The Effects of Carbonation and Elevated Temperatures on the Properties of Magnesium-Based Mortar

Abstract

Reactive MgO cement can substitute Portland cement in constructions for numerous purposes. While hydration of MgO yields a low-strength material, incorporation of additives, CO₂ and heat curing can improve strength development. This study highlights how curing conditions and additives affect reactive MgO cements under elevated temperatures, providing key insights for enhancing sustainable construction materials with lower carbon footprints. Therefore, mortar mixtures were carbonated in a CO₂ incubator for 3–14 days utilizing reactive MgO cement as a binder. Sodium carbonate and sodium bicarbonate were separately incorporated into mixtures to investigate their influence on compressive strength and microstructure. Elevated temperatures were also tested on carbonated magnesium-based material. Carbon dioxide curing enhanced compressive strengths and the reference carbonated group reached 20.1 MPa at the 14th day of curing. Sodium carbonate reduced the hydration rate and compressive strength, while air-cured mortar with sodium carbonate solution had the lowest compressive strength. However, sodium bicarbonate increased the strength of air-cured specimens. The specimens heated to 50 °C demonstrated a 35% increase in compressive strength, while temperatures of 100 °C or above led to a decline in strength. Hydration of MgO produced brucite and carbonation formed dypingite in the specimens’ microstructure. Temperature rise led to the disintegration of dypingite and rthe reappearanceof MgO.

1. Introduction

Over the past decade, global production of Portland cement (PC) has exceeded 4.4 billion tons [1]. Approximately 7% of world energy use and 7–8% of global carbon emissions are attributed to this industry [2,3]. Consequently, numerous recent investigations have concentrated on the advancement of low-carbon emission and energy-efficient cement alternatives. The manufacturing of reactive MgO cement (RMC) is one of the effective alternatives identified in these investigations. RMC necessitates lesser energy use due to its ability to be calcined at reduced temperatures compared to PC [4]. RMC can be produced from the by-products generated during seawater desalination procedures and can also be extracted from seawater in various ways [5,6]. Moreover, magnesium carbonate binders derived from non-fossil MgO sources have garnered considerable attention for their capacity to sequester CO₂ rather than emit fossil CO₂ during manufacturing [7,8]. There are two separate methods available for carbonation-based strength development: curing in a CO₂ chamber for pre-cast elements, and in situ hydration involving the reaction of MgO with Mg-carbonates in the presence of water [7,8,9,10,11,12]. The carbonation in CO₂ chambers facilitates substantial CO₂ sequestration, with 1 ton of MgO capable of sequestering 1.1 tons of CO₂, and such a process is deemed appropriate for the fabrication of pre-cast masonry units, particularly bricks [13]. The hydration of MgO/hydrated magnesium carbonate phase blends under atmospheric conditions usually results in reduced CO₂ binding in the cement while enabling the on-site production of mortar and concrete [8,10,14].

The utilization of reactive MgO as a cement substitute presents certain challenges, despite its potential benefits. Prior research indicates that mechanical strength decreases as MgO content increases, which is linked to alterations in porosity and hydration products [15]. Furthermore, the morphological and mechanical properties of the products arising from the hydration and carbonation of RMC fluctuate according to the differing factors of hydration and carbonation [16,17,18,19]. Subsequent to these processes, the magnesium compounds such as MgCO₃·3H₂O (nesquehonite), 4MgCO₃·Mg(OH)₂·4H₂O (hydromagnesite), and 4MgCO₃·Mg(OH)₂·5H₂O (dypingite) can be synthesized and the strength of these compounds is mostly influenced by the conditions of the carbonation environment [20]. Similar to PC concrete, carbonation products fill the voids in the material, enhancing its strength. Studies suggest that the compound MgCO₃-3H₂O possesses a dense structure, and mixtures containing MgCO₃·3H₂O exhibit greater ultimate strength compared to other carbonate products [21]. Unluer and Al-Tabbaa [20] assert that the conversion of MgO into the MgCO₃·3H₂O compound during carbonation results in a solid volume 2.34 times greater than that of Mg(OH)₂, characterized by needle-like elongated features in its shape [21]. This leads to a reduction in void volume, thereby enhancing stiffness and strength significantly. MgCO₃·3H₂O is more readily acquired when the ambient pH is approximately between 7.5 and 9.0 and the temperature ranges from 40 °C to 65 °C [22]. Nonetheless, at temperatures beyond 50 °C, the stability of MgCO₃·3H₂O diminishes, leading to its transformation into other forms such as 4MgCO₃·Mg(OH)₂·4H₂O and 4MgCO₃·Mg(OH)₂·5H₂O [16]. Although more stable structures are generated post-transformation, these microstructural alterations may affect the ultimate strength. Consequently, it is essential to observe alterations in the microstructure and to possess knowledge regarding the final product to attain the anticipated ultimate strength.

