Mechanical Characteristics of Clay-Based Masonry Walls

Abstract

The building sector is under increasing pressure to lower its environmental impact, prompting renewed interest in raw soil as a low-carbon and locally available material. This study investigates the mechanical and thermal properties of clay-based masonry walls through a comprehensive experimental program on earthen mortars, bricks, and their interfaces, considering both stabilized and non-stabilized formulations. Compressive, bending, and shear tests reveal that strength is strongly influenced by mortar composition, hydration time, and the soil-to-sand ratio. The addition of 5–7.5% cement yields modest gains in compressive strength but increases the carbon footprint, whereas extended pre-hydration achieves similar improvements with lower environmental costs. Thermal characterization of the studied samples (SiO₂ ≈ 61.2 wt%, Al₂O₃ ≈ 11.7 wt%, MgO ≈ 5.1 wt%) revealed that SiO₂-enriched compositions significantly enhance thermal conductivity, whereas the presence of Al₂O₃ and MgO contributes to increased heat capacity and improved moisture regulation. These findings suggest that well-optimized clay-based mortars can satisfy the structural and thermal requirements of non-load-bearing applications, offering a practical and sustainable alternative to conventional construction materials. By reducing embodied carbon, enhancing hygrothermal comfort, and relying on locally available resources, such mortars contribute to the advancement of green building practices and the transition towards low-carbon construction.

1. Introduction

Buildings use 40% of the world’s energy, making the construction industry one of the biggest contributors to greenhouse gas emissions [1]. Moreover, this industry contributes 10% of global greenhouse gas (GHG) emissions, of which 52% are attributable to concrete [1]. Significant CO₂ emissions are produced when clinker and clay are burned to extremely high temperatures, especially during the cement manufacturing process [2,3].

To reduce the environmental impact of building, recycled waste [4,5] and geo- and bio-based materials [3,6,7,8,9,10] are increasingly being employed to offset or reduce the carbon footprint.

These basic materials are substituted for cement, aggregates, and other building materials in several applications [11,12,13].

Soil, a building material used for thousands of years, is valued for its durability and widespread availability [2,3,11,14,15,16]. It consists of a variable mixture of clay, silt, sand, and sometimes gravel, with clay acting as a binder, similar to cement in concrete. Soil offers several advantages for sustainable, cost-effective, and durable housing [6]. Due to its binding properties, clay-based soil can be used in applications such as mortar, plaster, filler bricks, and load-bearing bricks. These materials can be molded, compacted, or stacked, and various soil construction techniques exist, including lightened soil [11], cob [16], rammed soil bricks [17], and soil plasters [6]. Soil construction is also compatible with other techniques, such as timber framing and block construction, and possesses notable fire-resistant properties. An example of a soil wall constructed with clay masonry blocks and clay mortar is shown in Figure 1.

With the rise in reinforced concrete, traditional knowledge of soil construction techniques, such as cob, masonry, and timber framing, has largely been lost. However, the growing demand for sustainable building materials has sparked renewed interest in raw soil and fly ash, thanks to their natural availability and minimal processing requirements [18,19,20]. More studies are needed to fully understand these materials in terms of their production processes, properties, and applications in modern construction. Masonry, which is one of the oldest ways to build structures, often uses blocks made from soil and is used in many types of buildings. However, masonry is naturally brittle and has different properties in different directions, which increases its vulnerability to earthquake damage. To properly assess how earthquake-prone masonry buildings are, it is important to use a method that considers the different factors that can affect their performance [11]. A thorough, multi-area approach is needed to evaluate how well masonry buildings can withstand earthquakes and to help with restoring them. This method should consider the different values of important factors while also following guidelines to protect historical buildings. Earlier studies have looked at how things like the strength of the materials, the ratio of height to thickness, and the type of mortar used affect the strength of masonry [12,13]. Many researchers [20,21,22,23,24] have also investigated using cob as a building material, mainly to improve its strength and stability.

Table 1. Summary of cob mixture compositions and their ratios.

Mix ID	Clay (%)	Sand (%)	Additives	Water Content (%)	Fiber Type & Content	Reference
Soil 0	75–85	15–25	–	20%	Straw, 2%	Hamard et al. (2016) [25]
Cob 5	66.67	33.33	–	–	Straw (36.3 kg/m3)	Alassaad et al. (2022) [6]
Yellow Soil	36	13.5	–	28%	–	Quagliarini et al.(2010) [26]
3DP Cob	19–20	80–81	–	22–28%	Straw, 2%	Gomaa et al. (2021) (a) [27] Gomaa et al. (2021) (b) [28]
M31	39.89	17.73	Cement 2.48%, Lime 7.98%	31.91%	–	Alqenaee et al. (2019) [29]
S1	–	–	–	25%	Hemp straw, 5%	Zeghari et al. (2021) [19]
Soil 0	21	18	–	24%	Wheat straw, 1.7%	Miccoli et al. (2019) [30]

below presents a simplified selection of cob mixtures derived from the literature review, focusing on key variables such as clay and sand content, additives, water content, and type of fibers used. These results illustrate the diversity of mix designs tailored to different performance goals, such as structural integrity, thermal behavior, or compatibility with 3D printing. The fiber-reinforced mixes often show higher water requirements, and additives like lime and cement are included to improve cohesion and printability.

