Influence of Wood Fiber on Mechanical and Thermal Insulation Properties of Lightweight Mortar

Abstract

To advance the development of green building materials and achieve high-value utilization of waste resources, this study investigates the mechanistic influence of incorporating waste wood fibers on the mechanical and thermal insulation properties of lightweight mortar. Five fiber contents were designed—0%, 0.4%, 0.8%, 1.2%, and 1.6%—to systematically evaluate their effects on compressive strength, flexural strength, and tensile bond strength, as well as thermal conductivity, pore structure, and microstructural interfaces. The results demonstrate that at low fiber dosages (particularly 0.4% and 0.8%), wood fibers can significantly enhance both the mechanical strength and thermal insulation performance of mortar. Specifically, at a fiber content of 0.8%, the 28-day compressive strength increased by 10.62%, and the flexural strength by 23.8%; the tensile bond strength reached its peak at 0.4%, with a 14.8% improvement. The lowest thermal conductivity recorded was 0.16 W/(m·K), accompanied by a remarkable 61.9% reduction in porosity compared to the control group. Low-field nuclear magnetic resonance (LF-NMR) analysis revealed that wood fiber incorporation markedly increased the proportion of capillary pores, reduced total porosity, and enhanced mortar compactness; scanning electron microscopy (SEM) observations further indicated that the honeycomb-like morphology and surface roughness of wood fibers substantially improved interfacial bonding performance and microcrack-bridging capacity. The findings suggest that an optimal fiber content—recommended to not exceed 0.8%—can synergistically improve the mechanical and thermal insulation properties of lightweight mortar, providing both theoretical support and practical guidance for its application in green building wall materials.

1. Introduction

In recent years, under the strategic guidance of the “dual-carbon” objectives, China’s construction industry has been accelerating its transition toward energy efficiency and environmental sustainability [1]. Among the key energy-saving materials, lightweight mortar—enhanced by the incorporation of lightweight aggregates such as expanded glass beads and expanded perlite—has been extensively employed in wall insulation systems due to its ability to markedly reduce matrix density and improve thermal performance [2,3,4]. However, lightweight aggregate mortars are often plagued by insufficient compressive strength and weak interfacial bonding [5,6]. More critically, issues such as stratification and segregation caused by the flotation of lightweight aggregates during construction, as well as plastic shrinkage cracking induced by their high water absorption, further constrain their practical engineering applications [7,8].

In recent years, natural plant fibers—sourced primarily from agricultural and forestry residues or other renewable resources such as hemp, coconut shells, palm fibers, and wood-processing by-products—have become increasingly accessible and cost-effective. With simple and environmentally friendly processing requirements, their overall cost is significantly lower than that of synthetic fibers, making them widely adopted as reinforcement materials in lightweight mortars [9,10,11]. Sari et al. [12] treated corn husk fibers (CHFs) with a 5% NaOH solution, effectively removing non-cellulosic components and exposing a greater proportion of cellulose structures. This modification enhanced fiber crystallinity, resulting in a substantial increase in tensile strength. Merta et al. [13] conducted wedge-splitting tests using three types of natural fibers—hemp, elephant grass, and wheat straw—and found that the extent of fracture toughness enhancement varied among fiber types, with hemp fibers exhibiting the most pronounced effect. Teixeira et al. [14] investigated the mechanical performance and fracture mechanisms of cementitious composites reinforced with palm and jute fibers, reporting that palm fibers displayed superior bonding when favorably oriented, whereas jute fibers, due to partial surface coverage, induced matrix porosity and compromised interfacial efficiency. Hwang et al. [15] examined the influence of varying contents of short coconut shell fibers on the mechanical performance of cement-based composites, revealing that increased fiber dosage markedly improved toughness and crack resistance. Similarly, Soltan et al. [16] demonstrated that incorporating 2 vol% short Curauá fibers enhanced the tensile strain-softening behavior of cementitious composites. Collectively, these studies indicate that different fiber types exert significant reinforcing effects on mechanical properties, with fiber dosage variation and targeted fiber application further optimizing toughness and interfacial performance.

