Evaluating the use of recycled sawdust in porous foam mortar for improved performance

2.1 Materials

The materials utilized in this experimental program were carefully selected to ensure they fit the application in FM. As the principal binder, Ordinary Portland Cement (OPC CEM1) complied with BS EN 197-1 [14] requirements. Tables 1 and 2 show that it was compatible with lightweight and structural applications because of its high calcium oxide content (62.76%), specific gravity (3.13), particle size (68 μm), and 28-day compressive strength (32 MPa). In order to ensure balanced grading and enhanced workability, natural river sand was utilized as the control aggregate. Its specific gravity was 2.58, and its fineness modulus was 2.31 (Table 3). When evaluating mortars amended with sawdust, this baseline mix served as a reference (Figure 1).

Figure 1

Process of turning raw sawdust into fine sawdust: (a) raw sawdust, (b) crushed sawdust, and (c) sieved sawdust.

The protein-based foaming agent, Noraite PA-1, generated stable, uniform foam with an expansion ratio of 25×, a specific gravity of 1.13, and a pH of 5.58. It enabled the target mortar density of 1,000 kg·m⁻³, a standard for lightweight applications. In order to balance workability and strength, a consistent water-to-cement ratio of 0.47 was used in all mixes, using water that met BS 3148 requirements.

Locally produced sawdust was used as the experimental sand substitute. It was cleaned, dried, crushed, and sieved to provide a consistent particle distribution with a fineness modulus of 1.79 (Figure 2). Its modest levels of calcium and magnesium oxides (Table 4) and high silicon dioxide concentration (69.05%) indicated possible pozzolanic qualities.

Figure 2

Physical appearance of fine sawdust and its sieve analysis in comparison to fine river sand.

Figure 3 shows the X-ray diffraction (XRD) pattern of sawdust employed in this study. Identification and characterization of cellulose using XRD were based on methods proposed by Park et al. [15] and French [16]. Sawdust principally comprises amorphous particles; nonetheless, the cellulose portion has some crystalline characteristics, as indicated by the XRD results. Three crystalline peaks can be seen in Figure 3 at 2-θ of 16°, 22.5°, and 34.5°. The initial peak at 16° gives a sign of its crystallized state. Nonetheless, it is commonly less intense and more expansive than the second peak, ascribed to the semi-crystalline characteristics of cellulose in wood. The second peak at 22.5° is the most significant, signifying the crystalline parts of cellulose. It is frequently extensive and scattered due to the incomplete crystallinity of cellulose in sawdust. The third peak at 34.5° implies a further crystallography sign for cellulose. The level of intensity is generally moderate, as the cellulose crystals in sawdust may lack precise order; yet it remains detectable.

Figure 3

XRD pattern of sawdust.

The XRD examination generally confirmed the existence of crystalline cellulose structures characteristic of lignocellulosic biomass. The peaks suggest that the sawdust maintains its fibrous, organic characteristics and does not exhibit pozzolanic reactivity in the conventional manner. Sawdust, in contrast to materials like fly ash or silica fume, does not possess the amorphous silica or alumina necessary for a productive pozzolanic reaction with calcium hydroxide during cement hydration. Minor interactions between organic components and hydration products are more likely to affect setting time and microstructure than to contribute to additional cementitious reactions. The article has been revised to clarify in the XRD and discussion sections that sawdust serves primarily as a lightweight, inert filler exerting physical effects on the mortar matrix, rather than functioning as a chemically reactive addition.

Sawdust replaced sand in increments of 0–25%, with its density (1,260 kg·m⁻³) and water absorption (0.6%) accounted for in the mix designs (Table 5). This methodical material preparation and characterization created a solid basis for assessing the advantages of adding sawdust to FM in terms of performance requirements, durability, and mechanical properties.

