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Article

Tuning the Carbonation Resistance of Metakaolin–Fly Ash-Based Geopolymers: The Dual Role of Reactive MgO in Microstructure and Degradation Mechanisms

1
Department of Architectural Engineering, Hunan Urban Construction College, Xiangtan 411100, China
2
College of Civil Engineering, Xiangtan University, Xiangtan 411105, China
3
Puyang Institute of Technology, Henan University, Puyang 457000, China
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(10), 549; https://doi.org/10.3390/jcs9100549
Submission received: 2 September 2025 / Revised: 22 September 2025 / Accepted: 2 October 2025 / Published: 7 October 2025
(This article belongs to the Special Issue Composite Materials for Civil Engineering Applications)

Abstract

Geopolymers, as a novel class of low-carbon and eco-friendly cementitious material, exhibit outstanding durability and promote the resource utilization of industrial solid wastes. However, as a promising alternative to ordinary Portland cement, its susceptibility to carbonation-induced degradation may limit its widespread application. To address this challenge, this study systematically examined the effects of magnesium oxide (MgO) content and the metakaolin-to-fly ash ratio on the carbonation performance, mechanical properties, pH value, and microstructures of metakaolin–fly ash-based (MF-based) geopolymer pastes. The findings revealed that an increase in the fly ash ratio correlated with a decline in the compressive strength of MF-based geopolymer pastes. Conversely, the incorporation of MgO significantly enhanced the compressive strength, with higher fly ash ratios leading to more substantial improvements in strength. Furthermore, the addition of MgO and fly ash effectively mitigated the penetration of carbonation and the associated decrease in the pH value of the MF-based geopolymer pastes. Specifically, compared to the control group without MgO (M8F2-0%), MF-based geopolymer pastes with 4% and 8% MgO additions exhibited reductions in carbonation depth of 69.4% and 80.4%, respectively, after 28 days of carbonation, while pH values were observed to be 1.22 and 1.15 units higher, respectively. Additionally, microscopic structural analysis revealed that the inclusion of MgO resulted in a reduction in pore size, porosity, and mean pore diameter within the geopolymer pastes. This improvement was mainly attributed to the promotion of hydration processes by MgO, leading to the formation of fine Mg(OH)2 crystals within the high-alkalinity pore solution, which enhances microstructural densification. In conclusion, the incorporation of MgO significantly improves the carbonation resistance and mechanical performance of MF-based geopolymers. It is recommended that future studies explore the long-term performance under combined environmental actions and evaluate the economic and environmental benefits of MgO-modified geopolymers for large-scale applications.