In addition, the industrial utilization rate of wood residues in China is estimated to be only about 17.4%, indicating that the vast majority of these residues are currently not employed for industrial purposes [17]. Unrecycled waste wood is typically either left in open-air stockpiles for natural degradation or incinerated as waste [18]. Such open-air storage not only consumes valuable land resources but also leads to the loss of organic constituents such as lignocellulose. Notably, natural plant fibers such as jute, palm, hemp, and coconut have been used to enhance the performance of cement mortars; however, systematic comparative studies among different fiber types remain limited. Wood fibers, by contrast, are more widely available and cost-effective, and their lightweight nature, porous structure, and low thermal conductivity make them well suited for improving both the thermal insulation and mechanical properties of lightweight mortars [19]. Nevertheless, publications directly comparing the performance of wood fibers with other long fibers in cement mortars are scarce. Therefore, this study focuses on elucidating the mechanisms of wood fiber reinforcement and determining its optimal dosage to address this research gap.

Therefore, this study employs waste wood fibers as reinforcing agents and systematically investigates, through dosage regulation, their effects on the mechanical properties, thermal conductivity, pore structure, and interfacial microcharacteristics of lightweight mortar. By integrating multi-scale testing techniques, including low-field nuclear magnetic resonance (LF-NMR) and scanning electron microscopy (SEM), the interaction mechanisms between wood fibers and the mortar matrix are elucidated. The findings are intended to provide both theoretical foundations and practical guidance for the engineering application of waste wood fibers in high-performance building mortars.

2. Experiment Overview

2.1. Experimental Materials

The water, P•O 42.5 ordinary Portland cement, manufactured sand, and expanded glass beads used in this study were all supplied by Hezhou Guangsha Environmental Protection Co., Ltd. (Hezhou, China). The physical properties of the water, P•O 42.5 ordinary Portland cement, manufactured sand, and expanded glass beads are presented in

Table 1. Physical properties of tap water.

Property	PH Value	Chemical Content (mg/L)	Carbonate Content (mg/L)	Bicarbonate Content (mg/L)
Value	7.3	51	65	60

,

Table 2. Physical properties of cement.

Specific Surface Area (m2/kg)	Setting Time (min)		Flexural Strength (MPa)		Compressive Strength (MPa)		Loss on Ignition (%)
	Initial setting	Final setting	3 d	28 d	3 d	28 d
364	158	256	5.7	8.5	27.5	53.6	3.16

,

Table 3. Physical properties of manufactured sand.

Apparent Density (kg/m3)	Loose Bulk Density (kg/m3)	Voids Ratio (%)	Content of Fine Particles (%)	Content of Mud Lumps (%)	Water Content (%)	Crushing Value (%)
2520	1510	40	10.9	1.2	20	2.3

and

Table 4. Physical properties of vitrified microspheres.

Property	Density (Kg/m3)	Compressive Strength (KPa)	Thermal Conductivity (W/(m·K))	Water Absorption Rate (%)	Surface Porosity (%)	Water Retention Rate (%)
Performance	102	196	0.042	16	95	93

, respectively. Representative appearances of the manufactured sand and expanded glass beads are shown in Figure 1a,b.

The wood fibers used in this study were supplied by Guangxi Huaqiang Building Materials Co., Ltd. (Beihai, China), which had pretreated them using a sodium hydroxide wet-immersion modification process (as shown in Figure 2). Under ambient temperature and pressure, the fibers were immersed in a 0.5% NaOH solution for 6 h, thoroughly rinsed with tap water, and then dried for subsequent use. The physical properties of the fibers are summarized in

Table 5. Physical properties of wood fiber.

Fiber Type	Water Absorption Rate (%)	Water Content (%)	Bulk Density (kg/m3)	Length to Diameter Ratio (%)
Wood Fiber	310.5	12	170	1.14–3.28

. The cellulose ether employed was BNE-119C (100,000 viscosity grade) produced by Benor(Beijing, China), while the redispersible latex powder was BNE-7030, also manufactured by Benor.

Figure 3 presents the SEM micrograph of the wood fibers at 5000× magnification. It can be clearly observed that the fiber surface exhibits pronounced roughness and irregularity, with widespread presence of lamellar or granular attachments and debris. Such surface characteristics are expected to exert a substantial influence on the interfacial interactions and bonding performance between the fibers and the cementitious matrix.

