2.1.1. Soils

1)Location of the Study Sites

The soils examined in this study are from the city of Moundou, located in southwestern Chad, approximately 450 km from the capital, Ndjamena. The city is situated between three bodies of water: Lake Taba to the north, the Logone River to the southeast, and Lake Wey to the west. The city is situated in a flat basin on the banks of the Logone River at 8.550˚ north latitude and 16.090˚ east longitude. Temperatures in the city range from 10˚C to 31˚C, with an annual average of 27˚C. The dry season lasts approximately 5 months (from November to the end of March), but its boundaries are highly variable. The city has a Sudanian-type semi-humid tropical climate with an average annual rainfall of 1127 mm. The soil types in Moundou consist primarily of three types characteristic of the Sudanese zone [24]:

- hydromorphic soils covered with recent and ancient alluvial deposits in the floodplains. These are beige, sandy-clay and silty soils that are frequently flooded during the rainy season.

- leached ferralitic soils on the non-flooded areas, which are gray-brown in color, very sandy on the surface, and sandy loam deeper down.

- red soils, which are sandy deeper down and covered with a lateritic crust, particularly on the upper parts of plateaus.

2)Sampling

For this study, three soil varieties were selected to examine their geotechnical profiles and determine their suitability for inclusion in mixtures used to manufacture earthen bricks. The clay was sourced from the teacher training college’s quarry, located at the following geographic coordinates: 8˚33'11.74"N, 16˚04'41.95"E. The laterite was extracted 80 cm below the natural ground level from the Koutou quarry, located at the northern entrance to Moundou at 8˚35'59.68"N, 16˚04’0.97"E. The sand comes from the Lac Wey quarry, located west of the city of Moundou. It was extracted from the riverbed at a depth of approximately 3 meters below the natural ground level. Its coordinates are 8˚33'14.9"N, 16˚03'10.59"E. The soil in all of these quarries has been mined for decades by local communities, primarily for the production of adobe bricks and roofing and wall coverings.

3)Geotechnical Characterization of Soils Used

Table 1 illustrates the different particle size classes of the three types of soil used in brick manufacturing.

Table 1 presents the particle size classes for three types of soil. Particles larger than 2 mm are negligible, with percentages passing through a 0.08 mm sieve of 77.08%, 45.7%, and 49.7%. All soils contain more than 35% fine particles (<80 μm)

Table 1. Different particle size classes of the three types of soil used.

and are therefore classified as fine soils according to the [25]. The soil at the teacher training college’s, classified as clayey or silty (77.08% passing through the 0.08 mm sieve), is dominated by fine particles. The lateritic soil at Koutou, with 45.7% fines, is rich in sand, classifying it as sandy with fines. The soil at Lac Wey, with 49.7% fines, lies on the boundary between coarse and fine soils and is classified as coarse. Using the results of Atterberg limit tests—specifically the plasticity index and the liquid limit—the Casagrande diagram ([26] and the USCS soil classification system [27]), these soils can be classified as follows: clay soil at teacher training college’s (MH), lateritic clayey sand from Koutou (SC), and silty-clayey sand from Lac Wey (SM-SC).

Knowing the nature of these three soil types, we determined the optimum moisture content using the modified Proctor test. The optimum moisture content (WOPM) is 18%.

4)Classification and Mineralogical Identification of Soils in Use

Table 2 presents the methylene blue values (MBV), the blue activity index (BAI), the specific surface area (SS), and the cation exchange capacity (CEC) of our soils.

Table 2. Classification and mineralogical identification of soils in use.

Table 2 shows that the analyzed soils, characterized by medium to low plasticity, have a blue activity index (BAI) below 3, indicating their inactivity. They consist mainly of kaolinite, with less illite, and exhibit a significant cation exchange capacity (CEC) typical of kaolinite/laterite soils. The results indicate that these soils do not possess the characteristics of highly expansive soils such as smectite or montmorillonite, but rather those of tropical lateritic soils dominated by weakly active kaolinite, revealing a clear consistency in the data.