1. Introduction

Geopolymers represent a novel class of low-carbon and green materials that can be extensively manufactured using industrial solid wastes, aligning with sustainable development principles [1,2,3,4]. However, the inherent complexity of geopolymers presents significant challenges for practical application. Durability issues, particularly carbonation, constitute a major challenge limiting the long-term service performance of geopolymer concrete structures [5,6,7,8]. In concrete, reinforcing steel is passivated by the high-alkalinity pore solution, forming a protective oxide film that inhibits corrosion [9,10,11,12]. However, carbonation reduces the pore solution pH due to carbonic acid attack. When the pH value falls below a critical threshold, the pore solution can no longer sustain the passivating film, significantly increasing corrosion risk [13,14,15]. In chloride-laden environments, carbonation further decomposes Friedel’s salt and accelerates chloride ion migration in concrete, promoting depassivation and corrosion initiation [16]. This vulnerability may be exacerbated in geopolymer concrete due to fundamental differences in hydration products compared to ordinary Portland cement (OPC) [17]. OPC hydration yields substantial Ca(OH)2, which buffers against pore solution alkalinity and Ca2+ depletion during carbonation. Conversely, geopolymer reaction products contain negligible Ca(OH)2 and rely primarily on free alkali in the pore solution to mitigate CO2 ingress [18,19,20,21]. Consequently, geopolymer durability differs markedly from OPC [22]. Furthermore, the diversity of aluminosilicate source materials used in geopolymers results in complex systems with significant variations in hydration products and gel structures.
While the carbonation behavior of OPC is well-established, geopolymers demonstrate a heightened sensitivity to carbonation, and their underlying carbonation mechanisms remain insufficiently understood. Unlike OPC, geopolymers form gels such as N-A-S-H, C-S-H, and/or C-A-S-H gels upon hydration, with minimal Ca(OH)2 formation [23,24,25]. Furthermore, their pore solutions contain high concentrations of alkali metal ions (Na+ or K+) [9,26]. Thus, the carbonation mechanisms observed in traditional OPC systems are not directly applicable to geopolymers. Robayo-Salazar et al. [27] reported high Na+ concentrations in geopolymer pore solutions, with Na2CO3 and NaHCO3 being primary carbonation products. This distinct pore solution chemistry fundamentally alters the carbonation process. The absence of a Ca(OH)2 buffer typically results in accelerated carbonation rates. Bakharev et al. [28] demonstrated that slag-based geopolymer pastes exhibited inferior carbonation resistance versus OPC pastes, manifesting as greater strength loss and increased carbonation depth under both NaHCO3 solution and high-concentration CO2 exposure. Mechanistically, carbonation detrimentally affected geopolymers, especially high-calcium formulations. The lack of Ca(OH)2 prevented protective CaCO3 formation, leaving them directly vulnerable to CO32− attack [29]. This contrast is further highlighted by Li et al. [24], who observed opposing trends in post-carbonation compressive strength between OPC paste and alkali-activated slag.
Given that carbonation risk significantly impedes the industrial adoption of geopolymers, researchers are actively exploring various admixtures to enhance carbonation resistance [30,31]. Abdalqader et al. [25] found that adding 10% MgO to fly ash–slag geopolymers effectively reduced carbonation depth. In alkaline media, MgO reacts to form finely dispersed Mg(OH)2 crystals, which refine the microstructure. It can also generate layered double hydroxide (LDH) phases that provide strong CO2 adsorption and buffering capacity, thereby significantly improving resistance to acidic degradation [32]. To address the inherent Ca(OH)2 deficiency directly, Lv et al. [18] incorporated Ca(OH)2 into geopolymers, successfully inhibiting carbonation depth progression and alkalinity loss; however, excessive additions markedly reduced post-carbonation compressive strength [33]. Other mineral admixtures, including silica fume, hydrotalcite, cement clinker, and calcium sulfate-based expansive agents, have also demonstrated potential for modulating the carbonation performance of geopolymers [34,35]. Overall, research on geopolymer carbonation remains limited compared to OPC systems, particularly given the compositional complexity and variability of geopolymer formulations [36,37]. There is a critical need to elucidate how specific precursor types and ratios influence microstructural evolution and chemical response during carbonation. In particular, the role of MgO in MK-FA blends, and the interaction between MgO and the aluminosilicate gel phases under carbonation conditions, require in-depth investigation.
To address these research gaps, this study systematically investigates the synergistic effects of MgO dosage and metakaolin/fly ash (MK/FA) ratio on the carbonation resistance of MF-based geopolymers. Accelerated carbonation tests were employed to investigate the effects of MgO content and the MK/FA ratio on compressive strength, carbonation depth, and pore solution alkalinity. Subsequently, mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR) were used to analyze pore structure and phase composition evolution pre- and post-carbonation. This work aims to provide new insights into the formulation design of MgO-modified MF-geopolymers with enhanced carbonation durability, highlighting the unique role of MgO beyond conventional stabilization mechanisms.

2. Experimental Programs

2.1. Raw Materials

In this study, metakaolin (MK) and fly ash (FA) were employed as silica-aluminate precursors to prepare metakaolin–fly ash-based (MF-based) geopolymers. The chemical compositions of MK and FA were characterized using X-ray fluorescence (XRF) spectrometry, with detailed mass fractions of constituent oxides provided in Table 1. Magnesium oxide (MgO) was used as a carbonization modification material, which was a white powder with a purity of over 99% (by mass). In addition, the specific surface area of fly ash metakaolin was 420 m2/kg and 1200 m2/kg, respectively.
The alkaline activator consisted of anhydrous sodium silicate powder (59.6 wt% SiO2, 21.6 wt% Na2O), sodium hydroxide flakes, and deionized water. The sodium silicate solution had an initial modulus (Ms = SiO2/Na2O) of 2.85, which was adjusted to a target modulus of 1.2 by adding sodium hydroxide flakes. The final activator contained 10% alkaline content (Na2O by mass).

