Strength behavior of laterite blocks enhanced with kaolin, metakaolin and sisal fiber at different fiber aspect ratios

Abstract

Laterite blocks offer a low-carbon alternative to conventional masonry units, but their wider application is constrained by low mechanical strength and brittle behavior. Pozzolanic stabilization using kaolin and metakaolin, combined with natural fiber reinforcement, presents a promising approach to enhancing strength performance; however, the combined influence of binder type, binder content, and fiber aspect ratio on laterite block strength remains insufficiently understood.

This study investigated the compressive and tensile strength behavior of laterite blocks stabilized with kaolin and metakaolin at binder contents ranging from 0% to 15%, with and without sisal fiber reinforcement at varying fiber contents and aspect ratios. Compressive and tensile strength tests were conducted on replicated specimens. Statistical analysis comprised descriptive statistics, HC₃ robust two-way ANOVA, Welch ANOVA, Games-Howell post hoc tests, and effect size estimation using partial eta-squared.

The binder type and content had statistically significant effects on both compressive and tensile strength, with strong interaction effects observed. Metakaolin-stabilized blocks consistently outperformed kaolin-stabilized blocks, achieving higher strength values across all binder contents. Optimal compressive strength occurred at approximately 9% binder for kaolin and 12% binder for metakaolin. Sisal fiber reinforcement significantly enhanced strength up to an optimum fiber content, beyond which strength declined. Optimal fiber contents were identified as approximately 0.75% for kaolin-stabilized blocks and 0.38% for metakaolin-stabilized blocks. Tensile strength exhibited greater sensitivity to fiber reinforcement than compressive strength.

The results demonstrate a strong synergistic effect between metakaolin stabilization and sisal fiber reinforcement when the fiber content and aspect ratio are optimized. Carefully designed metakaolin-sisal systems substantially improve the mechanical viability of laterite blocks, supporting their use as sustainable, low-carbon masonry materials.

Keywords:
laterite blocks; kaolin; metakaolin; sisal fiber; fiber aspect ratio; compressive strength; tensile strength.

1. Introduction

The construction industry is a major contributor to global greenhouse gas emissions, largely due to the extensive use of energy-intensive materials such as Portland cement and fired clay bricks (Singh et al., 2024). In response to growing environmental and economic concerns, there has been increasing interest in alternative construction materials that are locally available, cost-effective, and environmentally sustainable (Yahia & Shahjalal, 2024). Laterite soil, which is widely distributed in tropical and subtropical regions, has gained attention as a viable material for low-carbon construction due to its abundance, low embodied energy (Ezreig et al., 2022), and suitability for earthen masonry applications. When processed into compressed laterite blocks, the material offers opportunities for reducing dependence on conventional masonry units while supporting sustainable housing development, particularly in resource-constrained regions (Kportufe et al., 2025). Recent reviews on compressed earth blocks (CEBs) confirm that earth-based masonry can reduce dependence on energy-intensive conventional materials when local soils, low-carbon stabilizers and appropriate compaction methods are used (Bailly et al., 2024; Turco et al., 2021). However, these materials require careful optimization because their mechanical performance is strongly affected by soil grading, stabilizer type, moisture control and curing conditions (Bahar et al., 2004; Bui et al., 2023).

Despite these advantages, the structural application of laterite blocks is often limited by their inherent mechanical weaknesses (Ezreig et al., 2022). Unstabilized laterite masonry typically exhibits low compressive and tensile strength, high brittleness, and poor resistance to crack initiation and propagation (Bhat et al., 2024). Similar limitations have been reported for unstabilized and weakly stabilized compressed earth blocks, where inadequate particle bonding, moisture sensitivity and low tensile capacity restrict structural application unless the soil matrix is stabilized or reinforced (Bailly et al., 2024; Riffi et al., 2026; Turco et al., 2021).These deficiencies are largely attributed to the heterogeneous nature of lateritic soils, variations in mineralogical composition, and weak interparticle bonding (Ezreig et al., 2022). Consequently, unstabilized laterite blocks frequently fail to satisfy the strength requirements for load-bearing applications, restricting their use to non-structural or low-stress environments (Mensah et al., 2022). Therefore, addressing these mechanical limitations is essential to expand the applicability of laterite blocks in sustainable construction (Ezreig et al., 2022).

One widely adopted approach to improving the performance of earthen materials is pozzolanic stabilisation (Ali & Atemimi, 2024). Pozzolanic materials react with calcium hydroxide in the presence of moisture to form cementitious compounds that enhance particle bonding and matrix densification (Golewski, 2022). Kaolin, a naturally occurring aluminosilicate clay, and metakaolin, its thermally activated form, have been investigated as stabilizing agents for lateritic soils due to their high silica and alumina content (Fekoua et al., 2023). Metakaolin, produced through the controlled calcination of kaolin, exhibits greater pozzolanic reactivity owing to its amorphous structure, often resulting in superior strength development (Geu et al., 2025). Metakaolin-based stabilization has been shown to improve the mechanical performance of lateritic soils and compressed earth blocks through pozzolanic reaction, pore refinement and matrix densification (Dabakuyo et al., 2022; Ebid, 2022; Sore et al., 2018). These findings support the use of calcined kaolin-based binders as lower-carbon alternatives to conventional cement stabilization in earthen construction systems. However, the effectiveness of both kaolin and metakaolin is strongly dependent on binder content, curing conditions, and interaction with the base laterite matrix, necessitating systematic evaluation to identify optimal stabilization levels (Ezreig et al., 2022).

In addition to chemical stabilization, fiber reinforcement has been recognized as an effective strategy for improving the tensile behavior and crack resistance of brittle construction materials (Mujalli et al., 2022). Natural fibers have attracted increasing attention as sustainable reinforcement materials due to their renewability, biodegradability, low cost, and favorable mechanical properties (Maiti et al., 2022). Among these, sisal fiber is particularly promising because of its relatively high tensile strength and good compatibility with earthen matrices (Choudhary et al., 2023). Fiber reinforcement can enhance load redistribution, delay crack propagation, and improve post-cracking behavior, thereby addressing one of the key limitations of stabilized laterite blocks (Accornero et al., 2022). Studies on fiber-reinforced earth blocks show that natural fibers can improve crack control, post-cracking behavior, flexural resistance and thermal performance when the fiber content is properly selected (Bouchefra et al., 2022; Kumar & Barbato, 2022; Labiad et al., 2023; Namango & Madara, 2014). In particular, sisal fiber has been identified as a promising reinforcement for compressed earth blocks because it can improve strength while maintaining the sustainability benefits of local earth-based materials (Labiad et al., 2023; Namango & Madara, 2014).

