PC production. Recycling of the emitted gases, waste heat, and CO2 during the production of RMC, which is already available in a number of RMC production plants in China, could also be incorporated to reduce the overall environmental impacts. Another improvement implemented in some plants could be the collection and sorting of dust generated during RMC production in the dust precipitator. This could be further enhanced with the recycling of the lowgrade magnesite tailings and the residual materials collected at the end of the production process into useful marketable products instead of their direct disposal (Pan et al., 2002; Yue et al., 2011). A technical point that was highlighted in this study was the lack of regional and local inventories on cement production, which led to the use of inventories obtained from different countries involving data based on national averages for comparison purposes. This presented a challenge for local decision makers as the regional variations in key factors such as fuel types, material properties, and technologies make it difficult to come up with a final conclusion that can be applied to different regions. For this reason, a more comprehensive global database presenting detailed information on the main parameters that control the production and final properties of RMC can facilitate better-informed LCA studies. In a further study (Ruan and Unluer, 2017a), the environmental impact of RMCbased concrete samples were analyzed and compared with that of RMC-PFA, RMC-GGBS, and PC samples. The use of SCMs (PFA and GGBS) within RMC formulations at up to 50% replacement rate of RMC enabled a reduction of the environmental impacts by revealing lower environmental loads than PC samples in most impact categories. The introduction of PFA and GGBS in RMC formulations decreased the CO2-eq emissions during the entire life cycle of the samples. Compared to PC samples, RMC, RMC-PFA, and RMC-GGBS samples led to up to 68% reduction in CO2-eq emissions. These reductions were more pronounced with an increase in

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Comparison of energy consumption during Portland cement (PC) production and extraction of MgO from seawater (Hassan, 2014)

Table 7.3

PC

RMC (MgO) Energy (GJ/tonne of PC)

Process

Energy (GJ/tonne of RMC)

Mining and crushing

0.04

Extraction

1.30

Pretreatment

0.06

Pretreatment

5  104

Calcination

4.69

Precipitation of Mg(OH)2

2.80

Finish grinding

0.08

Calcination

4.50

Total

4.87

Total

8.60

Process

the PFA and GGBS contents that replaced RMC, associated with the low embodied CO2 emissions of these SCMs, along with the ability of some of their phases to carbonate. When compared with PFA, the use of GGBS within RMC samples led to a lower climate change impact because of the higher CO2 sequestration potential enabled by their higher CaO contents. Other than the dry-route, another viable option for the production of RMC is its extraction from reject brine or seawater (wet-route). A comparison of the production of RMC through this route with the production of PC in terms of the energy required is provided in Table 7.3. The total energy required for the production of one tonne of RMC is around 8.6 GJ, which is higher than the energy required for an equivalent amount of PC (Hassan, 2014). A majority of this energy is associated with the precipitation of the Mg(OH)2 in solid form and its calcination for the production of RMC in the final step. This is mainly because Mg(OH)2 precipitates contain a lot of free water and the dewatering of Mg(OH)2 through a physical approach such as filtering is very difficult and not effective. Therefore, considerable amounts of energy is used to first dehydrate the Mg(OH)2 precipitates via calcination (Shand, 2006). The energy required during this process can be reduced if a locally sourced reject brine obtained from the waste stream of desalination plants is used instead of seawater to avoid the need for extraction (Dong et al., 2017, 2018). Although promising from a technical perspective, further advancements in reducing the energy required in each step of this process are needed for this route to be more environmentally friendly than existing PC production.

7.3.6

Recycling of carbonated samples

The carbonate phases that form in RMC systems can decompose back into MgO/RMC when subjected to elevated temperatures, as shown in the thermodynamic cycle in

Carbon dioxide sequestration in magnesium-based binders

O 2 +H

on

ly

1. Mixing

Hydration: MgO + H2O → Mg(OH)2

5. Recycling

End product

Calcination: MgCO3·3H2O → MgO + CO2 + 3H2O

155

2. Casting

Furnace

3. Curing

Carbonation: Mg(OH)2 + CO2 + 2H2O → MgCO3·3H2O

4. Testing

Figure 7.9 The closed-loop recycling process of carbonated RMC samples.

