Magnesium cements and their carbonation curing: a state-of-the-art review

Abstract

The Portland cement (PC) production industry is a key contributor of CO₂ emission. The demand of cement is mounting day by day due to the rapid infrastructure development in the world. Consequently, CO₂ discharge from the construction sector is continuously increasing and accounts for about 8% of the total CO₂ emission, which becomes a global concern nowadays. Wide applications of eco-friendly cements can significantly reduce the CO₂ release. Therefore, use of magnesium cements (MCs) might be a promising solution to ease such concern. As a rapid hardening cement, MCs can be characterized as low-carbon due to their lower embodied energy and carbon storage ability during the service. This review mainly summarizes the findings of previous studies related to the carbonation performances of PC blended with magnesia and MCs products, and particularly, the influence of Accelerated carbonation curing (ACC) process on the properties of MCs and corresponding CO₂ sequestration performance. The effects of ACC on mechanical strength, hydration and mineral carbonation mechanisms, pore structures, pore solution pH and thermal properties are discussed. The limitations of existing research are also discussed, which may provide the directions for future research and development of MC material products.

摘要

波特兰水泥（PC）生产行业是二氧化碳排放的主要贡献 者。由于基础设施的快速发展，水泥的需求日益增加。因此，建筑业的二氧化碳排放量持续增加，约占二氧化碳排放总量的8%，成为当今全球关注的问题。环保水泥的广泛应用显着减少CO₂排放。因此，使用镁水泥（MCs）可能是缓 解这种担忧的一个有希望的解决方案。作为一种快硬水泥，MC因其较低的内含能源和使用期间的碳储存能力而具有低 碳特征。本次综述主要总结了PC 与氧化镁和MCs共混后的碳酸化性能，特别是加速碳酸化固化（ACC）过程对MC性 能和相应CO₂封存的影响。讨论了 ACC 对机械强度、水合作用和矿物质碳化机制、孔隙结构、孔隙溶液 pH 值和热性 能的影响。还指出了现有研究的局限性，这可能提供MC材料产品未来的研发方向。

1 Introduction

The increasing trend of CO₂ gas in atmosphere has threaten the living quality on the earth because the growing rate of CO₂ in air is directed to the global climate change as well as environmental imbalance [1]. Most of the reasons for CO₂ accumulation in atmosphere are human activities such as production of daily necessary goods, manufacture of raw materials for construction industry. The literature studies reported that cement industry is the second-largest emitter of CO₂ among the aforesaid sources, which annually accounts for around 25% of global industrial emissions [2] and about 8% of the global emissions [1, 3,4,5,6] and contributes substantially to global warming [5]. More specifically, PC production is one of the key contributors of CO₂ emission [7]. It is noted that PC has been used as an important construction material since ancient era [2]. The demand of PC is still growing due to the rapid infrastructure development worldwide [2]. It is reckoned that about 4.8 billion joule energy is required to yield one ton of PC [8]. In addition, the worldwide annual manufacture of cement was around 3.5 billion metric tons in 2020 [9, 10] and this number is projected to approach 4.83 in 2050 [10]. As a consequence, it may cause the increase of mean temperature of the whole world by 1.4 to 5.8 °C [11]. Furthermore, the PC industries release CO₂ mainly from two ways like combustion of fuels and calcination process of CaCO₃ (limestone) to form CaO (CaCO₃ + heat → CaO + CO₂ ↑). It is calculated that nearly 0.5 kg of CO₂ is discharged for per kg clinker production [12] and PC contains above 80% of clinker [4], which directs the abundant release of CO₂ in air from PC manufacture process. Moreover, a research also stated that about 500 billion cubic yards concrete is produced per year worldwide for infrastructure development [13] and PC is used as a major fraction of this concrete quantity. Therefore, PC concrete production also has a great impact on global warming through emitting the CO₂ to atmosphere.

The magnesium cements (MCs) are widely known as ‘CO₂-negative’ cements for the absorption of atmospheric CO₂ [14,15,16] and form the carbonated magnesium products through reacting with the main component magnesia (MgO) [16]. The applications of MCs have gained increasing popularity as a construction material to reduce the CO₂ discharge or capture CO₂ from air in context of climate change [17, 18]. Most notably, recent studies have revealed the excellent properties of magnesium cements (MC) over PC, in terms of mechanical strength properties, high-temperature stability [19], CO₂ uptake capacity, fire resistance, compatibility with industrial byproducts, stabilization of toxic contaminants in the hydration products [14, 20,21,22]. More importantly, the light-burnt (calcined at 600–1000°C) and hard-burnt (calcined at 1000–1400°C) MgO manufacturing processes causes lower CO₂ emission and energy consumption compared to OPC [23]. This indicates that the use of light-burned magnesia (also known as reactive magnesia) can be a possible source of CO₂ savings by reducing energy requirements [16]. Another important logic for proving eco-friendly MgO is that the potential of MgO for circular use [24]. Calcined MgO dispersed over the earth surfaces can absorb CO₂ to build carbonate mineral [23]. Then the carbonate crystals can be re-gathered and calcined again to obtain the MgO. Such process may lead to the reduction of minimum 2–3 Gt CO₂ per year and help suppress global warming [23].

Carbonation denotes the chemical response of cementitious materials to CO₂ over the service life or the test protocol period [25]. According to the previous study [26], carbonation curing stimulates the strength gain and CO₂ storage of cement materials. Gruyaert et al. [27] report that carbonation leads to the notable changes on the voids, pore size distribution and hydration chemistry of solid cement matrices. Additionally, Soroushian et al. [28] narrated that carbonation curing may improve the durability of plain concrete as a result of the development of carbonate-rich reaction products. Xuan et al. [29] and Zhang et al. [30] explained that carbonation can also improve the performance of fibre-reinforced cementitious composites and recycled aggregate concrete. In the recent years, ACC has been broadly used as an effective CO₂ storage for cementitious composites [31]. It facilitates a rapid carbonation system to realize substantial CO₂ capture capacity of cement materials through the chemical reactions, which can also mimicry the natural carbonation process by controlling the carbonation factors [31, 32]. The ACC is not only a method to arrest atmospheric CO₂, but also an approach to efficiently modify the hydration products, microstructures and enhance the maturity of early age strength properties of mortars and concretes [31, 33]. It is important to note that the ACC process is more economical, environmentally and technically advantageous than natural carbonation [33]. Currently the ACC system has also explored for MC composites for storing and neutralizing the CO₂.

This paper presents a review and extensive discussion of the carbon curing/carbonation in comparison with PC counterpart in terms the following issues: 1) impacts of high CO₂ emission to environment for high production of PC; 2) carbonation mechanisms and CO₂ uptake of PC composites; 3) carbonation mechanisms of MCs; 4) CO₂ sequestration enhancement of PC incorporated with MgO; and 5) influence of carbonation curing on the physico-mechanical and microstructural properties of MCs. Finally, conclusions and future perspectives are high-lighted. This review is expected to be able to demonstrate a roadmap for further development of carbonated MCs properties and for widespread applications of carbonated MC products in construction sectors in an eco-friendly way.

