Accelerated Carbonation as a Potential Alternative for Autoclaved Fiber Cement Material—A Comparison in Macro and Micro Scale

Abstract

This study investigates accelerated carbonation as a low-energy alternative to autoclave curing in the production of fiber cement composites reinforced with lignocellulosic fibers. The effects of both curing routes on physical–mechanical performance, durability, and microstructural evolution were systematically evaluated before and after 25 wetting–drying cycles. Carbonation-cured composites achieved mechanical performance comparable to autoclaved materials, while exhibiting higher bulk density (≈1.37–1.38 g/cm³) and a reduction of approximately 15% in total void volume. Water absorption values were up to 17% lower than those of autoclaved counterparts. After accelerated aging, both systems showed stable mechanical properties, with increases in modulus of elasticity of approximately 21% (autoclaved) and 26% (carbonated), indicating ongoing hydration and densification processes. Thermogravimetric analysis revealed carbonation degrees of approximately 16–17%, corresponding to CO₂ uptake values of up to 35.8 kg/m³ of fiber cement. X-ray diffraction confirmed the consumption of portlandite and the formation of calcium carbonate phases, contributing to pore refinement and matrix densification. Microstructural observations indicated improved fiber–matrix interaction in carbonated composites due to the precipitation of carbonation products at the interface, whereas autoclaved materials exhibited signs of fiber degradation associated with hydrothermal curing. These effects were reflected in higher deformation capacity and specific energy retention in carbonated systems. Overall, accelerated carbonation represents a promising alternative to autoclave curing, delivering comparable mechanical performance while enhancing fiber durability, refining pore structure, and enabling CO₂ sequestration within the cementitious matrix.

1. Introduction

Fiber cement is a widely used composite material consisting of a Portland cement-based matrix reinforced with fibers such as cellulose pulps, synthetic fibers, and supplementary cementitious materials including silica powder and fly ash. These components are typically processed through slurry-dewatering and pressing techniques, followed by curing under controlled conditions to ensure adequate mechanical performance and durability [1,2]. In industrial practice, autoclave curing remains the dominant method, enabling the formation of crystalline calcium silicate hydrate (C–S–H) phases, particularly tobermorite, which is responsible for the high mechanical strength of fiber cement products [1,3].

The autoclaving process involves hydrothermal conditions, typically requiring temperatures between 160 and 180 °C and pressures ranging from 7 to 12 bar for several hours [4,5,6,7,8,9,10,11,12]. While effective in promoting the formation of mechanically robust phases, this curing route is associated with high energy consumption and operational costs, raising concerns regarding its environmental and economic sustainability [13]. In addition, exposure to elevated temperature and pressure may adversely affect the integrity of lignocellulosic fibers, potentially compromising the long-term mechanical performance and durability of fiber-reinforced cementitious composites.

In this context, accelerated carbonation has emerged as a promising alternative curing strategy for cement-based materials. This process involves the reaction between CO₂ and alkaline hydration products, such as Ca(OH)₂, leading to the formation of calcium carbonate phases that can densify the matrix and modify its pore structure [14,15,16,17]. Unlike natural carbonation, which is limited by the low atmospheric CO₂ concentration (~0.04%), accelerated carbonation can be achieved within hours under controlled CO₂-rich environments, significantly enhancing reaction kinetics [17].

Previous studies have demonstrated that carbonation curing can improve the physical and mechanical properties of fiber cement composites. For instance, increases of up to 33% in mechanical strength have been reported under optimized carbonation conditions [18]. Additionally, the formation of carbonation products within the pore network contributes to pore refinement, reduced permeability, and improved durability [14,19]. Another relevant aspect is the potential of carbonation to reduce the alkalinity of the cementitious matrix, which may mitigate the degradation of lignocellulosic fibers and improve fiber–matrix compatibility over time [20].

Despite these advances, a comprehensive comparison between autoclave curing and accelerated carbonation in fiber cement systems remains limited, particularly concerning their combined effects on microstructural evolution, interfacial behavior, and durability after aging processes. Most existing studies focus on isolated curing conditions or specific performance aspects, lacking a systematic evaluation of how these curing routes influence both matrix densification and fiber integrity simultaneously.

Furthermore, the mechanisms governing the interaction between hydration and carbonation reactions in fiber cement systems are not yet fully understood. The evolution of phase assemblage, pore structure, and interfacial properties depends on multiple factors, including CO₂ concentration, temperature, relative humidity, and initial moisture content [17,21]. Developing a deeper understanding of these coupled processes is essential for optimizing curing strategies and tailoring the performance of fiber-reinforced cementitious composites.

In addition to performance-related aspects, accelerated carbonation offers the possibility of incorporating CO₂ into cementitious materials, contributing to carbon capture and utilization strategies in the construction sector [22]. This dual function (enhancing material properties while enabling CO₂ sequestration) positions carbonation curing as a potential pathway toward more sustainable fiber cement production.

Thus, this study aims to evaluate accelerated carbonation as an alternative to autoclave curing in the production of fiber cement composites reinforced with lignocellulosic fibers. The investigation focuses on the comparative analysis of physical–mechanical properties, durability under wetting–drying cycles, and microstructural evolution, including phase composition and fiber–matrix interaction. The formulations were designed considering the typical chemical requirements of each curing route. Silica-rich compositions are commonly employed in autoclaved systems to promote tobermorite formation, whereas carbonation-cured systems require higher availability of calcium-bearing phases to maximize carbonate precipitation. By systematically assessing these aspects, this work seeks to provide new insights into the role of carbonation in improving composite performance while preserving fiber integrity and reducing the environmental impact associated with conventional curing processes.