Fig. 7.4. Similar to the production of RMC from magnesite through the dry-route, the calcination of HMCs that form during the carbonation of RMC samples enables their decomposition into the starting material, MgO. This can process (Fig. 7.9), then be repeated via the hydration and carbonation of RMC in new mixes, thereby facilitating the recycling and reuse of old samples in the preparation of new samples. Through this approach, RMC-based carbonated samples can be fully recycled and reused at the end of their service life by being subjected to thermal reprocessing that can facilitate the conversion of the carbonate phases back into the starting material. The complete recycling and reuse of RMC samples presents not only environmental benefits because of the elimination of the extraction of virgin raw materials and need for landfill area for storing demolished concrete but also economic advantages associated with the reuse of materials within a circular economy. Results from a preliminary study (Sonat et al., 2017a) on the influence of the two most important factors, calcination temperature (700e1100
C) and residence time (2e6 h), on the mechanical properties of recycled RMC samples indicated the direct relationship between calcination conditions and performance of the recycled samples. Calcination of samples at 800e900
C for 2 h led to the highest strength results because of the complete decomposition of carbonate phases, while maintaining a sufficient level of reactivity for RMC. Although the recycled samples demonstrated 15%e20% lower strengths than the original samples, this was attributed to the changes in the reactivity of RMC, aggregate properties, and aggregate-cement paste interface. Robustness of the recycling process was also demonstrated via five repetitive calcination cycles applied on the same batch. Successful separation of aggregates from the surrounding cement paste before the recycling process could resolve issues associated with performance and lower the total energy requirement by only subjecting the cement paste to calcination.

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Carbon Dioxide Sequestration in Cementitious Construction Materials

These findings clearly indicated the feasibility of the recycling and reuse of carbonated RMC samples in the preparation of new samples within a closed system, without necessitating the use of any additional natural resources. The main issues identified during this recycling process included the changes in the reactivity of RMC and aggregate properties that were altered due to the elevated temperatures used during calcination. The weakening and disintegration of aggregates were observed under high temperatures, which was accompanied with a thin layer of residual paste on their exteriors. The formation of this layer weakened the aggregate-paste interface and resulted in a higher water absorption, higher porosity, and lower specific gravity (Evangelista and de Brito, 2010; Padmini et al., 2009; Katz, 2003). These changes caused a decline in the performance of the recycled samples when compared to the initial mix (Tabsh and Abdelfatah, 2009; Evangelista and De Brito, 2007). Some potential solutions for maintaining sample performance after the recycling process could involve the separation of the aggregates from the paste before recycling (Sonat et al., 2017a), removal of the adhered mortar via grinding, and presoaking (Shi et al., 2015) or its strengthening via various methods such as carbonation (Shi et al., 2015; Zhang et al., 2015a,b). In addition to the mechanical properties, the environmental impact of the recycling process plays a key role in its successful implementation on a large scale. The main components that must be considered during this process are the energy and CO2 emissions associated with (1) each material used within the mix design, (2) calcination process, and (3) capture and sequestration of CO2 during the curing process. Even if CO2 is obtained from flue gases emitted by power plants, it may need to be separated from other gases and purified for an efficient carbonation process. These may be accompanied with the emissions associated with the separation of the aggregates from the paste, which will also result in energy use and CO2 emissions. If the CO2 emitted during the production of RMC is completely sequestered via the carbonation of RMC samples during curing (Ruan and Unluer, 2016; Liska and Al-Tabbaa, 2009; Dung and Unluer, 2016; Unluer and Al-Tabbaa, 2014b, 2013) and the coarse aggregates are separated from the binder before recycling takes place, the recycling and reuse of RMC samples can be considered as a sustainable alternative to existing practices. Furthermore, considering the additional benefit of the complete elimination of raw material (e.g., magnesite) extraction, the recycling process can present an advantage from an environmental standpoint, while maintaining a comparable performance with the unrecycled samples. Further investigation on the recycling and reuse of RMC samples need to be performed to determine the optimum conditions for the calcination of carbonate phases, as well as the curing conditions. Separation of aggregates from the binder phase before recycling seems to be the key factor in achieving a balance between the environmental impacts and performance and verifying the feasibility of this process from an environmental standpoint. Nevertheless, the ability of RMC systems to provide a medium for the permanent sequestration of CO2 while enabling strength development presents an advantage amongst many other processes that generate CO2. The reuse of CO2 in the development of sustainable construction products enables its safe storage within thermodynamically stable systems, adding further value to the same volume of the raw

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157

materials initially entering the manufacturing process. This process, in theory, can be repeated infinitely within the same batch, which can contribute to sustainable construction agendas (Sonat et al., 2017a).