2.2 FM mix design

The FM mix design sought to evaluate the impact of using sawdust in different ratios in place of fine sand. A protein-based foaming agent and precise material proportions were used to produce air gaps, achieving the desired density of 1,000 kg·m⁻³. This makes it suitable for lightweight building applications, such as partitioning and insulation. Workability, strength, and durability were balanced in the design by using a set water-to-cement ratio of 0.47 and a cement-to-sand ratio of 1:1.5. Sawdust was used in six different blends, substituting fine sand at weight percentages of 0, 5, 10, 15, 20, and 25%. The control mix (F0) contained only fine sand, while subsequent mixes incrementally incorporated sawdust (Table 5).

In order to maintain mix consistency and desired density, sawdust has to be adjusted due to its lower density (1,260 kg·m⁻³) and higher water absorption (0.6%) in comparison to fine sand (1,690 kg·m⁻³, 0.5%). A 25× expansion ratio was used for the foaming agent so as to ensure the stability of the mixture and the uniform distribution of air gaps. The preparation involved a dry mixing of cement, sand, and sawdust. Water and a foaming agent were then added to produce a stable, constant foam. The mixtures were poured into molds and allowed to be cured in line with accepted procedures. Figure 4 shows the FM specimens in steel molds after casting. Moreover, the workability, mechanical properties, durability, and microstructure of each mix were evaluated to determine the optimal quantity of sawdust substitution that balances functional performance and environmental benefits. Thus, this approach provided useful data regarding sawdust’s sustainable application in lightweight construction.

Figure 4

FM specimens in steel molds after casting.

3.1 Fresh properties

The fresh properties of the FM mixes were determined just after mixing to measure the workability and setting times. The density was also determined. The slump flow test was used to determine workability according to ASTM C230 [17], and the mortar’s spread diameter was also noted. The target slump flow diameter, which is typical for FM applications, was chosen between 210 and 250 mm. The projected effect of sawdust’s water-absorbing capacity on flow behavior led to the recording and correlation of any deviations from this range with its composition.

The fresh density of the FM mixtures was measured using BS EN 12350-6 [18]. An accurate density measurement was ensured in the mixes considering the fact that the materials’ composition was altered by the addition of sawdust. To record the start and final setting times, setting time was measured using a Vicat device in accordance with BS EN 196-3 [19]. The results of these experiments provided crucial information regarding the mixtures’ initial performance, which influences application and handling.

3.2 Mechanical properties

The tests for modulus of elasticity (MOE), splitting tensile strength, flexural strength, and compressive strength were used to evaluate mechanical performance. 100 mm³ cubes were subjected to compressive strength tests in accordance with BS EN 12390-3 [20]. The average of three specimens per mix was used to reflect the findings of the tests conducted on the samples after 28 days of curing.

Flexural strength was measured using 100 × 100 × 500 mm prisms in accordance with BS EN 12390-5 [21], while splitting tensile strength tests were conducted on cylindrical specimens measuring 100 × 200 mm, following BS EN 12390-6 [22]. Static loading tests, in accordance with ASTM C469 [23], were used to measure the MOE using cylindrical specimens under uniaxial compression. A thorough understanding of the structural performance of FM with different sawdust contents was made possible by these mechanical tests.

3.3 Durability properties

Tests on gas permeability, porosity, and water absorption were used to evaluate the mixes’ durability. Water absorption tests were performed on cylindrical specimens measuring 50 mm in diameter and 100 mm in height in accordance with BS EN 1881-122 [24]. The samples were immersed in water for 24 h, and the percentage increase in mass was recorded.

The vacuum immersion method outlined in RILEM TC-14 CPC11 [25] was used to measure porosity. To determine the material’s pore volume, cylindrical specimens with dimensions of 50 mm in diameter and 50 mm in height were vacuum-saturated. Gas permeability tests were carried out on similar-sized specimens using the method developed by Cabrera and Lynsdale [26], which involved measuring the rate of gas flow through the material under controlled pressure conditions. These tests collectively evaluated the impact of sawdust on the resistance of FM to moisture ingress and gas diffusion.