2. Materials and Methods

The experimental program consisted of five stages: raw material characterization, fiber cement production, curing process, accelerated aging, and physical–mechanical and microstructural characterization of the composites (Figure 1). These stages were designed to evaluate the influence of the curing methods on the physical, mechanical, and microstructural behavior of the fiber cement composites.

2.1. Characterization of Raw Materials

Ordinary Portland cement (OPC), ASTM C150 Type III, was used as the primary binder in the production of fiber cement composites. This cement was selected due to its high early reactivity, which is essential for both hydration and carbonation processes. In carbonation-cured systems, the availability of calcium hydroxide (Ca(OH)₂), formed during hydration, plays a key role as a precursor for calcium carbonate formation. Class F fly ash was incorporated as a supplementary cementitious material, characterized by a combined content of SiO₂, Al₂O₃, and Fe₂O₃ higher than 70%. Its use contributes to particle packing and long-term pozzolanic reactions, while also influencing carbonation kinetics by modifying pore structure and reducing the availability of free calcium phases. Silica powder, with a high SiO₂ content (~99%), was used in autoclaved formulations to promote the formation of calcium silicate hydrate phases, particularly tobermorite (Ca₅Si₆O₁₆(OH)_2.4H₂O), under hydrothermal conditions [23].

A carbonate-based filler (limestone) was also incorporated in both formulations. In addition to improving particle packing, this material may act as a nucleation site for carbonate precipitation during carbonation curing, contributing to matrix densification. Lignocellulosic fibers were used as reinforcement, consisting of cellulose pulp and recycled textile fibers (jeans pulp). These fibers play a fundamental role in crack bridging and energy absorption mechanisms. However, their performance is strongly dependent on the chemical environment of the matrix, particularly pH and ionic transport. Thus, the interaction between the fibers and the cementitious matrix is expected to differ significantly depending on the curing process, especially when comparing hydrothermal conditions with CO₂-rich environments.

The chemical composition of the raw materials, determined by X-ray fluorescence (XRF), is presented in

Table 1. Chemical composition of OPC, fly ash, silica powder and carbonate-based filler used to produce fiber cement boards.

Oxides	%-wt
	OPC	Fly Ash	Silica Powder	Limestone
SiO2	19.40	62.50	99.00	3.99
Al2O3	4.41	26.50	<1.00	1.03
Fe2O3	3.83	4.36	<0.05	0.42
CaO	63.1	1.13	<0.05	43.90
MgO	1.31	0.52	<0.05	7.60
SO3	3.00	0.16	<0.05	0.13
Na2O	0.30	0.30	<0.05	-
K2O	0.39	1.26	<0.05	0.22
TiO2	0.38	1.70	<0.05	0.06
P2O5	0.20	0.26	<0.05	0.05
MnO	0.06	<0.05	<0.05	0.09
SrO	0.05	<0.05	<0.05	<0.03
LOI	3.87	1.48	0.04	42.4

LOI: Loss on Ignition—1200 °C during 3 h.

. The particle size distribution was measured using laser diffraction (Mastersizer 2000, Malvern Panalytical Ltd., Malvern, UK) with isopropanol as dispersing medium, and the results are shown in Figure 2 and

Table 2. Particle size distribution of the raw material used to produce the samples.

Raw Material	D10 (µm)	D50 (µm)	D90 (µm)
OPC	6.89	22.44	66.31
Fly ash	3.89	35.61	137.32
Silica powder	3.24	24.87	72.13
Carbonate-based filler	2.20	11.70	51.90

.

Crystalline components formed during the OPC hydration and after the autoclaved and carbonated curing processes were identified through X-ray diffraction analysis. A miniFlex 600 X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) was used during this analysis. The raw materials used to produce carbonated and autoclaved fiber cement were characterized by X-ray diffraction and the results are presented at Figure 3.

The X-ray diffraction patterns of the raw materials used in the production of fiber cement composites are presented in Figure 3. The OPC diffractogram exhibits characteristic peaks associated with alite (C₃S), calcite (CaCO₃), and quartz (SiO₂), confirming the typical mineralogical composition of Portland cement. The presence of alite indicates a high potential for early hydration, leading to the formation of calcium silicate hydrate (C–S–H) and portlandite (Ca(OH)₂), which is particularly relevant for subsequent carbonation reactions. The fly ash shows diffraction peaks predominantly related to quartz and mullite phases, indicating a high degree of crystallinity. These phases are known for their low reactivity at ambient temperature, suggesting that the contribution of fly ash to the system is mainly associated with physical effects, such as particle packing and pore structure modification, rather than early-age chemical reactivity. This behavior is expected to differ under autoclave curing, where elevated temperature and pressure may enhance the dissolution of aluminosilicate phases. The silica powder presents sharp and well-defined diffraction peaks corresponding to quartz, confirming its highly crystalline nature. Its role in the system is primarily related to providing a reactive silica source under hydrothermal conditions, promoting the formation of tobermorite in autoclaved composites.

The carbonate-based filler exhibits diffraction peaks associated with calcite, indicating the presence of pre-existing carbonate phases in the system. These phases may influence the interpretation of thermogravimetric results, particularly in the quantification of carbonation degree, and can also act as preferential sites for nucleation and growth of secondary carbonate phases during accelerated carbonation.

The mineralogical composition of the raw materials highlights a system combining reactive calcium sources (OPC), relatively inert or slowly reactive aluminosilicates (fly ash and silica), and pre-existing carbonates. This combination is expected to play a key role in governing the balance between hydration and carbonation reactions, as well as the development of the pore structure and mechanical performance of the composites.