2 Carbonation process in PC materials

2.1 Carbon capture method

In the recent years, the global leaders have been raising their voices for saving this planet as a living place through minimizing the environmental pollution occurred by CO₂ emission from different sources [34]. So, the scientists and environmentalists have also been attempting to explore the new technology on the reduction of CO₂ emission at the sources level of cement production as well as capturing and storage of CO₂ [35]. The worldwide total use of captured CO₂ is below 0.5% of the total releases for chemical feedstock. Consequently, new solutions are needed to be developed for CO₂ capture and utilization [36]. The literature studies [35, 37, 38] stated that carbon capture and utilization (CCU) technique for concrete can be an effective technique to attaining the goal of carbon neutrality, which is capable of storing CO₂ expelled from burning fossil fuels. It is a mineral sequestration process, where minerals chemically store CO₂ in the form of solid carbonates under the carbonation system [3, 35]. Kamal et al. [3] described the process of CO₂ mineral storage in concrete, which can minimize the carbon footprint and provide a cycle of carbon flow to attain a better eco-friendly construction practice. This method not only preserve the CO₂ in the inner structure of concrete, but also improves the concrete properties. It is calculated that around 0.1 to 1.4 gigatons of CO₂ can be stored in concrete using CCU by the year 2050 [38, 39]. Hanifa et al. [40] and Seifritz [41] illustrated that CO₂ mineral storage (i.e. mineral carbonation) is an acceleration system of weathering carbonation or natural carbonation where CO₂ is fixed indelibly as a stable crystal structure of carbonate through thermodynamically reactions:

$$\mathrm{CaO }+\mathrm{ C}{{\text{O}}}_{2} \to \mathrm{ CaC}{{\text{O}}}_{3} +\mathrm{ Heat}$$

(1)

Furthermore, Wang et al. [42] adopted a novel and cost-effective conception where the CO₂-rich industrial flowing exhaust gas was utilized to carbonize the RMC and binary cement consisted with 50% RMC and 50% PC. Their investigations revealed that carbonation curing under the CO₂-rich industrial exhaust gas improved strength properties of both RMC pastes and binary cement pastes.

2.2 Carbonation mechanisms

Possan et al. [1] and Wang et al. [43] narrated that calcium hydroxide (Ca(OH)₂) is formed as a saturated liquid state under the physical-chemical neutralization process of cement, which finally precipitates as CaCO₃ by interacting with CO₂. In this reaction, CO₂ is consumed from air. When the cement paste is exposed to CO₂ in concrete, CO₂ dissolves in pore solution through dissociation and form H₂CO₃ (hydrogen carbonate). A series of reactions occur to develop calcite (CaCO₃), shown below [1, 43]:

$$\mathrm{Ca\, }{({\text{OH}})}_{2} =\mathrm{ C}{{\text{a}}}^{2+} + 2({{\text{OH}}}^{-})$$

(2)

$${{\text{CO}}}_{2}({\text{g}}) + {{\text{H}}}_{2}\mathrm{O }= {{\text{H}}}_{2}{{\text{CO}}}_{3}$$

(3)

$${{\text{H}}}_{2}{{\text{CO}}}_{3} = {{\text{H}}}^{+}+{HCO}_{3}^{-}$$

(4)

$${HCO}_{3}^{-} = {{\text{H}}}^{+}+ {CO}_{3}^{2-}$$

(5)

$${{\text{Ca}}}^{2+}+ {CO}_{3}^{2-} =\mathrm{ CaC}{{\text{O}}}_{3}({\text{s}})$$

(6)

Figure 1 illustrates the basic carbonation mechanisms of PC, where black arrows designate the dissolving process of cement clinker particles and precipitation of hydration products. In addition, small red arrow symbols characterize the softening of CO₂ toward the pore solution, while the big red arrow signs direct the dispersion of CO₂ toward the pores of the PC paste [44]. These process results the development of calcium carbonate, which contribute to decrease the pore spaces and increase the strength properties of PC composites. It should be mentioned that the carbonation degree and mechanisms of cement clinker primarily rely on the proper dispersion of CO₂ into the cement material and the associated carbonization reactions [45].

Fig. 1

Basic carbonation process in PC materials [45]

Moreover, Possan et al. [1] also explains the carbonation gradually transfers from surfaces into the inner structure of concrete as shown in Fig. 2, forming a carbonate. Development of such layer is directly associated to the ingress of CO₂ into concrete.

Fig. 2

Progress of carbonation in concrete over the service life: a a typical section of a reinforced concrete and b formation of carbonate for ingress of CO₂ into concrete (Adapted from [1])

2.3 CO₂ uptake measurement

The CO₂ uptake denotes the carbonation degree and carbon storage capacity of the cementitious materials [25]. Generally, this term is used to ensure a better CO₂ releases balance through measuring the CO₂ uptake (%) [46]. As CO₂ uptake is an important parameter of carbonation process, global researchers have been calculated in different ways. He et al. [25] estimated the CO₂ uptake from TG-DTG analysis using the following equation:

$${({\text{X}})\,}_{{\text{CO}}2\mathrm{\, uptake}} = {({\text{X}})}_{\mathrm{Mass\, loss\, at\, }500-1000\,\mathrm{^\circ{\rm C} }}-{({\text{NA}})\,}_{\mathrm{Mass\, loss\, at\, }500-1000\,\mathrm{^\circ{\rm C} }}$$

(7)

Where, (X)_{Mass loss at 500–1000 °C} indicates the mass loss of carbonated specimens at 500–1000°C and (NA) _{Mass loss at 500–1000 °C} specifies the noncarbonated reference specimens 500–1000°C.

Fang and Chang [47] calculated the CO₂ uptake (%) considering the mass curve method using the relation presented here:

$${{\text{CO}}}_{2}\mathrm{ \,uptake \,}(\mathrm{\%}) = \frac{{CO}_{2\, absorbed}}{{Mass}_{WHCP}} \,\mathrm{x\, }100$$

(8)

Here, CO_{2 absorbed} was measured from the total mass variation in the carbonation method and deducting the mass of CO₂ which amount was not participated in the carbonation reaction at conducted chamber.

Xian et al. [48] proposed a mass gain method for estimating the CO₂ uptake considering the expression as below:

$${{\text{CO}}}_{2}\mathrm{ \,uptake\, }(\mathrm{\%}) = \frac{{m}_{2} + {m}_{water}-{m}_{1} }{{m}_{cement}}\mathrm{ \,x }\,100\mathrm{ \,\%}$$

(9)

Here, m₁ indicates the mass of sample prior carbonation, m₂ denotes the mass of sample after carbonation, m_water designates the mass of water recorded from carbonation curing chamber, and m_cement refers the mass of dry cement.

2.4 Applications of carbonation in PC composites

Hanifa et al. [40] summarized the CO₂ uptake performances from the literature studies which were conducted on different cement materials as summarized Table 1. It has been well confirmed from the recorded information in Table 1 that CO₂ uptake can significantly rise in cement materials through increasing the CO₂ concentration, ingestion pressure and curing period. It is interesting to mention that the chemical composition and microstructural characteristics of cement pastes are changed in carbonation weather because of the precipitation of CaCO₃. As a consequence, precipitated CaCO₃ play a filling role, decrease the cavities of the concrete structure, and enhance the mechanical as well as durability properties [49].