3.4 Microstructural analysis

After 28 days of curing, representative fragments of the FM samples were selected and oven-dried at 60°C for 24 h to remove residual moisture without damaging the microstructure. The dried samples were then carefully fractured to expose a fresh surface and subsequently mounted on aluminum stubs using carbon adhesive tape. To ensure proper conductivity and minimize charging effects during SEM observation, the mounted samples were sputter-coated with a thin layer of gold (Au) using a vacuum sputter coater. SEM imaging was then conducted at an accelerating voltage of 15–20 kV to examine the morphology and pore structure of the FM matrix.

Using SEM for microstructural examination, the internal structure of the mortar and the interfacial zones between sawdust particles and the cement matrix were detected. The SEM analysis was performed in accordance with ISO 16700 [27] guidelines, using a Quanta FEG 650 electron microscope. Samples measuring 15 × 15 × 15 mm were prepared from each mix, coated with a conductive layer, and examined at a magnification of 150×. The SEM images provided qualitative insights into the dispersion of sawdust particles, pore distribution, and the effects of sawdust on matrix homogeneity.

This experimental program ensured that all critical aspects of FM performance were systematically evaluated. Moreover, the experiments were intended to give a thorough understanding of the material’s behavior in real-world scenarios, in addition to measuring the impact of sawdust as sand replacement.

4.1 Fresh properties

Figures 5 and 6 show the slump flow and setting times for the mortar mixes. The fresh properties of FM mixes were significantly influenced by the inclusion of sawdust. There was a gradual reduction in workability with increasing sawdust content, as shown in the slump flow test results. The control mix (F0) achieved a slump flow diameter of 250 mm, consistent with typical FM specifications [28]. However, mixes with 25% sawdust (F25) recorded a reduced slump flow of 210 mm, indicating a 16% decrease in workability. The reason for this decline is that sawdust’s hygroscopic qualities allowed it to absorb water from the mixture and reduce flowability, which aligns with findings in related studies [29].

Figure 5

Slump flow of mixes.

Figure 6

Setting time of mixes.

Recent density studies demonstrated a steady decrease with the rise in sawdust content. The control mix (0% sawdust) exhibited a fresh density of around 1,000 kg·m⁻³, whereas the mix with 25% sawdust substitution (F25) demonstrated a decreased density of 925 kg·m⁻³. This trend is ascribed to the markedly reduced specific gravity of sawdust in relation to natural fine sand, resulting in a lighter overall mixture. The reduction in fresh density indicates possible advantages for lightweight building applications where a diminished dead load is advantageous.

The initial and final setting times were prolonged with the increase in sawdust content. The delay, averaging approximately 10 min for elevated sawdust levels, is presumably attributable to the presence of lignocellulosic elements in the sawdust. These organic compounds can disrupt cement hydration events by adsorbing water and partially interacting with calcium ions, leading to reduced hydration kinetics. The minor retardation suggests that sawdust may function similarly to other plant-derived additives that affect setting time by chemical interaction with the binder matrix. These results align with prior research indicating delayed setting when cellulose-based materials are present [15,16]. The observed alterations in fresh properties indicate that sawdust significantly affects the density and hydration characteristics of FM. These impacts must be considered while optimizing mix design for certain performance criteria.

4.2 Mechanical properties

The mechanical performance of the FM mixes was systematically evaluated to understand the impact of sawdust substitution on compressive strength, flexural strength, splitting tensile strength, and MOE. The findings clearly indicate that strength decreases as sawdust concentration rises, which can be attributed to variations in density, matrix porosity, and interfacial bonding. Moderate substitution levels, however, continued to provide satisfactory results, especially for non-structural applications.