2.2. Sample Design

In this study, a systematic experimental investigation was conducted on the influence of wood fibers on the properties of lightweight mortar. Based on practical engineering requirements, the volume fraction of wood fibers relative to the total mortar volume was used as the variable, with five dosage levels set at 0%, 0.4%, 0.8%, 1.2%, and 1.6%. Preliminary tests indicated that at a dosage of 2%, the mortar exhibited performance trends similar to those observed at 1.2%–1.6%, while workability and fiber dispersion deteriorated noticeably. Therefore, the maximum dosage was limited to 1.6% to ensure both practicality and engineering feasibility. For ease of identification, the specimens were designated as JC (control group), WF4, WF8, WF12, and WF16, where “WF” denotes the wood fiber series, and the accompanying numeral represents the corresponding fiber content (%). The detailed mix proportions of each mortar group are provided in

Table 6. Experimental design of wood fiber vitrified microsphere lightweight mortar.

Sample	Cement (g)	Mechanical Sand (g)	Glass Microspheres (g)	Water (g)	Cellulose (g)	Expansive Agent Powder (g)	Fiber (%)
JC	350	500	150	400	2.5	4	0
WF4	350	500	150	400	2.5	4	4
WF8	350	500	150	400	2.5	4	8
WF12	350	500	150	400	2.5	4	12
WF16	350	500	150	400	2.5	4	16

.

2.3. Test Method

2.3.1. Mechanical Property Test

In this study, the mechanical properties of the lightweight mortar specimens were tested at curing ages of 7 and 28 days under standard curing conditions to evaluate their performance over time. Considering that the incorporation of wood fibers makes the early-age strength highly sensitive to humidity and prone to significant fluctuations, the analysis focused primarily on the long-term (28-day) strength to more accurately reflect the material’s actual performance. The compressive and flexural strengths of the cement mortar were evaluated in accordance with the Methods of Testing Cements—Determination of Strength (ISO Method) (GB/T 17671-2021) [20]. The preparation procedure was as follows: the admixture was first dissolved in the mixing water and then poured into the mixing bowl, followed by the addition of cement. The mixer was run at low speed for 30 s while the wood fibers were gradually introduced. To ensure proper dispersion of the fibers within the cement paste, the fibers were added manually in small increments to prevent clumping and excessive water absorption. Subsequently, the mixture of manufactured sand and expanded glass beads was added, and mixing continued according to the standard cement mortar mixing protocol. For fiber-reinforced specimens, an additional 30 s of high-speed mixing was applied to further improve homogeneity. Finally, the mixture was cast into prismatic molds measuring 40 mm × 40 mm × 160 mm, with three specimens prepared for each group. Testing was conducted using a WANCE computer-controlled universal testing machine in accordance with the three-point bending method specified in GB/T 17671-2021, with a span of 100 mm and a loading rate of 50 N/s. Compressive strength tests were carried out on the same equipment under static loading at a rate of 2.4 kN/s. For each data point, the average value of three specimens was calculated, and the standard deviation was determined to assess repeatability.

2.3.2. Tensile Bond Strength

The tensile bond strength of cement mortar represents a critical performance indicator, as it reflects the mortar’s ability to maintain adhesion under tensile stresses. This property directly governs the structural integrity and service life of buildings. The evaluation of bond strength was conducted in accordance with the Standard for Test Methods of Basic Properties of Building Mortar (JGJ/T 70-2009) [21].

2.3.3. Thermal Conductivity Testing Method

The thermal conductivity of cement mortar reflects its ability to conduct heat during the process of heat transfer, thereby directly influencing the thermal insulation performance and energy efficiency of buildings. The testing procedure was carried out in strict accordance with the Measurement of Steady-State Thermal Resistance and Related Properties of Thermal Insulation Materials—Heat Flow Meter Apparatus (GB/T 10295-2008) [22], which specifies the standardized methodology for determining material thermal conductivity.

2.3.4. LF-NMR Measurements

In this study, a low-field nuclear magnetic resonance (LF-NMR) analyzer was employed to systematically investigate the pore distribution within mortar after the incorporation of plant fibers.

2.3.5. Scanning Electron Microscopy (SEM)

According to the procedures described in Section 2.3.1 and Section 2.3.2, hydration product samples of neat cement paste at the specified ages were prepared and then examined using a scanning electron microscope (SEM, Sigma 300 model, manufactured by Carl Zeiss AG, Germany, Oberkochen) to observe and analyze their microstructure.