7.3.7

Magnesium silicate cements

In addition to the production of MgO from magnesite (dry-route) and its extraction from brine or seawater (wet-route) discussed in detail in earlier sections, magnesium silicates, such as olivine and serpentine, can also serve as a suitable source for MgO (Flatt et al., 2012; Lackner, 2003; Schneider et al., 2011). What makes magnesium silicates attractive is their abundant presence worldwide, with reserves of over 10,000 billion tonnes. These resources are extractable via open pit surface mining, similar to limestone in terms of methodology and cost. A low-carbon binder containing RMC, pozzolans, and HMCs was recently developed by relying on the production of RMC from magnesium silicates (Novacem Limited, 2010; Vlasopoulos and Bernebeu, 2014). The proposed process took place in three stages: (1) carbonation of magnesium silicates under elevated temperature and pressure (170
C and 15 MPa), producing magnesium carbonate and SiO2, as shown in Eqs. (7.16) and (7.17); (2) calcination of magnesium carbonate at 700
C to produce RMC, during which the CO2 generated was recycled back in the initial step; and (3) use of RMC to produce HMCs. During this process, the hydration of RMC was modified by the addition of HMCs produced by specialized reactor technology. These HMCs were considered as high CO2 sinks as they contain 300e500 kg CO2 per metric tonne of carbonate. Their formation was shown to enable high strength development along with lower CO2 emissions (50e100 kg CO2 per tonne of cement) and reduced energy requirements, claimed to be equivalent to 60%e90% of the energy required by PC. The low temperatures used during this process could also enable the use of fuels with low energy content or carbon intensity (Stuart and Nikolaos, 2010). Mg2SiO4 þ 2CO2 / 2MgCO3 þ SiO2

(7.16)

Mg3Si2O5(OH)4 þ 3CO2 / 3MgCO3 þ 2SiO2 þ 2H2O

(7.17)

Differing from the slow carbonation of magnesium silicates in nature, the proposed approach accelerated this process through the use of the high pressure reaction of supercritical CO2 and finely ground magnesium silicate rocks. In any similar sequestration scenario, the need to purify CO2 obtained from flue gases before it is pressurized for its reaction with magnesium silicates could reduce the overall efficiency of the process. However, the suggested approach made use of the concentrated CO2 produced from the calcination of magnesite, eliminating the need to concentrate CO2 before its reaction with magnesium silicates. The development of this binder was preceded by the attempts to store CO2 in the form of thermodynamically stable and environmentally inert solid carbonates of magnesium and calcium under different schemes in Finland since the mid-2000s (Teir et al., 2007a,b,c, 2009; Zevenhoven and Teir, 2004; Teir, 2008). Although this approach did not require the separation of magnesite and silica, this could potentially present a challenge in the cement context

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Carbon Dioxide Sequestration in Cementitious Construction Materials

as the magnesite produced at the end of the first step was then calcined to produce RMC, necessitating a minimum level of impurities. The presence of any residual silica during this process could result in the formation of magnesium silicates, reversing the intended reaction sequence. Although the global reserves of magnesium silicate minerals are more than adequate to capture all the CO2 emitted because of human activities, certain issues need to be addressed for the application of these binders on a commercial scale. The grinding of olivine to the high fineness needed to achieve high levels of reactivity requires high amounts of energy as it has a Mohs hardness of 6.5e7 (Gartner and Hirao, 2015). Alternatively, the hydration of olivine in nature results in the formation of serpentine, with a lower hardness of 3e5 (King, 2012). Although the theoretically low CO2 emissions offered by these binders obtained from magnesium silicates can potentially present an effective method for the reduction of CO2 within the cement industry, limited information is currently provided on their production parameters, details of the exact mix designs, and their performance in the long term. Further research is needed to comprehend the reaction mechanisms and the properties of these binders, increase the efficiency of the production cycle, and identify solutions to reduce the primary energy requirement per tonne of cement produced.