2.2. Experimental Design and Specimen Preparation

To address the trade-off between the high reactivity of metakaolin (MK) and the lower reactivity, yet beneficial compositional variability of fly ash (FA), MK was employed as the main precursor, with FA used to partially replace MK at substitution levels ranging from 0 to 40 wt%, thus forming metakaolin–fly ash (MF-based) geopolymer. Additionally, MgO (4–8 wt% of total solids) was incorporated to modify the geopolymer matrix, which was a strategy aimed specifically at enhancing carbonation resistance through microstructural refinement and chemical stabilization. This approach not only differentiated our work from previous studies focused primarily on slag- or fly ash-only systems but also allowed us to evaluate the synergistic effects of MgO with varying MK/FA ratios, which remained largely unexplored in the existing literature. All mixtures were prepared with a constant water-to-binder ratio of 0.63. Detailed mix proportions and experimental parameters of MF-based geopolymers are summarized in Table 2.
Cubic paste specimens (50 × 50 × 50 mm3) were cast by weighing FA and MK according to the mix proportions in Table 2. To ensure statistical reliability, three replicates were prepared for each testing condition, with results presented as average values. The raw materials were combined with a pre-hydrated alkaline activator (aged for 24 h) and mixed in a planetary mixer. The mixture underwent low-speed mixing for 3 minutes, followed by high-speed mixing for an additional 3 minutes to ensure homogeneity. The preparation process of MF-based geopolymer pastes is plotted in Figure 1. The slurry was then poured into molds and cured in a chamber at 22 °C and 95% relative humidity (RH) for 24 h. After demolding, the specimens were sealed with plastic film and returned to the conditioning chamber (20 ± 2 °C, 95 ± 2% RH) until 28 days of age. Accelerated carbonation tests were initiated during this period.

2.3. Accelerated Carbonation Procedure

Following the Chinese Standard (GB/T 50082-2024), specimens were preconditioned at 20 °C and 70% RH for 48 h before accelerated carbonation testing. One surface of each specimen was exposed to carbonation, while the remaining surfaces were sealed with paraffin wax. The carbonation chamber maintained a CO2 concentration of 20 ± 2%, a temperature of 25 ± 3 °C, and an RH of 70 ± 5%. Specimens were subjected to carbonation for durations of 0, 1, 3, 7, 14, 21, and 28 days, with subsequent analyses performed at each interval.

2.4. Testing Methods

2.4.1. Compressive Strength

A hydraulic universal testing machine was used to evaluate the compressive strength of uncarbonated specimens and those that had been carbonated for 28 days. Three replicates per group were tested, and the average value was reported as the compressive strength for that group.

2.4.2. Carbonation Depth

Carbonated specimens were split along their central axis using a dry-cutting saw. A 1% phenolphthalein ethanol solution was sprayed onto the freshly exposed surface. The carbonation front (non-pink region) was measured at ten equidistant points using a vernier caliper (0.1 mm precision). The average distance from the exposed surface was recorded as the carbonation depth.

2.4.3. Material Alkalinity

After measuring carbonation depth, the remaining half of each specimen was sectioned into five depth intervals: 0–10 mm, 10–20 mm, 20–30 mm, 30–40 mm, and 40–50 mm from the exposed surface. Each segment was dried at 50 °C to constant mass, ground, and sieved through a 200-mesh sieve (75 μm aperture). The pH of the pore solution was determined via solid–liquid extraction. Powdered samples were mixed with deionized water (1:3 mass ratio), soaked for 1 hour, and the supernatant pH was measured using a Metrohm 818 Titrino Plus potentiometric titrator (Herisau, Switzerland; ±0.01 accuracy).

2.4.4. MIP, XRD, and FTIR Measurements

Fragments from the 0–10 mm depth of 28-day cured and carbonated specimens were immersed in anhydrous ethanol to arrest hydration, dried, and subjected to mercury intrusion porosimetry (MIP) using a US Conta PoreMaster 33 series aperture analyzer. The XRD tests were carried out using a Rigaku Ultima IV series X-ray diffractometer from Japan with the scanning range set at 5–70° to analyze the powder in the 0–10 mm fraction of the 28-day cured age and 28-day carbonized age samples that had passed through a 200 mesh standard sieve.
Additionally, FTIR testing was performed using a Japanese Shimadzu IRTracer-100 series Fourier transform infrared spectrometer by mixing the powder with KBr and pressing it into a tablet for analysis within a scanning range of 400 to 4000 cm−1, with a resolution of 4 cm−1.