3. Results and Discussion

3.1. Compressive Strength Test

Figure 4 illustrates the effects of varying wood fiber dosages on the compressive and flexural strengths of lightweight mortar specimens at 7 and 28 days. The results indicate that at 7 days, the compressive strengths of mortars with 0.4%, 0.8%, 1.2%, and 1.6% fiber contents decreased by 37.4%, 40.8%, 46.7%, and 72.3%, respectively, compared with the control group. This decline is primarily attributed to the high water absorption capacity of wood fibers, which sequesters a considerable amount of free water during the early mixing stage, thereby delaying cement hydration and hindering early strength development [23].

With prolonged curing, the water absorbed within the wood fibers may gradually be released, potentially providing an “internal curing” effect that promotes further cement hydration and helps compensate for the early strength loss. This hypothesis is consistent with the conclusions of Chakraborty et al. [24] on polymer-modified jute fiber mortars, although differences in the absorption–release capacity and interfacial behavior of wood fibers may lead to variations in the extent of strength recovery. At 28 days, the compressive strength increased by 3.9% and 10.6% for fiber contents of 0.4% and 0.8%, respectively, relative to the control, whereas the strengths decreased by 6.5% and 36.8% at dosages of 1.2% and 1.6%. The maximum strength enhancement was observed at 0.8% fiber content. This improvement can be ascribed to the formation of a fiber-reinforced skeleton structure within the mortar, which enhances the compressive stability of the matrix. At dosages of 0.8% and below, fibers are uniformly dispersed, effectively filling voids, improving compactness, and providing structural reinforcement under compressive loading [25].

Conversely, when the fiber content exceeds 1.2%, agglomeration readily occurs, leading to non-uniform fiber distribution, the introduction of additional pores, and the disruption of matrix continuity and homogeneity. These defects result in stress concentration and a subsequent reduction in strength [26]. Therefore, the optimal fiber dosage should be controlled within 0.8% to achieve a balanced enhancement of mechanical performance and structural stability.

3.2. Flexural Strength Test

Figure 5 illustrates the influence of wood fibers on the flexural strength of mortar. Unlike compressive performance, the improvement in flexural strength was more pronounced at early ages, particularly at 7 days and under low fiber dosages (≤0.8%). At fiber contents of 0.4% and 0.8%, the flexural strength increased by 25.4% and 29.2%, respectively, compared with the control group. In contrast, when the dosage reached 1.2% and 1.6%, the flexural strength decreased by 34.6% and 38.5%, respectively. At low dosages, the tensile properties and good dispersion of wood fibers help form a uniform reinforcing network within the mortar, effectively sharing tensile stresses and suppressing microcrack propagation under flexural loading. This is consistent with the findings of Zhao et al. [27], although the improvement observed in this study was more pronounced, likely due to differences in fiber surface roughness, interfacial bonding, and length distribution.

As the curing age extended to 28 days, further cement hydration enhanced the interfacial bonding between fibers and the matrix, leading to additional improvements in flexural strength. The 0.8% group exhibited the greatest enhancement, with a 23.8% increase over the control, indicating that the reinforcing effect remained stable and reliable [28]. However, at dosages exceeding 1.2%, pronounced fiber agglomeration occurred, resulting in localized “bridging” structures and excess porosity, which compromised the microstructural integrity and led to a decline in flexural performance [29].

Therefore, to achieve desirable crack resistance and toughness in lightweight mortar, the fiber content should be controlled at no more than 0.8%.

3.3. Tensile Bond Testing of Mortars

Figure 6 presents the influence of wood fibers on the tensile bond strength of lightweight mortar. At 14 days, specimens containing 0.4% and 0.8% fiber exhibited increases of 34.7% and 43.1%, respectively, compared with the control group, indicating that low fiber dosages can markedly enhance interfacial bonding performance. This study shows that at low dosages, wood fibers significantly enhance interfacial bonding performance. This trend aligns with the findings of Awwad et al. [30] on industrial hemp fibers, but here the wood fibers exhibited superior interfacial bonding performance, likely due to their smaller aspect ratio and better dispersion, which promote a more uniform three-dimensional reinforcing network within the matrix.