The high temperature can adversely affect the hydration products of RMC by causing dehydration and decarbonation of critical phases like hydrated magnesium carbonates and brucite, which compromises strength. The initial reactivity and purity of RMC, as well as curing conditions, determine the extent of hydration and strength gain before thermal exposure. Effective management of these parameters and prevention of high-temperature exposure beyond critical decomposition thresholds are crucial for preserving the mechanical performance of RMC-based materials [23]. Hay et al. have shown that RMC-based concretes exhibit good thermal stability up to approximately 300 °C; however, they lose significant mechanical integrity beyond this temperature due to phase decomposition [24]. Research examining carbonated MgO-based engineered cementitious composites indicated that thermal exposure up to 500 °C influences mechanical integrity and microstructure, providing insights into temperature resilience [25]. In another study, the mechanical performance and microstructural evolution of MgO-based binder systems at elevated temperatures have been analyzed, revealing the hydration-retarding effects of MgO that affect durability [26]. Furthermore, the use of additives like biochar in reactive magnesium oxide calcium aluminate cement mixtures has been investigated to improve carbonation and associated temperature-dependent performance and it was concluded that biochar serves as a sustainable, bio-renewable addition that enhances carbonation efficiency and provides environmental advantages for reactive MgO-based cements by facilitating carbon sequestration and diminishing dependence on conventional cementitious materials [27]. Moreover, research on reactive magnesium oxide cement cured under ambient and accelerated conditions has enhanced comprehension of its chemical and mechanical durability when subjected to elevated temperatures [28].

Based on the aforementioned literature summary, it is believed that the microstructural formations in RMC, a potential environmentally friendly alternative to PC, are temperature-sensitive due to the carbonation hardening process. Furthermore, the impact of elevated temperatures on the mechanical properties of the material remains ambiguous. The present study systematically examines the microstructural changes and mechanical properties of reactive MgO cement when exposed to high temperatures. Effort is directed toward describing temperature-induced alterations of magnesium carbonate phases, particularly dypingite and associated hydrates, and their influence on strength development. The impact of precursor characteristics and carbonation processes on the strength and thermal response of reactive MgO cement-based materials is analyzed. The findings seek to enhance comprehension of the temperature sensitivity of carbonation hardening in reactive MgO cement and to guide the development of durable, sustainable, low-carbon cementitious alternatives appropriate for various construction applications.

2. Materials and Methods

In experimental studies, three mortar series were produced: a reference mortar group including solely RMC, and two RMC-based mortar groups, each comprising the precursor substances sodium carbonate and sodium bicarbonate, respectively. The fresh mortars were placed into 5 cm-cubic molds and subsequently subjected to air and accelerated CO₂ curing (ACC) for 3 and 14 days, after which their compressive strength was determined. Further investigations were carried out on the reference specimens to understand the effect of high temperatures on the mechanical performance of magnesium-based carbonated materials. Additionally, microstructural analyses, TGA, and XRD were conducted on the specimens.