Over the years, researchers have devoted considerable effort to studying masonry built from raw, compacted soil blocks combined with mortar formulated to match their properties. Much of this work has focused on the mechanical behavior of the bricks, the mortar, and the critical junction where the two meet. To assess these characteristics, a variety of experimental methods are employed, including compression tests, flexural strength assessments, bucket shear tests, and the Casagrande method [31]. Despite these investigations, the brick–mortar interface remains insufficiently understood. This point of contact is frequently identified as the most fragile element in an earthen wall, as it brings together materials with inherently different mechanical responses. The way this interface performs can significantly influence the overall durability and load-bearing capacity of the structure [17,24,25]. However, the scarcity of focused studies on its behavior leaves notable gaps in our understanding, limiting the development of reliable models for predicting structural performance.

Mechanical behavior is only one part of the picture. The thermal performance of earthen materials is equally important, especially in terms of energy efficiency. Properties like thermal conductivity and specific heat capacity determine how effectively the material moderates indoor temperatures, reducing reliance on heating and cooling systems [26,27,28]. By improving thermal comfort and lowering energy demand, these characteristics make earthen construction a compelling choice for sustainable building design.

In this context, this study aims to experimentally investigate the mechanical behavior of blocks, mortar, and their interface in masonry, while also examining the thermal properties of earthen materials—particularly specific heat capacity and thermal conductivity—and their influence on indoor comfort and energy efficiency. By addressing the knowledge gaps related to the block–mortar interface, the work seeks to improve the structural performance of raw soil masonry. Ultimately, this research provides insights that may support the future adoption of environmentally sustainable building materials by optimizing the thermal and mechanical properties of earthen construction systems.

2. Materials and Experimental Methods

2.1. Materials

In the case of non-stabilized soil mortar, the raw materials used are soil, sand, and water, whereas in stabilized soil mortar, cement is added to the mix. Mortar formulation is critical, as it significantly influences the mechanical characteristics of masonry walls [20,21,32,33,34]. Therefore, it is essential to develop suitable formulations for both stabilized and non-stabilized mortars.

The soil used for brick production was sourced from Bechmezzine, in northern Lebanon. The sand used was a standardized construction-grade sand with a grain size of up to 4 mm. Figure 2 shows the soil sample used for the experimental tests. Several tests were conducted on different mortar formulations to determine the optimum sand-to-soil ratio (S/T). This ratio must remain within specific limits: formulations with excessive sand content produce weak, powdery mortars unsuitable for construction, while those with excessive clay content result in overly sticky mortars that are difficult to work with and prone to shrinkage cracking [21,32,33,34,35,36,37]. Based on our experimental testing, the optimum sand-to-soil ratio was determined to be 30:70 (sand: soil) for non-stabilized mortar. This ratio provided the best balance between workability, compressive strength, and bond performance at the brick–mortar interface. Ratios with higher sand content produced weak mortars with poor cohesion, whereas higher soil content resulted in sticky mortars prone to shrinkage and difficult to place.

This image illustrates the texture and composition of the material before any preparation or treatment.

Soil classification was performed using standard geotechnical methods in accordance with applicable norms. Particle size distribution was analyzed separately for fine particles using laser diffraction (LS 13 320, Beckman Coulter) and for coarse particles (>80 μm) following standard procedures. Particle size distribution was analyzed separately for fine particles using laser diffraction (LS 13 320, Beckman Coulter Inc., Brea, CA, USA) and for coarse particles (>80 μm) following standard procedures. Additionally, the soil’s plasticity was assessed using Atterberg limits to calculate the plasticity index (PI). The methylene blue value and particle size analysis were also used to classify the soil as sandy, silty, or clayey [17,18,24].

The chemical composition of the soil was further analyzed using X-ray fluorescence (XRF). A Cu microfocus source and a parabolic multilayer mirror on the primary beam were utilized in the Inel Equinox 3500 spectrometer. An Amptek X-123SDD Silicon Drift detector was positioned 10 mm above the sample to ensure high sensitivity, even for low atomic number elements. The results of the characterization tests, including Atterberg limits and methylene blue value, are summarized in

Table 2. Atterberg limits and methylene blue values of the crushed soil.