The lignocellulosic fibers used as reinforcement consisted of wood pulp and recycled textile fibers (jeans-based). The dimensional characterization of the fibers was carried out to determine their length and width distributions, as well as their aspect ratio. Fiber dimensions were measured through optical microscopy and image analysis. For each fiber type, a representative number of individual fibers was analysed to obtain statistically reliable distributions. The length and width of each fiber were determined, and the aspect ratio was calculated as the ratio between length and diameter.

The results are presented in Figure 4, which shows the relative frequency distributions of fiber length and width, and in

Table 3. Average length, width, and aspect ratio of lignocellulosic fibers used as reinforcement in fiber cement composites.

Fiber	Fiber Length (mm)	Fiber Width (µm)	Aspect Ratio
Wood pulp	2.35	35.3	66.6
Recycled textile fiber	0.4	11.3	35.4

, where the average values are summarized. Wood pulp fibers exhibited an average length of approximately 2.35 mm and an average width of 35.3 µm, resulting in an aspect ratio of 66.6. In contrast, recycled textile fibers presented significantly smaller dimensions, with an average length of 0.4 mm and width of 11.3 µm, corresponding to an aspect ratio of 35.4.

The use of fibers with distinct geometrical characteristics allows the evaluation of their combined effect on the physical–mechanical performance of fiber cement composites produced under different curing conditions.

2.2. Production of Fiber Cement

Fiber cement boards were produced at laboratory scale using a slurry-dewatering process followed by pressing. The formulations adopted for autoclaved and carbonated systems are presented in

Table 4. Formulations used to produce fiber cement boards.

Raw Material	%wt 1
	Autoclaved	Carbonated
OPC	36	50
Fly ash	25	36.5
Silica powder	25.5	0
Carbonate-based filler	5.5	5.5
Recycled textile fiber	4	4
Wood pulp	4	4
Total	100	100

¹ % masses of dry raw materials (without added water in the formulation).

. Initially, lignocellulosic fibers (cellulose pulp and recycled textile fibers) were dispersed in water at 3000 rpm for 5 min to promote adequate fiber separation and minimize agglomeration. Subsequently, the dry raw materials (OPC, fly ash, silica powder, and carbonate-based filler) were gradually added to the fiber suspension and mixed at 2500 rpm for 2.5 min to ensure homogeneous distribution of solid particles within the slurry.

The resulting mixture was then poured into a casting box, where a vacuum pressure of approximately −0.8 bar was applied to remove excess water and promote particle consolidation. This dewatering step is essential to achieve uniform fiber dispersion and matrix densification. After dewatering, the green boards were subjected to pressing at 3.2 MPa for 5 min to further reduce porosity and improve interfacial contact between fibers and the cementitious matrix. The production procedure is summarized in

Table 5. Summary of the processing parameters used for the production of fiber cement boards.

Procedure	Parameter	Time
Dispersion of cellulose pulp in water	3000 rpm, room temperature	5 min
Dry materials mixed with water and pulp cellulose	2500 rpm, room temperature	2.5 min
Casting	Pouring into moulds manually and application of vacuum at −0.8 bar	0.5 min
Pressing	3.2 MPa	5 min
Initial curing	Saturated air at 45 °C	12 h

, and a schematic representation of the slurry-dewatering method is shown in Figure 5 Following pressing, the boards were sealed and submitted to an initial curing stage under saturated air at 45 °C for 12 h, as presented in Figure 6. This step was performed to promote early hydration reactions and provide sufficient mechanical integrity for handling prior to the final curing processes. Figure 6 presents fiber cement boards (200 × 200) mm: after casting (Figure 6A) and sealed boards for the initial curing (Figure 6B).

Figure 5. Slurry-dewatering method used to produce autoclaved and carbonated fiber cement samples. (a) Mix of raw materials; (b) Pre-vacuum process; (c) Manual compacting process; (d) Vacuum; (e) Final compacting.

Figure 5. Slurry-dewatering method used to produce autoclaved and carbonated fiber cement samples. (a) Mix of raw materials; (b) Pre-vacuum process; (c) Manual compacting process; (d) Vacuum; (e) Final compacting.

Figure 6. Fiber cement boards (200 × 200) mm: (A) after casting; (B) sealed boards for the initial curing.

Figure 6. Fiber cement boards (200 × 200) mm: (A) after casting; (B) sealed boards for the initial curing.

2.3. Curing Process

After the initial curing stage, the fiber cement boards were subjected to two distinct curing regimes: autoclave curing and accelerated carbonation. These curing routes were selected to enable a direct comparison between conventional hydrothermal processing and a CO₂-based alternative.

2.3.1. Autoclave Curing

Autoclave curing was performed in a pressure reactor system (Parr Instrument Company, Moline, IL, USA, model 4552, 7 L capacity). The boards were exposed to hydrothermal conditions at 175 °C under a pressure of 7 bar for 6 h. These conditions were selected to promote the formation of crystalline calcium silicate hydrate phases, particularly tobermorite, which are typically associated with improved mechanical strength in fiber cement materials. After the autoclave treatment, the samples were removed from the reactor and allowed to cool to room temperature under ambient conditions.