2.1.2. Cement

In our study, we used CPJ-CEM II 32.5 composite Portland cement, consisting of 65% clinker and 35% additives (limestone, slag, or fly ash). It is manufactured locally by the Société Nationale de Ciment (SONACIM).

2.1.3. Formulations and Manufacturing of Stabilized Earth Bricks

1)Formulations

The composition of the mixtures used in this study is presented in Table 3.

Based on these material proportions, we used three (3) formulations to manufacture cement-stabilized, compressed earth bricks. They are summarized in Table 4.

Table 3. Composition of the various material mixtures.

Table 4. Material formulations for the manufacture of stabilized bricks.

2)Manufacturing

As part of our study, we initially prepared masonry blocks measuring 30 × 15 × 13 cm³ to analyze the mechanical properties of the bricks. Subsequently, in accordance with the experimental conditions for the thermal tests, we were required to modify the size of our brick samples. We therefore manufactured bricks measuring 8 × 5 × 4 cm³. However, all test specimens (30 × 15 × 13 cm³ and 8 × 5 × 4 cm³) were manufactured using the same methods and formulations, with the exception of their molds, which were different. They were produced using the conventional method (mixing, compaction, and demolding). Initially, the natural clay was sun-dried, manually crushed, and then sieved before being combined with the binders (cements) in accordance with the standard [28] (Figure 1(a)). The materials were first mixed dry, according to the various formulations listed in Table 4. Subsequently, a quantity of water corresponding to the optimal water content was instantly added to the mixture, followed by mixing (Figure 1(b)). The required amount of mixture was weighed and placed into the mold (Figure 1(c)). To increase the density of the material and bring the solid grains closer together, the static compaction technique is used. For this study, we used a lightweight manual press with a pressing force of 5 to 10 metric tons and a production rate of 300 to 600 blocks per day (Figure 1(d)).

Figure 1. Test specimen preparation: (a) sieving the ground material; (b) mixing the material; (c) filling the mold with the material; (d) manual compaction press.

The maximum curing period is set at 28 days. However, for the mechanical tests, the stabilized earth bricks were tested after curing periods of 3, 7, and 28 days. In total, 18 brick samples were used for the mechanical tests (6 for the 4% cement formulation, 6 for the 6% cement formulation, and 6 for the 8% cement formulation). For the bricks used in the thermophysical measurements, the maximum curing period was set at 28 days. The samples were then dried for 24 hours in an oven to completely remove all traces of moisture. Subsequently, green sandpaper was used to sand these elements in order to obtain flat surfaces that ensure optimal contact with the heating element (Figure 2). Twelve (12) brick samples were used for the thermal tests (4 for the 4% cement formulation, 4 for the 6% cement formulation, and 4 for the 8% cement formulation).

Figure 2. Bricks that have been dried and sanded with green sandpaper.

3)Curing of theBlocks

Curing involves keeping the material in a sufficiently humid environment for a sufficient period of time to maximize chemical reactions. The use of hydraulic binders such as cement requires a curing period to allow hydration reactions to occur under optimal conditions, thereby improving the binder’s effectiveness. In our case, after demolding, the manufactured blocks are kept in a humid atmosphere at room temperature in the laboratory and are covered with a curing bag made of plastic film to maintain humidity (Figure 3(a)). After a curing period, the test specimens are removed and stored in a temperature-controlled room at 20˚C and 50% relative humidity for drying. The maximum curing time for our test specimens is 28 days. The test specimens are then marked and subjected to various tests (Figure 3(b)).

Figure 3. Preparation of test specimens: (a) curing using plastic wrap; (b) test specimens marked for testing.

2.2.1. Characterization of Thermophysical Properties

1)Physical Properties

a)Bulk Density

The hydrostatic weighing method, in accordance with the standard [29], was used to determine the density of the stabilized and compressed bricks. The principle involves weighing a sample that has been dried at 105˚C in an oven until a stable mass is achieved, then coating its entire outer surface with paraffin, and finally weighing it in open air. The paraffin-coated bricks are then submerged in water. Given that paraffin has a density of 0.88 g/cm³, the apparent density ( ρ a ) of the dry bricks is calculated using Archimedes’ principle and equation (1).

m s : mass of the sample; V : volume of the sample; m p : mass of the dry, paraffin-coated sample; m ′ p : mass of the paraffin-coated sample submerged in water; ρ w : density of water (1 g/cm³); ρ p : density of the embedding paraffin (0.88 g/cm³).