2.1. Materials and Mix Proportions

RMC powder with 98% purity was used as a binder in the mixtures. The purity of the powder was verified via XRF analyses and the results confirmed the high purity of magnesium oxide. The oxide concentrations were as follows: MgO 99.2%, P₂O₅ 0.4%, CaO 0.2% and other oxides 0.2%. The particle size distribution curve of magnesium oxide can be seen in Figure 1. BET analysis showed that the surface of MgO was 24.87 m²/g and the density of MgO was measured as 3.69 g/cm³. Acid neutralization was performed to measure the reactivity of RMC. Phenolphthalein, a pH indicator, and a 1.0 M acetic acid solution were utilized to assess the reactivity of RMC used in the study. The measurements showed that the color change, representing neutralization, occurred within 20–25 s, suggesting a high reactivity associated with a lower neutralization duration.

In mortar mixtures, two different precursors, namely sodium carbonate (Na₂CO₃) and sodium bicarbonate (NaHCO₃), in powder form, were incorporated in order to understand their effects on the carbonation of RMC. Solutions were prepared with these precursors provided that the molarity of the solutions was 0.12 M. Prewashed and graded river sand was used in the mortar mixtures. The designation of the mixture groups and their mix properties are given in

Table 1. Mix proportions.

Group	w/b	RMC	Water	0.12 M Na2CO3	0.12 M NaHCO3	River Sand	Air Curing (Days)		CO2 Curing (Days)		Heat Exposure (°C)
		(kg/m3)					3	14	3	14	50	100	200	400
R3A	1.3	430	560			860	+
R14A								+
R3C									+
R14C										+
R14C50										+	+
R14C100										+		+
R14C200										+			+
R14C400										+				+
SC3A	1.3	430		567		860	+
SC14A								+
SC3C									+
SC14C										+
SBC3A	1.3	430			565.5	860	+
SBC14A								+
SBC3C									+
SBC14C										+

. 5 × 5 × 5 cm³ cubic specimens were cast for compressive strength measurements [29]. The specimens were kept in their curing medium after production for 24 h and then they were demolded and labeled. After demolding, the specimens were returned to their curing medium. A similar production process was followed for paste specimens to be used for microstructural investigations. The paste specimens, cast into 50 mL Falcon tubes, were produced with the same water-to-binder ratio as the mortar specimens in order to eliminate the effects of sand particles on the microstructural analyses, especially in XRD analysis.

2.2. Curing Conditions

Specimens were subjected to ACC and air curing. Half of the specimens of each mixture group were cured in a carbon dioxide incubator at 10 ± 0.2% carbon dioxide concentration, 25 ± 3 °C temperature, and 80 ± 5% RH; the remaining half was kept in a conditioning chamber at 25 ± 3 °C, 65 ± 10% RH. Specimens cured in a carbon dioxide incubator were designated with the letter C, and the air-cured specimens were designated with the letter A. All of the specimens were kept in their curing medium for 3 and 14 days. Following the curing periods, compression tests, XRD, and thermal gravimetric analyses (TGA) were conducted.

2.3. Heating Procedures

The effects of high temperatures on both the mechanical and the microstructural properties of the carbonated RMC-based materials (on the reference group, R) were investigated on the specimens subjected to the heating regimes shown in Figure 2. The heating rate was 5 °C/min during the heating procedures, and the specimens were kept in the furnace for 2 h at the maximum temperature. After the heating process, the specimens were left in the furnace till the furnace temperature dropped to room temperature. Before the heating, the initial mass of the specimens was measured and the final mass of the specimens was determined following the cooling.

2.4. Mass Change Monitoring and Compression Tests

The mass of 5 × 5 × 5 cm³ cubic specimens was measured after demolding and at the end of their curing periods and heating processes. After curing and heating periods, compressive strength tests were conducted on the cubic mortar specimens by using a uniaxial testing machine with a loading rate of 0.60 MPa/s.

2.5. Powder Preparation and TGA/XRD Protocols

The specimens were ground using a mortar and pestle and then sieved through a 150 μm sieve. The powder samples were used to conduct TGA and X-ray diffraction (XRD) analyses. In TGA, the sample mass was 17 ± 5 mg and the temperature was increased linearly from ambient temperature to 1000 °C with a heating rate of 10 °C/min under a nitrogen flow of 60 mL/min. Identical samples were used to characterize the crystalline phases of hydration and carbonation products. In XRD analysis, the diffraction patterns were obtained between 5° and 80° with a step size of 0.04°, the anode material was Cu, and the intended wavelength was K-α (with generator settings 40 mA and 45 kV).

