Exploring Mechanisms of Hydration and Carbonation of MgO and Mg(OH)2 in Reactive Magnesium Oxide-Based Cements

Reactive magnesium oxide (MgO)-based cement (RMC) can play a key role in carbon capture processes. However, knowledge on the driving forces that control the degree of carbonation and hydration and rate of reactions in this system remains limited. In this work, density functional theory-based simulations are used to investigate the physical nature of the reactions taking place during the fabrication of RMCs under ambient conditions. Parametric indicators such as adsorption energies, charge transfer, electron localization function, adsorption/dissociation energy barriers, and the mechanisms of interaction of H2O and CO2 molecules with MgO and brucite (Mg(OH)2) clusters are considered. The following hydration and carbonation interactions relevant to RMCs are evaluated: (i) carbonation of MgO, (ii) hydration of MgO, carbonation of hydrated MgO, (iii) carbonation of Mg(OH)2, (iv) hydration of Mg(OH)2, and (v) hydration of carbonated Mg(OH)2. A comparison of the energy barriers and reaction pathways of these mechanisms shows that the carbonation of MgO is hindered by the presence of H2O molecules, while the carbonation of Mg(OH)2 is hindered by the formation of initial carbonate and hydrate layers as well as presence of excessed H2O molecules. To compare these finding to bulk mineral surfaces, the interactions of the CO2 and H2O molecules with the MgO(001) and Mg(OH)2 (001) surfaces are studied. Therefore, this work presents deep insights into the physical nature of the reactions and the mechanisms involved in hydrated magnesium carbonates production that can be beneficial for its development.


■ INTRODUCTION
Increasing carbon dioxide (CO 2 ) emissions are currently one of the most serious environmental challenges. 1 Cement manufacturing, and specifically the manufacture of ordinary Portland cement (OPC), is the source of ∼5%−7% of global greenhouse gas emissions. 2 Limestone (CaCO 3 ), the conventional feedstock for OPC manufacturing, is excavated, crushed, and sintered with other materials in a cement kiln at temperatures reaching ∼1450°C to produce clinker. During the calcination of CaCO 3 , CO 2 is directly emitted (i.e., CaCO 3 → CaO + CO 2 ), causing ∼50%− 60% of the total emissions from OPC production. 3 From the standpoint of sustainable development, the cement industry is seeking alternatives to reduce CO 2 emissions while maintaining the same performance. 4 Among the proposed alternative binders, Mg-based cements have attracted attention for their promise as partial replacements for OPC. 5 When magnesium oxide (MgO) is derived from Mg silicates (e.g., olivine and serpentine), less environmental and economic impact is generated. 6 The net CO 2 emissions from the carbonation of these binders may be ∼73% lower than OPC 7 and, therefore, may potentially lead to the formation of carbonnegative cements. Moreover, the lower production temperature of reactive MgO-based cement (RMC) compared to that of OPC (i.e., 700−1000°C vs 1450°C), and its potential to gain strength through its reaction with CO 2 , have attracted special attention. 7 Considering the need for the rapid development of carbon capture and utilization technology, 8 the main advantage of RMCs produced from Mg−Si minerals in concrete formulations is their ability to absorb and permanently store CO 2 in the form of stable carbonates during the carbonation process when MgO is sourced from low-CO 2 feedstocks. 9 In such processes, MgO reacts with water (H 2 O) to form brucite (Mg(OH) 2 ), which generally has a weak and porous structure. 5,10 However, hydrated MgO has a strong ability to absorb CO 2 and produce carbonated products at a strength useful for construction purposes. 11 In other words, the dissolution of MgO through hydration results in the formation of Mg(OH) 2 , which is then carbonated according to the following reaction and produces a range of hydrated magnesium carbonates (HMCs): Mg(OH) 2 + CO 2 + 2H 2 O → MgCO 3 ·3H 2 O. Nesquehonite (MgCO 3 ·3H 2 O) is the most commonly obtained HMC, yet other phases such as hydromagnesite (4MgCO 3 ·Mg(OH) 2 ·4H 2 O), dypingite (4MgCO 3 ·Mg(OH) 2 ·5H 2 O), and artinite (MgCO 3 ·Mg(OH) 2 · 3H 2 O) can also be present. 12,13 Recent experimental studies have examined the formation of HMCs through the hydration and carbonation of RMC. In particular, improvement of the hydration and mechanical performance of carbonated MgO-based systems has been observed with the introduction of various hydration agents at different concentrations. 14 In this way, the simultaneous use of magnesium acetate at 0.05 M and carbonate seeds (up to 1% of cement content) improved mechanical performance of carbonated RMC concrete mixes. 15 However, investigation of the physical nature of mechanisms involved in the reactions of HMC production is still immature. One of the reasons for this is the limitation of available experimental methods for the determination of such processes occurring at the nanoscale in bulk materials.