Parameter	LL (%)	PL (%)	PI (%)	MBV (g/100 g)	USCS
Soil	27.9	23.3	3.7	1.45	Low plasticity silt
Sand	22.7	21.0	2.5	0.77	Silty sand with gravel

LL: Liquid Limit; PL: Plastic Limit; PI: Plastic Index; MBV: Methylene Blue Value and USCS: Unified Soil Classification System.

.

Table 3. Chemical composition of materials by XRF.

Percentage of Each Component (%)	Soil
Silicon Dioxide (SiO2)	61.14 ± 2.5
Aluminum Oxide (Al2O3)	11.39 ± 0.52
Iron (III) Oxide or Ferric Oxide (Fe2O3)	8.38 ± 0.42
Magnesium Oxide (MgO)	5.08 ± 0.21
Potassium Oxide (K2O)	3.17 ± 0.11
Titanium Dioxide (TiO2)	4.08 ± 0.16
Calcium Oxide (CaO)	3.27 ± 0.105
Sodium Oxide (Na2O)	2.11 ± 0.094
Manganese Oxide (MnO)	1.16 ± 0.087
Phosphorus Pentoxide (P2O5)	1.14 ± 0.084
Sulfur Trioxide (SO3)	<0.1 ± 0.004

presents the major oxide composition of the single soil source used for all mortar formulations in this study, providing detailed chemical information relevant to its mechanical and thermal behavior.

The water content while laying is also an important parameter and was studied as part of the parametric analysis. After examining compressive, bending, and shear stresses, the effect of water content was thoroughly evaluated. Water should be added in such a way that the mortar achieves a plastic consistency, allowing proper placement and adhesion without segregation or excessive shrinkage. This consistency is commonly evaluated in practice during mixing and by standardized tests, such as the flow table test (NF EN 1015-3) [38].

Five types of mortar mixes were prepared and cast into cylindrical molds with a diameter of 7 cm and a height of 14 cm. Each mold was filled in two consecutive layers to ensure uniform compaction and eliminate entrapped air. Excess air was removed by tapping the mold with a hammer (25 times) after filling the first layer; then the mold was filled with the second layer and tapped again (25 times) (Figure 3).

The experimental program was designed to assess the effect of soil content and cement stabilization on the mechanical and thermal behavior of mortars and masonry assemblies. Two groups of specimens were prepared: Group 1 representing traditional earthen masonry techniques as qualitative references, and Group 2 consisting of systematically varied stabilized and non-stabilized mortars to evaluate their influence on mechanical and thermal performance.

Table 4. Mix proportions of stabilized and non-stabilized mortars used in the tests.

Series	Type	Composition (% by Weight)	Notes
Ref	Stabilized mortar	50% Soil/45% Sand/5% Cement	Rolled sand (0–4 mm)
M1	Stabilized mortar	67% Soil/28% Sand/5% Cement	Same materials as Ref; only proportions changed
M2	Non-stabilized	70% Soil/30% Sand	Standard mix; reference for comparison
M3	Non-stabilized	64% Soil/36% Sand	Variation to observe effect of increased soil content
M4	Non-stabilized	60% Soil/40% Sand	Further variation with soil content

summarizes the proportions of soil, sand, and cement used in all mortar formulations [27,32].

To investigate the influence of soil content in mortar on the shear and flexural performance of brick assemblies, the experimental program was structured into two distinct groups of specimens. Group 1 consisted of traditional assemblies designed to reproduce common earthen construction practices. In this group, doublets of fired clay bricks (BE) and raw mud bricks (RE) were bonded with a simple mortar containing 70% soil and 30% sand (

Table 5. Composition and characteristics of mortars used in traditional brick masonry.

Series	Mortar Type	Soil (%)	Sand (%)	Cement (%)	Notes
Backed soil (BE)	Soil mortar	70	30	0	Fired clay bricks with soil mortar
Mud + Soil (RE)	Soil mortar	70	30	0	Raw mud bricks with soil mortar

). These specimens were not intended for systematic mechanical analysis but rather to serve as a qualitative reference, highlighting the performance of conventional earthen techniques without stabilization.

Group 2, in contrast, formed the central focus of this study. It comprised doublets constructed with stabilized and non-stabilized soil mortars, prepared using both fired clay bricks and cut compressed soil blocks (½ BTC). The mortar formulations were carefully varied to explore the role of soil-to-sand ratio and cement content: Ref. (50% soil, 45% sand, 5% cement), A1 (70% soil, 30% sand), A2 (75% soil, 25% sand), and A3 (67.5% soil, 25% sand, 7.5% cement), as summarized in

Table 6. Mix design and curing conditions for soil and stabilized mortar samples.