With continued curing to 28 days, the overall bond strength further increased; however, the differences between dosages diminished. Relative to the control, the 0.4%, 0.8%, and 1.2% groups showed enhancements of 14.8%, 5.1%, and 8.3%, respectively. Among these, the 0.4% dosage maintained the most pronounced reinforcing effect, while the improvement at 0.8% stabilized. In contrast, the 1.6% group exhibited a 19.0% reduction in bond strength compared with the control at 28 days, highlighting that excessive fiber content can cause agglomeration and introduce additional porosity, thereby disrupting interfacial continuity and weakening adhesion.

Accordingly, the optimal fiber dosage is recommended to be maintained around 0.8%, ensuring a balance between significant early-age enhancement and long-term structural stability.

3.4. Thermal Conductivity Test

As shown in Figure 7, the regulation of thermal conductivity in lightweight mortar by wood fibers exhibited a nonlinear trend. At a dosage of 0.4%, the thermal conductivity reached its minimum value of 0.16 W/(m·K), representing a 23.8% reduction compared with the control group, thereby demonstrating a remarkable improvement in thermal insulation. This enhancement can be attributed to the hydrophilic nature of wood fibers, which promotes moisture evaporation and the formation of micropores, while their hollow structure and random distribution disrupt heat transfer pathways, effectively lowering thermal conduction efficiency [31].

For the 0.8% dosage group, the thermal conductivity was measured at 0.17 W/(m·K), a 19.0% reduction relative to the control, indicating that good insulation performance was still maintained. When the dosage increased further to 1.2%, the thermal conductivity rose to 0.20 W/(m·K), approaching the baseline value. This suggests that fiber agglomeration and heterogeneity in the pore structure compromised the original thermal resistance system, thereby weakening the insulating effect. At 1.6% dosage, the thermal conductivity slightly decreased to 0.19 W/(m·K), but the improvement was limited, with only a 9.5% reduction compared to the control.

These findings reveal that low fiber dosages (≤0.8%) effectively enhance the thermal insulation performance of lightweight mortar, whereas excessive incorporation can disrupt the microstructural stability and diminish thermal efficiency. Therefore, it is recommended that the wood fiber content be maintained within the range of 0.4%–0.8% to achieve an optimal balance between thermal resistance and structural stability.

3.5. T₂ Spectrum of Mortar

To quantitatively elucidate the regulatory mechanism of plant fibers on the pore structure of lightweight mortar, low-field nuclear magnetic resonance (LF-NMR) technology was employed to characterize the pore features of the specimens. This technique, based on the principle of proton nuclear magnetic resonance, analyzes the spin behavior of hydrogen nuclei in saturated water to obtain information such as porosity, pore size distribution, and fluid states within the material. It offers the advantages of strong non-destructiveness, operational simplicity, and suitability for in situ measurements.

In the NMR signal, the transverse relaxation time (T₂) serves as a key parameter for evaluating pore characteristics. Its relaxation process is jointly influenced by bulk fluid relaxation (T_2B), surface relaxation (T_2S), and diffusion relaxation (T_2D), as expressed in Equation (1):

$${\displaystyle \frac{1}{T_2}}={\displaystyle \frac{1}{T_{2B}}}+{\displaystyle \frac{1}{T_{2S}}}+{\displaystyle \frac{1}{T_{2D}}}$$

(1)

Under the experimental conditions of this study, the diffusion effect can be considered negligible, and the relaxation of bulk free water is comparatively weak; therefore, the transverse relaxation time (T₂) is predominantly governed by surface relaxation. Typically, shorter T₂ times correspond to smaller pore sizes, whereas longer T₂ times indicate larger pores. Consequently, the T₂ distribution curve can be effectively employed to capture the specific influence of fiber incorporation on the pore size distribution and overall pore structure of mortar. The relaxation time can be approximately expressed as

$${\mathrm{T}}_2\approx {\mathsf{\rho }}_2{\displaystyle \frac{\mathrm{V}}{\mathrm{S}}}$$

(2)

In Equation (2): T₂ denotes the transverse relaxation time, ρ₂ is the transverse surface relaxivity, V represents the pore volume, and S is the pore surface area.