The effectiveness of fiber reinforcement, however, is not solely dependent on fiber type or dosage, but is strongly influenced by fiber geometry, particularly the fiber aspect ratio. The fiber aspect ratio governs stress transfer efficiency (Gonabadi et al., 2025), crack-bridging capacity, and fiber-matrix interaction. Previous studies on natural fiber-reinforced compressed earth blocks indicate that fiber length and geometry influence both the mechanical strength and thermal behavior because longer fibers may improve crack bridging, while excessive length or content may cause poor dispersion, fiber balling and increased voids (Laibi et al., 2018; Riffi et al., 2026; Turco et al., 2021). Therefore, the fiber aspect ratio should be treated as a design variable rather than a secondary material characteristic. While appropriate fiber aspect ratios can significantly enhance mechanical performance, excessive fiber content or unfavorable aspect ratios may lead to fiber agglomeration, poor dispersion (Bayraktar et al., 2023), and reduced matrix cohesion, ultimately diminishing strength. Identifying an optimal fiber aspect ratio is therefore critical to maximizing the benefits of fiber reinforcement in stabilized laterite composites (Sani & Otaghsaraei, 2023).

Although previous studies have examined pozzolanic stabilization and natural fiber reinforcement independently, limited research has systematically investigated their combined influence on the mechanical performance of laterite blocks (Salaou et al., 2022). In particular, comparative assessments of kaolinand metakaolin-stabilized laterite blocks reinforced with sisal fiber at varying fiber aspect ratios remain scarce. Although previous studies have examined metakaolin-stabilized compressed earth blocks (Dabakuyo et al., 2022; Sore et al., 2018) and sisal or other natural fiber-reinforced earth blocks (Labiad et al., 2023; Namango & Madara, 2014), fewer studies have combined pozzolanic stabilization with fiber aspect-ratio optimization in laterite block systems. This creates a specific knowledge gap concerning the interaction between binder reactivity, fiber geometry and strength development. Furthermore, many existing studies lack rigorous statistical evaluation of interaction effects between binder type, binder content, and fiber reinforcement (Ma et al., 2022). This gap limits the development of performance-based guidelines for the design of sustainable laterite masonry units.

This study aims to evaluate the compressive and tensile strength behavior of laterite blocks stabilized with kaolin and metakaolin and reinforced with sisal fiber at different fiber aspect ratios. The specific objectives are to: (i) determine the effect of kaolin and metakaolin content on the strength of laterite blocks; (ii) identify the optimum binder content for each stabilizing material; (iii) assess the influence of sisal fiber content and aspect ratio on the strength response of the optimized kaolinand metakaolin-stabilized blocks; and (iv) examine the interaction between binder type, binder content and fiber reinforcement using robust statistical analysis. The study provides experimental evidence for developing stronger and more sustainable laterite masonry units for low-carbon construction.

2. Materials and methods

The experimental procedures were conducted with reference to recognized material and masonry testing standards. Particle size distribution was determined using sieve analysis procedures consistent with ASTM D6913/D6913M. Compressive strength testing of the masonry block specimens was conducted with reference to BS EN 772-1 and ASTM C140/C140M, while indirect tensile strength testing was adapted from ASTM C496/C496M because the specimens were laterite block units rather than standard concrete cylinders. The adopted standards were used to improve the reproducibility, reliability and comparability of the experimental program.

2.1 Materials

2.1.1 Laterite soil

The laterite soil used in this study was obtained from a naturally occurring lateritic deposit representative of soils commonly found in tropical regions. The soil was air-dried, crushed, and sieved to remove oversized particles prior to use. Laterite is characterized by a high content of iron and aluminum oxides, with variable proportions of silica and clay minerals, which influence its mechanical behavior and suitability for stabilization. The processed laterite soil served as the primary matrix material for the production of compressed laterite blocks in all experimental mixes.

The physical and engineering properties of the laterite soil were determined in accordance with relevant standards. Particle size distribution was conducted following BS 1377:2022 and ASTM D6913/D6913M, while the Atterberg limits (liquid limit, plastic limit, and plasticity index) were determined in accordance with BS 1377-2:2022 and ASTM D4318. The specific gravity of the soil solids was determined following BS 1377-2:2022 and ASTM D854, and compaction characteristics (optimum moisture content and maximum dry density) were evaluated in accordance with BS 1377-4:2022 and ASTM D698. Soil classification was carried out using the Unified Soil Classification System (USCS) in accordance with ASTM D2487 and the AASHTO Soil Classification System (AASHTO M145).

2.1.2 Kaolin and metakaolin

Kaolin was employed as a natural pozzolanic stabilizing material due to its aluminosilicate composition and fine particle size. Metakaolin was produced through the thermal activation of kaolin by calcination at controlled temperatures to induce dehydroxylation and enhance pozzolanic reactivity. The transformation of kaolin into metakaolin results in an amorphous structure with increased reactivity towards calcium hydroxide, leading to improved cementitious bonding within the laterite matrix. Both kaolin and metakaolin were incorporated as partial stabilizing binders at varying percentages by weight of the laterite soil, allowing for a comparative assessment of their effectiveness in improving mechanical strength (Figure 1).

Figure 1
Particle size distribution of kaolin and metakaolin stabilizers.

The chemical composition of the kaolin and metakaolin was determined at Ghana Standard Board Authority in Accra using X-ray fluorescence (XRF) analysis in accordance with ASTM C114, while mineralogical characterization was performed using X-ray diffraction (XRD). The production of metakaolin involved calcination of kaolin at a controlled temperature range of 650-800°C, consistent with the recommendations of ASTM C618 for natural pozzolanic materials. The physical properties of the kaolin and metakaolin, including specific gravity and particle size characteristics, were determined following ASTM D854 and ASTM D6913/D6913M, respectively.