Series	Mortar Type	Soil (%)	Sand (%)	Cement (%)	No. of Samples
Ref.	Soil mortar	50	45	5	5
A1	Soil mortar	70	30	0	5
A2	Soil mortar	75	25	0	5
A3	Soil stabilized mortar	67.5	25	7.5	5

. This systematic design allowed us to isolate the effects of stabilization and mix proportions on bond behavior.

While the traditional BE and RE series of Group 1 provide a baseline for comparison, the results presented in the following sections focus primarily on Group 2 (Ref., A1, A2, A3), since these specimens were specifically developed to evaluate how controlled modifications in mortar composition influence the structural performance of clay-based masonry.

For each formulation (Ref, A1–A3), five samples were prepared. All samples were tested in compression, bending, and shear. For thermal and hygrothermal tests, three samples per formulation were tested. The experimental program, illustrated in the flowchart below (Figure 4), outlines the sequence of concrete preparation and testing, with all mechanical and thermal tests conducted.

2.2. Experimental Methods

2.2.1. Mechanical Properties

Compressive Strength Test

The compressive strength of the mortar formulations was evaluated using cylindrical specimens with dimensions of 7 cm in diameter and 14 cm in height. Prior to testing, all samples were air-dried under controlled laboratory conditions to ensure consistent moisture content. The test was conducted in accordance with the NF EN 1015-11 standard [39], which specifies the procedure for determining the compressive strength of hardened mortars.

A uniaxial compressive load was applied vertically to each specimen at a constant rate of 1 kN/s using a hydraulic press (Figure 5). The loading continued until visible cracking and structural failure occurred (Figure 6). Immediately after the test, the residual water content of each specimen was measured and recorded to support further analysis of the relationship between moisture content and mechanical performance.

Bending Test

In parallel, a standardized flexural test was conducted under controlled laboratory conditions. The brick doublets were rigidly fixed, and a load was applied through a lever system connected to the clamp. A thin resilient layer was inserted between the clamp and the brick surfaces to ensure uniform stress distribution and avoid localized crushing. Rotations along the other axes were blocked to preserve test integrity.

The flexural behavior of the brick–mortar assemblies was evaluated using a laboratory-controlled test protocol derived from the NF EN 1052 standards. The procedure was designed in reference to field-oriented adaptations reported in NF EN 1052-2 [41] and NF EN 1052-5 [42], but was conducted exclusively under controlled laboratory conditions.

In this setup, brick doublets were rigidly clamped, and a bending moment was applied through a lever system connected to the clamp until bond failure occurred at the mortar joint. A thin resilient layer was placed between the clamp and the brick surfaces to ensure uniform stress transfer and to avoid local crushing. Rotations along the other axes were restrained to maintain test integrity.

At failure, three parameters were recorded:

• The weight of the upper element with mortar (W);
• The applied load through the lever system (F₁);
• The clamp weight (F₂).

The flexural strength was calculated using the following equation:

$${\mathrm{F}}_{\mathrm{w}\mathrm{i}}={\displaystyle \raisebox{1ex}{$\left[{\mathrm{F}}_1\times {\mathrm{e}}_1+{\mathrm{F}}_2\times {\mathrm{e}}_2-\left(\left(\raisebox{1ex}{$2$}\!\left/ \!\raisebox{-1ex}{$3$}\right.\right)\mathrm{d}\left({\mathrm{F}}_1+{\mathrm{F}}_2+\raisebox{1ex}{$\mathrm{W}$}\!\left/ \!\raisebox{-1ex}{$4$}\right.\right)\right)\right]$}\!\left/ \!\raisebox{-1ex}{$\mathrm{Z}$}\right.}$$

where

• $Z={\displaystyle \frac{{b·d}^2}{6}}$
• b is the average joint width (mm);
• d is the average thickness of the tested sample (mm);
• F₁ is the maximum applied force (N);
• F₂ is the clamp weight (N);
• W is the weight of the upper element with mortar (N);
• e₁ is the distance between the applied load and the tensioned face of the test body (mm);
• e₂ is the distance between the clamp’s center and the tested sample face (mm).

A schematic representation of the measurement parameters is provided in Figure 7.

Finally, the shear stress values derived from this flexural test were compared to those obtained from direct shear tests following NF EN 1052-3 [43].

Shear Test

The same doublets were used in the bending test. In this test, the bucket was pulled directly over the doublets (the doublet is clamped using a clamp) following standard NF EN 1052-3 [43]. After that, the bricks were put in the bucket to find out the maximum weight that these bricks could carry (Figure 8). The bucket is used to apply the shear test to soil doublets.