Accordingly, the pore radius r can be expressed as

$$\mathrm{r}\approx {\mathsf{\rho }}_2{\mathrm{F}}_{\hspace{0.25em}\mathrm{s}}{\mathrm{T}}_2$$

(3)

In Equation (3), ${\mathrm{F}}_{\mathrm{s}}$ is a dimensionless geometric shape factor of the pore structure. In this study, the internal pores were assumed to be cylindrical, for which ${\mathrm{F}}_{\mathrm{s}}$ = 2. When the surface relaxivity ${\mathsf{\rho }}_2$ is taken as 5 nm/ms, the relaxation times obtained for mortar specimens align well with the pore radius at all scales [32].

Furthermore, following the pore-scale classification method commonly adopted in cementitious materials, the pore structure can be divided into four categories according to pore radius: macropores (r > 1 μm), capillary pores (0.1 μm < r ≤ 1 μm), transitional pores (0.01 μm < r ≤ 0.1 μm), and gel pores (r ≤ 0.01 μm) [33]. This classification system was also employed in the present study.

As illustrated in Figure 8, after the incorporation of wood fibers into lightweight mortar, the T₂ relaxation spectra consistently exhibited a characteristic three-peak distribution, corresponding to gel pores, capillary pores, and macropores, respectively. Compared with the control group (0%), all fiber-reinforced mortars showed a leftward shift in the signals, indicating a reduction in pore size and an overall densification of the pore structure. At a dosage of 0.4% (WF4), the main peak (gel pores) was significantly intensified, while the secondary peaks of capillary and macropores were notably diminished. This suggests that, at this dosage, the fibers were uniformly dispersed within the matrix, effectively filling larger voids and forming a dense multiscale structure, representing the most pronounced pore refinement effect. When the dosage increased to 0.8% (WF8), the main peak continued to intensify and the proportion of gel pores further increased; however, a slight resurgence of the capillary pore peak was observed, indicating the onset of fiber agglomeration, which began to impair pore continuity, although the overall pore-regulating capacity remained satisfactory. At 1.6% dosage (WF16), the macropore peak reappeared prominently, suggesting severe fiber agglomeration that resulted in the re-emergence of large pore structures, thereby compromising matrix compactness and diminishing the pore-regulating effect.

In summary, wood fibers effectively refine the pore structure of lightweight mortar, particularly at a dosage of 0.4%, where the proportion of gel pores is maximized and the formation of capillary and macropores is suppressed. Beyond 0.8% dosage, however, the benefits are progressively offset by agglomeration effects. Therefore, maintaining the fiber content within the range of 0.4%–0.8% is recommended to achieve comprehensive optimization of pore structure alongside mechanical and thermal performance.

Variations in relaxation time can effectively reflect the trends of porosity within mortar [34]. Figure 9 presents the relaxation characteristics of lightweight mortar modified with different wood fiber dosages. After the incorporation of fibers, the porosity of all mortar groups exhibited a “decrease–increase” trend with increasing fiber content. Specifically, the porosity decreased in the order SJ (0%) > WF16 (1.6%) > WF8 (0.8%) > WF4 (0.4%). In particular, the 0.4% fiber-content group showed a markedly lower porosity than the control group (approximately 61.9%, estimated based on model assumptions), indicating that an appropriate amount of wood fiber can effectively fill the voids between aggregates, form a denser fiber skeleton, reduce heat-conduction pathways, and thereby enhance both the thermal insulation and mechanical strength of the mortar. However, as the fiber content increased further, agglomeration effects became more pronounced, leading to the formation of localized voids. Consequently, porosity increased again, compactness declined, and both thermal insulation and mechanical performance were adversely affected.

Therefore, the wood fiber dosage should be carefully controlled to maintain optimal porosity regulation, ensuring simultaneous enhancement of both insulation efficiency and mechanical strength.

Previous studies have demonstrated that capillary pores and macropores together constitute the dominant portion of the mortar pore system, and their volumetric proportion largely determines the overall porosity distribution. More importantly, they are the primary factors influencing the thermal insulation performance of mortar [35]. Figure 10 presents the effects of varying wood fiber dosages on the relative contents of capillary pores and macropores in lightweight mortar. At low fiber dosages (0.4% and 0.8%), the relative content of capillary pores increased markedly by 14.89% and 7.52%, respectively, while the proportion of macropores decreased correspondingly. This indicates that wood fibers preferentially fill the mesopores formed by the packing of expanded glass beads, leading to the formation of a new capillary pore network. Such structural optimization not only enhances the mechanical properties of mortar but also improves its thermal insulation performance.