7.4

Future trends and recommendations

To enable its successful implementation, it is important to acknowledge the major limitations associated with the use of RMC as a binder and provide recommendations to turn existing limitations into potential advantages. In addition to identifying the main benefits and drawbacks of RMC, this section aims to highlight the current gaps in the research performed so far, which will play a key role in the development of RMC formulations on a commercial scale. The background, characterization, and performance of different grades of MgO in a range of applications are summarized in Table 7.4. The most well-known use of MgO within the construction industry is as an expansive additive in dam construction and applications such as MgO boards and insulation materials using MOC, MOS, and MAP cements. Compared to the vast amount of PC produced, the volume of MgO used is relatively low as these are fairly niche applications. Although extensive research has been performed on carbonated RMC mixes, the use of RMC in commercial applications including masonry blocks has been limited. To achieve a significant impact, the industry must be presented with clear and compelling advantages of using RMC, which will present a sufficient driver for increased RMC production, making it feasible on a commercial scale. Advances in the production process and the introduction of carbon capture and storage through the carbonation of RMC obtained from vast resources (reject brine or magnesium silicates) can produce by-products in appropriate quantities and costs to make low-carbon RMC commercially viable. The main production route for RMC is currently through the calcination of magnesite. Although magnesite resources are widely available, the majority of deposits are located in China and North Korea. This not only limits global access but also keeps

Different uses, compositions, characteristics, and applications of MgO-based cementitious products

Table 7.4

Characteristics

Main applications

Magnesium oxychloride (MOC) cements

MgO and magnesium chloride

High fire resistance, low thermal conductivity, resistance to abrasion, high transverse and crushing strengths, resistance to static charge accumulation, insecticidal property, outstanding appearance

Boards, rendering wall insulation panels, flooring and decking, fire protective coatings, grinding and polishing stones

Magnesium oxysulfate (MOS) cements

MgO and magnesium sulfate

High fire resistance, low thermal conductivity, high dimensional stability, compatibility with steel reinforcement

Lightweight insulation panels in floors, walls, and roof insulations

Magnesium phosphate (MAP) cements

MgO (hard- or dead-burned) and phosphoric acid

High early strength, high water resistance, good adhesive properties, affinity for cellulose, antibacterial properties

Industrial flooring, fire protection, insulation panels, rapid repair of deteriorated roads, bridge decks, highways, tunnels, pavements, airport runways, industrial floors, solidification and stabilization of radioactive wastes, clinical applications

MgO-based expansive additives (MEA)

MgO (hardburned)

Prevention of concrete thermal cracking, reduction of costs associated with temperature control measures, speeding up of construction

Expansive additive for the compensation of thermal shrinkage of mass concrete used in dam constructions

Carbonated magnesium cements

MgO (lightburned)

Ability to fully carbonate and gain strength, recyclability at the end of use

Potential for use in cement-based applications

Magnesium silicate cements

MgO (lightburned), pozzolans and HMCs

Large reserves of raw materials (magnesium silicates), recycling of CO2 generated during production, high final strength

