Evaluating the influence of plant-based and synthetic fibers on freeze-thaw resistance in cementitious composites

ABSTRACT

Enhancing the freeze–thaw durability of cementitious materials is a key challenge for sustainable construction, particularly in cold-climate applications. Within the broader research field of fiber-reinforced cementitious composites, this study contributes to the ongoing shift toward eco-friendly reinforcement strategies by comparatively evaluating natural plant-based fibers and conventional synthetic fibers. The influence of date palm, dis, alfa, hemp, and polypropylene fibers on the freeze–thaw resistance of metakaolin-based mortars was systematically investigated. Six mortar formulations were tested, each containing fibers at 0.1% by total volume. Compressive and flexural strengths, porosity, dynamic elastic modulus, and thermal conductivity were evaluated before and after freeze-thaw cycles. Results showed polypropylene fibers provided high initial compressive strength (~62 MPa), though significant stiffness reduction (~16%) occurred after cycling. Hemp fibers initially delivered superior flexural strength (~7.5 MPa) but exhibited substantial degradation (~32% loss) after 120 cycles. Alfa and date palm fibers presented intermediate performance with moderate mechanical deterioration, whereas dis fibers maintained relatively stable compressive strength (~45 MPa). Natural fibers generally increased porosity post-cycling but showed reduced thermal conductivity, reflecting structural damage. Overall, the study confirms that fiber incorporation significantly influences the freeze-thaw durability of mortars, with polypropylene fibers offering enhanced long-term stability compared to natural alternatives.

Freeze-thaw durability; Cementitious mortars; Plant fibers; Polypropylene fibers

1. INTRODUCTION

Cementitious materials such as mortars and concretes are fundamental building blocks in modern construction, possessing excellent compressive strength and versatility. However, their durability under harsh environmental conditions, particularly freeze-thaw cycles, remains a significant challenge for construction applications in cold regions. The freeze-thaw resistance of cementitious materials is a critical durability parameter that directly influences the service life and structural integrity of buildings and infrastructure in cold climate regions [¹].

Freeze-thaw deterioration in mortars is a complex physicochemical process involving multiple degradation mechanisms. Upon freezing, the volumetric expansion of pore water induces hydraulic pressures that exceed the tensile strength of the matrix, causing internal microcracking [²]. Repeated freeze-thaw cycles exacerbate this damage, leading to scaling and structural degradation. Several theories explain this behavior: the hydraulic pressure theory attributes damage to pressure buildup from unfrozen water during ice formation; the osmotic pressure theory highlights water migration toward ice fronts due to chemical potential gradients; the critical air content theory underscores the role of entrained air voids in mitigating internal stress; and the crystallization pressure theory focuses on the mechanical stress from ice crystal growth against pore walls [³,⁴,⁵,⁶]. Together, these mechanisms drive the progressive deterioration of the mortar’s microstructure. The mechanical properties of the mortar matrix, particularly tensile strength and elasticity, determine its capacity to withstand internal stresses before failure [⁷, ⁸]. These interrelated factors create a complex durability profile that varies with material composition and environmental exposure conditions.

The incorporation of fibers into mortar matrices has emerged as a promising strategy for enhancing freeze-thaw durability [⁹, ¹⁰]. Fibers fundamentally alter the mechanical behavior of cementitious composites by bridging microcracks and redistributing internal stresses, which significantly modifies their response to the destructive forces generated during freeze-thaw cycles [¹¹, ¹²]. This fiber-matrix interaction creates a composite system with improved tensile capacity, strain-hardening behavior, and energy absorption characteristics [¹³].

Synthetic fibers, particularly polypropylene (PP), are widely used in cementitious materials to improve freeze-thaw durability due to their chemical stability and reliable mechanical performance. Studies have shown that PP fibers significantly reduce mass loss and strength degradation during freeze-thaw cycles by controlling microcracking and redistributing internal stresses [¹⁴,¹⁵,¹⁶]. Their effectiveness depends on parameters such as fiber length, volume fraction, and geometry, which influence workability, bonding, and crack resistance, ultimately enhancing the material’s resilience under cyclic freezing conditions. Despite their effectiveness, synthetic fibers raise sustainability concerns due to their petroleum-based origin, high energy consumption, and carbon emissions [¹⁷, ¹⁸]. Their end-of-life disposal may also contribute to microplastic pollution. These environmental drawbacks have driven interest in natural plant-based fibers as renewable, biodegradable alternatives with promising performance potential [¹⁹].