2.1.3 Sisal fiber

Sisal fiber was used as a natural reinforcement material to enhance the tensile behavior and crack resistance of the stabilized laterite blocks. The fibers were sourced from mature sisal plants and processed to obtain uniform fibers free from impurities. Sisal fibers were characterized by their length, diameter, and corresponding aspect ratio, defined as the ratio of fiber length to diameter. Fiber lengths were selected to achieve different aspect ratios while maintaining consistent fiber properties across mixes. The fibers were incorporated at varying fiber contents by weight of the composite to evaluate the influence of fiber dosage and aspect ratio on the mechanical performance of both kaolinand metakaolin-stabilized laterite blocks.

The fibers were manually cleaned, cut to the required lengths, and air-dried prior to mixing. Due to the unavailability of specialized fiber-testing facilities, direct determination of tensile strength and Young's modulus was not conducted in this study. Consequently, reference values reported in literature for sisal fibers were adopted for discussion and comparison purposes where necessary. Therefore, the experimental program focused on evaluating the effect of fiber inclusion on the engineering performance of the stabilized laterite blocks rather than on the intrinsic mechanical properties of the fibers themselves.

2.2 Mix design and specimen preparation

The mix design for this study was developed to systematically evaluate the effects of binder type, binder content, and sisal fiber reinforcement on the mechanical performance of laterite blocks. Kaolin and metakaolin were used as stabilizing binders at contents ranging from 0% to 15% by weight of laterite soil. The binder contents investigated were 0%, 3%, 6%, 9%, 12% and 15% by dry weight of laterite soil. For fiber-reinforced specimens, sisal fiber was incorporated into the optimum kaolin and metakaolin binder systems identified from the unreinforced mixes. Kaolin-stabilized specimens were reinforced at fiber contents of 0.25%, 0.50%, 0.75% and 1.00%, while metakaolin-stabilized specimens were reinforced at 0.13%, 0.25%, 0.38% and 0.50%. Five replicate specimens were prepared for each mix configuration. The mixtures were compacted in 140mm x 100mm x 100mm molds using an electrically operated hydraulic press and compacted under a constant compaction force for all mixtures. The optimum water content obtained during the compaction factor test was used to achieve workable consistency and comparable compaction between mixes. Specimens were demolded after 24 hours and labelled according to binder type, binder content, fiber content and fiber aspect ratio. To investigate the influence of fiber reinforcement, sisal fibers were incorporated into selected mixes at varying fiber contents by weight of the composite. Fiber lengths were adjusted to achieve different fiber aspect ratios while maintaining consistent fiber properties. The selected fiber contents and corresponding aspect ratios were chosen based on preliminary trials and existing literature to ensure adequate fiber dispersion and effective fiber-matrix interaction without compromising workability or cohesion of the fresh mix.

Control mixes without fiber reinforcement were prepared for both kaolinand metakaolin-stabilized laterite blocks at each binder content level. These control specimens provided a baseline for assessing the individual effects of binder type and binder percentage on compressive and tensile strength. Fiber-reinforced mixes were then produced by introducing sisal fibers into the optimized binder systems, allowing for direct comparison between unreinforced and fiber-reinforced composites.

For specimen preparation, the required quantities of laterite soil and binder were dry-mixed to achieve a uniform blend. Sisal fibers were then gradually added and evenly distributed within the dry mixture to minimize fiber agglomeration. Water was subsequently introduced to obtain a workable consistency suitable for molding. The resulting mixtures were compacted into block molds under controlled conditions to ensure uniform density and specimen geometry. All specimens were demolded after initial setting and subjected to a consistent curing regime prior to mechanical testing.

The fiber aspect ratio was controlled by varying fiber length while maintaining a constant fiber diameter, ensuring that observed strength responses were attributable to changes in fiber geometry and content rather than material variability.

2.3 Curing and testing procedures

After molding, all laterite block specimens were demolded after 24 hours and cured for 28 days under laboratory air curing conditions. The curing procedure was kept constant for all mixtures to ensure that differences in mechanical performance were attributable to binder type, binder content, sisal fiber content and fiber aspect ratio rather than variations in curing history. Compressive strength testing was conducted in accordance with BS EN 772-1 / ASTM C140/C140M. Prior to testing, specimen dimensions were measured and the bearing surfaces were checked to ensure proper alignment. Each specimen was placed centrally in the compression testing machine of 2000 KN capacity at the Highways Laboratory, Takoradi. The maximum failure load was recorded, and the compressive strength was calculated by dividing the peak load by the loaded cross-sectional area. Tensile strength was assessed to examine the resistance of the laterite blocks to cracking and tensile stresses. Given the brittle nature of earthen materials, tensile strength testing was carried out using an indirect method suitable for masonry and composite materials. Specimens were positioned in accordance with the selected testing procedure, and load was applied gradually until failure. The tensile strength was determined from the peak load at failure, providing insight into the effectiveness of binder stabilization and fiber reinforcement in improving tensile performance.

All strength tests were conducted on multiple replicate specimens for each mix configuration to ensure reliability of the results. The mean values obtained from these tests were used for subsequent statistical analysis.

2.4 Statistical analysis

Statistical analyses were conducted to evaluate the effects of binder type, binder content, and sisal fiber reinforcement on the mechanical strength of laterite blocks. Descriptive statistics, including mean values, standard deviations, and 95% confidence intervals, were computed for compressive and tensile strength to provide an initial summary of the data and to facilitate comparison between mix configurations.

To assess the influence of binder type (kaolin and metakaolin) and binder content on strength in the absence of fiber reinforcement, HC₃ robust two-way analysis of variance (ANOVA) was employed. The use of robust ANOVA was justified by the presence of unequal variances and the relatively small sample sizes, which may violate the assumptions of conventional ANOVA. The HC₃ estimator was selected to provide heteroskedasticity-consistent standard errors and more reliable inference under these conditions. Interaction effects between binder type and binder content were also examined to determine whether the strength response differed between kaolinand metakaolin-stabilized systems.