Theoretical approaches with predictive capabilities, such as those based on the density functional theory (DFT), show a high capability for determining the most stable atomic structures and exploring the physical and chemical properties of these finite systems. 16−19 Computational approaches have been successfully utilized to investigate in depth the mechanisms related to the formation of HMCs. For instance, the structure, formation energy, and electronic properties of four commonly exposed surfaces of nesquehonite crystal have been studied by using DFT-based calculations. 16 In another computational work the activity and selectivity of MgO surfaces for CO 2 conversion have been studied. 20 In particular, the adsorption and dissociation of CO 2 , as well as its subsequent hydrogenation to HOCO and HCOO, on various MgO surfaces, have been investigated. It has been shown that the direct dissociation of CO 2 on MgO is thermodynamically unfavorable because of high reaction energy, while hydrogenation of CO 2 to HCOO by hydride H is more feasible on MgO. DFT simulations have also been utilized to compare the adsorption and activation reaction mechanisms of CO 2 and H 2 molecules on hydrogen-assisted MgO(110), pure Ni(111), and Ni/MgO interfaces. 21 Computational methods have also been applied for a deeper exploration of the effects of various promoters and dopants upon CO 2 adsorption on the MgO−CaO(100) surface. 22 Theoretically supported experimental infrared-based studies have been performed to identify the structure of the CO 2 species adsorbed on the various MgO surface. 23 It has been shown that the active site toward CO 2 , which is a Lewis acid, differs from that for the deprotonating adsorption of Brønsted acids. Another experimentally supported computational study provided a comprehensive study of the CO 2 adsorption on the MgO and Mg(OH) 2 surfaces. 24 It has been found that chemisorption of CO 2 on the MgO surface is facilitated by the presence of H 2 O.
Because the reaction degrees of MgO and Mg(OH) 2 are relatively low (ca. 50%), they reduce the effectiveness of CO 2 utilization to form a cementitious binder. 25 Furthermore, because the transformation of HMCs shows mixed diffusion and reaction-limited control, and it proceeds through the production of metastable intermediates, the specifics of nesquehonite conversion to other HMCs remain unclear. The conversion of these metastable intermediates also raises concerns about the durability of cement. 26 Therefore, insights into the potential reactions in the MgO/H 2 O/CO 2 system, and an understanding of the nature of kinetic hindrance in MgO and Mg(OH) 2 carbonation and hydration at the atomic level, are of immediate interest.
In this work, the physical nature of the mechanisms for HMC production on MgO and Mg(OH) 2 nanoclusters is considered by using DFT calculations. Clusters are collections of atoms that act as a link between gases and bulk phase materials (liquids and solids). They are considerably large to be considered as molecules while considerably small to be classified as liquids or solids, and almost all of the atoms in a cluster are on or near its surface, making them a good choice for considering surface reactions. 27 In addition, robust reactions at oxide surfaces, such as the exchange rates of H 2 O molecules on the surface, can be reliably predicted by using molecular simulation methods. 28 Here, the interaction of these nanoclusters of potentially promising RMC raw materials with ambient molecules (H 2 O and CO 2 ) is considered. The mechanism of the following reactions is investigated: carbonation of MgO, hydration of MgO, carbonation of hydrated MgO, carbonation of Mg(OH) 2 , hydration of Mg(OH) 2 , and hydration of carbonated Mg(OH) 2 . Notably, even though through-solution dissolution−precipitation reactions are often the dominating reactions in HMC synthesis, surface carbonation can become important to the overall carbonation kinetics by hindering further reactions, including dissolution. Understanding the mechanisms of these reactions is accomplished by calculating adsorption energy, charge transfer, electron localization function, and adsorption/ dissociation energy barriers of H 2 O and CO 2 upon reactions with the MgO and Mg(OH) 2 clusters. To gain further insights into the difference between MgO and Mg(OH) 2 clusters and bulks, the interactions between the surfaces of bulk MgO and Mg(OH) 2 with H 2 O and CO 2 molecules are also investigated. The results also shed light on the underlying reason for the hindrance of carbonation of MgO and Mg(OH) 2 that has been previously observed experimentally. Therefore, the results of this work reveal the mechanisms that take place during HMC production that can further facilitate the development of their production.