Natural fibers differ structurally from synthetic ones, featuring cellulose microfibrils embedded in lignin and hemicellulose matrices. This hierarchical composition governs their mechanical behavior, with cellulose contributing to tensile strength and stiffness, while lignin and hemicellulose influence durability and fiber-matrix bonding [²⁰]. However, challenges remain. The hydrophilic nature of natural fibers can cause swelling, weakening the matrix interface. Moreover, cement’s high alkalinity (pH >12.5) degrades hemicellulose and lignin, threatening long-term durability [²¹, ²²]. Mechanical property variability also complicates their consistent application [²¹, ²³]. To address these issues, various treatments—alkalization [²⁴], silane [²⁵], and acetylation [²⁶]—have been used to reduce water absorption and improve alkali resistance. Modifying the cement matrix with pozzolanic materials can also lower alkalinity and preserve fiber integrity.

Applying natural fibers specifically for freeze-thaw resistance enhancement represents an emerging research frontier with promising preliminary results. Few studies have investigated the performance of natural fibers under freeze-thaw conditions, though literature remains less extensive than that of synthetic fibers. For instance, ZAID et al. [²⁷] observed a 34% increase in freeze-thaw durability index in concrete reinforced with 0.3% jute fibers. DENG et al. [²⁸] reported that incorporating 0.9% sisal fibers reduced soil deformation under freeze-thaw cycles by 59.26%. BERKOUCHE et al. [²⁹] found that adding up to 0.2% flax fibers by volume improved the freeze-thaw resistance of foam geopolymers, along with enhanced thermal insulation.

The comparative analysis summarized in Table 1 highlights that previous studies have predominantly focused on synthetic fibers (such as PVA, PP, and carbon) or on a limited range of natural fibers, often under restricted experimental conditions. Many investigations report improved freeze–thaw resistance and mechanical performance; however, they are frequently constrained by a low number of freeze–thaw cycles, narrow fiber dosage ranges, high material costs, or the absence of direct comparisons between natural and synthetic reinforcements. In addition, several studies emphasize either mechanical performance or environmental exposure without providing a comprehensive assessment that integrates mechanical, physical, thermal, and microstructural responses.

Table 1
Comparative summary of previous studies on fiber-reinforced cementitious composites under freeze–thaw conditions and identified research gaps.

WORK	CONTRIBUTION	CRITICISM
[30]	Demonstrated superior freeze-thaw resistance of PVA fibers compared to PP fibers.	Limited to 300 cycles; does not explore long-term effects.
[31]	Showed that hemp and flax fibers reduce deterioration and improve energy absorption.	Performance compromised with high fly ash content.
[32]	Found carbon fibers to have the highest frost resistance; used ABAQUS modeling for predictions.	High cost and potential dispersion issues.
[33]	Composites with hemp, flax, and fly ash were formulated for cold region infrastructure.	Did not compare with synthetic fibers.
[11]	Investigated the effect of freeze–thaw cycling on mechanical properties of polyethylene fiber and steel fiber reinforced concrete	Focused only on PP and steel fibers; lacks comparison with other synthetic fibers.
[34]	Explored the use of plant straw fibers in low alkaline magnesium oxychloride cement.	Limited to 50 freeze-thaw cycles; lacks comprehensive mechanical property analysis.
[35]	Analyzed the impact of chloride environment on PVA-ECC frost resistance.	Focused only on PVA fibers; lacks comparison with other fibers.
[29]	Investigated the effect of flax fiber on thaw-freeze resistance of foam geopolymer composites	Does not address other fiber types.

Consequently, a clear knowledge gap remains regarding the comparative freeze–thaw durability of diverse plant-based fibers versus conventional synthetic fibers under extended cycling conditions. The present study addresses this gap by offering a systematic and comparative evaluation of four natural fibers—date palm, dis, alfa, and hemp—for enhancing the freeze-thaw resistance of cementitious mortars. Date palm fibers, widely available as agricultural by-products in arid and semi-arid regions, have been studied for their positive effects on the mechanical behavior of mortars, particularly in improving impact resistance and flexural strength [³⁶], [³⁷]. While their role in freeze-thaw durability remains relatively unexplored, their demonstrated effectiveness in reinforcing cement matrices suggests promising potential. Dis fibers, obtained from Ampelodesmos mauritanicus, have received limited attention in the context of cementitious applications, though existing research indicates their suitability for mortar reinforcement, particularly in enhancing tensile and flexural performance [³⁸]. Their natural durability and compatibility with cementitious systems position them as an interesting candidate for further study. Alfa fibers, or esparto grass, have been traditionally used in building materials and have shown potential in improving crack resistance and toughness in cement-based composites [³⁹]. Their integration into mortar mixes has also been associated with improved post-cracking behavior and reduced shrinkage and enhanced thermal insulation [³⁹, ⁴⁰], which may contribute positively to freeze-thaw performance. Hemp fibers, among the most extensively researched natural reinforcements, have demonstrated substantial benefits in mortar and concrete applications, including improved tensile and flexural strength [³²], enhanced ductility, and crack-bridging capacity [⁴¹]. Although most studies have focused on mechanical reinforcement, only limited research has addressed their behavior under freeze-thaw conditions. Taken together, these fibers offer a diverse set of properties and prior applications that justify a systematic investigation of their comparative performance under freeze-thaw cycling, particularly when benchmarked against conventional synthetic fibers such as polypropylene.