2.3.2. Accelerated Carbonation Curing

Accelerated carbonation was conducted in a climatic chamber (Espec Corp., Osaka, Japan, model EPL-4H) with controlled temperature, relative humidity, and CO₂ concentration. The curing process was carried out at atmospheric pressure (1.0 atm), with CO₂ concentration of 20% and relative humidity of 60% for 6 h. The carbonation parameters were selected based on previous studies involving fiber cement carbonation curing, which demonstrated that relative humidity around 60% provides adequate moisture for CO₂ dissolution while maintaining sufficient pore accessibility for gas diffusion. CO₂ concentration of 20% was adopted to accelerate carbonation kinetics without causing excessive superficial densification, whereas atmospheric pressure was selected to simplify the process and improve industrial applicability [10]. The boards were exposed to a CO₂-rich environment under controlled temperature and humidity conditions, allowing the diffusion of CO₂ into the porous structure and its reaction with alkaline hydration products. This process promotes the formation of calcium carbonate phases and modifies the pore structure of the cementitious matrix. The carbonation parameters were defined based on preliminary studies to ensure effective CO₂ uptake and homogeneous curing of the boards.

After both curing processes, the boards were immersed in water for 5 min to promote surface rehydration and then stored under laboratory conditions until testing. The physical, mechanical, and microstructural characterizations, as well as the accelerated aging tests, were initiated at 7 days after production. A schematic representation of the curing processes is shown in Figure 7 and Figure 8.

2.4. Accelerated Ageing Test

The durability of the fiber cement composites was evaluated through accelerated aging cycles based on repeated wetting and drying processes [24]. A total of 25 cycles were applied to both autoclaved and carbonation-cured boards. Each cycle consisted of two stages (i) immersion in water at (25 ± 3) °C for 18 h, followed by (ii) drying in an oven at 60 °C for 6 h. This procedure was designed to simulate environmental exposure conditions involving moisture variation and thermal effects, which are known to influence the long-term performance of fiber cement materials. The wetting stage promotes water ingress into the porous structure, potentially activating residual hydration reactions and facilitating ion transport within the matrix. The subsequent drying stage induces moisture removal and thermal stresses, which may affect both the cementitious matrix and the fiber–matrix interface. After completing the 25 cycles, the specimens were conditioned at laboratory temperature prior to physical, mechanical, and microstructural characterization. A schematic representation of the accelerated aging procedure is illustrated in Figure 9.

2.5. Thermogravimetry Analysis

To evaluate the effectiveness of carbonation, thermogravimetric analyses were performed and then the determination of the Carbonation Degree (CD) and the amount of CO₂ absorbed were calculated. The degree of carbonation evaluates the amount of alkaline compounds that can react with CO₂, to form carbonates, concerning the theoretical total content of these compounds available for this reaction. Samples of the fiber cement boards were crushed and evaluated on a TG/DSC STA449 F3 (NETZSCH-Gerätebau GmbH, Selb, Germany) thermogravimetric balance with a heating rate of 10 °C/min and N₂ atmosphere (50 mL/min). The absorbed CO₂ values were determined by TG mass loss analysis from approximately 550 °C to 1000 °C, not accounting for the number of carbonates present in the composites before CO₂ exposure, then multiplied by the residual mass after 1000 °C resulting in the values in non-volatile basis expressed in percentage [25]. The intrinsic carbonates present in the fiber cement were subtracted and the carbonation degree (CD) was determined according to Equation (1).

$$CD={\displaystyle \frac{\left(C-C_0\right)}{\left(C_{max}-C_0\right)}}100$$

(1)

in which C_max is the required CO₂ to react with the available oxides in the cement, then forming CaCO₃ (Equation (2)). C is the amount of CO₂ in the carbonated samples and C₀ the amount of CO₂ in non-carbonated fiber cement boards [26].

$$C_{max}=0.785\left(CaO-0.56Ca\left(C{O}_3\right)-0.7\times S{O}_3\right)+1.091MgO+0.71N{a}_{2}O+0.468K_{2}O$$

(2)

Equation (2) presumes that the whole Na₂O, CaO, K₂O and MgO formed in the cementitious matrix react with the CO₂, forming carbonates. It excludes the amount of CaO combined as CaCO₃ and sulphates [27]. For the calculations of the degree of carbonation, non-carbonated samples were produced to establish a parameter of intrinsic carbonation related to the raw materials and the naturally occurring carbonation between the alkaline species of the inorganic matrix and atmospheric CO₂. These samples were designated as REF. This parameter was also applied in the work of Filomeno et al. [10].

2.6. Physical–Mechanical Tests

The bulk density, water absorption, and total volume of voids of the autoclaved and carbonated boards were determined using the procedures described in the international standard ASTM C 948-81 [26].

Four-point bending tests were conducted on an EMIC DL30000 universal testing machine with a 5 kN load cell and a deflectometer (Instron Corporation, Norwood, MA, USA, 30 mm max deformation, 0.0001 mm accuracy) to measure central displacement. The test parameters included a span of 135 mm between bottom supports and 45 mm between top supports, with a loading speed of 5 mm/min. Modulus of rupture (MOR), limit of proportionality (LOP), modulus of elasticity (MOE), and specific energy (SE) were calculated following the methodologies of Savastano et al. [28] and RILEM recommendations [29]. The mechanical properties (modulus of rupture—MOR (Equation (3)), modulus of elasticity—MOE (Equation (4)), limit of proportionality—LOP (Equation (5)), and specific energy—SE (Equation (6))) were evaluated by flexural test in a universal testing machine EMIC DL 30000. The specimens were cut into (160 × 40 × 5) mm³ and submitted to the four-point bending tests. The bottom and top spacings used in the tests were 135 and 45 mm, respectively. The displacement speed was 5 mm/min, the samples were tested 8 days after production. The mechanical properties were calculated according to Azevedo et al. (2024) [30].