3. Results and Discussion

3.1. Compressive Strength

Accelerated carbonization tests were conducted on MF-based geopolymers with varying MgO contents, and the compressive strength was measured before and 28 days after carbonization. As plotted in Figure 2, the compressive strength of MF-based geopolymer pastes decreased progressively with increasing fly ash content. Specifically, in the MF-based geopolymer, increasing fly ash content from 0% to 40% reduced compressive strength from 37.6 MPa to 33.2 MPa, which reported a decrease of approximately 11.7%. However, the incorporation of MgO significantly enhanced the compressive strength of geopolymer pastes, and this strengthening effect became more pronounced as the fly ash content increased. For the pure metakaolin-based geopolymer system, adding 4% and 8% MgO increased compressive strength by 1.9% and 4.8%, respectively. In MF-based systems with 20% fly ash content, 4% and 8% MgO additions increased compressive strength by 8.6% and 13.9%, respectively. When the fly ash content increased to 40%, the compressive strength increased by 6.0% and 31.3% after adding 4% and 8% MgO, respectively. This amplified strengthening effect at higher fly ash levels is attributed to highly reactive MgO promoting fly ash hydration, while Mg(OH)2 crystals formed during MgO hydration fill internal pores and refine the microstructure, thereby enhancing macroscopic mechanical properties [31,38].
Comparing the compressive strength of geopolymer systems with different MgO content before and after carbonization, it was found that the strength of the samples increased after carbonization. For pure metakaolin-based geopolymers, the compressive strength of samples containing 0%, 4%, and 8% MgO increased by 10.4%, 11.7%, and 10.9%, respectively, compared to the uncarbonized samples after carbonization. In the MF geopolymer system with a fly ash content of 20%, the compressive strength of the samples without MgO content and those with 4% and 8% MgO increased by 3.0%, 6.1%, and 7.7%, respectively, after carbonization. When the fly ash content was increased to 40%, the corresponding increases were 9.6%, 10.2%, and 6.4%, respectively. The improvement in post-carbonation strength may be related to the effective sequestration of CO2 by the hydration reaction-generated hydrotalcite-like phase in the slurry, while the formation of amorphous magnesium-containing hydrates (M-S-H gel) further optimized the microstructure of the geopolymer, thereby enhancing its macroscopic mechanical properties [19].

3.2. Carbonation Depth

Figure 3 illustrates the impact of MgO content on the carbonization depth of MF-based geopolymer pastes. The results revealed that as the carbonization age increased, the carbonization depth of all geopolymer pastes demonstrated an upward trend. However, at the same carbonization age, the specimens containing MgO exhibited significantly lower carbonization depths compared to those specimens without MgO. This enhanced resistance was attributed to two mechanisms: (1) the formation of magnesium-containing layered double hydroxides within the paste, which possess high CO2 adsorption capacity, and (2) the direct reaction of unreacted MgO particles with CO2 [36,39].
In the case of pure metakaolin-based geopolymer pastes with MgO contents of 0%, 4%, and 8%, the carbonization depths of the M10F0-0%, M10F0-4%, and M10F0-8% specimens after 28 days of carbonization were 33.4 mm, 12.6 mm, and 5.2 mm, respectively. In comparison to the geopolymer samples without MgO (M10F0-0%), the carbonation depths of the geopolymer pastes with 4% and 8% MgO additions decreased by 62.3% and 84.4%, respectively. For MF-based geopolymer pastes with 20% fly ash content, the carbonization depths of the M10F0-0%, M10F0-4%, and M10F0-8% specimens after 28 days of carbonization were 25.5 mm, 7.8 mm, and 5.0 mm, respectively, when the MgO content was 0%, 4%, and 8%. Compared to the geopolymers without MgO (M8F2-0%), the carbonation depths of the MF-based geopolymer pastes with 4% and 8% MgO additions were reduced by 69.4% and 80.4%, respectively. For MF-based geopolymer pastes with 40% fly ash content, the carbonization depths of the M10F0-0%, M10F0-4%, and M10F0-8% specimens after 28 days of carbonization were 15.4 mm, 9.6 mm, and 7.4 mm, respectively, when the MgO content was 0%, 4%, and 8%. When compared to the control group without MgO (M6F4-0%), the carbonation depths of the specimens with 4% and 8% MgO additions decreased by 37.7% and 51.9%, respectively. Furthermore, as the fly ash content increased, the carbonization depth of the MF-based geopolymer pastes decreased. Specifically, increasing the fly ash content from 0% to 40% reduced the carbonation depth of MF-based geopolymer pastes from 33.4 mm to 15.4 mm, as demonstrated in Figure 3.