In light of the limitations identified in previous studies, particularly the restricted range of fiber types, limited freeze–thaw cycles, and the lack of direct comparison between natural and synthetic reinforcements, this research aims to clearly position and extend current knowledge on fiber-reinforced cementitious composites. The present study provides a systematic and comparative evaluation of the freeze–thaw resistance of metakaolin-based mortars reinforced with several plant-based fibers (date palm, dis, alfa, and hemp) and benchmarks their performance against conventional polypropylene fibers. The effects of prolonged freeze–thaw cycling are assessed through a combined analysis of mechanical properties (compressive and flexural strengths, dynamic elastic modulus), physical characteristics (density and porosity), thermal behavior (thermal conductivity), and microstructural evolution using SEM observations.

By integrating mechanical, physical, thermal, and microstructural perspectives within a single experimental framework, this work directly addresses the existing knowledge gap concerning the durability and limitations of natural fiber-reinforced mortars under freeze–thaw exposure. The outcomes of this study provide clear performance benchmarks and mechanistic insights, supporting the rational use of plant-based fibers as sustainable alternatives to synthetic fibers in cementitious materials intended for cold-climate applications.

2. MATERIALS AND METHODS

2.1. Materials

All The different plant fibers used in this study are Alfa, Diss, Date Palm, Hemp, and Hemp Shiv. These fibers were used with a length of 1.5 cm. SEM observations (Figure 1) reveal clear differences in the surface morphology of the investigated fibers. Natural fibers exhibit rough, irregular, and fibrillated surfaces that can enhance mechanical interlocking with the cementitious matrix but may also promote water absorption [⁴²], [⁴³]. These morphological features are expected to play a key role in governing crack-bridging efficiency and freeze–thaw durability. Table 2 presents the physical, chemical, and mechanical properties of the plant and polypropylene fibers.

Figure 1
SEM micrographs of the different plant fibers used in the study.

Table 2
Physical, chemical, and mechanical properties of plant-based and polypropylene fibers.

FIBER TYPE	ALFA [44]	DISS [45]	DATE PALM [46]	POLYPROPYLENE [47]	HEMP [48]
Absolute density (kg/m3)	1400	1230	1383.3	900	1580
Lignin content (%)	24	/	35 ± 5	/	6
Cellulose content (%)	45	30–40	43 ± 2	/	56.1
Hemicellulose (%)	24	4.8–0.08	8 ± 2	/	10.9
Ash content (%)	2	/	/	/	/
Tensile strength (Mpa)	248.49	173.53	375.33	450	619.25
Maximum strain (mm)	0.705	0.433	1.178	/	0.682
Water absorption (%)	100	88–90	132.5	/	158
DIAMETER (µm)	883.07	900–2480	204.8–450	24.5	110
Length (cm)	1.5	1.5	1.5	1.5	1.5

2.2. Mortar mix design and experimental testing

A total of six mortar formulations were prepared, each incorporating the same fiber content of 0.1% by total volume. This fiber dosage was chosen as an optimal compromise between mechanical performance, homogeneous fiber dispersion, and workability of the fresh mortar. Previous studies have shown that low fiber volume fractions effectively enhance crack-bridging and durability while avoiding fiber agglomeration, excessive porosity, and increased water demand, which are particularly detrimental under freeze–thaw conditions [²⁹, ³⁸]. The binder content (cement + metakaolin) was fixed at 450 kg/m3, and the same granular skeleton, water content, and superplasticizer dosage were used across all mixes. The details of the mortar mix compositions are summarized in Table 3.

Table 3
Mix compositions of prepared mortars.