$$MOR={\displaystyle \frac{\left[3P_{max}\left(L_{inf}-L_{sup}\right)\right]}{2w{h}^2}}$$

(3)

$$MOE=\left[{\displaystyle \frac{\left(276\times L_{inf}^3\right)}{\left(1296w{h}^3\right)}}\right]\left({\displaystyle \frac{P}{\delta }}\right)$$

(4)

$$LOP={\displaystyle \frac{\left[3P_{lop}\left(L_{inf}-L_{sup}\right)\right]}{2w{h}^2}}$$

(5)

$$SE=\left[{\displaystyle \frac{1}{\left(hw\right)}}\right]\int P\left(\delta \right)d\delta$$

(6)

where P_max is the maximum load, L_inf is the major span between the supports, L_sup is the minor span between the two loading points, w and h are the specimen width and depth, respectively. P is the load and δ is the deformation. P_lop is the load at the upper point of the linear portion of the load versus deflection curve.

The results were expressed as mean values accompanied by standard deviation. Statistical analysis was performed using one-way analysis of variance (ANOVA), followed by Tukey’s post hoc test at a significance level of p < 0.05, allowing the identification of statistically significant differences between formulations and curing conditions. S.A.S 9.3 (Statistical Analysis System) software was used during this comparison.

2.7. Microstructural Analysis

X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to investigate phase evolution in samples acquired before and after each carbonation condition. XRD patterns were collected using a LA-60 diffractometer (HORIBA Ltd., Kyoto, Japan) with CuKα radiation (λ = 1.5406 Å) (40 kV, 30 mA), scanning from 10° to 70° at 2°/min. The crystalline phases were identified using the Crystallography Open Database (COD).

Scanning electron microscopy (SEM) analysis was performed on the fractured samples to observe the dispersion of the lignocellulosic reinforcement in the inorganic matrix and on the polished surface to examine the effect of carbonation and accelerated aging in the region near the vegetal fibers (transition zone). A Hitachi TM3000 tabletop scanning electron microscope (Hitachi High-Technologies Corporation, Tokyo, Japan) was used for evaluation using backscattered electrons.. The samples for microstructural analysis were prepared and polished as described by Urrea-Ceferino et al. [31].

3. Results

3.1. Physical Properties

The bulk density of autoclaved and carbonated fiber cement boards, before and after accelerated aging, is presented in Figure 10. The carbonated composites exhibited higher density values, reaching approximately 1.37–1.38 g/cm³, compared to 1.31–1.32 g/cm³ for autoclaved samples. This increase is primarily attributed to the formation of carbonation products, mainly calcium carbonate, which precipitate within the pore structure and contribute to matrix densification. These results were also observed in the work of Filomeno et al. [10], in which the authors investigated different curing regimes of fiber cement reinforced with vegetal fibers and found that the apparent density values increased after accelerated carbonation. After accelerated aging, a slight increase in density was observed in carbonated samples, which may be associated with continued hydration and secondary carbonation reactions occurring during the wetting–drying cycles. These processes promote additional solid phase formation within the pore network, further enhancing matrix compactness.

The apparent void volume results (Figure 11) corroborate this result, showing a reduction of approximately 15% in carbonated composites compared to autoclaved ones. This decrease is directly related to the precipitation of carbonate phases within capillary pores, leading to pore refinement and reduced connectivity. As a consequence, the transport pathways for water and aggressive agents are restricted, which is beneficial for durability. The influence of these microstructural changes is further reflected in the water absorption results (Figure 12). Carbonated samples exhibited water absorption values approximately 17% lower than those of autoclaved composites after aging. This reduction is consistent with the lower porosity and refined pore structure induced by carbonation. In contrast, autoclaved materials, although mechanically robust due to tobermorite formation, retain a more open pore structure, which facilitates water ingress. Additionally, the presence of unreacted hydration products in the carbonated system, as indicated by the measured carbonation degree (~16–17%), suggests that not all portlandite was consumed during curing. During the aging cycles, the reactivation of hydration reactions and the subsequent carbonation of newly formed Ca(OH)₂ may further contribute to pore filling and reduced water absorption. The results indicate that accelerated carbonation promotes a more efficient pore refinement mechanism compared to autoclave curing, leading to higher density, reduced porosity, and lower water absorption. These changes play a critical role in enhancing the durability and long-term performance of fiber cement composites.

3.2. Mechanical Properties

The mechanical performance of the autoclaved and carbonated fiber cement composites, before and after accelerated aging, is presented in Figure 13, Figure 14, Figure 15, Figure 16 and Figure 17. The MOR results (Figure 13) indicate that both curing routes led to comparable flexural strength levels, with no significant reduction after accelerated aging. Autoclaved boards exhibited a slight increase in MOR after aging, whereas carbonation-cured composites showed only minor variation, indicating stable load-bearing capacity over time. The modest increase in MOR observed for autoclaved materials after aging can be associated with the continued hydration of unreacted clinker phases during the wetting–drying cycles, leading to the additional formation of strength-contributing products. In contrast, the carbonated system, which undergoes significant densification during curing, already presents a more stabilized microstructure, resulting in limited variation in MOR after aging.