For fiber-reinforced mixes, where heterogeneity of variance was more pronounced, Welch ANOVA was applied to evaluate the effect of fiber content and corresponding aspect ratio on compressive and tensile strength within each binder system. When significant main effects were detected, Games-Howell post hoc tests were performed to identify specific pairwise differences between fiber content levels. This post hoc approach was selected due to its robustness to unequal variances and unequal sample sizes.

In addition to hypothesis testing, effect sizes were estimated using partial eta-squared (ηp²) to quantify the magnitude of the effects of binder type, binder content, and their interaction on mechanical strength. Reporting effect sizes provided insight into the practical significance of the observed differences beyond statistical significance alone. All statistical analyses were conducted at a significance level of 5%.

3. Results

3.1 Compressive strength summary

Table 1 presents the descriptive statistics for the compressive strength of laterite blocks stabilized with kaolin and metakaolin, with and without sisal fiber reinforcement. For both materials, unstabilized laterite blocks (0% binder) exhibited the lowest compressive strength, with a mean value of 0.796 N/mm², indicating limited load-bearing capacity in the absence of stabilization.

Table 1
Descriptive statistics for compressive strength (N/mm²).

Material	Binder (%)	Fiber type	Fiber (%)	n	Mean	SD	95% CI
Kaolin	0	None	0	5	0.796	0.092	[0.738, 0.878]
Kaolin	3	None	0	5	0.808	0.039	[0.780, 0.840]
Kaolin	6	None	0	5	1.114	0.033	[1.090, 1.140]
Kaolin	9	None	0	5	1.834	0.092	[1.770, 1.916]
Kaolin	9	Sisal	0.25	5	2.026	0.079	[1.968, 2.090]
Kaolin	9	Sisal	0.5	5	2.114	0.046	[2.078, 2.150]
Kaolin	9	Sisal	0.75	5	2.2	0.049	[2.160, 2.236]
Kaolin	9	Sisal	1	5	2.08	0.035	[2.050, 2.106]
Kaolin	12	None	0	5	1.618	0.099	[1.544, 1.700]
Kaolin	15	None	0	5	1.268	0.079	[1.208, 1.326]
MetaKaolin	0	None	0	5	0.796	0.092	[0.738, 0.880]
MetaKaolin	3	None	0	5	1.08	0.161	[0.956, 1.216]
MetaKaolin	6	None	0	5	1.162	0.049	[1.128, 1.202]
MetaKaolin	9	None	0	5	1.85	0.131	[1.756, 1.964]
MetaKaolin	12	None	0	5	1.912	0.076	[1.856, 1.970]
MetaKaolin	12	Sisal	0.13	5	2.129	0.093	[2.066, 2.211]
MetaKaolin	12	Sisal	0.25	5	2.208	0.052	[2.168, 2.248]
MetaKaolin	12	Sisal	0.38	5	2.39	0.067	[2.342, 2.444]
MetaKaolin	12	Sisal	0.5	5	2.14	0.029	[2.118, 2.162]
MetaKaolin	15	None	0	5	1.658	0.118	[1.566, 1.746]

For kaolin-stabilized laterite blocks without fiber reinforcement, compressive strength increased with increasing binder content up to 9%, reaching a mean value of 1.834 N/mm², before declining at higher binder contents (Figure 2). A similar trend was observed for metakaolin-stabilized blocks, although higher compressive strengths were achieved at corresponding binder levels, with the maximum mean value of 1.912 N/mm² recorded at 12% metakaolin content. This indicates a generally superior strength performance of metakaolin compared to kaolin across the investigated binder range.

Figure 2
Effect of binder content on compressive strength of laterite blocks without fiber reinforcement.

The inclusion of sisal fiber resulted in a marked increase in compressive strength for both binder systems at their respective optimal binder contents. For kaolin-stabilized blocks at 9% binder content, compressive strength increased from 1.834 N/mm² in the unreinforced mix to a maximum mean value of 2.20 N/mm² at 0.75% fiber content, followed by a slight reduction at higher fiber dosage. Similarly, metakaolin-stabilized blocks at 12% binder content exhibited an increase in compressive strength from 1.912 N/mm² to a peak mean value of 2.39 N/mm² at 0.38% fiber content, after which a decline was observed at higher fiber content.

Overall, the descriptive statistics indicate that both binder type and fiber reinforcement significantly influence compressive strength, with metakaolin-sisal composites demonstrating the highest compressive strength values among all mix configurations. The presence of clear peak values suggests the existence of optimal binder-fiber combinations for maximizing compressive performance.

3.2 Tensile strength summary

Table 2 summarizes the descriptive statistics for the tensile strength of kaolinand metakaolin-stabilized laterite blocks with and without sisal fiber reinforcement. Unstabilized laterite blocks exhibited low tensile strength, with mean values of 0.15 N/mm², reflecting the brittle nature of untreated laterite masonry.

Table 2
Descriptive statistics for Tensile Strength (N/mm²).

Material	Binder (%)	Fiber type	Fiber (%)	n	Mean	SD	95% CI
Kaolin	0	None	0	5	0.15	0.016	[0.138, 0.162]
Kaolin	3	None	0	5	0.104	0.011	[0.096, 0.114]
Kaolin	6	None	0	5	0.108	0.004	[0.104, 0.110]
Kaolin	9	None	0	5	0.108	0.004	[0.104, 0.110]
Kaolin	9	Sisal	0.25	5	0.124	0.011	[0.116, 0.134]
Kaolin	9	Sisal	0.5	5	0.148	0.009	[0.141, 0.154]
Kaolin	9	Sisal	0.75	5	0.173	0.026	[0.153, 0.193]
Kaolin	9	Sisal	1	5	0.143	0.004	[0.139, 0.146]
Kaolin	12	None	0	5	0.14	0.014	[0.130, 0.152]
Kaolin	15	None	0	5	0.13	0.012	[0.120, 0.138]
MetaKaolin	0	None	0	5	0.15	0.016	[0.138, 0.162]
MetaKaolin	3	None	0	5	0.124	0.013	[0.114, 0.134]
MetaKaolin	6	None	0	5	0.138	0.013	[0.130, 0.150]
MetaKaolin	9	None	0	5	0.172	0.031	[0.148, 0.196]
MetaKaolin	12	None	0	5	0.26	0.028	[0.238, 0.282]
MetaKaolin	12	Sisal	0.13	5	0.3	0.022	[0.282, 0.318]
MetaKaolin	12	Sisal	0.25	5	0.308	0.019	[0.292, 0.322]
MetaKaolin	12	Sisal	0.38	5	0.352	0.008	[0.346, 0.358]
MetaKaolin	12	Sisal	0.5	5	0.304	0.026	[0.284, 0.324]
MetaKaolin	15	None	0	5	0.228	0.015	[0.218, 0.240]