COMPONENT	CM	MPP	MAF	MDS	MDP	MHE
Cement (kg/m3)	315	315	315	315	315	315
Metakaolin (kg/m3)	135	135	135	135	135	135
Sand (kg/m3)	1350	1350	1350	1350	1350	1350
Water (kg/m3)	250	250	250	250	250	250
Superplasticizer (kg/m3)	6	6	6	6	6	6
Fiber (kg/m3)	/	0.91	1.40	1.20	1.25	1.35

Mortar with dimensions of 4 × 4 × 16 cm3 and 5 × 10 cm2 cylindrical specimens were prepared following the guidelines stipulated in NF EN 206-1 standard [⁴⁹]. After an initial curing period of 24 hours, the specimens were removed from their molds and subsequently cured in water at a constant temperature of 20 °C. Property evaluations were carried out on the hardened mortar samples after a curing duration of 28 days (Figure 2).

Figure 2
Specimens exposure to freeze-thaw cycling in climate chamber.

Experimental tests included assessments of compressive and flexural strengths according to standard NF EN 196-1 [⁵⁰] before and after exposure to 60 and 120 freeze-thaw cycles. Freeze-thaw cycles were conducted following NF P18-424 standard [⁵¹], with each cycle consisting of freezing at -18 ± 2 °C for 4 hours followed by thawing at 20 ± 2 °C for another 4 hours (Figure 2). Additionally, the dynamic modulus of elasticity was evaluated using ultrasonic pulse velocity measurements as specified in ASTM C597 [⁵²]. Density measurements were conducted in compliance with ASTM guideline [⁵³]. Thermal conductivity tests were measured using a Hot Disk TPS 1500 thermal conductivity meter [⁵⁴]. Water-accessible porosity was determined in accordance with NBR 9778 guideline [⁵⁵]. These latter tests (dynamic modulus, density, thermal conductivity, and porosity) were carried out on samples prior to and after completing 60 freeze-thaw cycles.

3. RESULTS

3.1. Compressive strength

Figure 3 highlights the effect of freeze/thaw cycles on the compressive strength of mortars containing 30% metakaolin, comparing a control sample without fibers (CM) and five formulations reinforced with different natural and synthetic fibers.

Figure 3
Compressive strength of different mortars subjected to freeze/thaw cycles.

The mortar without fibers (CM) undergoes a progressive loss of strength with increasing freeze/thaw cycles (approximately 52 MPa at 0 cycles, 49 MPa at 60 cycles, and 47 MPa at 120 cycles), which is mainly attributed to microcrack initiation and propagation induced by ice crystallization pressure and hydraulic stresses within the pore network [⁵⁶, ⁵⁷]. This behavior confirms the inherent susceptibility of plain cementitious matrices to freeze–thaw deterioration.

The mortar reinforced with polypropylene fibers (MPP) exhibits the best overall performance, with a high initial strength (around 62 MPa) and a moderate strength loss (≈10 MPa after 120 cycles). This improved performance is associated with the chemical inert and hydrophobic nature of PP fibers, which effectively limit water ingress, restrain microcrack development, and redistribute internal stresses during freezing and thawing. The relatively moderate strength loss observed in MPP suggests that PP fibers act primarily as micro-reinforcement, delaying crack coalescence rather than completely preventing damage accumulation.

Mortars reinforced with alfa (MAF), date palm (MDP), and hemp (MHE) fibers exhibit intermediate performance, reflecting a balance between beneficial crack-bridging effects and adverse durability-related mechanisms. Although these plant-based fibers enhance the initial compressive response by improving stress transfer within the matrix, their hydrophilic character promotes water absorption, swelling, and subsequent fiber–matrix debonding under cyclic freezing conditions. Among them, hemp fiber mortar (MHE) shows the most pronounced strength loss after 120 cycles, which can be linked to its hollow and porous internal structure, leading to higher local porosity and increased susceptibility to freeze–thaw-induced damage [⁵⁸].

The Diss fiber mortar (MDS) maintains a relatively stable compressive strength close to 45 MPa throughout the cycles, indicating limited sensitivity to freeze–thaw action. However, its lower initial strength and minimal evolution suggest weak fiber–matrix adhesion and reduced load-transfer efficiency, resulting in a marginal reinforcement effect. In contrast, polypropylene fibers, due to their dimensional stability and low affinity for moisture, provide more consistent mechanical performance under aggressive environmental exposure, in agreement with previous findings reported by ZHANG et al. [⁵⁹].

These results indicate that while natural fibers can contribute to initial mechanical enhancement, their long-term effectiveness under freeze–thaw conditions is strongly constrained by moisture sensitivity, alkaline degradation, and induced porosity. Synthetic polypropylene fibers remain more efficient in preserving compressive strength due to their superior physicochemical stability in cyclic freezing environments.