Table 1 CO₂ uptake (%) by PC composites

3 Properties of MCs and their carbonation reactions

3.1 Reactive magnesia cement

Reactive magnesia cement (RMC) is a well-known cement and utilized alone as a binder or partial replacement in other cements like PC. RMC can be mixed by 2–3% in PC production for denser microstructures, increased strength, better corrosion and fire resistance, stabilization of the contaminants [42, 54, 55]. Additionally, RMC can chemically capture CO₂ and improve the cementing properties under the ACC. The literature studies exemplify that during the carbonation period, CO₂ is dissolved and ionized in the cement matrix, and then precipitates as anhydrous and hydrated carbonates in the pore spaces of the matrix, boosting the microstructural solidity and structural integrity [42, 56, 57]. Moreover, RMC contains about 85% of active MgO and CO₂ uptake up to 92%, where PC shows around 50.4% [42]. The following reactions occur under the ACC process and hydrated magnesium carbonates are formed as follows [58,59,60,61,62,63]:

$$\mathrm{MgO }+\mathrm{ C}{{\text{O}}}_{2} \to \mathrm{ MgC}{{\text{O}}}_{3}\, ({\text{Magnesite}})$$

(10)

$$\mathrm{MgO }+ {{\text{H}}}_{2}\mathrm{O }\to \mathrm{ Mg\, }{({\text{OH}})}_{2} \,({\text{Brucite}})$$

(11)

$$\mathrm{Mg\, }{({\text{OH}})}_{2} +\mathrm{ C}{{\text{O}}}_{2} + 2{{\text{H}}}_{2}\mathrm{O }\to \mathrm{ MgC}{{\text{O}}}_{3} \,.3{{\text{H}}}_{2}\mathrm{O\, }({\text{Nesquehonite}})$$

(12)

$$2\mathrm{Mg\, }{({\text{OH}})}_{2} +\mathrm{ C}{{\text{O}}}_{2} + 2{{\text{H}}}_{2}\mathrm{O }\to {{\text{Mg}}}_{2}{{\text{CO}}}_{3}{({\text{OH}})}_{2} \,.3{{\text{H}}}_{2}\mathrm{O\, }({\text{Artinite}})$$

(13)

$$5\mathrm{Mg\, }{({\text{OH}})}_{2} + 4{{\text{CO}}}_{2} \to {{\text{Mg}}}_{5}{({{\text{CO}}}_{3})}_{4}{({\text{OH}})}_{2} \,.4{{\text{H}}}_{2}\mathrm{O\, }({\text{Hydromagnesite}})$$

(14)

$$5\mathrm{Mg\, }{({\text{OH}})}_{2} + 4{{\text{CO}}}_{2} + {{\text{H}}}_{2}\mathrm{O }\to {{\text{Mg}}}_{5}{({{\text{CO}}}_{3})}_{4}{({\text{OH}})}_{2} \,.5{{\text{H}}}_{2}\mathrm{O\, }({\text{Dypingite}})$$

(15)

Brucite is the key hydration product of RMC. Its strength is enhanced during the carbonation process and MgCO₃ is formed, which is a thermodynamically stable mineral. A literature study [64] stated that hydromagnesite and Nesquehonite crystals are developed with the increases of carbonation age through the transition of artinite. Zarandi et al. [65] noted that Nesquehonite is also converted into dypingite for more carbonation intensity. Overall, these carbonation products improve the compactness and interlocked network structures of the carbonated RMC. As a consequence, the strength is well developed into the carbonated RMC [56, 65, 66].

RMC is produced with low energy consumption and possesses good CO₂ capture performance and its diverse applications are expanding day by day in the construction sectors. The Fig. 3 illustrates schematically the carbonation reaction steps of magnesia towards the carbonated products.

Fig. 3

Carbonation mechanisms of magnesia (Adapted from [11])

3.2 Magnesium phosphate/potassium cement

The MPC/MKPC is broadly known as a rapid hardening cement and mostly used for quick repairing of roads and concrete structures [67, 68]. MPC is shaped by the interactions of dead-burned magnesia (alkaline in nature) and ammonium phosphate (or potassium phosphate) salts (acidic in nature), which is commonly recognized as acid-based reactions. It is to be noted that MPC gains popularity because of its excellent properties such as fast setting, high early strength, superior interfacial bonding to convention concrete, resistant to fire and low drying shrinkage [69,70,71,72]. The main hydration products of MPC is struvite, which is developed as follows [72, 73]:

$$\mathrm{MgO }+\mathrm{ K}{{\text{H}}}_{2}{{\text{PO}}}_{4}(\text{potassium dihydrogen phosphate}) + 5{{\text{H}}}_{2}\mathrm{O }\to \mathrm{ MgKP}{{\text{O}}}_{4}. \,6{{\text{H}}}_{2}\mathrm{O\, }({\text{K}}-{\text{struvite}})$$

(16)

$$\mathrm{MgO }+\mathrm{ N}{{\text{H}}}_{4}{{\text{H}}}_{2}{{\text{PO}}}_{4}(\text{ammonium dihydrogen phosphate}) + 5{{\text{H}}}_{2}\mathrm{O }\to \mathrm{ MgN}{{\text{H}}}_{4}{{\text{PO}}}_{4}. \,6{{\text{H}}}_{2}\mathrm{O\, }({\text{struvite}})$$

(17)

Due to the rapid hydration of MPC/MKPC, considerable quantities of dead-burned MgO particles may remain at unreacted. Consequently, when the CO₂ curing is employed in solid MPC/MKPC specimens, CO₂ is reacted with free MgO particles and brucite (Mg(OH)₂), forming similar carbonated minerals as of RMC such as MgCO₃, MgCO₃.3H₂O, ((Mg)₅(CO₃)₄(OH)₂.5H₂O), and ((Mg)₅(CO₃)₄(OH)₂.4H₂O) [35]. The porosity and structural stiffness of MPC/MKPC are enhanced after developing these carbonated crystals [74].

3.3 Magnesium oxychloride cement (MOC)

The MOC is comprised of the interactions of MgO and MgCl₂ (magnesium chloride) solution [75]. The hydration phases, physico-mechanical, and microstructural characteristics of MOC are principally varied by the mix combinations of MgO and MgCl₂ in aqueous solution. At room temperature, the hydration phases development of MOC can be illustrated by the below chemical reactions [76]:

$$5\mathrm{MgO }+\mathrm{ Mg}{{\text{Cl}}}_{2} + 13{{\text{H}}}_{2}\mathrm{O }= 5{\text{Mg}}{({\text{OH}})}_{2} \,.{{\text{MgCl}}}_{2} \,.8{{\text{H}}}_{2}\mathrm{O\, }(\mathrm{Phase\, }5)$$

(18)

$$3\mathrm{MgO }+\mathrm{ Mg}{{\text{Cl}}}_{2} + 11{{\text{H}}}_{2}\mathrm{O }= 3{\text{Mg}}{({\text{OH}})}_{2}\, .{{\text{MgCl}}}_{2}\, .8{{\text{H}}}_{2}\mathrm{O\, }(\mathrm{Phase\, }3)$$

(19)