However, when the fiber dosage increased to 1.6%, the relative content of capillary pores continued to rise, yet macropores still remained predominant. Although this configuration may be beneficial for thermal insulation, it adversely affects mechanical performance. In particular, an excessive proportion of macropores can induce air convection, thereby increasing heat loss and diminishing insulation efficiency.

In summary, the appropriate incorporation of wood fibers can optimize the pore structure of lightweight mortar, achieving a balance between mechanical performance and thermal insulation, while also mitigating issues associated with the particle size distribution of expanded glass beads and their weak interfacial bonding.

3.6. SEM Analysis of the Interface Between Plant Fiber and Lightweight Mortar

To investigate the influence of different plant fibers on the microstructure of the interfacial transition zone (ITZ) in lightweight mortar, scanning electron microscopy (SEM) was employed. The microstructural observations are presented in Figure 11A,B. As shown in Figure 11A, the surface of the control mortar (JC) exhibited a relatively fine, irregular granular morphology. The interior of the mortar matrix was primarily composed of lamellar and irregular cement hydration products. The ITZ displayed relatively uniform bonding, without any evident reinforcing structures or complexity at the interface.

As shown in Figure 11B, the incorporation of wood fibers induced pronounced changes in the microstructure of lightweight mortar. The SEM images reveal that the fibers formed a highly porous, honeycomb-like network within the matrix, with considerable internal porosity. These honeycomb structures markedly enhanced the thermal insulation performance of the mortar. During the hydration curing process, the inherent porosity and strong hydrophilicity of the fibers allowed them to absorb large amounts of mixing water at early stages, leading to swelling. Subsequently, as the fibers gradually released water, shrinkage occurred. This “swelling–desorption” cycle generated numerous irregular voids and macropores around the fibers. Moreover, the rough surface and irregular cross-sections of the fibers further promoted the formation of pores. Collectively, these structural features optimized the internal pore distribution, effectively suppressing heat conduction and thereby improving thermal insulation performance.

It is noteworthy, however, that if the absorption–desorption cycle of the fibers is not properly controlled, early-age cracking may occur, adversely affecting the mechanical properties of the mortar. Hence, in practical applications, the dosage of wood fibers should be carefully regulated to preserve the thermal insulation benefits without compromising the structural integrity and long-term stability of the mortar.

4. Conclusions

This study investigated the effects of wood fiber incorporation on the mechanical and thermal insulation properties of lightweight mortar, with particular attention to compressive strength, flexural strength, tensile bond strength, and thermal conductivity. In addition, low-field nuclear magnetic resonance (LF-NMR) and scanning electron microscopy (SEM) were employed to analyze the pore structure and interfacial transition zone microstructure. The key conclusions are as follows:

• Appropriate wood fiber dosages (0.4%–0.8%) significantly improved the mechanical properties of lightweight mortar. At 28 days, compressive strength increased by up to 10.6%, while flexural strength and tensile bond strength were enhanced by approximately 23.8% and 14.8%, respectively. These improvements can be attributed to the fibers’ ability to fill voids, form reinforcing skeleton structures, and bridge microcracks.
• Wood fibers exhibited a positive effect on the thermal performance of mortar. At a dosage of 0.4%, the thermal conductivity decreased to a minimum of 0.16 W/(m·K), owing to the fibers’ hollow and porous structures combined with their uniform dispersion, which optimized pore morphology, reduced heat conduction pathways, and suppressed convective heat transfer.
• Pore structure analysis revealed that low fiber dosages markedly reduced total porosity (by as much as 61.9%) while simultaneously increasing the proportion of capillary pores. This dual effect contributed to both improved thermal insulation and enhanced mechanical strength. However, when the fiber dosage reached or exceeded 1.2%, agglomeration effects introduced structural heterogeneity and stress concentrations, leading to performance deterioration.
• The optimal wood fiber content in lightweight mortar should be kept below 0.8%, within which it can significantly improve mechanical and thermal insulation properties while promoting the reuse of forestry residues and supporting green building. Future work will verify these findings on larger elements and under on-site conditions to assess feasibility and long-term performance.