3.3. pH Value

Alkalinity testing was performed on powder sampled from the 0–10 mm depth section extending inward from the exposed surface of the specimens. Figure 4 presents the pH values measured for each group of MF-based geopolymer pastes at different carbonation ages. A gradual decline in pH value was observed for all MF-based geopolymer pastes as the carbonation age increased. This is because as the geopolymer sample continues to be exposed to the CO2 environment, the concentration gradient between the interior and exterior of the slurry causes CO2 to continuously diffuse inward, reacting with the hydration products of geopolymers, ultimately leading to a gradual decrease in the alkalinity of the pastes [13,15]. Notably, the pore solution pH decreased significantly during the first 7 days of carbonation, after which the rate of decrease slowed and gradually stabilized. This stabilization primarily occurs because ions within the geopolymer pore solution reached an equilibrium state after 28 days of carbonation. Consequently, the final pore solution pH demonstrated no significant difference between the metakaolin-based geopolymer paste and the MF-based geopolymer pastes containing either 4% or 8% MgO.
During the initial carbonation stage, pH values for all geopolymer pastes ranged from 11.92 to 12.31. After 28 days of carbonation, the alkalinity of unmodified MF-based geopolymer paste fell below 10.5. This decrease in pH value was directly proportional to the depth of carbonation, as indicated in Figure 3. The incorporation of MgO significantly reduced the rate of pH decline. This was primarily attributed to MgO effectively delaying the ingress of external CO2, thereby slowing the carbonation rate. For the pure metakaolin-based geopolymer paste, when the MgO content was 0%, 4%, and 8%, the pH values after 28 days of carbonation were 10.36, 10.81, and 10.76, respectively, which decreased by 1.65, 1.11, and 1.22 compared to those of specimens before carbonation. For the MF-based geopolymer paste with 20% fly ash content, when the MgO content was 0%, 4%, and 8%, the pH values after 28 days of carbonation were 9.78, 11.00, and 10.93, respectively, which decreased by 2.40, 0.96, and 1.13 compared to those of specimens before carbonation. For the MF-based geopolymer paste with 40% fly ash content, when the MgO content was 0%, 4%, and 8%, the pH values after 28 days of carbonation were 10.49, 11.32, and 11.33, respectively, which decreased by 1.74, 0.93, and 0.98 compared to those of specimens before carbonation. These results demonstrated that MgO modification maintained the pore solution pH around 11 after 28 days of carbonation. Additionally, increasing the fly ash content reduced the initial pH of the uncarbonated paste. When the fly ash content increased from 0% to 40% in the MF-based geopolymer pastes, the pH value exhibited a slight decreasing trend (approximately 0.2).