4.2.1 Compressive strength

A crucial factor in assessing mortar’s structural performance is its compressive strength. In Figure 7, the compressive strength is shown, and in Figure 8, the percent change in the compressive strength is shown. The control mix (F0), which is typical for FM, has a 28-day compressive strength of 9.8 MPa. When the maximum substitution level of 25% (F25) was attained, the compressive strength decreased by 33.7% to 6.5 MPa. The sawdust particles have a lesser density and stiffness compared to fine sand, and this eventually led to a decrease in the compressive strength [6]. Moreover, the matrix was also weakened because of the microstructure’s increased porosity, especially at higher sawdust substitution levels. This is consistent with earlier studies that reported similar declines in mechanical performance when incorporating cellulose-rich biomass into cement-based materials [9]. Despite the strength reduction, the values achieved at moderate replacement levels remain within acceptable ranges for non-structural and lightweight construction applications. These results emphasize the trade-off between mechanical properties and sustainability when incorporating recycled organic materials into cementitious composites.

Figure 7

Compressive strength of mixes.

Figure 8

Percentage of change in compressive strength.

ASTM C796/C796M and analogous standards indicate that FMs and cellular concretes designed for non-structural or lightweight construction applications generally necessitate compressive strengths between 2 and 7 MPa, contingent upon their specific application (e.g., insulating material layers, wall partitions, void filling). With a 15% replacement of sawdust, the FM attained a compressive strength of around 2.82 MPa at 28 days, satisfying or surpassing the established criteria, while simultaneously reducing density and encouraging the utilization of sustainable materials. Consequently, the 15% threshold is recognized as a performance-sustainability trade-off juncture, in which mechanical properties are maintained within acceptable parameters for non-load-bearing applications.

4.2.2 Flexural strength

The flexural strength results and rate of variation in the values are shown in Figures 9 and 10, respectively. Generally, flexural strength measures a material’s capacity to tolerate bending pressures, and it is essential in applications where the mortar is subjected to flexural stresses. The control mix’s (F0) flexural strength at 28 days was 3.5 MPa. As the sawdust concentration increased, this value gradually dropped, reaching 3.2 MPa for F5 and 2.8 MPa for F15. The observed flexural strength for F25 was 2.75 MPa, which is 21.5% less than the control mix. This decrease is related to sawdust particles disrupting cohesiveness within the mortar matrix, which acts as a weak spot. A similar assertion was made in a related study [11]. However, considering the very slight reduction in flexural strength, the material might still have sufficient ductility for various uses [30].

Figure 9

Flexural strength of mixes.

Figure 10

Percentage of change in flexural strength.

4.2.3 Splitting tensile strength

The splitting tensile strength of the FM mixes and the percentage change in the values are shown in Figures 11 and 12, respectively. This is a crucial factor in comprehending how cracks spread in the matrix. For the control mix, the splitting tensile strength was 2.2 MPa. As the quantity of sawdust increased, the splitting tensile strength decreased in the same trend as the flexural and compressive strength values. Mix F25 indicated a tensile strength of 1.8 MPa, the lowest and 18.2% lower than the control. The corresponding tensile strengths of mixes F5 and F15 were 2.0 and 1.85 MPa. Compared to the relatively little decline in tensile strength, the reductions in compressive and flexural strengths are more pronounced. This pattern suggests that the dispersion of tensile stresses across the matrix is less affected by sawdust substitution since the particles are uniformly distributed at moderate substitution levels [11].

Figure 11

Splitting tensile strength of mixes.

Figure 12

Percentage of change in splitting tensile strength.

4.2.4 Modulus of elasticity

The stiffness and the ability of the mortar to deform elastically under load have been measured using the MOE tests (Figure 13). The MOE for the control mix was measured at 15 GPa. As the sawdust content increased, this value progressively decreased, dropping to 13 GPa for F10 and 11 GPa for F25. There was a decrease in the MOE values, and this was attributed to the higher number of voids and reduced rigidity. Moreover, the results were supported by SEM analysis, which showed discontinuities in the matrix, and this could be attributed to higher sawdust content added. Despite this decline, the mortar’s elastic behavior remained suitable for lightweight applications where flexibility and reduced density are prioritized [31].