Despite the comparable MOR values, the stress–strain behavior (Figure 17) revealed clear differences in deformation capacity between the curing methods. Autoclaved composites exhibited lower strain at failure and a steeper post-peak response, indicating a more brittle response of the material. This response is consistent with the hydrothermal conditions applied during autoclaving, which may promote partial degradation of lignocellulosic fibers, inducing surface defects and weakening the fiber–matrix interface. As a consequence, the efficiency of stress transfer and the post-cracking contribution of the fibers are reduced. In contrast, carbonation-cured composites showed higher deformation capacity and a more pronounced post-cracking regime, indicating more effective fiber bridging and progressive load transfer. These results may be associated with improved fiber–matrix interaction, likely related to the precipitation of carbonation products at the interface, promoting better fiber anchorage within the matrix.

After accelerated aging, a reduction in deformation capacity was observed for both systems, particularly in carbonated composites. This decrease can be associated with the formation of additional hydration products during the wetting–drying cycles, leading to increased matrix stiffness and reduced deformability, as well as progressive degradation of lignocellulosic fibers under cyclic exposure conditions. These effects contribute to reduced fiber anchorage and lower post-cracking reinforcement efficiency.

The LOP results (Figure 14), which represent the transition from elastic behavior to the onset of matrix microcracking, showed distinct responses after aging. Autoclaved composites exhibited an increase of approximately 4.75% after aging (from 13.16 MPa to 13.82 MPa), indicating a higher stress required to initiate cracking. Such response may be associated with the steeper initial slope observed in the stress–strain curves and can be attributed to continued hydration of residual cementitious phases during the wetting–drying cycles, resulting in matrix densification and enhanced stiffness.

In contrast, carbonation-cured composites showed a reduction of approximately 11% in LOP after aging, reaching values close to 9.05 MPa. This decrease suggests that the elastic regime of these materials became more sensitive to interfacial changes induced by cyclic exposure. In the stress–strain curves, this behavior is reflected by an earlier deviation from linearity, indicating the onset of microcracking at lower stress levels. This response may be associated with localized interfacial adjustments, such as partial debonding or stress redistribution at the fiber–matrix interface.

The MOE results (Figure 15) further support these observations. After aging, both systems exhibited significant increases in stiffness, with increments of approximately 21% for autoclaved and 26% for carbonated composites. These increases are directly reflected in the steeper elastic slopes of the stress–strain curves and indicate progressive matrix stiffening. These findings indicate that additional solid phases formed during aging contributed to matrix stiffening, including continued hydration of residual clinker phases and secondary carbonation reactions involving newly formed portlandite. The slightly higher increase observed in carbonated systems suggests a more pronounced pore refinement effect during the aging process.

The most distinct differences between curing methods were observed in the SE results (Figure 16), which represent the energy absorption capacity and are directly related to the area under the stress–strain curve. Autoclaved composites exhibited lower SE values compared to carbonated materials in all conditions, indicating a more brittle mechanical response. Although an increase of approximately 19% in SE was observed after aging, the overall energy absorption capacity remained limited. This result is consistent with stress–strain curves characterized by a sharp post-peak load drop and limited deformation capacity.

In contrast, carbonation-cured composites maintained higher SE values and exhibited a more pronounced post-cracking regime, characterized by gradual load reduction and increased deformation capacity. This result suggests more effective fiber bridging and improved stress transfer across cracks. The enhanced performance may be associated with improved fiber–matrix interaction, likely related to carbonate precipitation at the interface, which may contribute to better fiber anchorage within the matrix.

As a result, while increases in MOE and, to a lesser extent, LOP reflect matrix stiffening, the evolution of SE and the shape of the stress–strain curves demonstrate that post-cracking behavior is primarily governed by fiber integrity and interfacial performance. Accelerated carbonation provided a more favorable balance between stiffness and toughness, whereas autoclave curing, despite promoting higher stiffness, resulted in reduced deformation capacity and lower energy absorption, likely associated with fiber degradation and less efficient interfacial bonding.

3.3. Thermogravimetry (TG)

The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of the REF (non-carbonated), carbonated, and carbonated-aged composites are presented in Figure 18. The results provide insight into the thermal stability of the materials and allow the identification and quantification of hydration and carbonation products. The initial mass loss observed between approximately 70 and 110 °C is attributed to the removal of physically adsorbed water and loosely bound water within the pore structure. A second mass loss event in the range of 300–420 °C is associated with the thermal degradation of lignocellulosic fibers, confirming their presence and contribution within the composite system. A distinct peak around 450 °C is observed in the REF sample and is assigned to the decomposition of portlandite (Ca(OH)₂). The absence or significant reduction in this peak in the carbonated samples indicates the consumption of portlandite during the carbonation process, which is consistent with the formation of calcium carbonate phases, as shown in Equation (7).

$$Ca{\left(OH\right)}_2+C{O}_2+x{H}_{2}O\to CaC{O}_3.n{H}_{2}O+(x-n)H_{2}O$$

(7)

In the carbonated and carbonated-aged samples, a pronounced mass loss is observed in the temperature range of approximately 650–780 °C, corresponding to the decomposition of calcium carbonate as indicated by Equation (8):

$$CaC{O}_{3\left(s\right)}\to Ca{O}_{\left(s\right)}+C{O}_{2\left(g\right)}$$

(8)

The total mass loss associated with this region reached approximately 23.4% for carbonated samples and 24.3% for carbonated-aged materials, indicating an increase in carbonate content after aging. The calculated carbonation degree (CD) values ranged from approximately 16% for carbonated samples to 17% after aging, while the corresponding CO₂ uptake increased from 33.6 to 35.8 kg/m³, presented in

Table 6. Carbonation degree of the carbonated and carbonated—aged boards.