The MOC products are usually inclined to CO₂ uptake, where hydrated magnesium carbonate like chlorartinite is formed (from Phase 5) by absorbing CO₂ [66]:

$$\mathrm{MgO }+ {{\text{H}}}_{2}\mathrm{O }=\mathrm{ Mg}{({\text{OH}})}_{2}$$

(20)

$$\mathrm{Mg\, }{({\text{OH}})}_{2} \to {\mathrm{ Mg}}^{2+} + 2{{\text{OH}}}^{-}$$

(21)

$${{\text{CO}}}_{2} + {{\text{H}}}_{2}\mathrm{O }\leftrightarrow {{\text{H}}}_{2}{{\text{CO}}}_{3} \leftrightarrow {{\text{H}}}^{+}+ {HCO}_{3}^{2-} \leftrightarrow 2{{\text{H}}}^{+} + {CO}_{3}^{2-}$$

(22)

$$2{{\text{Mg}}}^{2+} + {CO}_{3}^{2-}+\mathrm{ O}{{\text{H}}}^{-} + {{\text{Cl}}}^{-} + 3{{\text{H}}}_{2}\mathrm{O }\to {{\text{Mg}}}_{2}({{\text{CO}}}_{3})\mathrm{ \,Cl}({\text{OH}}).\, 3{{\text{H}}}_{2}\mathrm{O\, }({\text{Chlorartinite}})$$

(23)

Phase 3 mineral also uptakes CO₂ and forms the new hydration product as below [77,78,79,80]:

$$3{\text{Mg}}{({\text{OH}})}_{2} \,.{{\text{MgCl}}}_{2} \,.8{{\text{H}}}_{2}\mathrm{O\, }(\mathrm{Phase\, }3) + 2{{\text{CO}}}_{2} \to \mathrm{ Mg}{({\text{OH}})}_{2}\, .{{\text{MgCl}}}_{2}\, . 2{{\text{MgCO}}}_{3} \,.6{{\text{H}}}_{2}\mathrm{O }+ 2{{\text{H}}}_{2}{\text{O}}$$

(24)

It is noted that strength properties of MOC composites are decreased in water environment [77] due to the decomposition of Phase 5 and Phase 3 minerals in contact of water, while both are key contributors of structural rigidness of MOC materials [78]. The crystal Phase 5 may be transformed into Phase 3 structure in the carbonation environment. The researchers observed that the new phase (Mg(OH)₂.MgCl₂. 2MgCO₃ 0.6H₂O) formed due to carbonation degrades less in water than the Phase 5 and Phase 3 minerals. In other words, the water-resistance performance of MOC is upgraded due to CO₂ uptake. Similarly, CO₂ sequestration of MOC enhances the strength and toughness properties due to formation of compact microstructures [80, 81]. Figure 4 illustrates the carbonation reaction stages of MOC with CO₂, where magnesium carbonate minerals are precipitated like chlorartinite.

Fig. 4

Theoretical illustration of MOC carbonation mechanisms [75]

3.4 Magnesium oxysulfate cement

The MOS is fabricated by the chemical responses of light-burned MgO and MgSO₄· 7H₂O (magnesium sulfate) solution, which concept in parallel with that of MOC [31]. As MOS is formed with lower energy consumption [82] and lower calcination-based magnesia [83], it is also known as eco-friendly cement. Previous research detected that the dominating hydration reaction product is 5Mg(OH)₂· MgSO₄· 7H₂O (5·1·7 phase) [84]. Another research stated that hydration phases of MOS can be defined as xMg(OH)₂· yMgSO₄·zH₂O, which can be designated as different phases. Formation reactions of these phases can be expressed as follows [85]:

$$3\mathrm{MgO }+\mathrm{ MgS}{{\text{O}}}_{4} + 11{{\text{H}}}_{2}\mathrm{O }\to 3{\text{Mg}}{({\text{OH}})}_{2}.\,\mathrm{ MgS}{{\text{O}}}_{4}.\, 8{{\text{H}}}_{2}\mathrm{O \,}(\mathrm{Phase \,}3-1-8)$$

(25)

$$5\mathrm{MgO }+\mathrm{ MgS}{{\text{O}}}_{4} + 7{{\text{H}}}_{2}\mathrm{O }\to 5{\text{Mg}}{({\text{OH}})}_{2}.\,\mathrm{ MgS}{{\text{O}}}_{4}. \,2{{\text{H}}}_{2}\mathrm{O\, }(\mathrm{Phase \,}5-1-2\mathrm{ \,or\, }3)$$

(26)

The durability properties of MOS matrices can be enhanced by employing carbonation curing. The CO₂ curing causes the strength improvement up to a limit at the early CO₂ sequestration and afterwards leads to a negative influence on the structural integrity [31]. Ba et al. [86] explained that MgCO₃ structure can be developed under carbonation curing of MOS through neutralizing the MgSO₄ and Mg(OH)₂ phases, which refine the microstructures, capillary porosity and improve the mechanical strength characteristics to a certain extent [31, 84]. The carbonation reaction of MOS can be written as follows [86]:

$$({\text{n}}+1)\,\mathrm{ Mg}{({\text{OH}})}_{2}\, \cdot {\text{MgSO}}4\, \cdot ({\text{m}}+1) {{\text{H}}}_{2}\mathrm{O }+\mathrm{ C}{{\text{O}}}_{2}\to {{\text{MgCO}}}_{3} +\mathrm{ nMg}{({\text{OH}})}_{2}\, \cdot {\text{MgSO}}4\, \cdot {{\text{mH}}}_{2}{\text{O}}$$

(27)

4 Properties changes and mechanisms of carbonated PC incorporating RMC

4.1 Improvement of physico-mechanical properties

As the RMC is very sensitive to CO₂ absorption, researchers have been exploring the CO₂ sequestration performance over a decade on the binary cement that combines PC and RMC. Chemical compositions of used PC and RMC are presented in Table 2. In the binary compositions, RMC is used as the partial substitution of PC, entitling the binary cement and exhibits some good facilities such as noteworthy carbonation efficiency and, comparatively rigid microstructures and durability [42, 87]. It is worth noting here that the carbonation performance of cement materials depends on the mix proportion of binders, water to cement proportion, pressurized concentration level of CO₂, exposed surface area, curing temperature and relative humidity. Figures 5, 6 and 7 represent the analyzed properties of carbonated binary cement reported by the literature studies. Figure 5 shows the enrichment of CS, apparent density, pore volume and porosity of the binary cement as compared with the carbonated control specimens (i.e., PC mixture). As is observed, the aforesaid properties were significantly improved. In addition, Fig. 6 illustrates the CO₂ uptake percentage of the binary and control PC samples, where it is quite clearly seen that CO₂ uptake (%) performance of the former was considerably increased and ranged from 21 to 35% at 7d to 56d carbonation ages. Moreover, Fig. 7 demonstrates the carbonation degree of examined samples. Pu and Unluer [88] observed the higher carbonation degree at the peripheral surfaces of binary cement matrices as compared to the inner portions of the carbonated samples.