It is essential to test the strength of the brick–mortar interface to identify the mortar offering the best bond (highest shear strength). The bucket shear test can be considered the first approach to shear the brick/mortar interface with a load-bearing strap, using the test developed by the LMDC laboratory [13,31].

The principle of this test is to determine the mass exerted on a masonry doublet, leading to its failure. One brick is fixed to the table by a clamp, while the other is connected to the first brick by the mortar bed and supports the strap carrying the seal. The strap is used to locate the force at the interface. The seal is filled with bricks.

The force applied by the strap to the equipment induces a combination of shear and tensile stresses because of the slight eccentricity between the strap and the brick/mortar interface at the table’s edge. It produced a lever arm effect, which was neglected in the first approach, as the probability of obtaining a shear failure is much greater [39,40]. A diagram showing the length and height (width) of the earthen doublets (earthen brick and earthen mortar) is presented in Figure 9.

The shear stress (τ) is calculated by dividing the force applied by the strap (F) over the sheared surface ($S_1$), which was obtained by determining the resultant of (b · h) using the following formula:

$$\tau ={\displaystyle \frac{F}{S_1}}$$

Three types of masonry doublets were produced for the shear test: Baked bricks and 70% E/30% S mortar, fired brick and sand-cement mortar, and BTC and 70% E/30% S mortar (in this study, the term “E” represents the soil clay, while “S” represents the sand).

2.2.2. Water Content

To assess the influence of moisture on the mechanical behavior of hardened mortars, the water content was measured at two key stages: during installation and at the time of breakage. These measurements were conducted on the same mortar formulations used in the compressive strength tests (Ref; M1; M2; M3; M4), ensuring consistency between mechanical and hygric evaluations.

The objective of this analysis was to better understand how water content during laying, as well as the moistening of the Compressed Earth Blocks (BTC) during installation, can affect the final performance of the mortar. For each formulation, two samples were collected—one during mortar placement and another after interaction with the BTC substrate. Each sample was weighed before and after oven-drying at 105 °C. The drying process continued until the weight stabilized, indicating the complete evaporation of free water. The initial water content was then calculated based on the difference between the wet and dry weights.

This procedure enabled a direct comparison of moisture levels among different formulations and their influence on compressive strength, particularly in relation to compressive strength.

2.2.3. Thermal Properties

Thermal Conductivity

The appropriate methodology for determining thermal conductivity depends on the nature, shape, and size of the samples studied. In this study, thermal conductivity measurements are performed using the Netzsch HFM 446 Lambda device, was sourced from NETZSCH-Gerätebau GmbH, Selb, Germany in accordance with the ISO 8301 standard [44]. This method involves subjecting the material, with a defined surface area (A) and thickness (e), to a temperature gradient and measuring the heat flux through it once thermal equilibrium is reached [45] (Figure 10).

The incoming and outgoing heat fluxes are measured using two heat flux sensors embedded in the hot and cold plates, which are in direct contact with the upper and lower surfaces of the sample, respectively. The positioning of the sensors within the plates follows the standard procedure (NF EN 12667:2001) [46] (p. 12) and does not affect the results, provided they are properly calibrated and in full contact with the sample. “Once equilibrium is reached and the heat flux stabilizes, the thermal conductivity of the material is calculated using Fourier’s law [47] with the equation below. The device has an accuracy of approximately 3%.

$$\lambda ={\displaystyle \frac{\mathrm{Q} \mathrm{e}}{\mathrm{A} \Delta \mathrm{T}}}$$

where

• λ: Thermal conductivity of the sample
• Q: Heat flux
• ΔT: Temperature difference across the sample (°C)
• e: Thickness of the sample (m)
• A: Surface area of the sample (m²)

Measurements were conducted on prismatic samples of 22 × 22 × 4 cm³ at 20 °C (Figure 10). These temperatures were selected based on literature references. Prior to testing, the samples were conditioned to the laboratory temperature and humidity (23 °C and 50% RH).

Specific Heat Capacity

The specific heat capacity is measured using the Differential Scanning Calorimetry (DSC) technique, in accordance with the NF EN ISO 11357-4 [48] standard. This technique is based on the difference in heat flux exchanged between a reference and a material sample under identical conditions during a temperature scan. The measurements were performed under a hydrogen (H₂) purge gas at a flow rate of 50 mL/min to enhance baseline stability and measurement sensitivity because of hydrogen’s high thermal conductivity.