Figure 13

MOE of mixes.

4.3 Durability properties

Durability is a crucial aspect when assessing the performance of FM for long-term applications, especially in environments exposed to moisture, temperature fluctuations, and other harsh conditions. In this study, water absorption, porosity, and gas permeability were examined to assess the durability of the FM mixes with different sawdust substitution amounts. These characteristics are crucial markers of a material’s resistance to environmental pressures since high porosity and water absorption can cause deterioration and shorten the material’s lifespan.

4.3.1 Water absorption

The water absorption is shown in Figure 14. The results showed that the control mix’s (F0) water absorption was 4.8%. The rate of water absorption rose in tandem with the sawdust concentration. Water absorption rose to 5.3% and 5.9% for F5 and F10, respectively. The mixes F15, F20, and F25 showed similar trends at 6.5, 7.3, and 8.2%, respectively. From F0 to F25, there was a 70.8% increase in water absorption. Moreover, the hygroscopic nature of sawdust particles and their tendency to retain moisture contributed to the increase in water absorption as the sawdust concentration increased [6]. Water may also enter the material through the increased number of voids in the mortar matrix caused by sawdust’s porosity [29]. Although this enhanced absorption may make sawdust-modified mortar less efficient in wet situations, it can still be used in dry or non-exposed conditions where water absorption is less of an issue [32].

Figure 14

Water absorption of mixes.

4.3.2 Porosity

Porosity is another vital durability parameter, influencing both the strength and the water absorption capacity of a material. Figure 15 shows the porosity results. The control mix (F0) exhibited a porosity of 15%, which is typical for FM mixes. As sawdust content increased, the porosity of the mixes also rose. F5 had a porosity of 16.3%, and by the time 25% sawdust was incorporated (F25), the porosity reached 20%, a 33% increase compared to the control mix. The presence of sawdust, which creates voids and air pockets in the mortar, is directly related to the increased porosity. This finding was also supported by SEM images, which revealed that the mix had more big voids at higher sawdust percentages. Although these spaces help the mortar be lightweight, they also weaken the internal structure of the substance. More porosity increases the mortar’s vulnerability to water absorption and may compromise its mechanical strength. The material’s capacity to withstand heat transfer may be enhanced by the increased voids, which also offers a possible advantage in terms of thermal insulation qualities [33].

Figure 15

Porosity of mixes.

4.3.3 Gas permeability

Gas permeability is an essential durability test that evaluates the ease with which gases, such as air or carbon dioxide, can penetrate the mortar. Figure 16 shows the gas permeability values. This property is particularly important for assessing the material’s performance in structures where air and moisture resistance are necessary. The controlled procedure described by Cabrera and Lynsdale [18] was used to examine the gas permeability of the FM mixes. Permeability clearly increased with increasing sawdust content, according to the data. Permeability was 0.32 × 10⁻¹² m² for the control mix (F0) and 0.40 × 10⁻¹² m² for the 25% sawdust mix (F25), a 25% increase. In mixes with more sawdust, the improved gas permeability is directly caused by the increased porosity and void spaces within the mortar matrix. Gases can more readily diffuse through the material through the voids [34]. While this may not be a significant issue in non-structural applications, it could potentially affect the performance of FM in conditions where airtightness or low permeability is required, such as in energy-efficient building designs.

Figure 16

Gas permeability of mixes.

4.4 Microstructural analysis

Figure 17 presents the SEM analysis of FM. The results revealed that sawdust addition affected the microstructure of the matrix. There was a uniform matrix with a solid cement paste-sand bond in the control mix (F0), thus showing evidence of good stability and strength. Moreover, lower sawdust levels (5–10%) showed dispersed voids and partial bonding with the cement paste with little impact on strength[35]. Nevertheless, substantial segregation, bigger voids, and poor bonding between sawdust particles and the matrix were seen at higher substitution levels (15–25%), which resulted in decreased strength and increased porosity. While sawdust’s silica content suggests potential pozzolanic activity at low levels, the weak interfacial bonding at higher contents undermined mechanical performance.