Table 2 Chemical compositions of PC and RMC used by previous studies for carbonation curing

Fig. 5

Properties improvement of carbonated PC composites blended with RMC: a compressive strength, b apparent density, c pore volume, and d porosity

Fig. 6

CO₂ uptake of carbonated PC materials comprising with RMC

Fig. 7

Carbonation degree of carbonated PC materials containing RMC

4.2 CO₂ sequestration mechanisms of PC and MgO binary system

The CO₂ sequestration is a process where CO₂ is chemically stabilized in the cement materials through converting into different crystals and entrapped forever [3]. This technique can play a crucial role in enhancing the cement materials properties and decreasing the carbon footprint in the world [3, 91]. Previous studies stated that free calcium ions (Ca²⁺) are available in PC matrices because of the dissolution of hydrated lime (Ca(OH)₂), and decalcification of CSH and ettringite [90]. In addition, Ca²⁺ ions may leach from calcium silicate anhydrous [92].

Similarly, the pore solution of RMC products contains Mg²⁺ ions. Consequently, the pore solution of binary cement having PC and MgO mixes comprise of both Ca²⁺ and Mg²⁺ ions, where Mg²⁺ ions interact with CaCO₃ and crystalize to magnesian calcite (Ca_xMg_1-xCO₃) as expressed below [90]:

$${{\text{Ca}}}^{2+} ({\text{aq}}) + {{\text{Mg}}}^{2+}({\text{aq}}) + {{\text{CO}}}_{3}^{2-}({\text{aq}}) \to {{\text{Ca}}}_{{\text{x}}}{{\text{Mg}}}_{1-{\text{x}}}{{\text{CO}}}_{3}\, ({\text{s}})$$

(28)

Pokrovsky [93] specified that the amount of magnesian calcite (e.g., CaMg(CO3)₂) in the carbonated hydration environment of binary cement is varied depending on the existing quantities of Ca²⁺ and Mg²⁺ ions in the pore solution. Furthermore, the magnesium carbonate crystal can develop through the reaction between Mg²⁺ ions and ${{\text{CO}}}_{3}^{2-}$ as follows [90, 94]:

$${{\text{Mg}}}^{2+}({\text{aq}}) + {{\text{CO}}}_{3}^{2-}({\text{aq}}) + {{\text{H}}}_{2}\mathrm{O }\to \mathrm{ MgC}{{\text{O}}}_{3} \,.3{{\text{H}}}_{2}\mathrm{O\, }({\text{s}})$$

(29)

The key factors influencing the CO₂ sequestration degree and capacity in the binary cement compounds can be classified into two types: (1) the dissolution amount and diffusion of Ca²⁺ and Mg²⁺ into the pore solution, and (2) the degree of dissolved CO₂ in pore water and its movement in the capillary pore networks of solid cement materials [90].

5 Influence of carbonation curing on MCs properties

5.1 Mechanical strength

Tables 3 and 4 summarize the existing findings on how ACC influences the strength properties of magnesium cements. Pu and Unluer [88] investigated that ACC led to 83% higher CS than the standard control specimens (i.e., without ACC). Jang et al. [95] reported that the CS values of the RMC samples increased proportionally with the CO₂ sequestration level. They also detected that the CS increased because of the formation of carbonates of hydromagnesite (Mg₅(CO₃)₄(OH)₂ 0.4H₂O) mineral that filled the internal cavities of porous microstructures. Table 2 also shows the better strength results of carbonated MPC matrices in relation to the control curing process. For instance, Jeon et al. [35] observed that CO₂ cured MPC specimens offered the higher CS and strain at the peak stress in comparison with the specimens cured in standard air. They demonstrated that the structural integrity was improved of carbonated matrices due to the densification of hydration environment through the reaction between the CO₂ and hydration products of dead-durned magnesia. Gao et al. [96] also recorded the similar findings on MPC materials and they explained an interesting thing is that fly ash-based MPC mixes exhibited the significant improved strength under ACC as compared to non-carbonated MPC samples. Table 3 illustrates the comparison of strengths of the carbonated and non-carbonated specimens for MOC and MOS composites. He et al. [77] stated that MOC paste comprising PFA (pulverized fuel ash) and cured in CO₂ showed the better strength and water-resistant performance than the control paste. They interpreted that this good performance was possibly due to the development of new carbonated phases. Moreover, Ba et al. [86] specified that carbonation curing improves the structural strength properties and toughness of MOS samples to some extent, as a result of lower water to cement ratio and refinement of meso-pores stimulated by the carbonation process. Similar results were reported by Zhang et al. [31], who described that hydration products of MOS may react with CO₂ to form new crystals, which contribute to the strength enhancement.

Table 3 Influence of ACC on strength properties of RMC and MPC composites

Table 4 Influence of ACC on strength properties of MOC and MOS composites

5.2 Pore structure

The influence of carbonation curing on the pore structures of magnesium cements are discussed in this section. It is observed that ACC changes the pores properties in the microstructures. Mo and Panesar [90] analyzed the pore size distribution (PSD) of PC incorporating RMC and GGBFS (ground granulated blast furnace slag) slag, which were cured at 99.9% concentration of CO₂ as displayed in Fig. 8. Their study revealed that lower pore volumes were found in carbonated specimens in relation to non-carbonated samples. Slag led to a coarser PSD, where RMC showed the opposite effect. Introduction of 10–20% RMC to PC paste reduced the pore sizes and total porosity of the blends in comparison to control matrices under the ACC system.

Fig. 8

Pore size distribution of carbonated and non-carbonated PC paste containing RMC and GGBFS at 56d [90]

Gao et al. [96] reported the MIP (Mercury intrusion porosimetry) analysis results of MPC paste blended with fly ash considering the ACC and standard air curing systems as exhibited in Fig. 9. It is seen from the cumulative pore size (CPS) and PSD curves that the porosity of pure MPC was increased to some extent after carbonization, where MPC having fly ash exhibited the porosity reduction and strength improvement in the ACC system. Fly ash promoted the formations of mineral and amorphous phases, which instigated the reduction of pore sizes and pore volumes.

Fig. 9

Pore size distribution analysis of MPC samples containing fly ash under carbonation and non-carbonation system (Adapted from [96])

Ba et al. [86] examined the pore structures of MOS cement specimens under the standard air and carbonation curing methods and the findings are presented in Fig. 10. It is quite clear that MOS sample prepared with the W/C of 0.5 showed the lower volume porosity (%) and significantly reduced pore sizes when cured in ACC system as compared to the standard air curing samples (Fig. 10). Furthermore, it is well evident from X-CT images (resolution of 38 µm) as shown in Fig. 11 that the meso-porous volume of hardened MOS paste having the W/C of 0.5 decreased in carbonation, specifically in the nearer regions of carbonized layer (in reconstructed images). Most notably, it was reckoned that the mean pore volume was about 12.74% in standard curing, where carbonization specimen presented refined microstructures and average pore volume of around 10.6% at 28d. For this reason, strength properties of MOS cement paste were enhanced to some extent under the ACC environment.

Fig. 10

Influence of ACC and non-carbonation system on porosity of MOS cement prepared with different W/C [86]

Fig. 11

Meso-porous structures of MOS cement sample: a transverse and longitudinal sections without carbonation and b transverse and longitudinal sections with carbonization [86]

Zhang et al. [31] also analyzed the micro-pore structure properties of MOS cement specimens employing the standard air curing and ACC systems. The obtained results are shown in Table 5. As it is seen, the MOS cement samples cured in ACC atmosphere exhibited the lower average pore sizes and considerably decreased gel pore (< 10 nm) and capillary pore sizes (10–100 nm) than the standard air cured matrices, indicating the compactness of MOS pastes obtained in the ACC system.