■ METHODS
The calculations were performed based on DFT using the Vienna ab initio simulation package 29 where the electron−ion interactions were simulated via the projector augmented wave method. 30 The generalized gradient approximation with the Perdew−Burke−Ernzerhof exchange-correlation function was employed. 31 The most energetically favorable MgO cluster has a cage-like configuration with T h symmetry that included six Mg 2 O 2 rings and eight Mg 3 O 3 to form a shortened octahedron with equivalent Mg and O vertices. 32 The system considered consisted of a MgO cluster placed in a cubic supercell with dimensions of 20 × 20 × 20 Å. A 3 × 3 × 3 k-point sampling was employed for structure optimization calculations, while a 1 × 1 × 1 k-point was used for electronic structure calculations. Mg(OH) 2 cluster consisting of nine units of Mg(OH) 2 33 was placed in a cubic cell with dimensions of 30 × 30 × 30 Å. A 1 × 1 × 1 Å k-point sampling was applied for all optimization and electronic structure calculations. The considered MgO and the Mg(OH) 2 slabs with the (001) cleaved-plane surface were selected based on the previous work. 34 A 2 × 2 × 1 Å and 1 × 1 × 1 Å k-point sampling was used for MgO and Mg(OH) 2 slabs, respectively.
The Journal of Physical Chemistry C pubs.acs.org/JPCC Article All systems considered were totally optimized to reach atomic forces and total energies less than 0.05 eV Å −1 and 10 −4 eV, respectively. A kinetic energy cutoff of 450 eV was set for all calculations. The van der Waals-corrected functional Becke88 optimization (optB88) 35 was adopted for the consideration of noncovalent chemical interactions between molecules and clusters. The adsorption energy of the molecule is given by the following equation: 36 where E molecule/cluster is the total energy of the cluster with the adsorbed molecule, E molecule is the total energy of the isolated molecule, and E cluster is the total energy of the bare cluster. Under this definition, the negative adsorption energy indicates an exothermic and favorable process. The electrons gained or lost are defined as the difference of valence electrons of an atom in the adsorbed system from the atom in a free molecule or a substrate, according to the equation Δq = q after adsorption − q before adsorption . The negative and positive values indicate electrons gained and lost, respectively. The charge transfer between the molecule and the cluster is given by the charge density difference (CDD) Δρ(r): cluster molecule cluster mol (2) where ρ cluster+molecule (r), ρ cluster , and ρ mol (r) are the charge densities of the cluster with the adsorbed molecule, the bare cluster, and the isolated molecule, respectively. The Bader analysis was used to calculate the charge transfer between the molecules and the clusters. 37 The Arrhenius equation is given by the following formula: where k is the rate constant, A is the pre-exponential factor, E b is the activation energy or the energy barrier for a reaction, R is the universal gas constant, and T is the absolute temperature. 38 The electron localization function (ELF) was calculated to obtain the distribution of electrons in the considered structures. The degree of charge localization in real space is depicted by the value of the ELF (between 0 and 1), where 0 represents a free electronic state and 1 represents a perfect localization. An isosurface value of 0.65 was adopted in this work. 39 The climbing image−nudged elastic band (CI-NEB) method 40 was used to obtain the reaction pathway of the molecule on the cluster. The AIMD simulations were performed at room temperature of 300 K. The simulation lasted for ∼5 ps with a time step of 1 fs, and the temperature was controlled by a Nose−Hoover thermostat. 41 ■ RESULTS AND DISCUSSION MgO Interaction with CO 2 and H 2 O. The interaction of the MgO cluster with the CO 2 molecule is considered to simulate the formation of MgO−CO 2 (MgCO 3 ) as the main precursor to HMCs. For this, various absorption configurations of the CO 2 molecule on the MgO cluster are considered (more details see Figure S1 in the Supporting Information). Figure 1a shows the lowest-energy configuration structure of the CO 2 molecule adsorbed on the MgO cluster combined with the CDD plot. In the most stable configuration, the O atom of the CO 2 molecule is bonded to the Mg atom of the MgO cluster. The length of the created Mg−O bond is 2.207 Å. The length of the C−O bond of the CO 2 molecule is elongated from 1.174 Å (bare CO 2 ) to 1.188 Å (CO 2 after adsorption on MgO). It is also found that the ∠(O−C−O) angle of CO 2 adsorbed on the MgO cluster decreases to 171.94°compared to 179.95°for