The samples, with dimensions of 20 × 20 × 4 cm³, are subjected to a temperature range of 15 °C to 25 °C, with a heating rate of 1 °C/min. The power difference is related to the specific heat capacity at constant pressure (Cp) through the equation below:

$$Q=M\times C\times \Delta T$$

where

• Q: Amount of heat (kJ)
• M: Mass of the sample (kg)
• C: Specific heat capacity (kJ/(kg·°C))
• ΔT: Temperature difference across the sample (°C)

3. Results and Discussion

3.1. Mechanical Results

3.1.1. Compressive Strength

The compressive strength of each mortar Rc is calculated by dividing the measured breaking force F (N) by the sample surface (mm²). Moreover, the mean resistance value Rc, the standard deviation σ, and the coefficient of variation) were calculated for the different sample series. A comparison of the compressive strength of all formulations is shown in Figure 11.

The compressive strength results presented in Figure 11 show a clear trend influenced by the composition of the samples. The reference sample (Ref), composed of 50% soil, 45% sand, and 5% cement, exhibited the highest compressive strength (~3.3 MPa), closely followed by sample M1 (~3.2 MPa), which contained 67% soil, 28% sand, and the same amount of cement. This indicates that for stabilized mortars, the soil/sand ratio has limited influence on strength, as cement plays a dominant role in enhancing mechanical performance. In contrast, samples with increasing soil content and no cement addition showed a progressive reduction in compressive strength, with values decreasing from 2.7 MPa (M2: 70% soil) to 2.5 MPa (M3: 64% soil) and 2.4 MPa (M4: 60% soil). The low dispersion of results for the tested cylinders (mean = 2.97 MPa, standard deviation = 0.08 MPa, coefficient of variation = 3%) reflects the reliability of the testing protocol, while other batches showed variations ranging from 4% to 8%, still within acceptable limits. These findings are consistent with the NF EN 1015-11 standard [40], confirming the expected order of magnitude for compressive strength in soil-based mortars. Overall, the addition of 5% cement significantly improves strength performance, while excessive soil content tends to weaken the material.

The compressive strength results reveal a relationship between the chemical composition and performance. Samples with higher calcium oxide (CaO) levels, such as stabilized mortars containing cement, exhibited greater compressive strengths because of the formation of additional binding phases. Conversely, samples with higher proportions of SiO₂ showed improved dimensional stability but slightly lower compressive strengths, as the sand component reduces the cohesive forces.

3.1.2. Bending Stress at the Brick Mortar

Figure 12 illustrates the bending strength at the block–mortar interface for four different mortar formulations: Ref, A1, A2, and A3. The reference mortar (Ref) recorded the lowest bending strength, approximately 0.23 MPa.

In contrast, all modified mortars (A1, A2, and A3) demonstrated improved performance:

• A1 achieved the highest strength, reaching around 0.28 MPa, which indicates enhanced bonding at the interface. This improvement can be attributed to the presence of additives such as fibers or pozzolanic materials that improve the mortar’s crack-bridging ability and ductility [30].
• A2 and A3 also showed increased bending strength, with values around 0.26 MPa and 0.24 MPa, respectively. The slightly lower result for A3 compared to A1 and A2 could be related to higher porosity or weaker bonding between the mortar and the block surface [45,49].

In all cases, the RE values (rammed soil) are lower than the BE values (backed soil), suggesting that the bending strength is reduced under more demanding or specific testing conditions, regardless of the formulation used.

In conclusion, all the modified mortars enhanced the tensile and flexural resistance at the block–mortar interface, which is essential for limiting crack formation and ensuring mechanical compatibility in masonry repair or strengthening applications [49]. These findings highlight the potential of well-designed mortar formulations to improve the mechanical performance of masonry structures under bending loads.

3.1.3. Shear Stress at the Brick–Mortar Interface

The bar chart (Figure 13) presents the shear strength at the block–mortar interface for different mortar formulations, both backed earth (BE) and rammed earth (RE) to environmental conditions.

The reference mortar (Ref) initially exhibited a shear strength of about 0.60 MPa, which dropped significantly to approximately 0.28 MPa after exposure—a reduction of nearly 53%, indicating high sensitivity to environmental factors.

Among the modified mortars:

• A1 showed the highest initial shear strength, reaching approximately 0.73 MPa, which is a clear improvement over the reference. After exposure, its strength dropped to about 0.35 MPa, but it still outperformed the Ref and other modified mixes in both conditions. This suggests that A1 not only enhances initial bonding but also retains a relatively better performance after degradation.
• A2 and A3 followed the same general trend. A2 started at around 0.65 MPa and dropped to 0.30 MPa after exposure, while A3 went from 0.62 MPa to 0.29 MPa. Though both formulations improved the initial shear strength compared to Ref, they remained vulnerable to environmental deterioration.