Table 5 Micro-pore structure properties of MOS specimens [31]

5.3 pH of pore solution

The pH variations of carbonated pore solution of the RMC and MOS compounds at different carbonation ages are presented in Fig. 12. It is seen that the pH of pore solutions ranges from 10.0 to 10.5 for non-carbonated (i.e. at air curing) RMC and MOS specimens, while reduced to the range of 10.0–10.3 because of carbonation curing under the 10% to 20% CO₂ environment. The reduction seems to be marginal. More importantly, the pore solution pH was not noticeably changed with the prolonged carbonation ages, possibly due to the pH buffering mechanism supplied by the brucite (Mg(OH)₂) mineral in the hydration products [99]. Hay and Celik [97] stated the similar finding that uncarbonated Mg(OH)₂ assists to balance the pH buffering issue in the carbonated MC samples.

Fig. 12

pH profiles of RMC and MOS at different carbonation ages

5.4 Hydration properties

5.4.1 Mineralogical formations

The influence of carbonation curing on the hydration reactions and mineralogical changes in contact to CO₂ are discussed in this section. Wang et al. [42] analyzed the crystal formation changes of RMC applying different concentration of CO₂ and the experimental results are displayed in Fig. 13. Figure 13a shows the gradually increasing contents of amorphous phase about 31.2% for 70% and 100% CO₂ curing at 1d duration, whereas the unreacted MgO content was decreased proportionally. Moreover, MgCO₃ and other secondary hydration products of RMC paste were increased with ages noticeably in 10% CO₂ environment (Fig. 13b), which indicates that low intensity of CO₂ curing needs adequate time to stimulate the hydration reactions for forming carbonation of RMC products.

Fig. 13

X-ray diffractions and mineral formation quantities analysis of RMC pastes with varying the CO₂ intensity and observation ages (Adapted from [42])

Jeo et al. [35] conducted XRD tests on carbonated and non-carbonated samples of MPC at 3d and 28d ages (Fig. 14). It was found that K-struvite (MgKPO₄ 0.6H₂O) developed as a main hydration product of MKPC pastes, whereas the high peaks of unreacted MgO and MgCO₃ were overlapped at 42.82° for standard air and carbonated samples. Additionally, it is quite evident from the results that few small peaks of Nesquehonite were detected in carbonated specimens at 3d and 28d matrices and such peaks were not noticed in air cured pastes, which signifies the chemical interactions of CO₂ and Mg (OH)₂ to form Nesquehonite.

Fig. 14

X-ray diffractions results MKPC pastes at a 3d and b 28d cured at standard air (4N) and carbonation conditions (4C) (Adapted from [35])

He et al. [77] performed the X-ray diffraction experiments on MOC paste containing 30% fly ash and pastes cured in > 99% CO₂. It is noted that carbonated MOC samples blended with fly ash showed better water-resistant capacity, due to the huge amorphous contents formed in the hydration region through the interaction between fly ash and MOC. Such formation was greatly enhanced by the ACC system. Consequently, the microstructural rigidity was improved and the intensity of Phase 5 (mineral) remained unchanged in the water media. Similarly, it is seen that the peak intensities of brucite (Mg (OH)₂), periclase and alumina were detected almost same even after 14d of water curing.

Li et al. [83] prepared the MOS comprised with fly ash (FA) and GGBFS and cured in standard air and CO₂. Then the samples were also cured in water up to 28 days and performed the XRD tests. The analytical results are exhibited in Fig. 15a, where high peaks of Mg (OH)₂ (brucite), Phase 5·1·7, magnesite (MgCO₃), and free MgO particles were noticed. It is also clearly seen that peak intensities of MgCO₃ were increased for CO₂ curing, because of the reaction of CO₂ and free MgO particles to produce MgCO₃ products (Fig. 15a). Noticeable modification of peaks intensities of Phase 5 was not observed for CO₂ curing. Additionally, intensities of brucite were noticed clearly after 28d in water region, which attributed to the interactions of water and MgO particles (Fig. 15b). Furthermore, the peaks of brucite were hardly marked after 28d of water immersion due to the solidification of CO₂ curing. It designates the sequestration of CO₂ that decreased the unreacted magnesia particles in the hardened MOS pastes.

Fig. 15

X-ray diffraction results of MOS blended with fly ash and slag cured at air and CO₂: a before water immersion and b after water immersion (Adapted from [83])

5.4.2 Microscopic observations

Wang et al. [42] inspected the microscopic pictures of air cured RMC and carbonated RMC pastes with 10% CO₂ as shown in Fig. 16. As detected, the standard cured specimen showed the rough surface and existence of free MgO particles in abundant quantities, whereas dense structure and regular sized hydration products were noted in the carbonated RMC samples. Similar characteristics were explored by Jang et al. [95] for light RMC pastes cured in 10% and 20% CO₂ as depicted in Fig. 17.

Fig. 16

SEM observations of RMC pastes at 1-d 0% CO₂ and 10% CO₂ system [42]

Fig. 17

SEM image of carbonated RMC paste: a RMC paste cured in 10% CO₂ and b RMC paste cured in 20% CO₂ [95]

It was observed in Fig. 17 that the number and sizes of hydromagnesite crystal was augmented in 20% CO₂ cured RMC paste (Fig. 17b) as compared to the 10% CO₂ cured sample (Fig. 17a). As a result, RMC specimen cured by 20% CO₂ presented the higher structural strength properties due to the filling of internal pores by hydromagnesite development [95].

Jeon et al. [35] analyzed the microscopic properties like micro-pores and micro-cracks of MKPC pastes considering the standard air and ACC systems. Their captured BSE images are shown in Fig. 18. It was specified that comparatively higher micro-cracks and pores were detected in the interfacial transition zone (ITZ) regions of air cured samples (i.e. MKPC-4N) than carbonated specimens (i.e. MKPC-4C). It is fairly clear that carbonation curing stimulated to form the solid microstructure through forming the carbonates minerals that reduces of cavities in the carbonated microstructures [35].

Fig. 18

BSE images of MPC pastes at 28d: a MKPC sample with air curing and b MKPC sample with CO₂ curing [35]

Li et al. [83] observed the SEM images of control MOS (Fig. 19) and MOS blended with FA 30% or GGBFS 30% pastes (Fig. 20) cured at air for 13d and 1d in CO₂. Then the samples were kept in water for 28d. The narrow-like minerals namely 5·1·7 phase having the diameter around 0.25 μm and height about 4 μm were noticed in the samples cured in air (Fig. 19a). After the 1-d carbonation, compactness of needle-like crystals was increased (Fig. 19b). In addition, micro-cracks were revealed in the microstructure of water cured matrices, which implies the degradation of MOS hydration products in contact with water.