Sample	Carbonation Degree (CD) (%)	Embodied CO2 (kg of CO2/m3 of Fiber Cement)
Carbonated	16	33.6
Carbonated—Aged	17	35.8

. This slight increase suggests that secondary carbonation occurs during the wetting–drying cycles, likely due to the formation of new portlandite through continued hydration, which subsequently reacts with atmospheric CO₂. The TG results also reveal that not all reactive phases were consumed during the initial carbonation curing, as evidenced by the moderate carbonation degree values. This residual reactivity plays a key role during aging, enabling further hydration and carbonation reactions that contribute to matrix densification and pore refinement, as previously observed in the physical and mechanical properties.

For the autoclaved samples (Figure 19), the thermal properties differ significantly. The mass loss between 70 and 200 °C is associated with water release from the interlayer structure of tobermorite (Ca₅Si₆O₁₆(OH)₂·4H₂O), and other hydration products formed under hydrothermal conditions. The absence of a pronounced portlandite decomposition peak confirms that most calcium hydroxide has been consumed during autoclave curing, primarily through the formation of calcium silicate hydrate phases. A minor mass loss event observed near 650–750 °C is attributed to the decomposition of intrinsic carbonates present in the raw materials or formed through limited exposure to atmospheric CO₂. The relatively low intensity of this peak compared to carbonated samples confirms the lower extent of carbonation in autoclaved systems. The TG analysis demonstrates that accelerated carbonation effectively promotes the formation of calcium carbonate phases through the consumption of portlandite, while autoclave curing leads to the formation of crystalline hydration products such as tobermorite. The presence of residual reactive phases in carbonated composites enables continued reactions during aging, contributing to further densification and improvement of material performance. These findings are consistent with the observed trends in density, porosity, and mechanical behaviour, reinforcing the role of carbonation as a key mechanism for microstructural refinement in fiber cement composites.

3.4. Phases Analysis

The X-ray diffraction patterns of the REF (non-carbonated), carbonated, and aged carbonated composites are presented in Figure 20. In all samples, the most intense reflections are associated with quartz and calcite, while lower-intensity peaks corresponding to alite and portlandite can also be identified. The presence of quartz is mainly related to the fly ash used in the formulations, whereas calcite is partially associated with the carbonate-based filler and the intrinsic carbonate content of the raw materials. A clear distinction between the non-carbonated and carbonated materials is observed in the relative intensity of the calcite and portlandite peaks. In the non-carbonated sample, portlandite reflections are evident, particularly at diffraction angles near 21°, 33°, and 34° 2θ, indicating the presence of Ca(OH)₂ formed during cement hydration. After accelerated carbonation, the intensity of these reflections decreases markedly, while the calcite peaks become more pronounced, especially around 22.5° and 29° 2θ. This phase evolution confirms that carbonation promoted the consumption of portlandite and the precipitation of calcium carbonate phases within the matrix. This result is in good agreement with the thermogravimetric data, which showed the disappearance or significant reduction in the portlandite decomposition peak and the increase in mass loss associated with carbonate decomposition. The increase in calcite peak intensity provides structural evidence that the curing process in a CO₂-rich atmosphere was effective in converting alkaline hydration products into stable carbonation products. This result was also found in the study by Dias et al. (2024), where the authors reported a decrease in the intensity of portlandite peaks and an increase in the intensity of calcite peaks due to the reaction products formed during curing in a CO₂-rich atmosphere [15].

The aged carbonated composites did not show major qualitative changes in phase assemblage compared to the non-aged carbonated samples, indicating that the main crystalline products formed during carbonation remained stable after the wetting–drying cycles. However, the persistence of low-intensity alite and portlandite peaks suggests that part of the cementitious system remained reactive after the initial curing stage. During accelerated aging, these residual phases may undergo further hydration, producing additional portlandite that can subsequently react with atmospheric CO₂. This interpretation is consistent with the slight increase in carbonation degree observed after aging and helps explain the continued pore refinement and the increase in stiffness detected in the physical and mechanical results.

The diffraction patterns of the autoclaved composites before and after aging are shown in Figure 21. In contrast to the carbonated system, the autoclaved materials are characterized by the appearance of tobermorite reflections, which are absent in the carbonated samples. Tobermorite is the main crystalline hydration product formed under hydrothermal conditions and is directly associated with the gain in mechanical strength of the autoclaved boards. Quartz reflections are also observed, mainly due to the silica powder and fly ash present in the formulation [2,28,32].

The presence of tobermorite confirms that the hydrothermal curing conditions were sufficient to promote the reaction between calcium-bearing phases and reactive silica sources, leading to the formation of a more crystalline calcium silicate hydrate structure. This phase assemblage explains the high mechanical strength and the relatively high stiffness of the autoclaved composites. At the same time, the absence of significant changes in the diffraction patterns after aging indicates that the wetting–drying cycles did not substantially alter the main crystalline phases formed during autoclaving.

Although autoclaving and carbonation both improve the performance of the composites, the XRD results show that they do so through distinct mechanisms. In the carbonated system, the dominant process is the conversion of portlandite into calcium carbonate, leading to pore filling, matrix densification, and improved fiber–matrix interaction. In the autoclaved system, the main mechanism is the formation of tobermorite, which increases matrix strength and stiffness under hydrothermal conditions.

These differences in phase evolution help explain the contrasting mechanical strength of the two systems. The formation of calcium carbonate in the carbonated boards is closely related to the lower void volume, reduced water absorption, and greater deformation capacity observed in the stress–strain curves, likely due to improved interfacial bonding and preservation of lignocellulosic fibers. In contrast, the formation of tobermorite in the autoclaved composites contributes to higher rigidity and strength, but the high temperature and pressure involved in the curing process may also promote degradation of the vegetal fibers, limiting post-cracking deformation and reducing energy absorption capacity.