3.4. Pore Structure

Figure 5 illustrates the changes in pore size distribution, porosity, and the most probable pore size of the M8F2-0% and M8F2-8% specimens before and after carbonation. Pore sizes in cementitious materials were typically categorized into four types: gel micropores (<10 nm), mesopores (10–50 nm), capillary pores (50–100 nm), and macropores (100–1000 nm) [1]. As depicted in Figure 5a, the pore size distribution of the M8F2-0% control group exhibited minimal variation before and after carbonation, with pore concentrations predominantly in the mesopore (10–50 nm) range. The changes in capillary and macropores were negligible. The absence of MgO in this group resulted in changes in porosity, primarily attributed to the transformation of Ca(OH)2 and CO2 infiltration during the carbonation process. The limited alteration in pore size may stem from the relatively loose initial microstructure of the control group, which meant that CO2 invasion and Ca(OH)2 transformation had a constrained impact on pore size distribution.
In contrast, after the introduction of MgO, the pore size distribution curve for the M8F2-8% specimens shifted leftward compared to the control group (M8F2-0%) prior to carbonation. This shift indicated a decrease in both capillary pores (50–100 nm) and macropores (>100 nm), suggesting an enhancement in the pore structure. This improvement manifested in the inclusion of MgO, which optimized the microstructure of the paste, ultimately reducing porosity and the most probable pore size. This enhanced porosity contributed to the enhancement of the compressive strength of the pastes, as demonstrated in Figure 2. The underlying mechanism involved MgO accelerating the hydration reaction and facilitating the formation of finer Mg(OH)2 crystals within the highly alkaline pore solution. These finer crystals were better dispersed throughout the paste, thereby enhancing the microstructure [40]. After 28 days of carbonation, the pore size distribution curve for the M8F2-8% specimens shifted significantly to the left, indicating a marked reduction in pore diameter.
Moreover, both the total porosity and the most probable pore diameter for the M8F2-0% and M8F2-8% specimens decreased following carbonation. Specifically, after 28 days, the porosity of the M8F2-0% and M8F2-8% specimens decreased from 21.8% and 20.2% to 19.9% and 18.2%, respectively. Concurrently, their most probable pore diameters diminished from 36.7 nm and 31.0 nm to 29.3 nm and 16.5 nm, respectively. This refinement of pores post-carbonation can be attributed to the continued participation of some Mg(OH)2 in the reaction within the carbonation environment, leading to the formation of expansive magnesium silicate hydrate (M-S-H) gel or a similar hydrotalcite phase [36]. Additionally, unreacted MgO particles can directly carbonize to produce magnesium carbonate (MgCO3) and hydrated magnesium carbonate. These carbonized products exhibit excellent bonding and filling characteristics, effectively refining the pores and reducing the overall porosity of the paste. Consequently, the macroscopic mechanical properties of the geopolymer incorporating MgO were enhanced after carbonation, aligning with the results of the compressive strength, as illustrated in Figure 5.

3.5. XRD Analysis

Figure 6 presents the XRD patterns of the M8F2-0% and M8F2-8% specimens, both before carbonization and 28 days post-carbonization. Within the 2θ range of 15 to 35°, the MgO-modified geopolymers exhibited several characteristic peaks indicative of an amorphous structure. The detected crystalline phases included quartz, mullite, and minor amounts of hydrotalcite and akermanite, along with (N,C,m)-A-S-H gel as the primary hydration products [26,41]. Notably, the hydration products from the MF-based geopolymer pastes did not contain calcium hydroxide or C-S-H gel, which were essential calcium reserves for counteracting carbonization. In contrast, the introduction of MgO to the MF-based geopolymer pastes resulted in a reduction in the intensity of the corresponding peaks, suggesting that MgO significantly enhanced the hydration reaction within the MF-based geopolymer pastes. This enhancement led to an increased number of amorphous gel phases, as reflected by the changes in the characteristic peaks. The phase composition of the M8F2-0% specimen remained largely unchanged before and after carbonization, though a small quantity of sodium salt crystals was observed following carbonization. In the hydration products of the MF-based geopolymer pastes with MgO addition, a distinct periclase diffraction peak was evident, indicating that MgO has not completely reacted. After carbonation, the MgO diffraction peak disappeared, indicating its reaction with CO2. Although hydrotalcite and akermanite were detected, their origin may be related to the raw materials rather than resulting solely from hydration. It should be noted that the expected carbonation product, such as magnesite (MgCO3), was not clearly detected by XRD, possibly due to its low crystallinity or content. This suggested direct carbonation of MgO may have formed poorly crystalline or amorphous phases.