Overall, these results highlight that all modified formulations (A1, A2, A3) are effective in improving initial bond strength, but none provided strong resistance to degradation after environmental exposure. The performance drop is consistent across all mixes and underlines the typical vulnerability of cementitious interfaces to moisture, thermal cycling, or chemical attack—as supported by durability studies in the literature [23].

In summary, formulation A1 appears to be the most promising option for applications requiring good early-age bonding and moderate durability. However, for long-term performance in harsh conditions, further improvements—such as protective coatings, additives, or formulation refinements—may be necessary to enhance the interface’s resistance to environmental stress.

The mechanical properties of the studied soil mortars and bricks are significantly influenced by their chemical composition. The primary constituents, as revealed by X-ray fluorescence (XRF) analysis (Table 2), include silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), and calcium oxide (CaO), among others. Silicon dioxide (SiO₂), which constitutes over 60% of the material, contributes to the rigidity and structural integrity of the bricks. The presence of aluminum oxide (Al₂O₃) enhances the plasticity of the mix, enabling better moldability, while calcium oxide (CaO) plays a crucial role in stabilizing the structure by forming calcium silicate hydrates (C-S-H) under certain conditions.

3.2. Water Content of the Different Mixtures

Figure 14 illustrates the water content of different mortar mixtures, including the reference mix (Ref) and four modified mixes (M1–M4). The reference formulation shows a water content of approximately 12%, which serves as a baseline for comparison. Mixtures M1 and M2 display similar water contents, slightly below and above the reference, respectively, indicating minor variations in water absorption or retention behavior. However, a noticeable increase is observed in M3, which reaches approximately 16%, the highest among all samples. M4 also demonstrates a relatively high-water content at around 14.5%. These elevated values in M3 and M4 may be attributed to the incorporation of highly porous additives, increased binder content, or fine particles that enhance the capillary water uptake of the mortar matrix [26,31,32]. High water content can be indicative of improved workability but may also signal increased porosity, which could negatively affect mechanical strength and long-term durability [24]. The results suggest that while M3 and M4 might offer better fresh-state properties or hydration dynamics, they may require additional considerations regarding waterproofing or durability treatments in practice. This trend underscores the importance of optimizing mix designs not only for mechanical performance but also for moisture-related behavior, especially in historical or exposed masonry applications.

3.3. Thermal Results

3.3.1. Thermal Conductivity

Figure 15 illustrates the results of thermal conductivity for the various examined mixture samples.

This bar chart illustrates the thermal conductivity [W/(m·K)] of five material samples, labeled as Ref, M1, M2, M3 and M4, with error bars representing the variability or uncertainty in the measurements. The thermal conductivity ranges between approximately 1.0 and 1.4 [W/(m·K)], with Ref exhibiting the highest value, close to 1.4 [W/(m·K)], and M2 showing the lowest value, just over 1.0 [W/(m·K)]. Samples M3 and M4 display comparable thermal conductivities, both slightly exceeding 1.2 [W/(m·K)], while M1 has a similar performance to Ref, indicating relatively high thermal conductivity.

The thermal performance of the soil-based materials is impacted by their chemical composition. Silicon dioxide (SiO₂) significantly affects the thermal conductivity of the materials, as its crystalline nature facilitates heat transfer. On the other hand, aluminum oxide (Al₂O₃) and magnesium oxide (MgO) improve the specific heat capacity, enabling the material to store more thermal energy. This feature is particularly advantageous for passive cooling and heating in buildings.

The thermal conductivity measurements (Figure 15) demonstrate that formulations with higher sand content, and thus higher SiO₂ levels, exhibit increased thermal conductivity. In contrast, the inclusion of calcium oxide (CaO) and ferric oxide (Fe₂O₃) in smaller amounts contributes to the reduction in thermal diffusivity, providing better insulation properties.

3.3.2. Specific Heat Capacity

Figure 16 presents the specific heat capacity [J/(kg·K)] of five material samples (Ref, M1, M2, M3, M4) with error bars showing measurement variability. Values range from approximately 1000 to 1200 [J/(kg·K)], with M4 and ref reaching the highest at 1150 [J/(kg·K)], followed by M3 1500 [J/(kg·K)]. M1 and M2 show slightly lower values, around 1050–1120 [J/(kg·K)].

The interaction between the chemical composition and hygrothermal properties is further exemplified by the materials’ water retention capabilities. Clay minerals containing aluminum silicates contribute to the regulation of indoor humidity through their capacity for moisture adsorption. This hygroscopic behavior allows the material to buffer fluctuations in relative humidity, improving indoor comfort. Moreover, the presence of magnesium oxide (MgO) and potassium oxide (K₂O) facilitates the formation of hydrophobic compounds. These reaction products reduce capillary water absorption, thereby enhancing the material’s resistance to moisture-related degradation and improving its durability in humid environments.