3. Results

The changes in RMC-based mortars were investigated regarding three different methods: ACC, use of precursors, and temperature exposure. Mass and compressive strength of the specimens were determined after curing and heating and the evaluations were made with the help of microstructural investigations. For mass and compression strength measurements, three specimens from each group were used and their average values with error bars representing a 95% confidence interval were shown in the figures.

3.1. Mass Changes

Mass change in the cubic specimen was shown in Figure 3. Air-cured specimens, especially the groups without precursors, showed higher mass reduction compared to the other groups. According to the results, precursors slowed down the water loss of the specimens and this may be attributed to the accelerating effect of precursors on the reaction between water and magnesium oxide [30]. Mass reductions in R3A and R14A were 14.3% and 18.2%, respectively. On the other hand, 4% and 1.2% mass reductions were measured in the SBC3A and SBC14A groups, respectively. ACC resulted in a mass increase after both 3 days and 14 days in each specimen group. The mass increase was more pronounced in the SBC14C group and was 4% of the initial mass of the group. However, to understand the extent of mass increase due to carbonation, the drying of the specimens should also be taken into account. For example, the mass change related to the carbonation of the reference mortar group can be calculated by using Equation (1). When both the mass loss of R14A and the mass increase of R14C are considered together, the mass change related to the carbonation of the reference groups is calculated to be around 21%. The sum of these values represents both the amount of carbon dioxide absorbed and the amount of free water prevented from evaporation due to the carbonation of the surfaces of the specimens. After the same procedure is applied to the specimens with precursors, the total mass increases are around 3% and 6% in the SC and SBC groups, respectively. According to the overall mass change results, it can be seen that the hydration of magnesium oxide was higher in the specimens with precursors, especially after 3 days of curing but the carbonation of the reference groups was much better.

$$\text{Mass change due to carbonation}=\text{Mass increase of R14C + Mass reduction of R14A}$$

(1)

The effect of high-temperature exposure on the mass of the specimens was also monitored and the results are given in Figure 4. The mass loss due to heating was the lowest in the specimens subjected to 50 °C compared to other temperature levels. As the heating temperature increased, the mass loss of the specimens increased at the same time and reached 25% at 400 °C. The extent of mass loss was higher at high temperatures because of the decomposition of microstructures in the specimens. These microstructural phases will be evaluated in the microstructural analyses section.

3.2. Compressive Strength Measurements

Compression tests were conducted on the 5 × 5 × 5 cm³ cubic specimens after each curing method and the results are given in Figure 5. Compressive strength of the specimens increased with curing age in both air-cured and carbon dioxide-cured specimens. In order to see the influence of carbonation on strength development, the average compressive strength values of the three groups after air curing and ACC were compared and calculated as 1.7 MPa and 5.4 MPa, respectively. At the end of 14 days of curing, the average compressive strength values of the same groups were 3.4 MPa and 13.9 MPa, respectively. Air-cured specimens showed a 100% increase and carbon dioxide-cured specimens showed a 157% increase in their compressive strength at the 14th day of production. Because of the fact that the strength development of air-cured specimens depends mostly on the hydration of magnesium oxide, while the strength development of carbon dioxide-cured specimens depends both on the hydration of magnesium oxide and carbonation [31]. A similar effect was observed when the strength development of carbon dioxide-cured specimens and air-cured specimens was compared at each curing age. Notably, 3 days of curing resulted in 217% strength development in the average compressive strength of the three groups. The strength development rate was 308% at the end of 14 days. When the results are evaluated considering the effects of precursors, it seems that sodium carbonate had an adverse effect on strength development after both air and ACC. On the other hand, sodium bicarbonate had a positive effect on the compressive strength development of 14 days of air-cured specimens. This may be attributed to the effect of sodium carbonates on the pH level of the solution. Sodium carbonate (Na₂CO₃) increases the pH level of the mixture and may slow down the production of magnesium hydroxide; on the other hand, the effect of sodium bicarbonate (NaHCO₃) on the pH level is limited, which may allow both hydration of magnesium oxide and carbonation of the resulting product (magnesium hydroxide) [32]. However, the positive contribution of the precursors to the compressive strength of the carbon dioxide-cured specimens was not observed. Therefore, the highest compressive strength value, 20.1 MPa, was achieved in the reference group after 14 days of ACC.