3.6. FT-IR Analysis

Figure 7 presents the FT-IR spectra of MM8F2-0% and M8F2-8% specimens before carbonation and after 28 days of carbonation. Analysis of specific functional groups within the hydration and carbonation products enables the assessment of qualitative material changes induced by carbonation. Overall, the phase composition of the MF-based geopolymer pastes remained largely unchanged following carbonation. Consequently, the functional groups associated with bound water, carbonates, and the gel structure exhibited no significant alterations. However, carbonation increased the polymerization degree of the silicate gel. Analysis of the pre-carbonation FT-IR spectra revealed absorption peaks at 3371 cm−1 and 3403 cm−1 for M8F2-0% and M8F2-8%, respectively, originating from O-H stretching vibrations of water molecules [42]. After 28 days of carbonation, these peaks shifted to 3368 cm−1 and 3367 cm−1. Peaks observed at 1654 cm−1 and 1647 cm−1 were attributed to O-H bending vibrations of bound water within the gel. Absorption peaks at 1458 cm−1 and 1363 cm−1 arose from asymmetric stretching vibrations of carbonate species, likely formed from unreacted residual NaOH present in the activator [6]. Following carbonation, these peaks shifted to higher wavenumbers (1503 cm−1 for M8F2-0% and 1385 cm−1 for M8F2-8%), consistent with substantial OH consumption to counteract CO2 ingress and generate carbonates.
Additionally, distinct peaks at 980 cm−1 (M8F2-0%) and 975 cm−1 (M8F2-8%) before carbonation correspond to asymmetric stretching vibrations of Si-O-Si and Si-O-Al bonds in the gel [43,44]. After 28 days of carbonation, these peaks shifted to 976 cm−1 and 973 cm−1, respectively, indicating a movement to lower wavenumbers. Incorporation of MgO induced a shift towards lower wavenumbers for these peaks in both pre- and post-carbonation specimens. The wavenumber of this peak served as an indicator of the gel’s polymerization state and degree within the matrix. MgO addition promoted the replacement of Al3+ by Si4+ during reaction, transforming aluminum-rich geopolymeric gel into a more stable silicon-rich structure [38,45]. This transformation exhibited lower porosity and higher strength in the geopolymer pastes, as illustrated in Figure 2 and Figure 5. This observation indicated that MgO not only enhanced the hydration reaction but also facilitated the formation of structurally more stable gels, thereby contributing to improved macroscopic material strength. Furthermore, absorption peaks at 856 cm−1 (M8F2-0%) and 835 cm−1 (M8F2-8%) were potentially associated with symmetric Si-O-Si stretching vibrations. Peaks occurring between 500 cm−1 and 800 cm−1 were attributed to in-plane bending vibrations of Si-O or Al-O bonds within the constituent SiO4 and AlO4 tetrahedral structural units [1,46].

4. Conclusions

This study investigates the impact of varying MgO and fly ash content on the carbonation performance of MF-based geopolymer pastes through accelerated carbonation tests. The carbonation mechanisms of MF-based geopolymer pastes at different ages were elucidated through XRD, FTIR, and MIP. The key findings are as follows:
(1) The compressive strength of MF-based geopolymer pastes gradually decreased with increasing fly ash content. Specifically, when the fly ash content rose from 0% to 40%, the compressive strength decreased by 11.7%. Conversely, the inclusion of MgO enhanced the compressive strength of the MF-based geopolymer pastes, with a more pronounced improvement observed at higher fly ash contents. For instance, at an 8% MgO content, the compressive strength of the M10F0-8%, M8F2-8%, and M6F4-8% specimens increased by 4.8%, 14.7%, and 31.3%, respectively, compared to the control group without MgO. Furthermore, carbonation led to a moderate strength gain in all specimens, with improvements ranging from 3% to 11%.
(2) As the carbonation age progressed, the carbonation depth of geopolymer pastes increased gradually, while the pH value of the pore solution declined. The incorporation of MgO and fly ash effectively mitigated both the progression of carbonation depth and the drop in pH value. For the MF-based geopolymer pastes with 20% fly ash content, the carbonation depths after 28 days were measured at 25.5 mm, 7.8 mm, and 5.0 mm for MgO contents of 0%, 4%, and 8%, respectively. In comparison to the geopolymer specimens without MgO (M8F2-0%), the additions of 4% and 8% MgO reduced the carbonation depth by 69.4% and 80.4%, respectively. After 28 days of carbonation, the pH values for M8F2-0%, M8F2-4%, and M8F2-8% were 9.78, 11.00, and 10.93, representing reductions of 2.40, 0.96, and 1.13 from their initial pre-carbonation values.
(3) The inclusion of MgO reduced the median pore diameter, total porosity, and most probable pore size. This effect was attributed to MgO accelerating the hydration reaction, which promoted the formation of finer Mg(OH)2 crystals in the highly alkaline pore solution. These crystals contribute to a more refined and homogeneous microstructure, thereby enhancing compressive strength. Furthermore, carbonation for 28 days also reduced both the porosity and the most probable pore size. Specifically, the porosity of the M8F2-0% and M8F2-8% samples decreased from 21.8% and 20.2% to 19.9% and 18.2%, respectively, while their most probable pore sizes reduced from 36.7 nm and 31.0 nm to 29.3 nm and 16.5 nm, respectively.
(4) The phase composition of the MF-based geopolymer pastes remained largely unchanged before and after carbonation, with the primary hydration products identified as N-A-S-H gel, mullite, hydrotalcite, and akermanite. The introduction of MgO reduced the intensity of the characteristic peaks for these phases, indicating that MgO significantly promoted the hydration reactions within the geopolymer system.