Potential for use in cement-based applications

159

Composition

Carbon dioxide sequestration in magnesium-based binders

Binder

160

Carbon Dioxide Sequestration in Cementitious Construction Materials

the price of RMC relatively high. One possible solution is to investigate alternative production processes. The relatively low availability of RMC can drive its sustainable production from magnesium silicates, which have abundant resources worldwide and can be easily extracted in a similar fashion to limestone. The environmental impacts of RMC production can be alleviated via its derivation from magnesium silicates and through the use of the carbonation potential of RMC in a range of construction products. Another suitable material for RMC production is waste brine obtained from desalination plants, which are likely to become more common in the future because of the rising demands of the growing world population on clean water supplies. The highly saline effluent from the desalination process has no economic value, but has adverse effects on the marine ecosystem as it is currently discharged back into the sea. This process disturbs the local water and sediment by introducing a multicomponent waste and increases the temperature, also endangering the marine organisms because of the residual chemicals mixed into the brine from the pretreatment process. Production of RMC from reject brine can provide a new application for this otherwise harmful waste effluent, which represents a potential raw material for the development of a range of construction materials. In this respect, the relatively limited use of RMC by the industry so far presents an opportunity for further innovation and impact. A well-planned research agenda that is coordinated with industry needs has the potential to profoundly influence current practice. Such an outcome will not only enable pioneering research in sustainable construction materials but also have global impact in terms of reduced environmental impact. Another barrier for the increased use of RMC is the insufficient documentation of applications and lack of reliable data compared to the high validation and market confidence in PC. The large and localized volume expansion and associated cracking caused by the late hydration of dead-burned MgO in PC limits MgO content to 5%e6%. However, unlike dead-burned MgO, RMC hydrates at a similar rate to PC, thereby eliminating these problems. This distinction between different MgO reactivities should be reflected in the codes of practice and national construction regulations in line with the extensive research performed in this area. The incorporation of RMC as a binder requires the revision of the current standards to induce flexibility and a performance-based approach within the industry. Similar to MgO boards, which compete with traditional gypsum-based drywalls in several countries because of the advantages they present in terms of fire safety, durability, mold and bacterial resistance, and humidity control, other applications involving RMC can gain acceptance because of the improvements they offer. As the contribution of RMC to the overall performance of concrete mixes is recognized, the industry will inevitably acknowledge their use in the future. To enable a more rapid progress towards achieving a sustainable built environment, all the parties involved in the construction industry need to be educated towards the application of these binders. The development of nontraditional binders on an industrial scale depends on the establishment of globally binding agreements on limiting the CO2 emissions associated with cement production and concrete usage. Because governments are one of the largest consumers of cement-based materials in line with their increasing infrastructure investments, their engagement in raising awareness and forming a strong coordination

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between all the parties involved in the built environment is imperative for the successful implementation of novel materials within the construction industry on a global level (Scrivener et al., 2016). The research performed on RMC blocks so far has successfully demonstrated their ability to achieve a dense microstructure, gain strength, and provide improved durability when subjected to the right curing conditions. Although the capability of RMC to carbonate and gain strength accordingly is well-established, certain challenges exist in achieving complete carbonation. One of the main issues to be resolved is the constraints in the progress of hydration and subsequent carbonation associated with the formation of a passivating layer from the completed hydration/carbonation reaction. This layer creates a physical barrier that inhibits further reaction. However, this limitation can be diminished via the adjustment of calcination parameters to produce RMC with a higher reactivity. Other solutions include the introduction of certain additives that enhance the hydration process, use of elevated reaction temperatures to increase the rate of the hydration and carbonation reactions, and the morphologic reconstruction of the brucite structure through its dehydroxylation. These improvements can lead to the formation of products with high surface areas, which can provide enhanced reactivity via the significantly increased gas-solid contact areas. Although they can develop some strength through hydration, RMC-based systems mainly rely on carbonation for a majority of their strength development. Unless used as an additive or a partial replacement of PC, the most immediate application of RMC in the cement industry can be through their use in the construction of precast elements, such as blocks, bricks, tiles, or other unreinforced units, the curing of which can be monitored in a factory environment. Although RMC blocks present a high potential for the sequestration of CO2 in concrete mixes, they rely on relatively high concentrations of CO2 (5%) to gain strength in a short time period that is compatible with the current market expectations. This necessitates the availability of a nearby CO2 source (e.g., cement plants or power stations using fossil fuels) to enable fast strength gain through the formation of carbonate phases under a relatively accelerated CO2 curing. The proximity of the production facility to a point-source CO2 emitter will also enhance the overall sustainability of the production process, while providing rapid strength gain without a dramatic increase in cost. Studies performed so far have shown the ability of RMC to fully carbonate within concrete block applications, in which CO2 from flue gases emitted by power plants can be permanently sequestered in the form of stable carbonation products. Alternatively, the carbonation reaction can be accelerated by increasing the CO2 pressure or the curing temperature, which is not only costly but also requires a higher energy. Changes in the mix design that allow for an interconnected pore structure can also facilitate faster diffusion of CO2 without increasing the overall cost or energy demand. Further reductions in CO2 emissions can be achieved through the introduction of high amounts of wastes owing to the low solubility of brucite, and supplementary cementitious materials such as PFA and GGBS, provided that a comparable performance as those of PC samples can be maintained. Whereas additional long-term research is needed to evaluate the carbonation of RMC under natural atmospheric exposure conditions, integration of these precast products into the current construction market can be relatively