R14C specimens were also subjected to heating at four different temperature levels: 50 °C, 100 °C, 200 °C, and 400 °C. After the specimens cooled down to room temperature, their residual compressive strength was determined, and the results are given in Figure 6. The compressive strength of the specimens increased 35% at 50 °C but at higher temperatures, the compressive strength of the specimens diminished. The reduction rates were around 14%, 37% and 78% at 100 °C, 200 °C, and 400 °C, respectively. Hay et al. reported a similar behavior at 50 °C and inferred that this strength increase was the result of hydration of the unhydrated RMC particles [24]. Between 50 and 100 °C and between 200 °C and 400 °C there were drastic strength deteriorations. At the first range, the compressive strength reduction rate was 37% and at the second range, the reduction was calculated as 64%. These temperature ranges correspond to the microstructural alteration temperatures which will be further investigated in the microstructural analyses section.

3.3. Microstructural Analyses

Powder samples were collected from paste specimens, which were subjected to the same conditions as the cubic specimens. TGA and XRD analyses were conducted on the powder samples and the effects of carbonation and high-temperature exposure on the microstructure of the magnesium-based material were examined. XRD patterns of the powder samples are given in Figure 7. Phase identification was performed using the ICDD Powder Diffraction Files (PDF). According to the results, the main phase of the air-cured reference samples was brucite (magnesium hydroxide, PDF No. 44-1482). However, the pattern changes in the air-cured samples incorporating precursors, especially in SBC samples. In SC samples, periclase (magnesium oxide, PDF No. 45-0946) peaks appeared in addition to brucite peaks. As it is mentioned above, sodium carbonate slows down the hydration of magnesium oxide particles and results in unhydrated RMC in the specimens. On the other hand, there was no periclase peak in air-cured SBC samples which indicates that a higher amount of hydration occurred in SBC samples compared to SC samples. Moreover, the brucite peaks were wider than the same peaks in the air-cured reference group showing the hydrated products were more amorphous in the SBC groups [33] which may influence the mechanical behavior of the specimens. Peaks representing the presence of dypingite (PDF No. 29-0857) were observed on the XRD pattern of carbonated samples [34]. The peaks on XRD patterns of R14C and R14C50 were almost the same without regard to the intensities. Therefore, the hypothesis about the hydration of unhydrated RMC particles was not substantiated by XRD examinations. As the temperature increased, the dypingite peaks disappeared and hydromagnesite peaks (PDF No. 25-0513) were observed on the XRD pattern of R14C200 due to the fact that dypingite starts losing mass around 50 °C and after 150 °C it releases a water molecule and turns to hydromagnesite [35]. The dehydration of brucite occurs below 400 °C and the recrystallization of magnesium oxide takes place in the specimens subjected to 400 °C [36]. The abrupt drop in strength upon exposure to 400 °C can be ascribed to this microstructural change.

Following TGA measurements, mass loss against temperature curves of the samples were plotted, and by computing the derivative of mass change over time in relation to temperature, derivative thermogravimetry (DTG) curves of the samples were derived, as illustrated in Figure 8. The peaks in DTG curves reflect sudden mass loss resulting from the release of water and carbon dioxide molecules. Once the temperature of the sample reaches 1000 °C, the residual material can be considered magnesium oxide only [34]. Consequently, the mass loss of the samples at certain temperature ranges corresponding to the prominent DTG peaks and the residual mass of the samples after TGA are presented in

Table 2. Mass reduction rates corresponding to DTG peaks.