The analysis of thermal property measurements shows that the error bars remain consistent across all samples, with no statistically significant deviations detected. This uniformity in variability indicates a high degree of data reliability. Within this context, the M3, M4, and reference formulations exhibited notably higher specific heat capacities, suggesting superior performance in applications where increased thermal energy storage and heat retention are desirable.

3.4. Measured Hygrothermal Properties of Traditional Construction Materials

Table 7. Mechanical and thermal properties of soil-based construction materials.

Construction Technique	Authors	Density [kg/m3]	Compressive Strength [MPa]	Specific Heat Capacity [J/(kg·K)]	Thermal Conductivity [W/(m·K)]
Adobe/Cob	Alassaad et al. (2022) [6]	1750–17,800	2.5–3	817.6–877.6	0.6–0.8
	Cagnon et al. (2014) [49]	1940–2070	3–7	950–1030	0.47–0.59
	Affan et al. (2023) [17]	1800–1820	3.1	800	0.52–0.41
Raw soil/Raw soil block	Porter et al. (2018) [50]	2064–2138	–	1321–1832	–
	Velasco-aquino et al. (2020) [51]	1340–2080	–	–	0.19–1.35
	Indekeu et al.(2017) [52]	1730	–	648	0.6–2.4
	El Mendili et al. (2025) [53]	1450–1600	3.6–6.8	1034–1389	0.48–0.69
	Present study	1500–1700	2.5–3.5	1.1–1.4	1050–1150

summarizes the measured thermal and mechanical properties of our developed formulations in comparison with values reported in the literature. The results indicate that the density of our samples (1500–1700 kg/m³) and their compressive strength ~2.5–3.5 MPa are within the ranges reported for adobe, cob, and compressed earth blocks. Thermal conductivity values ~1.1–1.4 [W/(m·K)]) are slightly higher than those found for traditional earthen materials, which can be attributed to the presence of stabilizers and the compacted mix design. The specific heat capacity ~1050–1150 [J/(kg·K)] confirms a good ability to store and release heat, consistent with the thermal mass behavior documented for soil-based construction. Overall, these results confirm that the developed formulations exhibit hygrothermal and mechanical performances comparable to traditional soil materials, while stabilization enhances mechanical strength without compromising their suitability for sustainable construction. summarizes the measured thermal and mechanical properties of our developed formulations in comparison with values reported in the literature. The results indicate that the density of our samples (1500–1700 kg/m³) and their compressive strength ~2.5–3.5 MPa are within the ranges reported for adobe, cob, and compressed earth blocks. Thermal conductivity values ~1.1–1.4 [W/(m·K)]) are slightly higher than those found for traditional earthen materials, which can be attributed to the presence of stabilizers and the compacted mix design. The specific heat capacity ~1050–1150 [J/(kg·K)] confirms a good ability to store and release heat, consistent with the thermal mass behavior documented for soil-based construction. Overall, these results confirm that the developed formulations exhibit hygrothermal and mechanical performances comparable to traditional soil materials, while stabilization enhances mechanical strength without compromising their suitability for sustainable construction.

4. Conclusions and Outlook

This study demonstrates that the composition and formulation of soil mortars play a critical role in determining the mechanical performance of rammed soil masonry. While soil mortars provide strong environmental benefits and satisfactory compressive strength, tensile and shear performance—particularly at the brick–mortar interface—remains a potential vulnerability. The experimental results show that incorporating a small proportion of cement (5%) significantly enhances compressive strength, whereas variations in the soil-to-sand ratio have a more moderate effect in stabilized mixes. Moreover, water content during mortar preparation strongly influences mechanical performance, highlighting the importance of careful control of mixing and processing parameters.

The study confirms that properly designed soil mortars can achieve a balance between mechanical efficiency and eco-efficiency, consistent with the low-carbon characteristics widely reported in the literature. The data collected also provides a baseline for further optimization of mortar formulations to meet specific structural and environmental performance requirements.

Future research will focus on expanding the experimental dataset to include larger sample sizes, enabling statistical determination of characteristic strengths. Additional investigations will examine workability (e.g., slump tests) in relation to water/clay ratios, flexural and shear behavior under realistic loading, and long-term durability under variable climatic conditions. Complementary studies will address thermal and acoustic performance, water resistance, and life-cycle assessment (LCA) to assess the overall sustainability and operational behavior of earthen constructions. Such research will facilitate the integration of optimized soil mortars into green building certification frameworks and support practical recommendations for sustainable construction.

Overall, the findings of this work contribute both to fundamental understanding and to practical applications of soil-based materials, highlighting their potential for low-carbon, durable, and resilient masonry solutions.