In kaolin-stabilized blocks without fiber reinforcement (Figure 3), tensile strength showed limited variation across binder contents, with mean values generally ranging between 0.104 and 0.140 N/mm². The introduction of sisal fiber at 9% kaolin binder content resulted in a noticeable improvement in tensile strength, increasing from 0.108 N/mm² in the unreinforced mix to a maximum mean value of 0.173 N/mm² at 0.75% fiber content, before decreasing at higher fiber dosage.

Figure 3
Effect of binder content on tensile strength of laterite blocks without fiber reinforcement.

Metakaolin-stabilized laterite blocks exhibited substantially higher tensile strength values compared to kaolin-stabilized blocks at equivalent binder contents. At 12% metakaolin binder content, the tensile strength of unreinforced blocks was 0.26 N/mm², which increased significantly with the inclusion of sisal fiber. The highest mean tensile strength of 0.352 N/mm² was recorded at a fiber content of 0.38%, followed by a reduction at 0.50% fiber content.

These results indicate that tensile strength is more sensitive to fiber reinforcement than compressive strength, with clear optimum fiber contents observed for both kaolinand metakaolin-stabilized laterite blocks. The descriptive trends further highlight the enhanced tensile performance of metakaolin-sisal systems relative to kaolin-sisal systems.

3.3 Effect of binder type and binder content on strength (no fiber)

The effects of binder type and binder content on the compressive strength of laterite blocks without fiber reinforcement were evaluated using an HC₃ robust two-way ANOVA. The results presented in Table 3 indicate that binder type had a statistically significant effect on compressive strength (F = 29.34, p < 0.001), while binder content exerted a highly significant influence (F = 226.72, p < 0.001). A significant interaction between binder type and binder content was also observed (F = 7.36, p < 0.001), indicating that the strength response to binder dosage differed between kaolinand metakaolin-stabilized systems.

Table 3
HC₃ Robust Two-way ANOVA for compressive strength (no fiber).

Effect	df	F	p
Material	1	29.34	< .001
Binder (%)	5	226.72	< .001
Material × Binder	5	7.36	< .001
Residuals	48

The HC₃ robust coefficient estimates for compressive strength are summarized in Table 4, with kaolin at 0% binder serving as the reference condition. Relative to the reference, increases in binder content generally resulted in significant positive effects on compressive strength, particularly at 6%, 9%, and 12% binder levels. The interaction terms further indicate that metakaolin exhibited a stronger strength response than kaolin at selected binder contents, notably at 3%, 12%, and 15% binder levels, where the interaction effects were statistically significant (p < 0.05).

Table 4
HC₃ Robust Coefficients for Compressive Strength (no fiber).

Term	Estimate	SE	t	p
(Intercept)	0.796	0.046	17.39	< .001
materialMetaKaolin	0	0.065	0	1
binder_f3	0.012	0.05	0.24	0.81
binder_f6	0.318	0.049	6.54	< .001
binder_f9	1.038	0.065	16.04	< .001
binder_f12	0.822	0.067	12.19	< .001
binder_f15	0.472	0.06	7.83	< .001
materialMetaKaolin:binder_f3	0.272	0.105	2.59	0.013
materialMetaKaolin:binder_f6	0.048	0.071	0.67	0.503
materialMetaKaolin:binder_f9	0.016	0.103	0.16	0.877
materialMetaKaolin:binder_f12	0.294	0.09	3.27	0.002
materialMetaKaolin:binder_f15	0.39	0.096	4.07	< .001

A similar analytical approach was applied to tensile strength, with HC₃ robust two-way ANOVA results presented in Table 5. Binder type had a highly significant effect on tensile strength (F = 118.8, p < 0.001), and binder content was also significant (F = 29.56, p < 0.001). The interaction between binder type and binder content was statistically significant (F = 16.66, p < 0.001), indicating that tensile strength development varied between kaolinand metakaolin-stabilized laterite blocks across binder levels.

Table 5
HC₃ Robust Two-way ANOVA for tensile strength (no fiber).

Effect	df	F	p
Material	1	118.8	< .001
Binder (%)	5	29.56	< .001
Material × Binder	5	16.66	< .001
Residuals	48

The HC₃ robust coefficient estimates for tensile strength (Table 6) show that, relative to kaolin at 0% binder, increasing binder content alone did not consistently enhance tensile strength in kaolin-stabilized blocks. In contrast, positive and statistically significant interaction effects were observed for metakaolin at higher binder contents, particularly at 9%, 12%, and 15%, demonstrating improved tensile performance in metakaolin-stabilized systems at these levels.

Table 6
HC₃ Robust Coefficients for Tensile Strength (no fiber).

Term	Estimate	SE	t	p
(Intercept)	0.15	0.008	18.97	< .001
materialMetaKaolin	0	0.011	0	1
binder_f3	-0.046	0.01	-4.72	< .001
binder_f6	-0.042	0.008	-5.11	< .001
binder_f9	-0.042	0.008	-5.11	< .001
binder_f12	-0.01	0.011	-0.94	0.35
binder_f15	-0.02	0.01	-2	0.051
materialMetaKaolin:binder_f3	0.02	0.014	1.41	0.166
materialMetaKaolin:binder_f6	0.03	0.013	2.28	0.027
materialMetaKaolin:binder_f9	0.064	0.019	3.32	0.002
materialMetaKaolin:binder_f12	0.12	0.019	6.2	< .001
materialMetaKaolin:binder_f15	0.098	0.015	6.65	< .001

Effect size estimates based on partial eta-squared (ηp²) are presented in Table 7. For compressive strength, binder content exhibited a very large effect size (ηp² = 0.96), followed by binder type (ηp² = 0.50) and the material-binder interaction (ηp² = 0.45). For tensile strength, large effect sizes were observed for binder type (ηp² = 0.77), binder content (ηp² = 0.80), and their interaction (ηp² = 0.67). These results indicate that both compressive and tensile strength are strongly influenced by binder selection and dosage, with interaction effects playing a substantial role in determining overall strength performance.