2)Measurement of Thermal Properties

In our study, we used the hot-wire/hot-plate method under transient conditions. The hot-wire method measures thermal conductivity, while the hot-plate method determines effusivity. The goal is to generate a slight local temperature increase in the material, which is assumed to be in thermal equilibrium, and to measure this temperature increase over a very short period of time (Figure 4).

Figure 4. Illustration of the hot-wire/hot-plate technique.

The measurement system used in this study was developed by the French Scientific and Technical Center for Building (CSTB) in accordance with the standards [30] and the RILEM AAC 11 - 13 recommendation. The equipment consists of thermal shock probes (a hot-wire probe to evaluate thermal conductivity and a hot-plate probe to measure effusivity) to be positioned between the two samples of the material to be characterized (Figure 5(a) and Figure 5(b)), an electronic data acquisition system, and graphical user interface software to control the experiments and analyze the results (Figure 5(c)). The shock probe is based on the principle of locally generating a slight increase in the material’s temperature (a few degrees above ambient temperature) and measuring this rise in temperature over time (over a period of a few minutes). The probe allows for measurements ranging from 20˚C to 100˚C. The temperature difference between the two sides of the sample is established using thermocouples of specific dimensions. The heat flux is measured by thermopiles in accordance with the standards [30]. Thermal conductivity is determined through mathematical signal processing integrated into the provided software. This technique offers numerous advantages, particularly its ease of implementation and the speed of the measurements. Indeed, since it is a transient method, the measurement time is consequently very short [31].

Figure 5. Experimental setup for the hot-wire/hot-plate method used to measure thermal properties: (a) hot-wire probe for thermal conductivity; (b) hot-plate probe for effusivity; (c) electronic data acquisition system.

Furthermore, it can be used not only for solids but also for liquids and other materials [32].

Thermal Conductivity

Thermal conductivity refers to a material’s ability to transfer heat by conduction when subjected to a temperature difference [33]. The method involves generating a slight local increase in temperature within the material (which is then considered to be at thermal equilibrium) and observing the rise in temperature over a very short period of time [34].

The following equation establishes a relationship between thermal conductivity and the change in temperature over time:

where: λ: Thermal conductivity in W·m⁻¹·K⁻¹; q: the applied heat flux in W/m; ΔT: the temperature difference in K; t: the test duration in s and c: constant.

ThermalEffusivity

Thermal effusivity represents the rate at which a material’s surface temperature changes (the sensation of cold or heat when touched). It describes how quickly a material absorbs heat. A lower effusivity coefficient results in faster heating of the material’s surface [33].

The equation below relates effusivity to temperature and time:

where Eff: thermal effusivity in w/m²∙K∙S^1/2; T: temperature in K; q: injected flux density in W∙m⁻²; t: time in s; R t : total resistance, i.e., the resistance of the probe and the contact resistance in K∙m²∙W⁻¹ and Cs is an intrinsic characteristic of the probe in w∙K⁻¹∙m⁻².

ThermalDiffusivity

Thermal diffusivity characterizes the rate at which heat propagates through the mass of a material. The lower this value, the longer it takes for the heat front to travel through the thickness of the material [33].

It is expressed by the following equation:

where λ is thermal conductivity in W·m⁻¹·K⁻¹; ρ is density in kg/m³; C p is specific heat in J/kg·K.

SpecificHeat

Specific heat measures the amount of energy required to raise the temperature of 1 kg of a material by 1 K.

It is determined by the formula:

where C p is the specific heat (J/kg·K); Q is the amount of heat absorbed; m is the mass of the sample; and ΔT is the change in temperature.

Thermal Resistance

The thermal resistance ( R t h ) of a wall depends on the thermal conductivity (λ) of the materials used and the thickness (e) of the layers of each material.