3.5. SEM Analysis

The microstructural features of the carbonated and autoclaved fiber cement composites, before and after accelerated aging, were analysed by scanning electron microscopy (SEM) using backscattered electron imaging. The results are presented in Figure 22, Figure 23, Figure 24 and Figure 25.

In the carbonated composites (Figure 22), the fibers are well embedded within a relatively dense matrix, with limited interfacial voids. The presence of spherical particles, associated with unreacted or partially reacted fly ash, is clearly observed. This is consistent with the lower reactivity of fly ash under the relatively mild conditions of carbonation curing. In addition, the matrix surrounding the fibers appears compact, suggesting that carbonation products precipitated within the pore structure and at the fiber–matrix interface.

Higher magnification images reveal the presence of fine reaction products filling voids and interfacial regions. These features are attributed to the precipitation of calcium carbonate phases, which contribute to pore refinement and improved interfacial bonding. This microstructural configuration supports the enhanced deformation capacity and higher specific energy values observed in the mechanical tests, indicating more efficient stress transfer and fiber bridging [30].

After accelerated aging (Figure 23), the carbonated composites maintain a relatively dense microstructure, although slight changes can be observed at the fiber–matrix interface. Some localized interfacial gaps and microcracks are visible, suggesting partial degradation or debonding of the fibers after cyclic exposure. Despite these changes, the overall matrix integrity is preserved, which is consistent with the stable MOR values and the moderate reduction in deformation capacity observed in the stress–strain behavior. Similar findings were reported by Azevedo et al. (2023) [33] in their study where the authors evaluated the use of curing in CO₂-rich environments to improve the mechanical properties of Low-Carb matrix composites reinforced with vegetable fibers. The authors concluded that the formation of carbonation products at the fiber-matrix interface promoted several improvements, including increased maximum deformation of the composites before failure, reduced water absorption, decreased void volume, and increased density, as observed in the results of this investigation [33].

In contrast, the autoclaved composites (Figure 24) exhibit a distinctly different microstructure. The matrix is characterized by lamellar and plate-like features associated with tobermorite formation, confirming the XRD results. A lower presence of intact fly ash particles is observed, indicating increased reactivity under hydrothermal conditions.

However, the fiber–matrix interface in autoclaved samples appears less cohesive compared to the carbonated system. The fibers show surface irregularities and defects, which can be attributed to degradation under high temperature and pressure conditions during autoclave curing. In several regions, interfacial voids and gaps are observed, indicating weaker mechanical anchorage of the fibers within the matrix.

These microstructural features explain the mechanical strength observed in autoclaved composites, particularly the lower deformation capacity and reduced specific energy values. The presence of a rigid matrix, combined with degraded fibers and weaker interfacial bonding, limits the efficiency of stress transfer after cracking, resulting in a more brittle response.

After accelerated aging (Figure 25), the autoclaved composites show limited changes in the overall phase morphology, consistent with the XRD results. However, further degradation of the fiber surfaces and enlargement of interfacial gaps can be observed, indicating progressive deterioration of the fiber–matrix interaction. This behavior contributes to the reduced post-cracking performance and confirms the sensitivity of lignocellulosic fibers to hydrothermal and cyclic environmental conditions.

SEM analysis highlights the fundamental differences in the microstructural development of the composites as a function of curing method. Accelerated carbonation promotes the formation of a dense matrix with improved fiber–matrix bonding due to the precipitation of carbonate phases, leading to enhanced deformation capacity and energy absorption. In contrast, autoclave curing results in a more crystalline and rigid matrix dominated by tobermorite formation, but with reduced interfacial efficiency due to fiber degradation.

These observations provide direct microstructural evidence supporting the mechanical and physical results, demonstrating that interfacial characteristics and fiber integrity play a key role in governing the performance of fiber cement composites under different curing regimes.

4. Conclusions

This study demonstrates that accelerated carbonation is a viable alternative to autoclave curing to produce fiber cement composites reinforced with lignocellulosic fibers, providing comparable mechanical performance while improving deformation capacity and reducing processing energy demand. The current evidence is mainly based on laboratory-scale specimens and limited short-term aging tests.

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Carbonation-cured composites achieved modulus of rupture (MOR) values comparable to autoclaved materials, indicating that high flexural strength can be obtained without hydrothermal curing;

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Despite similar strength levels, carbonated composites exhibited higher deformation capacity and specific energy, reflecting improved post-cracking behavior and more efficient fiber bridging;

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Autoclave curing led to a more rigid but brittle response, associated with tobermorite formation and partial degradation of lignocellulosic fibers under high temperature and pressure;

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Accelerated carbonation promoted pore refinement and matrix densification through the formation of calcium carbonate, while also enhancing fiber–matrix interfacial bonding, which is critical for energy absorption and crack control;

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After accelerated aging, both systems maintained stable MOR values; however, a reduction in deformation capacity was observed, attributed to matrix stiffening and progressive fiber degradation, particularly affecting post-cracking performance;

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The carbonation process enabled the production of fiber cement composites without the need for hydrothermal curing, representing a significant reduction in energy consumption and a more sustainable processing route.

Consequently, accelerated carbonation represents a promising pathway for the development of high-performance and more sustainable fiber cement materials, particularly for applications involving lignocellulosic reinforcement, where fiber integrity and interfacial performance are critical. Although the formulations differed according to the requirements of each curing route, the results provide important insights into the technological feasibility of accelerated carbonation as a potential alternative curing strategy.