the bare CO 2 . Table S1 combines the results for the adsorption energy E ads and charge transfer Δq between the CO 2 molecule and the MgO cluster. It is shown that E ads of the CO 2 molecule on the MgO cluster is −0.42 eV. According to the CDD plot (see Figure 1a), the CO 2 molecule acts as an acceptor to the MgO cluster with the charge transfer from the surface to the molecule of 0.092 e (see Table S1), which can be attributed to the basicity of the MgO cluster, as it can donate a pair of nonbonding electrons following the Lewis base role. 21 The observed elongation of the C−O bond and the enhanced charge transfer between the cluster and molecule suggest a strong interaction between them. The high electronegativity of O atoms of the molecule can be the driving force for the observed charge transfer compared to that of Mg atoms of the cluster. However, the ELF analysis (see Figure 1b) shows that electron density is  To deeper understand the interaction of the CO 2 molecule with the MgO cluster, density of states (DOS) and local density of states (LDOS) analyses of CO 2 −adsorbed MgO are performed (see Figure 1c). The bare MgO cluster has higher HOMO and HOMO−1 states than the CO 2 molecule, which indicates its tendency to oxidize the molecule, whereas the CO 2 molecule possess LUMO and LUMO+1 states, which verifies its ability to gain electrons. Moreover, strong overlapping of LUMO and LUMO+1 states is observed upon the interaction between the molecule and the cluster, suggesting a strong interaction between them. In addition, AIMD simulations are conducted to study the interaction of the CO 2 molecule with the MgO cluster at room temperature. The AIMD calculations (see Movie S1) confirm the possibility of the chemisorption of CO 2 on the MgO cluster at room temperature and suggest a low energy barrier E b for the reaction, as it is proposed from the E a and charge transfer calculations. Therefore, the chemisorption process of CO 2 on MgO is further considered.
The chemisorbed configuration of CO 2 is chosen based on the AIMD-obtained configuration (see Figure S2). In that case, the length of the Mg−O bond formed between the cluster and the molecule is 2.080 Å, which is shorter than that in the physisorbed state (2.207 Å). The length of the newly formed Mg−O bond in the chemisorbed configuration is 2.092 Å. The C−O bond lengths of the CO 2 molecule are 1.269 and 1.266 Å, which are significantly longer than those of the CO 2 in its physisorbed state (1.188 Å). This indicates that C−O bonds of CO 2 are highly elongated upon it interaction with Mg atoms. The ∠(O−C−O) angle of 179.95°of bare CO 2 decreases to 129.69°for CO 2 adsorbed on the MgO cluster. The CDD plot (see Figure 1d) and the Bader charge transfer analysis (see Table  S1) predict that CO 2 is an acceptor to MgO as it accumulates 0.117 e from the MgO cluster. The amount of charge transferred from MgO to chemisorbed CO 2 is higher than that from MgO to physisorbed CO 2 (see Table S1). Furthermore, E ads of CO 2 on MgO in its chemisorbed state is −1.05 eV (see Table S1), which is more than twice higher that of CO 2 physisorbed on MgO.
From Figure 1e, which shows ELF of CO 2 chemisorbed on MgO, it is seen that electron localization located on the C−O bond formed between CO 2 and MgO. In addition, strong electron redistribution is observed on O atoms of CO 2 , suggesting the formation of covalent bonds between the molecule and the cluster while the C−O covalent bonds of the CO 2 molecule remain stable. That contribute to the depletion of electrons from the surface to the molecule as it is observed in the CCD plot in Figure 1d.
According to the DOS and LDOS plots in Figure 1f, there is a strong hybridization of the HOMO, HOMO−1, and LUMO+1 states of the MgO cluster and the CO 2 molecule, indicating a strong interaction between them and signifying the possibility of chemisorption of the CO 2 molecule on the MgO cluster. The AIMD simulations also suggest that the chemisorption of CO 2 on MgO is favorable (see Movie S1 and Figure S2). Thus, the possible reaction mechanism for the transformation process for the CO 2 molecule on the MgO cluster from physisorbed to chemisorbed state is further studied through the NEB approach.  Table  S2). It seems that the O atom of CO 2 has a high tendency to oxidize the Mg atom of the cluster. This oxidation is expedited at IM3 by the approach and further bonding of the C atom of the molecule to the O of the cluster, which leads to a drop of E b to 0.001 eV. At FS, the second O atom of CO 2 is bonded to the Mg atom of the cluster, and E b further drops to −0.617 eV, which suggests the reaction is exothermic.