It is calculated as follows:

with e = 20 cm, the standard wall thickness. In fact, the thermal conductivity λ was measured on test specimens measuring 8 × 5 × 4 cm. Therefore, the 4-cm-thick test specimen is used solely to measure λ, which is an intrinsic property of the material. Thermal resistance R was then calculated for a standard wall thickness of 20 cm, in accordance with standard practice in civil engineering, to reflect the actual performance of the bricks in use, since this value allows for a comparison of the insulating performance of the different formulations under real construction conditions.

2.2.2. Dry Compressive Strength Test

After drying, a batch of test specimens is subjected to a simple compression test to determine their compressive strength in the dry state. The compressive strength of cement-stabilized blocks is measured either on a masonry unit (block and joint), on a half-block, or on a whole block [20]. In this study, compressive strength was measured according to the method developed by [35] on a whole block. The procedure involves placing a block on a compression press and loading it until failure in accordance with the standard [36] (Figure 6). The maximum load reached is recorded, and the compressive strength is determined using Equation 7.

R c : Compressive strength (MPa); F : compressive failure force (N); and A c : area over which the force is applied (mm²).

Figure 6. Compression test setup.

The thermophysical results for 12 samples of cement-stabilized earth bricks are presented in Table 5 and Table 6.

Table 5. Thermophysical parameters of cement-stabilized earth bricks.

The average thermal parameters are summarized in the following table:

Table 6. Average values of the thermophysical properties of cement-stabilized earth bricks.

Table 7 and Table 8 show the dry compressive strengths R c (in MPa) as a function of different mixtures (in % cement) and curing time (days).

Table 7. Values of the dry compressive strengths of cement-stabilized earth bricks.

Table 8. Average values of the dry compressive strengths of cement-stabilized earth bricks.

The bulk density increases with the cement content, rising from 1859 to 1863 kg/m³, representing a 5.9% increase when the cement content ranges from 4% to 8%. In fact, when cement comes into contact with water, it undergoes a hydration reaction that produces calcium silicate hydrates (C-S-H) and portlandite. These hydration products have two effects:

- They fill the interstitial voids between the clay, sand, and laterite particles.

- They coat the particles, creating a denser matrix.

It is this pore-filling mechanism that explains the increase in density: Indeed, bulk density and porosity are inversely related:

bulk ρ = solid ρ × (1 − porosity)

When cement hydrates and fills the pores, porosity decreases, which mechanically leads to an increase in bulk density.

The bulk density results obtained in this study (1759 - 1863 kg/m³) are largely consistent with those reported in the literature for stabilized soil blocks, which generally range between 1700 and 2100 kg/m³ [6][7][10].

Thermal conductivity (λ) increases significantly with the cement content, rising from 0.601 to 0.690 W/m·K, which corresponds to a 14.8% increase between 4% and 8% cement.

This result is fully consistent with the scientific literature. Several studies demonstrate that increasing the percentage of cement in stabilized earth bricks leads to an increase in thermal conductivity [11]-[15]. This trend can be explained by two main mechanisms:

1) Densification of the matrix: cement hydration products (hydrated calcium silicates, C-S-H) fill the interstitial voids, reducing porosity and thus the air content (an excellent insulator, λ ≈ 0.026 W/m∙K) in favor of a more conductive solid phase [11]-[13].

2) Replacement of laterite: Laterite has lower thermal conductivity than cement hydrates [16], and its proportion decreases from 16% to 12% in formulations F4 through F8.

Thermal effusivity (Eff) increases with the cement content, rising from 1202 to 1328.5 w/m²·K·S¹/² (+10.5%). Effusivity characterizes a material’s ability to exchange heat with its environment under transient conditions. High effusivity indicates greater thermal inertia: the material absorbs more heat during the day and releases it at night.

Furthermore, research on cement-stabilized, compressed earth bricks reports a similar trend, with an increase in effusivity from 800 to 1500 w/m²·K·S^1/2 [17]; [18].

Thermal diffusivity (D), which characterizes the rate at which a heat front propagates, increases slightly from 2.53 to 2.71 × 10⁻⁷ m²/s (+7.1%). This increase is consistent with the rise in thermal conductivity, which outweighs the slight change in specific heat.