3.2. Flexural strength

Figure 4 presents the evolution of flexural strength as a function of fiber type and the number of freeze–thaw cycles, highlighting the strong sensitivity of tensile-related properties to cyclic environmental loading. All mortars exhibit a progressive reduction in flexural strength with increasing cycles, reflecting degradation of matrix cohesion and weakening of fiber–matrix interfacial bonding due to repeated freezing-induced stresses. The control mortar without fibers (CM) shows the lowest flexural capacity and a clear decline from approximately 5.8 MPa to 4.9 MPa after 120 cycles, confirming the brittle nature and limited crack resistance of unreinforced cementitious matrices.

Figure 4
Flexural strength of different mortars subjected to freeze/thaw cycles.

Among the reinforced mortars, hemp fiber mortar (MHE) exhibits the highest initial flexural strength, reaching about 7.5 MPa, which exceeds that of polypropylene-reinforced mortar (MPP, ~6.5 MPa). This improvement is attributed to the rough surface morphology, high aspect ratio, and favorable spatial distribution of hemp fibers, which enhance mechanical interlocking and crack-bridging efficiency under flexural loading [⁶⁰]. However, after 120 freeze–thaw cycles, the flexural strength of MHE decreases markedly to approximately 5.1 MPa, corresponding to a loss of nearly 32%. This pronounced deterioration can be explained by the hydrophilic nature of hemp fibers, leading to water absorption, swelling–shrinkage cycles, and progressive alkaline degradation, which weaken the fiber–matrix interface and reduce load transfer efficiency, as demonstrated by KASHTANJEVA et al. [⁶¹].

In contrast, polypropylene fiber mortar (MPP) displays more stable behavior, with flexural strength decreasing from about 6.5 MPa at 0 cycles to approximately 5.5 MPa after 120 cycles. This limited reduction highlights the chemical inertness, low water absorption, and dimensional stability of PP fibers, allowing them to maintain effective crack control under repeated thermal cycling. Mortars reinforced with date palm (MDP) and alfa (MAF) fibers also show relatively high initial flexural strengths (around 7.0 MPa), followed by moderate strength losses after freeze–thaw exposure, indicating an intermediate durability governed by a balance between crack-bridging benefits and moisture-induced degradation mechanisms.

The Diss fiber mortar (MDS) presents the lowest flexural performance among the reinforced mixes, suggesting limited intrinsic tensile strength of the fibers and insufficient fiber–matrix interaction to effectively restrain crack propagation. Overall, these results demonstrate that plant-based fibers can significantly enhance initial flexural strength, particularly in tension-dominated loading, but their long-term performance under freeze–thaw cycling is strongly dependent on their physico-chemical stability and resistance to moisture and alkaline environments. Synthetic polypropylene fibers, although slightly less effective initially, provide superior long-term flexural stability, in agreement with previous findings on fiber-reinforced cementitious composites exposed to harsh environmental conditions [⁵⁹].

3.3. Failure mechanism

Figure 5 illustrates the flexural failure modes of the investigated mortars after 60 freeze–thaw cycles, providing qualitative insight into the damage mechanisms induced by cyclic freezing and the role of fiber reinforcement in controlling crack propagation. The control mortar without fibers exhibits a sudden and brittle failure characterized by a single, well-defined crack, which is typical of aged cementitious matrices lacking any crack-bridging mechanism. This behavior confirms the limited post-cracking capacity and low energy absorption of plain mortars after freeze–thaw exposure.

Figure 5
Visual aspect of the failure mechanism of the fiber-reinforced mortars.

In contrast, all fiber-reinforced mortars display comparatively tougher failure modes, reflecting the ability of fibers to bridge cracks and delay their propagation. The polypropylene-reinforced mortar (PP) shows the most ductile response, maintaining structural integrity after bending with only a narrow, non-penetrating crack. This behavior highlights the elastic nature, high tensile capacity, and chemical stability of PP fibers, which allow them to sustain deformation and effectively transfer stresses across cracks even after freeze–thaw cycling [⁶²]. Similarly, the Diss fiber mortar (DS) exhibits a relatively controlled crack pattern, suggesting adequate fiber–matrix interaction and a certain degree of crack-arrest capability.

Conversely, mortars reinforced with alfa (AF), date palm (PD), and hemp (HE) fibers present more pronounced cracking, often accompanied by fiber pull-out, interfacial debonding, or wider crack openings. These observations indicate a reduction in crack-bridging efficiency, which can be attributed to moisture-induced swelling, alkaline degradation, and weakening of the fiber–matrix interface during freeze–thaw cycles. This interpretation is consistent with SEM observations (Section 3.8), which reveal degraded interfaces and increased microstructural damage in these composites.