Fig. 19

Microscopic observations of MOS paste cured in water for 28d considering air and CO₂ sequestration treatment: a control with air and b control with CO₂ [83]

Fig. 20

Microscopic observations of MOS paste cured in water for 28d considering air and CO₂ sequestration treatment: a MOS+ fly ash 30% with CO₂ curing, and b MOS+ slag 30% with CO₂ curing [83]

On the other hand, solid MOS combinations containing supplementary materials like FA or GGBFS and treated with CO₂ (Fig. 20), unveiled the denser structure in comparison to standard air cured samples. It was possibly due to the development of the amorphous phases, nanocrystalline C-S-H, M-S-H gel and HCMs in the hydration regions for including FA and GGBFS [100,101,102]. For this reasons, the metrics of carbonated MOS incorporating FA and GGBFS attained the better durability properties [83].

5.4.3 Thermal properties

Wang et al. [42] conducted thermogravimetry analysis (TGA) tests on RMC pastes cured in air and different CO₂ concentrations at various hydration ages. As illustrated in Fig. 21, 1-d carbonated RMC samples lost more mass than the ambient air cured specimen. No significant transformation was seen of brucite (Mg(OH)₂) into HMCs phases in the specimens cured at low CO₂ concentration such as 10% and 40%, whereas notable changes were detected at high CO₂ intensities (i.e. 70% and 100% concentration). Additionally, in comparison to 1-d carbonated RMC paste, mass loss peak of 3-d CO₂ cured paste shifted from 370 °C to 390 °C. Furthermore, it was reported that the total mass loss dropped around 22.8% for 7d of 10% CO₂ specimens. Similar result was observed for 1d aged specimens cured in 100% CO₂ [42].

Fig. 21

TGA test results of RMC pastes (Adapted from [42]): a mass loss curves cured at 1-d in different CO₂ intensities and DTG curves corresponding to them, b mass loss curves for 10% CO₂ curing at different periods and DTG curves corresponding to them

Jeon et al. [35] performed the thermogravimetric analysis on the different aged carbonated MKPC pastes and the attained data are presented in Fig. 22. It was revealed that more mass loss occurred for carbonated samples than the air cured counterparts at the same ages. Existence of more water in the K-struvite minerals might cause the decrease of mass below 200 °C [103]. Decomposition of HMCs and MgCO₃ was detected at around 560 °C [97]. Finally, it was stated that carbonated MKPC samples exhibited the higher peaks of dehydration with increasing the curing ages, while the air cured matrices showed no change.

Fig. 22

Thermogravimetric analytical curves of carbonated (MKP-4C) and air (MKP-4N) cured MKPC specimens at different ages [35]

Figure 23 represents the DSC/TG results on MOS specimens employing air and carbonation curing process directed by Li et al. [83]. FA and GGBFS were also added in MOS pastes. It was reported from DSC curves that the control sample showed mass loss ranging from 7.48% to 8.73% in the heat range of 50 °C to 180 °C. In the other side, carbonated MOS sample revealed a higher mass loss for heat about 180 °C than the reference sample. It was also seen that carbonated samples showed first decomposition at higher temperature as compared to air cured specimens, which was possibly due to the effects of forming Mg (OH)₂ and HMCs crystal-like structure [97, 104]. Consequently, CO₂ treatment could improve the water-resistance performance of MOS matrix. It is clearly seen from the DSC curves of MOS comprising FA30 and GGBFS30 that carbonation curing influenced their way of dehydration and decomposition. An exothermic peak was spotted in FA30 and GGBFS30 blended samples at about 850 °C due to crystallization of the M-S-H gel (i.e., MgSiO₃ compound). It is recognized that M-S-H gel has an insoluble structure and can improve the water proofing properties of MOS cement [105].

Fig. 23

DSC-TG curves of air and carbonation cured MOS samples: a before immersion into water and b after 28d of water immersion [83]

6 CO₂ sequestration in RMC

Up to now, a large number of existing studies are found for measuring the CO₂ sequestration in RMC composites, whereas very few studies are available for MOC matrices in the literature. Figure 24 summarizes the CO₂ sequestration (%) information from published papers on RMC. All the specimens were cured in the ACC system under different CO₂ concentration levels, temperatures and RH (%). Then the sequestration percentages were measured through the thermogravimetry analysis. It is seen that the sequestration (%) ranged from about 4% to 46% at early ages to 28d for RMC solid specimens. It signifies that for 1 ton of RMC materials, ∼40–460 kg CO₂ could be the effectiveness of offsetting CO₂ absorption via the use of RMC as an alternate binder, in spite of the variation of CO₂ sequestration quantity may vary due to the different mix designs of raw materials proportions, testing ages, curing conditions etc. For example, Hay and Celik [106] applied the supercritical CO₂ pressure under 9.5 MPa at 35 ± 2 ^◦C condition and noticed that around 8.8% CO₂ sequestration was achieved at 4 h of experiment, which was very high amount considering the short carbonation age. It seems that supercritical CO₂ pressure suppressing the primary nucleation seeding and development of carbonates in the higher intensity of carbonation degree [95, 107].

Fig. 24

CO₂ sequestration (%) of RMC composites

Power et al. [75] examined the CO₂ sequestration of MOC-based particle boards for long periods. They reported that 1 kg CO₂/m² was sequestrated within 15 years, where the passive carbonation degree was about 0.07 kg CO₂/m²/year. Furthermore, Jankovský et al. [80] put forward an estimation that Phase 3 (3Mg(OH)₂.MgCl₂ 0.8H₂O) mineral of MOC can uptake CO₂ around 211 g/kg while 331 g/kg for Phase 5 (5Mg(OH)₂.MgCl₂ 0.8H₂O) crystal.

7 Comparative sustainability assessment of MgO and PC composites

The LCA of construction materials prepared with PC and RMC amalgams was conducted to analyze the sustainability issue of MCs composites [66, 108, 109]. McQueen et al. [23] demonstrated that MgO along with high-purity CO₂ is produced from the burning process of magnesite (MgCO₃). Then this MgO reacts with atmospheric CO₂ through spreading over the earth surface to form carbonate minerals again, which is recollected and re-calcined [110]. Therefore, a magnesia loop can be formed to facilitate a sustainable way to keep the environment balance. Table 6 illustrates the comparative analysis of sustainability of MgO and PC products considering different factors. It is noticeable that RMC and hard-burned magnesia (HBM) are manufactured at lower temperature than OPC, which leads to lower energy cost [23]. Although the incineration process of MgO releases little higher CO₂ as compared to PC, the overall MgO production shows around 73% lower net CO₂ emission than PC in the context of carbonation capability. In addition, Ruan and Unluer [108] studied the LCA considering the environmental impacts of RMC and PC manufacturing processes. They concluded that MgO presents fairly lesser impact on the whole ecological quality and capitals, when compared to PC. On account of the strength ratio modification (i.e. strength performance), previous research reported MgO manufacture results in about ∼13% and ∼32% smaller CO₂ intensity and impact to human health in relation to PC, correspondingly. Finally, it can be concluded that the use of MgO composites generates a lower ecological impact in terms of the total service life performance [14].

Table 6 Sustainability evaluation of PC and magnesia over the service life

8 Discussion and limitation of existing research

Magnesium cements have been known as low-carbon construction materials and drawn increasing attention globally from researchers and industrial investors for their lucrative properties over the PC composites. It is well-known that MCs respond well to CO₂ curing and show significant carbonation degree to uptake CO₂. Therefore, a lot of research has been conducted broadly over the last few years on the carbonation performance of MCs composites to demonstrate their environmental sustainability.