Furthermore, our results are consistent with the values for compressed and stabilized earth bricks, particularly those reported in the studies by [17] and [6], which range from 1.5 to 3 × 10⁻⁷ m²/s.

The specific heat remains virtually constant (1357.76 to 1367.54 J/kg·K) (+0.7%). This increase remains small due to the small variation in cement content (4% to 8%) and the high proportion of clay fraction (60%) in the formulations. A material with high specific heat capacity is equivalent to a material with good thermal inertia capable of improving thermal comfort in buildings. The following parameters influence the increase in specific heat capacity:

a- Densification

An increase in the cement content contributes to the densification of the matrix and to an increase in thermal storage capacity.

b- Mineralogy

Cement hydration products possess significant heat capacity, contributing to a slight increase in specific heat. However, this increase remains moderate due to the relatively low variation in cement content (4% to 8%) and the largely constant clay content (60%).

c- The clay fraction

Clay materials naturally possess high thermal inertia and good heat storage capacity, which limits the increase in specific heat.

Our results are consistent with the findings of [10][17], who obtained specific heat values ranging from 800 to 1,500 J/kg·K.

The thermal resistance ( R t h ) decreases from 0.33 to 0.28 m²∙K/W (−15.2%), due to the increase in thermal conductivity λ ( R t h = e / λ ). A lower thermal resistance indicates poorer insulating performance of the brick.

It should be noted that the thermal resistance R t h was calculated for a standard wall thickness of 20 cm, in order to reflect the actual performance of the bricks in a real-world application. This value allows for a comparison of the insulating performance of the different formulations under real construction conditions. Meanwhile, the thermal conductivity λ was measured on an 8 × 5 × 4 cm³ test specimen—that is, for a thickness of 4 cm—since λ is an intrinsic property of the material.

Furthermore, our results are consistent with those of [17] and [10], which show that cement stabilization of compressed earth bricks improves their mechanical properties but reduces their thermal insulation performance.

The results in Table 7 and Table 8 show a gradual increase in dry compressive strength as a function of cement content and curing time. This increase in strength between 3 and 28 days is primarily due to the ongoing cement hydration reactions, which lead to the formation of a cementitious matrix (C-S-H) responsible for the densification of the material. Two effects are observed in these results. First, the compressive strength R_c increases with the cement content (R_c 8% > R_c 6% > R_c 4%). Second, the compressive strength R_c increases with curing time (R_c 28 days > R_c 7 days > R_c 3 days).

Analysis of the trend (resistance gain)

Gain from 3 to 28 days (R_c4%→R_c8%)

4% cement:

1.78 → 3.51 MPa (+97%)

6% cement:

2.34 → 4.44 MPa (+89.74%)

8% cement:

3.03 → 6.08 MPa (+100%)

The material’s strength nearly doubles between 3 and 28 days

Overview of Selected Standards and Studies on the Classification of Stabilized Clay Bricks

In the field of stabilized clay bricks, several reference standards are used to classify these bricks based on whether they meet minimum construction requirements and whether they can be used for load-bearing walls (Table 9-11). The following tables summarize these classifications.

Table 9. Classification according to the CraTerre recommendation [37].

Table 10.Classification based on selected African studies ([37]-[40]).

Table 11.Classification according to the ASTM standard [37].

Changes in the Standards for our cement-stabilized earth bricks

a.Changes Based on Treatment Duration

At 3 days

The cement hydration is still incomplete; therefore, the strengths remain low.

At 7 days

Formulation B (4% cement) has already reached the minimum threshold for several stabilized earth bricks;

At 28 days

All formulations meet the requirements of the standards cited above.

b.Changes Based on Mechanical Strength

All formulations exceed the minimum compressive strength of 2 MPa generally recommended by several standards for stabilized clay bricks;The 6% and 8% formulations meet the requirements for bricks used in load-bearing walls;The 8% formulation provides very high compressive strength and can be used in sustainable construction.

Improving mechanical strength comes at the expense of thermal insulation.