The observed failure mechanisms are in good agreement with the mechanical, thermal, and microstructural results discussed previously. Polypropylene and Diss fibers provide better resistance to freeze–thaw-induced damage by maintaining effective crack control, whereas natural fibers (particularly hemp, alfa, and date palm) exhibit higher sensitivity to moisture and environmental aging, leading to reduced mechanical efficiency and more brittle failure behavior.

3.4. Dynamic modulus

Figure 6 shows the evolution of the dynamic modulus of elasticity of the investigated mortars before and after 60 freeze–thaw cycles, providing quantitative insight into stiffness degradation and internal damage accumulation. A reduction in dynamic modulus is observed for all mixtures, confirming that repeated freezing and thawing induces microcracking and loss of elastic integrity within the cementitious matrix. The control mortar (CM) exhibits a pronounced decrease from about 32 GPa to 24.8 GPa, indicating its limited intrinsic resistance to freeze–thaw action in the absence of any crack-bridging reinforcement.

Figure 6
Evolution of the dynamic modulus of the different studied mixtures.

The polypropylene fiber-reinforced mortar (MPP) shows a decrease in dynamic modulus from approximately 22 GPa to 18 GPa, corresponding to the highest relative stiffness loss among the reinforced mixes. Although polypropylene fibers are hydrophobic and generally associated with improved freeze–thaw durability, as indicated by ZAIMOGLU et al. [⁶³], this result suggests that the elastic modulus is strongly governed by fiber–matrix interfacial stiffness. Weak interfacial bonding or localized microcracking around smooth PP fibers may reduce stress transfer efficiency, leading to a more pronounced loss of dynamic stiffness under cyclic loading.

In contrast, mortars reinforced with natural fibers, particularly hemp (MHE) and date palm (MDP), exhibit the lowest modulus losses after 60 cycles, decreasing from about 27 GPa to 25.4 GPa and from 26.7 GPa to 24.2 GPa, respectively. This limited reduction indicates better preservation of elastic continuity, which can be attributed to the crack-bridging capacity and energy-dissipation mechanisms provided by lignocellulosic fibers. Their relatively rough surfaces and mechanical anchorage may help restrain microcrack propagation and limit water penetration, as also reported by MANIAN et al. [⁶⁴]. Alfa (MAF) and Diss (MDS) fiber mortars show intermediate behavior, with dynamic modulus decreasing from 35 GPa to 23.5 GPa and from 34 GPa to 25.3 GPa, respectively. The larger reduction observed for MAF may be related to higher water absorption and fiber swelling, whereas the comparatively stable response of MDS suggests a more favorable balance between fiber stiffness and interfacial bonding. Overall, these results indicate that, despite their hydrophilic nature, certain natural fibers can effectively mitigate stiffness degradation under freeze–thaw cycling, contributing to improved mechanical durability of metakaolin-based mortars and aligning with current trends toward sustainable eco-materials.

3.5. Water-accessible porosity

Figure 7 presents the evolution of water-accessible porosity of the metakaolin-based mortars before and after 60 freeze–thaw cycles. A general increase in porosity is observed for all mixtures, confirming that cyclic freezing promotes microcrack formation and enlargement of the pore network due to the pressure generated by ice crystallization within capillary pores. The control mortar (CM) shows only a slight increase in porosity, from approximately 18.5% to 18.9%, indicating moderate internal damage in the absence of fibers.

Figure 7
Porosity evolution of the various studied mixes.

The polypropylene fiber-reinforced mortar (MPP) exhibits a more noticeable increase in porosity, from about 18.62% to 19.1%. Although polypropylene fibers are hydrophobic, their smooth surface and limited interfacial bonding may reduce their ability to effectively seal or restrain microcrack opening, leading to progressive pore connectivity under freeze–thaw action. This behavior suggests that porosity evolution is governed not only by water absorption but also by fiber–matrix interfacial efficiency.

Mortars reinforced with plant-based fibers show higher porosity increases after cycling. The porosity of MAF rises from approximately 18.53% to 19.38%, while MDS and MDP exhibit the most pronounced increases, from about 18.5% to 20.1% and from 19.6% to 20.12%, respectively. This behavior can be attributed to the lignocellulosic nature of these fibers, which promotes water absorption, swelling, and subsequent shrinkage during freeze–thaw cycles, thereby inducing interfacial debonding and additional microvoid formation. Similar mechanisms have been reported for inadequately protected natural fibers in cementitious matrices by BELKADI et al. [⁶⁵].