The current review primarily focuses on the changes of engineering properties and CO₂ sequestration performances of the MCs composites employing the ACC process. According to the information conveyed in the previous sections, noteworthy reduction was observed of porosities, improvement of density and strength properties, and CO₂ sequestration in PC materials comprising with RMC due to the carbonation curing. Carbonated products are formed through stimulating the hydration reactions of RMC and formation of hybrid Ca-Mg products. These carbonated crystals refine the pore structures and improve the structural integrity of PC composites. It is quite obvious that the inclusion of RMC into the PC mixtures enhances the carbonation degree and CO₂ uptake capacity. As a consequence, the pore volume and porosity of the binary cement compositions are reduced, leading to the improved engineering properties. In addition, ACC method also significantly influence the microstructural as well as hydration reaction properties of MCs compounds, due to the dispersion of CO₂ within the micropores system and thus dense carbonate network. Consequently, better engineering performances (including better water resistance) are achieved in carbonated MCs than the non-carbonated. Furthermore, low carbon emission-based construction materials can be manufactured with MCs. Net CO₂ emissions can be considerably minimized through the use of low-calcined MgO and optimizing the; mix design, water to cement proportion, binder to aggregate ratio, carbonation degree and carbonation curing process [66].

However, further research is needed for better understanding the engineering properties of MCs. Although numerous papers have been published on the properties of carbonated RMC, relatively few research articles are found on the characteristics of carbonated MPC/MKPC, MOC, and MOS composites as compared in Table 7.

Table 7 Engineering and microstructural properties of carbonated MCs analyzed by literature studies

It is shown in Table 6 that the properties like pore solution pH, CO₂ sequestration (%), water proofing performance, and pore structures of carbonated MPC/MKPC, MOC and MOS compounds with and without supplementary cementitious materials need further exploration. It is commonly recognized that MC binders show good strength properties in comparison to their PC counterparts, for example CS ranges in RMC 20-70 MPa, > 40 MPa for MPC (at 3 h), 60–150 MPa for MOC and 25-45 MPa for MOS [66]. Therefore, it should be a good step forward to more explore and improve the properties of MCs through carbonation process for eco-friendly practical applications.

9 Conclusions

This paper has presented a comprehensive state-of-the-art review on the previous studies related to the carbonation mechanisms of MCs, the properties change under the carbonation curing and CO₂ sequestrations. Additionally, the influence of carbonation on the binary cement (PC and MgO) properties is also discussed here. The review can lead to the following concluding remarks:

• (1) Significant efforts have been spent over the past two decades on the storage of CO₂ in PC materials through applying the mineral storage process. It is widely accepted that CaCO₃ is precipitated as a carbonation product in PC and formed by the interaction between CaO and CO₂. About 0.1 to 1.4 gigatons of CO₂ can be stored in PC concrete using CCU by the year 2050.

• (2) Inclusion of different dosages of RMC into PC mixtures enhances its density and strength properties, and reduces its porosity and pore sizes. Also, presents a good viability to improve the CO₂ uptake performances of the PC and RMC blended composites in ACC system. Carbonated products are formed through stimulating the hydration reactions of RMC and form the hybrid Ca-Mg products. These carbonated crystals refine the pore structures and improve the structural integrity of PC composites. It was noted that addition of 10–50% RMC into PC composition increases the CO₂ uptake (%) performances by around 20% –35% and carbonation degree about 10%-60%. For these advantages, the applications of MC have been considerably increased over the last few years and gained increasing popularity as a construction material to reduce the CO₂ discharge or capture CO₂ from air in the context of climate change.

• (3) It was noticed that carbonation curing advances the structural integrity and toughness of MCs samples. Consequently, carbonated MC composites presented the better strength and water-resistance properties than the non-carbonated ones.

• (4) Carbonated MC specimens revealed the lower pore volumes in relation to non-carbonated samples. The ACC promoted the formations of minerals and amorphous phases, which instigated the reduction of pore sizes and pore volumes. It was also reported that the MCs comprising with supplementary materials such as fly ash exhibited the lower gel pore (< 10 nm) and capillary pore sizes (10–100 nm) than the standard air cured matrices, where fly ash promoted the amorphous phases under the CO₂ curing.

• (5) X-ray diffraction analysis showed that ACC greatly influences the hydration reactions of MC materials. Carbonation curing stimulates to form the minerals like Nesquehonite, MgCO₃, secondary hydration products and more amorphous contents, which refines the meso-pores and increase the densification of microstructures. Most notably, the peak intensity of MgCO₃ is increased by CO₂ curing because of the reaction of CO₂ and free MgO particles. Brucite was also found disappeared gradually from MCs due to the solidification in the carbonation environment. It specifies the CO₂ sequestration hardened MCs composites.

• (6) Relatively few studies were found in the literature on the estimation of CO₂ sequestration percentage of MC materials employing the ACC system. It was observed that about 4% to 46% CO₂ was sequestered in the RMC mixtures at 28d. The magnitude of CO₂ sequestration mainly depends on the mix compositions of raw materials in MC matrices, quantities of supplementary materials, carbonation intensities and curing ages.

10 Recommendations for further studies

In spite of the many studies available on the carbonation of RMC materials, they are still a relatively new topic in comparison to the carbonation of PC. Research on the engineering, microstructural and CO₂ uptake properties of carbonated MPC/MKPC, MOC, and MOS are still very limited. Hence, there is a great opportunity to explore the properties and CO₂ sequestration performances of MC mortars and concretes employing the ACC system. The future studies on the carbonation of MCs may be conducted from the following aspects:

• 1) Accelerated carbonation studies can be conducted on MCs blended with supplementary materials (SMs) such as silica fume, red mud, bauxite, calcine clay and limestone. As these SMs play a great role in improving the properties of MCs.

• 2) The effects of carbonation on the pore solution pH and corrosion probability of steel reinforced MC concretes containing SMs should be further clarified.

• 3) Long term durability study can be performed on carbonated MC specimens under accelerated exposure schemes like high alkaline and acidic environment.

• 4) Advanced material characterization techniques like Raman spectroscopy, nucleic magnetic resonance, and micro-X-ray computed tomography can be performed in order to gain deep insights into carbonated MCs samples.

• 5) CO₂ sequestration study can be implemented using the wet carbonation as well as carbonation on fresh mixtures of MCs composites.

• 6) Life cycle assessment on MCs like MPC, MOC and MOSC composites can be studied for sustainable field applications in future.

• 7) Influence of carbonation on the engineering properties of magnesium cement concrete can be monitored.

11 Nomenclature

ACC Accelerated carbonation curing

CCU Carbon capture and utilization

CSH Calcium silicate hydrate

CO2 Carbon dioxide

CS Compressive strength

FS Flexural strength

HMCs hydrated magnesium carbonates

LCA Life cycle assessment

MCs Magnesium cements

MPC/MKPC Magnesium phosphate/potassium cement

MOC Magnesium oxychloride cement

MOS Magnesium oxysulfate cement

PC Portland cement

RMC Reactive magnesia cement