To summarize, the elongation of the C−O bond and the decrease of the ∠(O−C−O) angle of the CO 2 molecule upon its chemisorption on the MgO cluster comparing to physisorption lead to an increase of E a . 24 In addition, higher charge transfer from the cluster to the CO 2 molecule during chemisorption stabilizes the adsorption of the CO 2 molecule on the cluster. 42 These results are well agreed with found low E b of exothermic transition of CO 2 from physisorbed state to chemisorbed state and with experimental observations confirming that the calcination of magnesite (MgCO 3 ) is an endothermic process. 5 Therefore, the chemisorption of CO 2 on MgO occurs favorably under the reaction conditions.
The reaction of H 2 O with MgO leads to the formation of Mg(OH) 2 , a phase that might also undergo carbonation, which results in the HMC formation. Hence, the hydration of the MgO cluster is also investigated. All possible absorption configurations of H 2 O on the MgO cluster are considered (see Figure  S3). According to Table S1, E ads for the most energetically favorable configuration of adsorbed H 2 O on the MgO cluster (see Figure 2a Table S1). The basicity of the MgO cluster facilitates the electrons transfer from the O atom of the cluster to the H 2 O molecule, while a higher electronegativity of the O atom of the molecule facilitates electron depletion toward H atoms. Such significant charge redistribution between the MgO cluster and the H 2 O contributes to its adsorption. 36 The ELF plot in Figure 2b shows  Table S2). Further reaction at IM2 and IM3 leads to the bonding of the H atom of the H 2 O molecule to the nearest O atom of the cluster and the consequent H 2 O dissociation at FS occurring with an energy release of 0.179 eV. A higher energy release during the carbonation (−0.617 eV) of the MgO cluster compared to that during hydration (−0.179 eV) of the MgO cluster indicates that the carbonation of the MgO cluster is a more exothermic process than its hydration. Therefore, the carbonated MgO is more thermodynamically stable. However, E b for carbonation of the MgO cluster is 0.235 eV, which is lower than E b of 0.245 eV for hydration of the MgO cluster. On the other hand, AIMD simulations suggest that the hydration of the MgO cluster passes faster than its carbonation (see Figure S4). Therefore, hydration and carbonation rates of the MgO cluster are compared based on the Arrhenius equation (eq 3), according to which the reaction rate depends on two factors: activation energy of the reaction and pre-exponential factor A. Therefore, besides the calculated E b , the A factor, describing the frequency of collisions between reactant molecules at a standard concentration, should be taken into consideration for the comparison of hydration and carbonation rates of the MgO cluster. The hydrolysis of the MgO cluster changes its structure due to a break of Mg−O bonds of the MgO cluster upon interaction with H 2 O, while the carbonation of the MgO cluster does not cause the alteration of the MgO cluster. This leads to a significant difference in the A factor for the hydration and carbonation of the cluster. As a result, the hydration of the MgO cluster is faster than its carbonation as it is shown by AIMD simulations (see Figure S4, Movie S1, and Movie S2). This observation is also in line with the fact that E ads of the H 2 O molecule (−0.95 eV) on the MgO cluster is more than 2 times lower than that of the CO 2 molecule (−0.42 eV) on the MgO cluster, which leads to faster hydration reaction. Faster hydration of the MgO cluster is also observed in AIMD simulations (see Figure S4)  O molecule dissociation on the MgO cluster is an exothermic process, and the carbonation of MgO is thermodynamically more favorable than its hydration. However, although the calculated E b for the hydration of the MgO cluster is higher than that for its carbonation, the hydration of the MgO cluster is found to be faster, as confirmed by the calculated E ads and AIMD simulations.