Despite this general trend, the hemp fiber mortar (MHE) shows a relatively moderate increase in porosity, from about 19% to 19.8%, suggesting improved fiber–matrix compatibility or a more effective crack-bridging mechanism that limits microcrack propagation. Overall, these results emphasize that porosity development under freeze–thaw conditions is strongly dependent on fiber type and interfacial behavior. Proper selection and treatment of natural fibers are therefore essential to control pore structure evolution and enhance the durability of eco-friendly cement-based composites exposed to extreme climatic conditions.

3.6. Density

Figure 8 illustrates the variation in apparent density of the mortars before and after 60 freeze–thaw cycles, reflecting the extent of internal damage and microstructural degradation. A general decrease in density is observed for all mixtures, which is primarily attributed to the development of microcracks, increased porosity, and loss of material compactness induced by cyclic freezing and thawing. The reference mortar (CM) shows a slight reduction in density from approximately 2180 kg/m3 to 2165 kg/m3, indicating limited internal deterioration and a relatively compact matrix despite the absence of fiber reinforcement.

Figure 8
Variation in mortar density before and after freeze–thaw cycles.

The polypropylene fiber-reinforced mortar (MPP) exhibits a more noticeable density decrease, from about 2167 kg/m3 to 2128 kg/m3. Although polypropylene fibers are hydrophobic and chemically stable, the observed reduction suggests that freeze–thaw-induced microcracking around fiber–matrix interfaces contributes to void formation and loss of compactness. Nevertheless, the density loss remains moderate compared to some plant-based fiber mortars, highlighting the dimensional stability of PP fibers.

Mortars reinforced with natural fibers show varying degrees of density reduction. The alfa fiber mortar (MAF) displays a limited decrease from approximately 2174 kg/m3 to 2166 kg/m3, suggesting relatively good dimensional stability and fiber distribution. In contrast, Diss (MDS), date palm (MDP), and hemp (MHE) mortars experience more pronounced density losses, decreasing from about 2168 kg/m3 to 2126 kg/m3, from 2152 kg/m3 to 2128 kg/m3, and from 2144 kg/m3 to 2125 kg/m3, respectively. These reductions can be attributed to the hydrophilic nature of lignocellulosic fibers, which absorb water, swell, and subsequently shrink during freeze–thaw cycles, leading to interfacial debonding and the formation of additional voids, as also reported by DRIDI et al. [⁶⁶].

Under freeze–thaw conditions, water trapped within fibers and surrounding pores further amplifies internal stresses, accelerating damage and reducing material compactness. The observed density variations correlate well with porosity evolution and mechanical degradation, confirming that fiber type, moisture sensitivity, and interfacial behavior play a crucial role in governing the physical durability of fiber-reinforced mortars exposed to harsh climatic cycling.

3.7. Thermal conductivity

Figure 9 illustrates the evolution of thermal conductivity of the mortars before and after 60 freeze–thaw cycles, highlighting the influence of fiber type and microstructural degradation on heat-transfer properties. All mixtures exhibit a decrease in thermal conductivity after cyclic exposure, which reflects the formation of microcracks, increased porosity, and loss of matrix continuity. The control mortar (CM) shows the highest thermal conductivity both before and after cycling, decreasing slightly from approximately 1.47 W/m·K to 1.456 W/m·K, indicating its relatively dense and compact microstructure.

Figure 9
Variation in mortar thermal conductivity before and after freeze–thaw cycles.

The incorporation of fibers leads to lower initial thermal conductivity due to the intrinsically insulating nature of both synthetic and plant-based fibers. The polypropylene fiber mortar (MPP) shows a moderate reduction from about 1.355 W/m·K to 1.34 W/m·K after 60 cycles, suggesting good dimensional stability and limited moisture uptake. This behavior is consistent with the hydrophobic character of PP fibers and their reported long-term thermal performance in aggressive environments [¹⁴].

Mortars reinforced with natural fibers exhibit more pronounced reductions in thermal conductivity after freeze–thaw exposure. The alfa fiber mortar (MAF) decreases from approximately 1.30 W/m·K to 1.23 W/m·K, while hemp (MHE) and date palm (MDP) mortars show similar trends, decreasing from about 1.30 to 1.22 W/m·K and from 1.28 to 1.24 W/m·K, respectively. These larger reductions indicate higher sensitivity to moisture ingress and freeze–thaw-induced damage, which increase porosity and disrupt heat-transfer pathways. Such behavior is commonly associated with the hygroscopic and biodegradable nature of lignocellulosic fibers, as reported by BELKADI et al. [⁶⁷].