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Carbon Dioxide Sequestration in Cementitious Construction Materials

straightforward. These units can be produced in any normal block manufacturing or precast plant, some of which are equipped with elevated CO2 curing chambers conventionally used to compensate for the early age drying shrinkage of PC-based blocks. The hydration and carbonation of RMC systems involves very complex stages comprising several phases and secondary products. The formation of a range of carbonation products were reported by several studies investigating the carbonation of RMC formulations. Until now, no studies have been performed on the stability and transformation of these HMCs within cement-based applications. Although this is believed to be due to the partial hydration of RMC and the presence of impurities, the reason for the occurrence of the different phases needs to be investigated and fully understood to acquire a comprehensive knowledge about the exact conditions that give rise to the individual carbonation components. This should include an investigation of stability, durability, and long-term performance of the prepared samples. Fully comprehending the carbonation mechanism of RMC systems can also facilitate their use in further applications currently involving PC. The properties of concrete are subsequently governed by the processes taking place at the molecular level. Thus, the durability and behavior of RMC samples during their service life strongly depends on their atomic structure. Further research must be performed to provide new knowledge on the otherwise poorly known molecular-scale mechanisms that operate during the formation, transformation, and stability of HMCs through the hydration and carbonation reactions. To facilitate the full understanding of the carbonation process, it is necessary to comprehend the factors that influence the properties of HMCs at nanometric and atomic levels. This can be achieved by investigating the hydration and carbonation processes and the changes in the pore structure through a multiscale approach, which will involve an in-depth study from the atomic level. These studies must be linked with the mechanical behaviors of RMC mixes, finally reaching the macrostructure of concrete. These findings will shed a light on the formation of different chemical phases and the conditions that lead to each phase at a molecular level. Understanding and controlling the parameters that affect the formation of these strength-providing materials will enable the adjustment of their properties according to the applications at hand. Overall, these outcomes can form a pathway for the development of a range of construction products with optimized sustainability potentials through the incorporation of RMC.

7.5

Conclusions

This chapter reviewed the production, properties, and applications of cementitious products containing MgO, with the goal of coalescing existing knowledge and providing improved understanding on the known capabilities and limitations. The main applications of MgO in construction materials are MOC cements used in fireresistant boards, MOS cements used in lightweight insulating panels, MAP cements used in fire protection and rapid repair materials, and MgO expansive additives that compensate for concrete shrinkage, particularly in dam construction. Other than these applications, the use of reactive MgO cement (RMC) in carbonated systems was

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163

discussed in detail. The parameters that control the future development of RMC, current state of the art, and gaps in existing literature were highlighted, supported by recommendations to turn limitations into potential advantages. RMC is produced from readily available minerals and high volume waste materials. When used as a building material, it can sequester CO2 while gaining strength, provides improved durability in extreme and aggressive environments, has a low sensitivity to impurities that enables the use of industrial by-products, and can be fully recycled at the end of its service life. Disadvantages include unfamiliarity of material properties and proposed formulations, insufficient documentation and lack of track record compared to the high validation and market confidence in PC. The impact of RMC on the built environment is highly dependent on the establishment of a regulatory landscape, which will promote improvements in the current production routes and performance. These are to be supported with the improvements to current production processes, identification and verification of clear performance benefits, and quantifiable environmental advantages over the whole life-cycle. Advancements in these areas will influence production costs and market price, incentivizing the industry to adopt the developed technologies. The relatively low availability of magnesite compared to limestone can drive the sustainable production of RMC from magnesium silicates and waste brine obtained from desalination plants, which can also alleviate the environmental impacts of current RMC production. Market confidence in RMC can increase in line with the extensive research performed in this area, highlighting the distinction between different MgO reactivities and corresponding capabilities. The promising results on the carbonation potential of RMC in a range of products can be further improved by using CO2 captured and recovered from flue gases emitted by power plants, thereby enabling its permanent sequestration in the form of stable carbonation products. This can extend the use of RMC in different structural applications, including the use of reinforcement that is compatible with lower pH systems (e.g., glass or polymer fibers). As the contribution of RMC to the overall performance of concrete mixes is recognized along with other compelling advantages concerning production and disposal, the industry can be incentivized to recommend RMC in building applications.

Acknowledgments This work has been supported by the Academic Research Fund Tier 1 (RG 113/14) provided by the Singapore Ministry of Education.

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