As it is found that hydration of MgO occurs faster than its carbonation, the CO 2 molecule interaction with the hydrated MgO cluster (previously found lowest-energy configuration of hydrated MgO is used) is studied. Several possible configurations of the CO 2 molecule on the hydrated MgO cluster are considered (see Figure S6). Figure 3a  The CDD plot in Figure 3a shows that the CO 2 molecule is an acceptor to the hydrated MgO cluster as there is a depletion of the electron on the Mg atom of the cluster and accumulation of electrons on the O atom of the CO 2 molecule. The Bader charge transfer analysis predicts that the amount of the charge transferred from the cluster to the molecule is 0.058 e. Importantly, E ads of the CO 2 molecule on the hydrated MgO cluster is −0.53 eV, which is lower than that of the CO 2 molecule on the bare MgO cluster. This suggests stronger bonding of the CO 2 molecule with the hydrated MgO cluster compared to the bare MgO cluster. The ELF plot in Figure 3b Figure S7). It is observed that the carbonation of bare MgO occurs slower than the carbonation of hydrated MgO due to the formation of OH groups on the MgO cluster during its hydration, which hinder the carbonation process.
To gain insights into the carbonation mechanism of hydrated MgO, the chemisorption process of CO 2 on it is considered. The lowest-energy configuration of chemisorbed CO 2 molecule on the hydrated MgO cluster (for more details see Figure S7) is shown in Figure 3d angle of the adsorbed CO 2 molecule is found to be 129.16°, which is lower than that of the CO 2 molecule in its physisorbed state. The CDD plot in Figure 3d shows that the charge is mostly distributed on the CO 2 molecule and partially on the O atom of the MgO cluster bonded to the C atom of the CO 2 molecule. The basicity of the hydrated MgO cluster drives the electron transfer from the molecule to the hydrated cluster. According to the Bader charge transfer analysis, the chemisorbed CO 2 molecule gains 0.086 e from the hydrated MgO cluster. Therefore, the amount of the charge transferred from the hydrated MgO cluster to the chemisorbed CO 2 molecule is higher than that from the hydrated MgO cluster to the physisorbed CO 2 molecule (see Table S1). The calculated E ads of −1.55 eV for the CO 2 molecule chemisorbed on the hydrated MgO cluster is higher than that of the CO 2 molecule chemisorbed on the bare MgO cluster (−1.05 eV). The ELF plot in Figure  An E b of 0.275 eV (see Table S2) for the transition of the CO 2 molecule from the physisorbed state to the chemisorbed state on the hydrated MgO cluster is calculated by the NEB approach (see Figure 3g). The transition involves the IM2 stage, where the O atom of the CO 2 molecule oxidizes the Mg atom of the MgO cluster, which leads to the drop of E b to 0.153 eV. This triggers an exothermic process of bonding the C and O atoms of the CO 2 molecule to the hydrated MgO cluster at the FS state via the IM3 (−0.800) state with the released energy of 1.028 eV. According to calculated reaction energies in the carbonation process of bare MgO (−0.617 eV) and hydrated MgO (−1.028 eV), carbonation of the hydrated MgO is thermodynamically more favorable. However, E b for the transition of CO 2 from the physisorbed state to the chemisorbed state on hydrated MgO (0.275 eV) is higher than that of CO 2 on bare MgO (0.234 eV). Therefore, CO 2 chemisorption on hydrated MgO is kinetically unfavorable. This matches the AIMD simulation results (see Figures S2 and S6), where the carbonation of bare MgO is faster than that of the hydrated MgO. Importantly, this verifies the fact that the initial hydration of MgO can hinder its carbonation. 15 In summary, the chemisorption of the CO 2 on the MgO cluster is found to be the most energetically favorable. The charge redistribution between the MgO cluster and the CO 2 molecule during the chemisorption 37,38 and the comparison of the energy released during carbonation of the bare and the hydrated MgO clusters suggest carbonation of the bare MgO cluster is faster than that of the hydrated MgO cluster, which uncovers the hindrance effect of H 2 O on the carbonation of MgO. The observed results are also supported by AIMD simulations (see Movie S4 and Figure S7). Mg(OH) 2 Interaction with CO 2 and H 2 O. In RMC reactions, the carbonation of Mg(OH) 2 leads to the production of a range of HMCs. 12,13 Therefore, the mechanism of the carbonation of Mg(OH) 2 is further studied. For that, several possible configurations of the CO 2 molecule and the Mg(OH) 2 cluster are examined (see Figure S8). The most favorable sites for the CO 2 molecule adsorption on Mg(OH) 2 are located at its edges. Figure 4a combines the atomic structure of the lowestenergy configuration and the CDD plot for the CO 2 molecule adsorbed on the Mg(OH) 2 cluster. In that case, the C atom of the CO 2 molecule is located below the O atom at the edge of the Mg(OH) 2 Table S1). This verifies the Lewis basicity of the Mg(OH) 2 cluster. According to Table S1, the E ads of CO 2 on Mg(OH) 2 is −0.69 eV.