The Diss fiber mortar (MDS) exhibits a relatively balanced response, with thermal conductivity decreasing from approximately 1.38 W/m·K to 1.28 W/m·K, suggesting improved fiber–matrix compatibility and a more stable internal structure compared to other plant-based fibers. Overall, these results demonstrate that although fiber incorporation enhances thermal insulation, the long-term thermal performance of fiber-reinforced mortars under freeze–thaw cycling is strongly governed by fiber durability, moisture sensitivity, and interfacial stability.

3.8. Scanning electron microscope

SEM images after 60 freeze–thaw cycles (Figure 10) reveal the microstructural effects of degradation on the various mortars containing 30% metakaolin. The fiberless mortar (CM) shows a heavily cracked surface and a rough texture, indicating advanced embrittlement. In contrast, fiber-reinforced mortars exhibit differing behaviors depending on the type of fiber used.

Figure 10
Microstructural analysis of the different studied mortars.

MPP reveals a denser microstructure with well-anchored fibers and minimal cracking, confirming its low porosity and good thermal performance retention, as supported by FERHAN et al. [¹⁴]. MDS shows a compact fiber–matrix interface with few voids, explaining its better thermal retention and resistance to cycling, which indicates good physicochemical compatibility between the fiber and the matrix.

On the other hand, MHE, MAF, and MDP mortars display clear signs of degradation: open pores, cracked fiber–matrix interfaces, and fiber detachment—particularly noticeable in MHE and MAF. These observations confirm the detrimental effects of water absorption and fiber swelling under cyclic conditions, as demonstrated by MOUDOOD et al. [⁶⁸].

Altogether, these results closely correlate with the decrease in thermal conductivity shown in Figure 9, illustrating that the dimensional stability of the fibers, their adhesion to the matrix, and their sensitivity to moisture are key factors in the thermal durability of mortars exposed to extreme environments.

4. CONCLUSION

This study provides a comprehensive assessment of the freeze–thaw durability of metakaolin-based mortars reinforced with natural plant-based fibers in comparison with conventional polypropylene fibers, with a particular focus on linking mechanical performance, physical degradation, thermal behavior, and microstructural evolution. The results demonstrate that fiber incorporation plays a critical role in governing damage mechanisms under cyclic freezing conditions, but the effectiveness strongly depends on the physicochemical nature of the fibers and their interaction with the cementitious matrix.

Synthetic polypropylene fibers exhibited superior stability under freeze–thaw exposure, effectively limiting crack propagation and preserving compressive and flexural performance despite some loss in elastic stiffness. Their hydrophobic and chemically inert nature makes them particularly suitable for applications requiring long-term durability in aggressive climatic environments. In contrast, natural fibers such as hemp, alfa, and date palm significantly enhanced the initial mechanical response, especially in flexure, due to their crack-bridging capability and mechanical anchorage. However, their long-term performance was constrained by moisture sensitivity, swelling–shrinkage effects, and alkaline degradation, which promoted interfacial damage, porosity increase, and stiffness loss during freeze–thaw cycling. Diss fibers showed a more stable mechanical response, suggesting a balanced interaction with the matrix, although their reinforcing efficiency remained limited.

From a durability perspective, the combined analysis of porosity, density, thermal conductivity, and SEM observations highlights that freeze–thaw-induced degradation is governed by microcrack development and fiber–matrix interfacial stability. While natural fibers contributed to improved thermal insulation, their durability under cyclic freezing remains a key challenge that must be addressed to ensure long-term performance.

Overall, this work underscores that plant-based fibers can be considered viable and sustainable alternatives for cementitious composites when durability requirements are carefully accounted for. Their effective use in cold-climate applications requires optimized fiber selection, possible surface treatments, and matrix modification strategies to mitigate moisture-related degradation. By providing comparative performance benchmarks and mechanistic insights, this study contributes to the rational design of eco-friendly fiber-reinforced mortars, supporting the transition toward more sustainable construction materials without compromising structural reliability.

5. ACKNOWLEDGMENTS

This research was conducted thanks to the joint collaboration between the Civil Engineering Laboratory at the University of Bordj Bou Arreridj and the L2MGC laboratory of CY Cergy Paris University in France.

DATA AVAILABILITY

Upon request, data will be provided.

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  » https://doi.org/10.1177/0731684418818893

Publication Dates

• Publication in this collection
  27 Apr 2026
• Date of issue
  2026

History

• Received
  29 Oct 2025
• Accepted
  12 Mar 2026