Table 7
Partial eta-squared (ηp²) for Binder × Material models (no fiber).

DV	Effect	ηp2
Compressive Strength	Material	0.5
Compressive Strength	Binder (%)	0.96
Compressive Strength	Material × Binder	0.45
Tensile Strength	Material	0.77
Tensile Strength	Binder (%)	0.8
Tensile Strength	Material × Binder	0.67

3.4 Effect of sisal fiber content on kaolin-stabilized laterite blocks (9% binder)

3.4.1 Compressive strength response

The effect of sisal fiber content on the compressive strength of kaolin-stabilized laterite blocks at 9% binder content was evaluated using Welch ANOVA, with the results presented in Table 8. The analysis revealed a statistically significant effect of fiber content on compressive strength (F = 7.47, p = 0.009), indicating that variations in fiber dosage led to meaningful differences in strength performance.

Table 8
Welch ANOVA results.

Dependent variable	n	F	df1	df2	p
Compressive strength (N/mm2)	20	7.47	3	8.65	0.009
Tensile strength (N/mm2)	20	6.25	3	7.86	0.018

Post hoc comparisons using the Games-Howell procedure (Table 9) identified significant differences between specific fiber content levels. The comparison between 0.25% and 0.75% fiber content showed a significant increase in compressive strength at the higher fiber dosage, with a mean difference of 0.174 N/mm² (95% CI: [0.035, 0.313], p = 0.018). In contrast, increasing fiber content from 0.75% to 1.00% resulted in a significant reduction in compressive strength, with a mean difference of -0.120 N/mm² (95% CI: [-0.209, -0.031], p = 0.012).

Table 9
Games-Howell post hoc comparisons (Significant only).

Comparison (fiber %)	Mean difference	95% CI	p(adj)
Compressive strength
0.25 vs 0.75	0.174	[0.035, 0.313]	0.018
0.75 vs 1.00	-0.120	[-0.209, -0.031]	0.012
Tensile strength
0.25 vs 0.50	0.024	[0.003, 0.045]	0.029
0.25 vs 0.75	0.049	[0.005, 0.093]	0.034

These results indicate that sisal fiber reinforcement enhances the compressive strength of kaolin-stabilized laterite blocks up to an optimal fiber content, beyond which further fiber addition adversely affects strength. Within the range investigated, a fiber content of 0.75% produced the highest compressive strength, highlighting the importance of fiber optimization in kaolin-based stabilized laterite systems.

3.5 Effect of sisal fiber content on metakaolin-stabilized laterite blocks (12% binder)

3.5.1 Compressive strength response

The effect of sisal fiber content on the compressive strength of metakaolin-stabilized laterite blocks (Figure 4 and Figure 5) at 12% binder content was evaluated using Welch ANOVA, with results presented in Table 10. The analysis showed that fiber content had a highly significant effect on compressive strength (F = 17.74, p < 0.001), indicating strong sensitivity of compressive performance to variations in fiber dosage.

Table 10
Welch ANOVA results.

Dependent variable	n	F	df1	df2	p
Compressive strength (N/mm2)	20	17.74	3	8.18	< .001
Tensile strength (N/mm2)	20	14.01	3	7.94	0.002

Figure 4
Effect of sisal fiber content on compressive strength of metakaolin-stabilized laterite blocks.

Figure 5
Effect of sisal fiber content on tensile strength of metakaolin-stabilized laterite blocks.

Games-Howell post hoc comparisons (Table 11) revealed statistically significant differences between specific fiber content levels. An increase in fiber content from 0.13% to 0.38% resulted in a significant improvement in compressive strength, with a mean difference of 0.261 N/mm² (95% CI: [0.094, 0.429], p = 0.005). Similarly, compressive strength at 0.38% fiber content was significantly higher than at 0.25%, with a mean difference of 0.182 N/mm² (95% CI: [0.059, 0.305], p = 0.007). However, further increasing fiber content from 0.38% to 0.50% led to a significant reduction in compressive strength, with a mean difference of -0.25 N/mm² (95% CI: [-0.366, -0.134], p = 0.002).

Table 11
Games-Howell post hoc comparisons (MetaKaolin @ 12% binder, Sisal).

Comparison (Fiber %)	Mean difference	95% CI	p(adj)
Compressive strength
0.13 vs 0.38	0.261	[0.094, 0.429]	0.005
0.25 vs 0.38	0.182	[0.059, 0.305]	0.007
0.38 vs 0.50	-0.25	[-0.366, -0.134]	0.002
Tensile strength
0.13 vs 0.38	0.052	[0.013, 0.091]	0.016
0.25 vs 0.38	0.044	[0.010, 0.078]	0.016
0.38 vs 0.50	-0.048	[-0.094, -0.002]	0.042

These results demonstrate that sisal fiber reinforcement enhances the compressive strength of metakaolin-stabilized laterite blocks up to an optimal fiber content, beyond which strength decreases. Within the investigated range, a fiber content of 0.38% yielded the highest compressive strength, indicating that metakaolin-stabilized systems require lower optimal fiber dosages compared to kaolin-stabilized systems.

4. Discussion

The present study investigated the strength behavior of laterite blocks enhanced with kaolin and metakaolin, with and without sisal fiber reinforcement, at different fiber contents and corresponding aspect ratios (Ayeni et al., 2022). The results demonstrate that strength development in stabilized laterite blocks is governed by a complex interaction between binder type, binder dosage, and fiber reinforcement, with distinct optimal combinations emerging for compressive and tensile performance (Salaou et al., 2022).