Sample	Mass Reduction (%)		Residual Mass (%)
	0–250 °C	250–500 °C
R3A	1.7	18.2	78.3
R3C	11.5	28.4	57.5
R14A	2.1	27.0	68.3
R14C	22.6	31.0	44.4
SC3A	2.3	26.2	69.1
SC3C	5.4	20.4	72.0
SC14A	2.0	26.0	69.6
SC14C	11.0	26.5	60.7
SBC3A	3.3	21.9	72.6
SBC3C	6.8	23.9	67.2
SBC14A	3.6	22.9	71.1
SBC14C	13.7	29.0	55.3
R14C50	20.6	30.7	45.8
R14C100	10.6	32.9	46.8
R14C200	7.1	36.4	53.8
R14C400	3.7	18.2	69.7

.

In air-cured specimens, there was only one significant peak representing the dehydroxylation of magnesium hydroxide at 370 °C [37,38,39]. On the other hand, several peaks appeared on the DTG curves of carbonated and heated specimens. Since mass loss of dypingite starts around 50 °C, the peaks below 250 °C represent the decomposition of dypingite and loss of free water [31,35]. The last peak on the DTG curves corresponds to the decarbonation of magnesium carbonate [40,41]. Therefore, the loss between 250 and 500 °C covers both the dehydroxylation of magnesium hydroxide and the decarbonation of magnesium carbonate. The results showed that the residual mass of R3A was the highest (78.3%), indicating that R3A had the lowest hydration and carbonation rate. At the end of 14 days of air curing, the residual mass decreased to 68.3% due to further hydration of magnesium oxide particles in R14A. Considering the dehydroxylation reaction of magnesium hydroxide given in Equation (2) [37,38,39], the hydration of magnesium oxide was almost complete in the R14A sample. Higher residual mass values can be an indication of unhydrated RMC particles. In carbon dioxide-cured SC samples, the extent of hydration and carbonation was lower compared to other samples and this can be attributed to the lowest compressive strength development in SC groups after carbonation. The mass loss below 250 °C increased significantly in the carbonated samples due to decomposition of dypingite. The highest amount of mass loss (22.6%) was recorded in the R14C sample at this temperature range. The mass losses of 14 days of carbon dioxide-cured samples were approximately 100% higher than the mass losses of 3 days of carbon dioxide-cured samples at this temperature range, showing that the carbonation continued after 3rd day of ACC but the rate of carbonation slowed down.

The decomposition formula of dypingite is given in Equation (3) [42]. In order to estimate the amount of dypingite in the carbonated specimens, Equations (2) and (3) can be used together, as it is shown in Equation (4). The R14C sample had the lowest residual mass (44.4%) and should have the highest amount of carbonation. According to Equation (4), 44.4/40 represents the molar ratio of magnesium oxide (molar mass of magnesium oxide: 40 g/mol) and the result is equal to α + 5β. Since the initial mass of the TGA sample is the sum of the mass of magnesium hydroxide and dypingite (assuming that there is no unhydrated magnesium oxide in the sample), 58α + 484β is equal to 100 (molar mass of magnesium hydroxide: 58 g/mol; molar mass of dypingite: 484 g/mol). The result of molar analysis showed that 90.3% of the R14C sample in mass was dypingite.

$${\mathrm{Mg}(\mathrm{OH})}_{2 (\mathrm{s})}\to {\mathrm{MgO}}_{ (\mathrm{s})}+{{\mathrm{H}}_2\mathrm{O}}_{ (\mathrm{g})}$$

(2)

$${4\mathrm{MgCO}}_3·{\mathrm{Mg}(\mathrm{OH})}_2·{5\mathrm{H}}_2{\mathrm{O}}_{ (\mathrm{s})}\to {5\mathrm{MgO}}_{ (\mathrm{s})}+{4\mathrm{CO}}_{2 (\mathrm{g})}+{6\mathrm{H}}_2{\mathrm{O}}_{ (\mathrm{g})}$$