The ELF plot in Figure 4b shows electron localization between the O atom of the CO 2 molecule and the Mg atom of the Mg(OH) 2 cluster, which characterizes electron transfer and strong bonding between the molecule and the edge of the cluster. The covalent bonding within the molecule also remains stable, as predicted by the charge localization on both the C−O bonds of the CO 2 molecule. The DOS and LDOS plots for the CO 2 −adsorbed Mg(OH) 2 cluster are shown in Figure 4c. The observed strong orbital hybridization of CO 2 and Mg(OH) 2 at the energy of −1.7 eV and in a range from −2 to −3.7 eV confirms the strong interaction between CO 2 and Mg(OH) 2 proposed by the charge transfer and ELF analysis. Figure 4d depicts the potential energy profile and atomic structures corresponding to the minimum-energy pathway for the carbonation of the Mg(OH) 2 cluster. It is shown that E b for the carbonation of Mg(OH) 2 is as low as 0.002 eV (TS in Figure  4d), which is equivalent to a spontaneous process at room temperature. To reach the chemisorbed state at FS (−0.303 eV), the CO 2 molecule passes through the IM2 state (−0.064 eV), where the C atom of the molecule bonds to the O atom of the cluster, and IM3 (−0.256), at which point the O atom of the molecule forms a bond with the Mg atom of the cluster. It is also noted that the carbonation of Mg(OH) 2 is a highly exothermic process.
In summary, the elongation of C−O bonds and the decrease of ∠(O−C−O) of the CO 2 molecule, along with the strong charge transfer between the molecule and the Mg(OH) 2 cluster, play a dominant role in CO 2 chemisorption on Mg(OH) 2 . Despite the chemisorption of CO 2 on Mg(OH) 2 occurring only at the edges, the chemisorption mechanism of CO 2 for Mg(OH) 2 is similar to that for MgO. In both cases, chemisorption of CO 2 is an exothermic process with low E b and significant energy release. However, the activation energy for the carbonation of Mg(OH) 2 (0.002 eV) is significantly lower than that for MgO (0.049 eV), confirming that carbonation of the Mg(OH) 2 is faster than that of the MgO. In turn, the energy released during the MgO carbonation (−0.617 eV) is about 2 times lower than that during the Mg(OH) 2 carbonation (−0.303 eV), which suggests that the carbonation of the MgO cluster is more thermodynamically favorable.
Mg(OH) 2 is often affected by aqueous environments; therefore, the interaction of the H 2 O molecule with the Mg(OH) 2 cluster can play a key role in HMC formation. From the studied configurations for that interaction of H 2 O with Mg(OH) 2 (see Figure S9 Table S1). E ads of the H 2 O molecule on the Mg(OH) 2 cluster is −0.74 eV (see Table S1). The ELF plot in Figure 4f shows insignificant electron distributions between the O atom of the H 2 O molecule and   Figure 4g, also suggest a weak interaction between the molecule and the cluster at −1.5, −1.8, and −2 eV and in the ranges from −2.2 to −2.5 eV and from −4.1 to −4.2 eV. In summary, it is found that the H 2 O molecule is located at the edges of the Mg(OH) 2 cluster. The calculated low E ads and weak charge transfer between the H 2 O molecule and the Mg(OH) 2 cluster suggest that H 2 O is physisorbed on Mg(OH) 2 . However, it is well-known that the presence of H 2 O facilitates the formation of HMCs in accessible pores during the carbonation process. 37 To investigate a mechanism of reaction of nesquehonite formation (Mg(OH) 2 + CO 2 + 2H 2 O → MgCO 3 ·3H 2 O), at the first step, the simultaneous interaction of the carbonated Mg(OH) 2 cluster and the H 2 O molecule (Mg(OH) 2 + CO 2 + H 2 O) is considered. At the second step, one more H 2 O molecule is introduced to the system studied at the first step (CO 2 + 2H 2 O + Mg(OH) 2 ). Although the natural process of the nesquehonite formation also includes nucleation and growth from species in solution, the studied reaction will still help to understand possible nucleation or growth paths of nesquehonite. At the first step, various configurations of one H 2 O molecule (see Figure  S10 Table S1, E ads of the H 2 O molecule on the Mg(OH) 2 cluster is −0.86 eV. The CDD plot