The superior performance of metakaolin relative to kaolin observed across both compressive and tensile strength responses can be attributed primarily to differences in particle size distribution and pozzolanic reactivity (Jing et al., 2023). As shown in Figure 1, metakaolin exhibits a finer grading than kaolin across most sieve sizes, resulting in a higher specific surface area and enhanced reactivity (Siline & Mehsas, 2022). This microstructural advantage promotes improved particle packing and more extensive formation of cementitious hydration products, which has been widely reported in metakaolin-stabilized soil and cementitious systems (Siline & Mehsas, 2022). This interpretation is consistent with previous studies, showing that metakaolin-based binders can enhance the strength and density of compressed earth blocks and lateritic soils by improving particle bonding and reducing porosity (Dabakuyo et al., 2022; Ebid, 2022; Sore et al., 2018). The statistically significant main effects and interaction effects identified in the HC₃ robust two-way ANOVA for both compressive and tensile strength (Tables 3 and 5) further confirm that binder type plays a critical role in determining strength outcomes, particularly when interacting with binder content.

Binder content was found to exert the strongest influence on compressive strength, as reflected by the very large effect size (ηp² = 0.96) reported in Table 7. For both kaolinand metakaolin-stabilized blocks, compressive strength increased with binder content up to an optimum range, beyond which marginal gains or strength reductions were observed (Tables 1 and 3; Figure 2). This behavior is consistent with previous studies on chemically stabilized lateritic and clayey soils, which report that insufficient binder content limits cementation, while excessive binder addition may induce brittleness, microcracking, or inefficient stress transfer within the matrix (Ezreig et al., 2022). The higher optimum binder content observed for metakaolin-stabilized blocks (approximately 12%) compared to kaolin-stabilized blocks (approximately 9%) reflects the greater pozzolanic demand associated with finer, more reactive materials (Fang et al., 2026).

Tensile strength followed a similar binder-dependent trend but exhibited greater sensitivity to binder type, as evidenced by the large effect size for material (ηp² = 0.77) and the significant interaction effects reported in Tables 5-7. Figure 3 clearly illustrates that metakaolin-stabilized blocks achieved substantially higher tensile strength across all binder contents compared to kaolin-stabilized blocks. This enhanced tensile response is indicative of improved interparticle bonding and reduced crack initiation propensity, which are critical limitations in unstabilized and weakly stabilized laterite masonry (Bhat et al., 2024).

The introduction of sisal fiber reinforcement led to pronounced improvements in both compressive and tensile strength, although the magnitude and nature of the response depended strongly on the fiber content and binder system. For kaolin-stabilized blocks at 9% binder content, fiber inclusion significantly affected compressive strength, with an optimal fiber content of 0.75% identified through Welch ANOVA and Games-Howell post hoc testing (Tables 8 and 9). The initial increase in strength with fiber addition can be attributed to enhanced crack bridging and stress redistribution, as widely reported for natural fiber-reinforced soil composites (Huang et al., 2022). This mechanism agrees with studies on sisal-, kenaf-, bagasseand doum fiber-reinforced earth blocks, where natural fibers improved tensile or flexural response by bridging cracks and increasing resistance to brittle failure (Bouchefra et al., 2022; Kumar & Barbato, 2022; Labiad et al., 2023; Laibi et al., 2018). However, the observed strength reduction at higher fiber contents suggests that excessive fiber incorporation disrupts matrix continuity, leading to poor compaction and fiber agglomeration, which negatively affects load transfer efficiency. Similar reductions at excessive fiber contents have been attributed to fiber clustering, increased void ratio, reduced matrix continuity and weak fiber-soil bonding, particularly where hydrophilic natural fibers are incorporated without sufficient dispersion control or surface treatment (Labiad et al., 2023; Riffi et al., 2026; Turco et al., 2021).

A similar but more pronounced fiber optimization behavior was observed in metakaolin-stabilized blocks at 12% binder content. The compressive strength response shown in Figure 4, supported by the statistically significant Welch ANOVA results in Table 10 and post hoc comparisons in Table 11, indicates that compressive strength increased significantly with fiber content up to 0.38%, beyond which strength declined. Notably, the optimal fiber content for metakaolin-stabilized blocks was lower than that for kaolin-stabilized blocks, suggesting that highly reactive binder systems require more conservative fiber dosages to maintain matrix integrity. This finding aligns with previous research indicating that dense, brittle matrices are more sensitive to fiber over-reinforcement (Hiralal Dhage & Khedkar, 2025; Lakshmaiya et al., 2025)

The tensile strength response to fiber reinforcement was even more pronounced, particularly in metakaolin-stabilized systems. Figure 5 shows a clear peak in tensile strength at 0.38% fiber content, with tensile strength increasing substantially relative to fiber-free specimens (Tables 2, 10, and 11). This behavior underscores the critical role of fibers in mitigating tensile weakness in stabilized laterite blocks, where crack initiation and propagation typically govern failure. The enhanced tensile performance observed at optimal fiber contents is consistent with established fiber reinforcement mechanisms, including crack bridging, pull-out resistance, and improved post-cracking behavior (Akbulut et al., 2025).

Comparatively, metakaolin-sisal systems consistently outperformed kaolin-sisal systems in both compressive and tensile strength, as evidenced by higher mean strength values (Tables 1 and 2), stronger statistical effects (Tables 3-7), and clearer optimization trends (Figures 4 and 5). This superior performance reflects the synergistic interaction between metakaolin’s high pozzolanic reactivity and the crack-bridging capacity of sisal fibers. The interaction terms reported in the robust coefficient tables (Tables 4 and 6) further support this interpretation, indicating that fiber effectiveness is enhanced in matrices with stronger binder-soil bonding.

The findings demonstrate that strength behavior in stabilized laterite blocks cannot be optimized through binder or fiber selection alone. Instead, optimal performance requires careful balancing of binder type, binder dosage, and fiber content to maximize beneficial interactions while avoiding adverse effects associated with over-stabilization or excessive fiber addition. The clear identification of optimal binder-fiber combinations through robust statistical analysis provides a strong basis for developing performance-based design guidelines for sustainable laterite masonry units.