Figure 17

SEM images of different LFC mixtures: (a) F0, (b) F1, (c) F2, (d) F3, (e) F4, and (f) F5.

The incorporation of sawdust in the FM mixture substantially affects the hydration process, resulting in a modified microstructure. Sawdust particles, owing to their low density and fibrous composition, generate supplementary voids within the mortar matrix, so augmenting the total porosity. This subsequently influences the formation of the toughened structure. The air spaces produced by the foam, along with the inclusion of sawdust, create a less dense matrix characterized by interconnecting pores. The SEM investigation indicated that increased sawdust content results in more irregular and bigger holes, potentially leading to diminished mechanical strength while improving thermal insulation qualities. The creation of the foam structure can be elucidated by the gas entrapment mechanism during mixing and the stabilization of air spaces by surfactants or admixtures, as articulated in the theories of [36] and [37]. The interaction between cement hydration products (calcium silicate hydrate, or C–S–H) and sawdust particles results in localized regions of reduced density, influencing the overall microstructure. This phenomenon has been examined within the framework of the gel-space ratio theory [38], which elucidates the equilibrium between hydration products and porosity in the cementitious matrix.

The ITZ, where sawdust particles mix with cement paste, is critical in influencing the overall mechanical properties and durability of FM. The SEM pictures reveal that the ITZ is more pronounced with increased sawdust content, as the sawdust particles serve as vulnerabilities within the matrix. The existence of these organic particles results in limited regions of diminished strength due to the weaker bonding between the cement hydration products (such as C–S–H) and the sawdust fibers. The feeble link within the ITZ is likely to contribute to the diminished compressive strength reported in the sawdust-modified mortars.

Additionally, the possible breakdown pathways of sawdust under various curing settings. Sawdust, as an organic substance, is subject to biodegradation, especially in situations with elevated moisture levels. Under humid curing circumstances, the cellulose and lignin in sawdust may undergo partial breakdown over time, potentially resulting in a decrease in the filler’s structural integrity. This deterioration can influence the stability and long-term resilience of the FM, especially in settings subjected to elevated humidity or cyclic wet-dry conditions. To address this, we have cited works such as [39], which investigate the impact of environmental exposure on organic fillers in cementitious materials. These findings highlight the need to optimize sawdust content and enhance bonding to maintain structural integrity in FM.

4.5 Sawdust long-term behavior in humid or biological environments

Although this study focused on the short- and medium-term performance of sawdust-based FM (up to 56 days), it is important to consider the potential long-term behavior of sawdust within cementitious systems, particularly in humid or biologically active environments. Sawdust, being an organic, lignocellulosic material, is inherently susceptible to biodegradation over time, especially when exposed to moisture, oxygen, and microbial activity. In humid conditions, water absorption by sawdust may not only lead to dimensional instability and increased porosity but could also accelerate microbial degradation, weakening the interfacial bond between the organic filler and the cement matrix.

Furthermore, under favorable biological conditions such as consistent moisture and moderate temperatures, sawdust may serve as a substrate for fungi or bacteria, potentially resulting in the slow breakdown of cellulose and hemicellulose components. This degradation could progressively compromise the mechanical integrity and durability of the mortar, especially in structural or load-bearing applications [40].

However, the alkaline nature of the cement matrix may offer some degree of protection against microbial activity by creating an inhospitable environment for many decaying organisms. Additionally, the encapsulation of sawdust within the dense matrix may limit oxygen access, thereby slowing down biological degradation. Previous studies [23,24] have reported that while initial mechanical strength may be reduced due to the inclusion of organic fillers, the long-term degradation is highly dependent on environmental exposure conditions and mixed design parameters [41].