Author Contributions

Methodology, S.L.; Formal analysis, D.J.; Investigation, S.L.; Data curation, S.L.; Writing—original draft, S.L. and D.J.; Writing—review & editing, S.L. and D.J.; Project administration, D.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article material. Further inquiries can be directed to the corresponding author.

Acknowledgments

This work was supported by the Natural Science Foundation of Hunan Province, China (Grant No. 2024JJ9067).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Raw materials and sample preparation of MF-based geopolymer pastes.
Figure 1. Raw materials and sample preparation of MF-based geopolymer pastes.
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Figure 2. The compressive strength of MF-based geopolymer pastes at various MgO contents after carbonation.
Figure 2. The compressive strength of MF-based geopolymer pastes at various MgO contents after carbonation.
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Figure 3. Carbonation depth of MF-based geopolymer pastes at various MgO contents.
Figure 3. Carbonation depth of MF-based geopolymer pastes at various MgO contents.
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Figure 4. The alkalinity of MF-based geopolymer pastes with various MgO contents.
Figure 4. The alkalinity of MF-based geopolymer pastes with various MgO contents.
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Figure 5. Pore size distribution and porosity of MF-based geopolymer pastes modified with MgO content.
Figure 5. Pore size distribution and porosity of MF-based geopolymer pastes modified with MgO content.
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Figure 6. XRD patterns of MF-based geopolymer pastes modified with MgO before and after carbonation.
Figure 6. XRD patterns of MF-based geopolymer pastes modified with MgO before and after carbonation.
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Figure 7. FT-IR spectra of MF-based geopolymer pastes modified with MgO before and after carbonation.
Figure 7. FT-IR spectra of MF-based geopolymer pastes modified with MgO before and after carbonation.
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Table 1. Chemical composition of metakaolin and fly ash (%).
Table 1. Chemical composition of metakaolin and fly ash (%).
MaterialSiO2Al2O3CaOFe2O3MgOK2ONa2OSO3TiO2Others
Metakaolin53.8043.20-1.100.820.450.18--0.45
Fly ash57.9631.143.023.860.522.03-0.640.520.31
Table 2. Design parameters and mix proportions of MF-based geopolymer pastes at various MgO contents.
Table 2. Design parameters and mix proportions of MF-based geopolymer pastes at various MgO contents.
Serial NumberMgO Content (%)MK (%)FA (%)Alkaline Content (%)Water/Binder Ratio
M10F0-0%0100010%0.63
M8F2-0%8020
M6F4-0%6040
M10F0-4%4100010%0.63
M8F2-4%8020
M6F4-4%6040
M10F0-8%8100010%0.63
M8F2-8%8020
M6F4-8%6040
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Li, S.; Ji, D. Tuning the Carbonation Resistance of Metakaolin–Fly Ash-Based Geopolymers: The Dual Role of Reactive MgO in Microstructure and Degradation Mechanisms. J. Compos. Sci. 2025, 9, 549. https://doi.org/10.3390/jcs9100549

AMA Style

Li S, Ji D. Tuning the Carbonation Resistance of Metakaolin–Fly Ash-Based Geopolymers: The Dual Role of Reactive MgO in Microstructure and Degradation Mechanisms. Journal of Composites Science. 2025; 9(10):549. https://doi.org/10.3390/jcs9100549

Chicago/Turabian Style

Li, Shuai, and Dongyu Ji. 2025. "Tuning the Carbonation Resistance of Metakaolin–Fly Ash-Based Geopolymers: The Dual Role of Reactive MgO in Microstructure and Degradation Mechanisms" Journal of Composites Science 9, no. 10: 549. https://doi.org/10.3390/jcs9100549

APA Style

Li, S., & Ji, D. (2025). Tuning the Carbonation Resistance of Metakaolin–Fly Ash-Based Geopolymers: The Dual Role of Reactive MgO in Microstructure and Degradation Mechanisms. Journal of Composites Science, 9(10), 549. https://doi.org/10.3390/jcs9100549

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