4.1 Limitations of the study

While the findings of this study provide clear insight into the strength behavior of kaolinand metakaolin-stabilized laterite blocks with sisal fiber reinforcement, certain limitations should be acknowledged. The experimental program was conducted using laterite soil sourced from a single location, and variations in mineralogical composition of lateritic soils from other regions may influence strength response. In addition, the number of specimens tested per mix configuration was limited, which, although addressed through the use of robust statistical methods, may affect the generalizability of the results. The study focused exclusively on short-term mechanical strength under controlled laboratory curing conditions and did not consider long-term durability or environmental exposure effects. This is important because previous studies have shown that fiber-reinforced earth blocks may exhibit improved mechanical or thermal performance while still requiring evaluation of water absorption, capillary suction, abrasion resistance and durability under wetting-drying exposure (Bailly et al., 2024; Labiad et al., 2023; Riffi et al., 2026). These factors should be examined in future studies to further validate the applicability of the proposed binder-fiber systems in field conditions.

5. Conclusions

This study examined the strength behavior of laterite blocks stabilized with kaolin and metakaolin, with and without sisal fiber reinforcement, at varying fiber contents and aspect ratios. The findings demonstrate that both binder type and fiber reinforcement play critical and interdependent roles in governing the mechanical performance of stabilized laterite masonry.

Binder content was identified as the dominant factor influencing compressive strength, with statistically significant effects and very large effect sizes observed for both kaolinand metakaolin-stabilized systems. Strength increased with binder content up to an optimum range, beyond which additional binder resulted in diminishing or adverse effects. Metakaolin consistently outperformed kaolin across all binder levels, achieving higher compressive and tensile strengths, which reflects its finer particle size distribution and greater pozzolanic reactivity.

Tensile strength exhibited greater sensitivity to binder type and fiber reinforcement than compressive strength. While kaolin-stabilized blocks showed limited tensile strength improvement with increasing binder content alone, metakaolin-stabilized blocks achieved substantially higher tensile strength, indicating improved interparticle bonding and reduced brittleness. These findings confirm the importance of binder selection when tensile performance is a critical design requirement.

The inclusion of sisal fiber significantly enhanced both compressive and tensile strength, provided that fiber content and aspect ratio were optimized. For kaolin-stabilized blocks at 9% binder content, an optimal fiber content of approximately 0.75% was identified, beyond which strength declined. In contrast, metakaolin-stabilized blocks at 12% binder content exhibited optimal performance at a lower fiber content of approximately 0.38%, highlighting the increased sensitivity of highly reactive binder systems to fiber over-reinforcement. These results underscore the necessity of tailoring fiber dosage to the binder system rather than applying uniform reinforcement levels.

Overall, the superior performance of metakaolin-sisal composites demonstrates a clear synergistic effect between pozzolanic stabilization and natural fiber reinforcement. Robust statistical analyses confirmed that interaction effects between binder type, binder content, and fiber reinforcement are substantial and must be considered in the design of stabilized laterite blocks. The study provides quantitative evidence that carefully optimized metakaolin-sisal systems can significantly improve the mechanical viability of laterite blocks for sustainable construction applications.

The findings of this research contribute to the development of low-carbon masonry materials by establishing performance-driven guidelines for the selection of binder type, binder dosage, and fiber content. These results support the broader adoption of stabilized laterite blocks as a structurally reliable and environmentally sustainable alternative to conventional masonry units.

6. Recommendations and future directions

Based on the findings of this study, several recommendations can be made for the practical application and further development of stabilized laterite blocks. Metakaolin is recommended as a preferred stabilizing binder for laterite blocks where improved mechanical performance is required, particularly in applications where tensile strength and crack resistance are critical. For fiber-reinforced systems, careful optimization of the sisal fiber content and aspect ratio is essential, since excessive fiber has shown to adversely affect strength. Therefore, the use of moderate fiber dosages tailored to the binder system is recommended to maximize the benefits of fiber reinforcement without compromising matrix integrity.

In practical block production, the results suggest that optimal binder-fiber combinations should be selected based on performance requirements rather than material availability alone. Specifically, metakaolin-stabilized laterite blocks incorporating low to moderate sisal fiber contents offer a promising balance between strength enhancement and material efficiency. These findings support the development of performance-based mix design guidelines for sustainable laterite masonry, particularly in low-carbon construction contexts.

Future research should extend the present research to evaluate the long-term durability of stabilized and fiber-reinforced laterite blocks under environmental exposure conditions, including wet-dry cycling, freeze-thaw action, and chemical attack. Investigations into the effects of fiber treatment methods, such as alkali or surface modification, may further improve fiber-matrix bonding and long-term performance. Future work should also examine fiber treatment and durability because recent reviews emphasize that natural fiber performance in compressed earth blocks depends not only on fiber type and content, but also on treatment method, fiber-matrix adhesion, moisture resistance and long-term environmental stability (Bailly et al., 2024; Riffi et al., 2026). Additionally, microstructural studies using techniques such as scanning electron microscopy and X-ray diffraction would provide deeper insight into the mechanisms governing binder hydration, fiber interaction, and crack propagation.

Further research is also needed to assess the structural performance of stabilized laterite blocks at the element and assembly levels, including masonry prism testing and full-scale wall behavior under axial and lateral loading. Life-cycle assessment and cost-benefit analysis would help quantify the environmental and economic advantages of metakaolin-sisal systems relative to conventional masonry materials. Finally, expanding the investigation to include other locally available natural fibers and hybrid reinforcement strategies may broaden the applicability of the findings and support the wider adoption of sustainable, low-carbon masonry technologies.

Funding information

No external funding was received for this research.

Data availability

All data generated or analyzed during this study are included in this published article.This manuscript is based on the PhD dissertation of Francis Adogla, entitled Effect of Kaolin and Metakaolin Stabilisation with Sisal Fibre Reinforcement on the Engineering Properties of Laterite Blocks, submitted in 2026 to the University of Skills Training and Entrepreneurial Development, Graduate Program in the Department of Construction Technology.

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Conflict of interests

The authors declare that they have no conflict of interest.

Associate Editor

Herlander Mata Fernandes Lima

Publication Dates

• Publication in this collection
  28 Sept 2026
• Date of issue
  Oct-Dec 2026

History

• Received
  04 Apr 2026
• Accepted
  07 July 2026