in Figure 5a shows that there is a depletion of electrons at the edge O atoms of the Mg(OH) 2 cluster and charge accumulation at the H atoms of the H 2 O molecule. The Bader charge transfer analysis shows that the H 2 O molecule gains 0.046e from the Mg(OH) 2 cluster, which confirms that H 2 O is a weak acceptor to Mg(OH) 2 (see Table  S1). In addition, the ELF plot in Figure 5b shows H 2 O is physisorbed on Mg(OH) 2 , as there is no electron density localization between the Mg(OH) 2  In addition, at the second step, the second H 2 O molecule is introduced to the Mg(OH) 2 + CO 2 + H 2 O system obtained at the first step (see Figure S11). The lowest-energy configuration of the H 2 O molecule on the Mg(OH) 2 + CO 2 + H 2 O system is shown in Figure 5d, where the second H 2 O molecule is also located at the edge of the carbonated Mg(OH) 2 Figure 5d displays the depletion of electrons at O atoms located at the edge of the Mg(OH) 2 cluster and charge accumulation at the H atoms of the H 2 O molecule. According to the Bader charge transfer analysis (see Table S1), the H 2 O molecule is a weak acceptor to the Mg(OH) 2 cluster with the charge transfer of 0.037e from the cluster to the molecule. The ELF plot in Figure 5e shows no election density localization between the second H 2 O molecule and the Mg(OH) 2 + CO 2 + H 2 O system, which means there is a weak interaction between them. The DOS and LDOS plots in Figure 5f also display a weak interaction of the H 2 O molecule and the cluster at the range from −2.8 to −3.5 eV.
AIMD simulations are used to investigate the reaction for the formation of HMCs via the interaction of the CO 2 and H 2 O molecules with the Mg(OH) 2 cluster (see Movie S5 and Figure  5g,h). As it is shown, the CO 2 and H 2 O molecules are bonded at the edges of the Mg(OH) 2 cluster, which suggests that the formation of HMCs starts at the edges of Mg(OH) 2 . In addition,   Figure 5h, the first CO 2 molecule can carbonate the Mg(OH) 2 cluster. However, after the bonding of the first CO 2 molecule to the cluster, the second CO 2 molecule is unable to bind to the carbonated Mg(OH) 2 cluster (Figure 5i,j). This suggests that the formation of an early layer of carbonates in RMC-based concrete formulations may limit the continuation of carbonation by forming a physical barrier that prohibits further interaction between Mg(OH) 2 and CO 2 . These limitations in carbonation of Mg(OH) 2 can cause large amounts of unreacted crystals leading to relatively low strength and porous microstructures. 15 Table S3). This indicates weak adsorption of CO 2 on the bulk MgO(001) compared to the physisorbed CO 2 on the MgO cluster (E ads = −0.42 eV). According to Figure 6b, 24 Figures 6c and 6d show the lowest-energy configurations of the CO 2 and H 2 O molecules on the Mg(OH) 2 (001) surface. Figure 6c indicates the location of the C atom of the CO 2 molecule is located above the Mg−O bond of the Mg-(OH) 2 (001) surface. The C−O bonds in the CO 2 molecule elongates from 1.174 Å (bare CO 2 ) to 1.177 and 1.178 Å, and ∠(O−C−O) changes from 179.95°to 179.00°. E ads of the CO 2 molecule on the Mg(OH) 2 (001) surface is as low as −0.25 eV, which is significantly lower than that of the CO 2 molecule on the Mg(OH) 2 cluster (−0.69 eV) (see Table S3). Figure 6d Table  S3). In addition, the distance between the H 2 O molecule and the Mg(OH) 2 (001) surface of 2.017 Å is longer than that between the H 2 O molecule and the Mg(OH) 2 cluster (1.957 Å). Similar to the case of MgO, the lower E ads of the CO 2 and H 2 O molecules on the Mg(OH) 2 cluster, compared to that on the Mg(OH) 2 (001) surface, suggests stronger interaction of these  ■ ASSOCIATED CONTENT

* sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.1c10590. Figure S1: various configurations of the CO 2 molecule on the MgO cluster; Figure S2: adsorption of CO 2 molecule on the MgO cluster; Figure Table S1: adsorption energy E ads and the amount of charge transfer Δq between the molecules and the clusters; Table S2: the energy barrier E b and the donor/ acceptor characteristics of the molecules on the clusters;