(3)

$$\mathsf{\alpha } {\mathrm{Mg}(\mathrm{OH})}_{2 (\mathrm{s})}+\mathsf{\beta } 4{\mathrm{MgCO}}_3·{\mathrm{Mg}(\mathrm{OH})}_2·{5\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{s})}\to (\mathsf{\alpha }+5\mathsf{\beta }){\mathrm{MgO}}_{(\mathrm{s})}+4\mathsf{\beta }{\mathrm{CO}}_{2 (\mathrm{g})}+(\mathsf{\alpha }+6\mathsf{\beta }){\mathrm{H}}_2{\mathrm{O}}_{(\mathrm{s})}$$

(4)

High-temperature exposure affected TGA results and as the heating temperature increased, the peaks below 250 °C on the DTG curves disappeared first. No peak was observed on the DTG curves of R14C200 and R14C400 below 250 °C because the dypingite-to-hydromagnesite alteration was complete in these samples [43]. There was a prominent reduction in the intensity of the peak representing dehydroxylation of magnesium hydroxide on the DTG curve of R14C400, indicating that most of the magnesium hydroxide turned to magnesium oxide during heating at 400 °C in the furnace [37,38,39]. Therefore, a drastic reduction in the compressive strength of R14C400 was obtained due to the severe microstructural deterioration in the specimens. The total mass loss values of R14C and R14C50 between 250 °C and 500 °C were measured as 31.0% and 30.7%, respectively. Since two values were very close, the hypothesis that the hydration of unhydrated RMC at 50 °C caused improvement in the compressive strength of the material was not supported by the findings of the study.

4. Conclusions

In this study, RMC was used in mortar mixtures, and the samples were subjected to carbonation in a carbon dioxide incubator for 3 and 14 days. To understand the effects of sodium-based precursors on both compressive strength development and the microstructure of the material, sodium carbonate and sodium bicarbonate agents were added to the mixtures. Additionally, the effects of high temperatures (50 °C, 100 °C, 200 °C, and 400 °C) on the properties of the carbonated magnesium-based material were investigated. Based on the measurements and analyses, the following conclusions were drawn:

• Curing time causes an increase in compressive strength after both air and ACC. However, the rate of strength enhancement decreases over time. Because ACC is more difficult to apply than other methods, optimization of the curing time is necessary and can be further investigated in the future.
• After 14 days of air curing, the compressive strength of RMC-based mortar (R14A) reached 3 MPa. On the other hand, ACC improved the compressive strength of the mortar, with the carbonated reference group achieving a value of 20.1 MPa after 14 days of curing (R14C), representing an increase of roughly 6.5 times. A comparable improvement was observed in the mortars employing precursors, demonstrating an approximate threefold increase in both groups.
• Sodium-based precursors had a negative impact on the carbonation and the strength development compared to the reference series. After 14 days of ACC, the lowest compressive strength was measured in the SC14C group, 7 MPa. This was attributed to the effect of sodium carbonates on the mixture pH. The decrease in pH might be the reason for the reduction in both hydration and carbonation rate which can be seen in TGA and XRD results. According to the literature, sodium bicarbonate less strongly affects the pH level of the mixture compared to sodium carbonate; therefore, its negative impact on strength was more limited in this study. The compressive strength of the SBC14C group was measured as 14.6 MPa. Moreover, air-cured samples incorporated with sodium bicarbonate yielded better strength results than the reference group.
• Temperature rise caused an increase in the material’s compressive strength at first, then resulted in a significant degradation. An increase was observed at 50 °C and it was around 35% compared to the strength of R14C. The residual strength was measured as 4.5 MPa after 400 °C heat exposure. In fact, this temperature does not cause significant deterioration in conventional PC materials. However, the decompositions in the microstructure of magnesium-derived materials are significantly completed at temperatures below 450 °C.
• The hydration of magnesium oxide produced brucite, which majorly governs the strength development of air-cured specimens, whereas carbonation led to dypingite formations contributing to the mechanical improvement of the material. On the other hand, temperature rise led to the disintegration of dypingite and the extent of decarbonation was the highest in the specimens subjected to 400 °C, resulting in the reappearance of magnesium oxide.

Use of reactive magnesium oxide directly influences the workability and cohesion of mortar. A similar mixture can be developed to use in 3D printing applications in order to produce free-form pre-cast structural components with the help of ACC.