The CCEB are based on mixtures with clay, laterite, sand and cement. We provide a brief overview of the intrinsic properties of the constituents used.

According to the Robert dictionary, clay is defined as “a clay rock, water-hungry, waterproof and plastic, known as clay”. In geotechnics, clay refers to any mineral of low particle size; the limit is set at 2 or 4 microns as the case may be. It is very sensitive to bad weather: it decreases in volume when it dries out and flows in the presence of moisture. Clay is indeed more or less plastic because it can form after superficial humidification or mixed with water a binder paste, easy to knead, to shape with precision, like a real modeling paste for children. This paste, when drying, contracts and splits. The important layers of clay are found in valleys, plains and streams. There are several families of clay soils but the most used are [7] [8] :

Black clays: they have low iron content; they are usually illites, montmorillonites and smectites;

White clays: they are composed of two layers namely a tetrahedral and another octahedral. Its crude formula ${\text{Si}}_2{\text{O}}_5{\left(\text{OH}\right)}_4$, less rich than illite, shows that it is composed only of aluminum, silicon, oxygen and hydroxide ion. It is a kaolinite type made of hydrated aluminum silicates and was discovered in China under “Kao Ling” or “high hills” near King Te Ching. It forms in hot and humid climates, especially tropical and subtropical, in drained (dry) environments by acidic Ph.

Clay is a word of Latin origin Argilla and also of Greek origin. Clayey rocks are present on almost the entire surface of the earth, but not all rocks of this type are identical. Indeed, depending on their formation conditions (mainly geological), their properties and composition vary and make each rock unique [9] .

The conditions that vary the properties and composition of these rocks are mainly: climatic or cosmic conditions (such as rain, heat, tide, etc.); the terrain and composition of the soil on which sediments and rocks that formed these clayey rocks are deposited, such as feldspar or basalt.

In general, clays are present a few centimeters deep in the earth. They can be found in almost all regions of Chad [10] . We used black montmorillonite clay, extracted from the quarry located at the teacher training college in Moundou, Chad ( Figure 1 ). According to the Google Maps application used on 12 May 2024 at 09:42, the geographical position indicated is 8˚33'11.74''N, 16˚04'41.95''E.

Laterites belong to the lateritic gravel family [11] [12] . They are hard, loose or indurated soils, rich in iron oxide or aluminium, resulting from the weathering of rocks in tropical climates. Laterisation is the process by which lateritic soils are formed. It is specific to hot, humid tropical regions and is influenced by a number of key factors: climate, topography, vegetation, parent rock and weather conditions.

Laterites are characterised by high levels of iron oxide (Fe₂O₃) and/or alumina (Al₂O₃) relative to the other components. In some laterites, the Fe₂O₃ content can exceed 80%, while the Al₂O₃ content is very low (a few per cent). In other cases, alumina content can be as high as 60%. Mineralogically, laterites consist of a mixture of goethite, hematite, kaolinite, quartz and residual minerals (ilmenites, rutiles, anatase, zircons, chromites, neoformed gibbsite) [13] .

The laterite used in this study comes from the Koutou quarry, located at the northern entrance to the town of Moundou in Chad ( Figure 1 ). According to Google Maps used on 12 May 2024 at 12 h 23 minutes, its position is: 8˚35'59.68''N, 16˚04'0.97''E. It is taken from 30 cm below the natural ground level.

In geotechnical terms, sand refers to granular materials made up of small particles with diameters ranging from 2 to 5 mm. The sand used in this study comes from the Lake Wey river, located to the west of the town of Moundou in Chad. It was taken from the riverbed about 3 m below the natural ground level. According to Google Maps, its position is 8˚33'14.9''N, 16˚03'10.59''E.

The hydraulic binder used is cement CPJ-CEM II/B 32.5, manufactured locally by the National Cement Company (SONACIM). It is composed of 80% to 85% clinker, 10% to 15% natural pozzolana and 5% gypsum [14] .

The various identification tests carried out at the laboratory of the National High School of Publics Works (NHSPW) in N’Djamena and the expressions of the results are presented below [15] [16] .

1) Density of Solid Particles: Standard NF P 94-054

The absolute density of solid particles is determined from Equation (1):

${\gamma }_s=\frac{{\gamma }_{w\left(m3-m1\right)}}{m2+m3-m1-m4}$ (1)

With the ${\gamma }_w$ density of water equal to 1 g/cm³; m1: the mass of the empty pycnometer; m2: mass of the pycnometer + distilled water; m3: mass of the pycnometer + soil sample and m4: mass of the pycnometer + soil sample + distilled water. The apparent density is calculated from Equation (2):

$\gamma =\frac{m}{v}$ (2)

2) Particle Size Analysis by Sieving: Standard NF P 94-056

The shape of the particle size curve indicates the degree of spread or uniformity of the particle size. This uniformity is expressed by the HAZEN $C_u$ coefficient defined by:

$C_u=\frac{d_{10}}{d_{60}}$ (3)

$d_{10}$: diameter corresponding to 10% cumulative sieving;

$d_{60}$: diameter corresponding to 60% cumulative sieving.

The curvature coefficient is defined by the following relationship:

$C_c=\frac{{\left(d_{30}\right)}^2}{\left(d_{10}\times d_{60}\right)}$ (4)

3) Atterberg Limits: Standard NF P 94-051

Atterberg limits consist of determining the moisture content of two consistencies of a coherent soil. Its purpose is to measure water contents and the identification of limits (liquid or plastic) to allow the classification of soils.

4) Sand equivalent: Standart NF EN 933-8

The test consists of pouring a sand sample and a small quantity of flocculant solution into a graduated cylinder and shaking so as to detach the clay coatings from the sand particles in the sample. After a prescribed settling period, the height of the flocculated clay is measured and the height of the sand in the cylinder is determined. The sand equivalent is the ratio of the sand height to the clay height multiplied by 100:

$ES=\frac{H_1}{H_2}\times 100$ (5)

where:

H₁: Height of flocculate.

H₂: Height of sediment.

There is no universally recognised standard for the manufacture of earth blocks, but techniques do exist to ensure their quality. According to standard NF XP P13-901, earth blocks must contain an average of 10% - 30% clay and at least 30% sand. The same standard allows cement to be added up to 10% of the total mass. Studies suggest that mixes with 30% sand and 70% clay are optimal for producing quality earth blocks. Larger quantities of clay can be used depending on the properties of the material and production techniques [17] [18] .

The optimum raw material proportions for this study are summarised below:

Using this formulation, a total of 24 sample bricks (8 bricks with 4% cement, 8 bricks with 6% cement and 8 bricks with 8% cement) with dimensions of 13 × 15 × 30 cm³ will be made for the experimental tests. The quantities of materials used are summarized in Table 1 .

The principle of water dosing is to determine the quantity of water needed to make a good wet mix for good compactness. To determine the quantity of water required, we will mix the earth fractions with a quantity close to the optimum content (Proctor tests on clay). We make four (4) bricks. The weight of each brick is known just after it is removed from the mould and its water content is noted. These two values (weight and water content) allow us to calculate the dry density (ρd) and the water content (ω). These two (2) values are used to draw a curve with the water content (ω) on the x-axis and the dry density (ρd) on the y-axis. The determination of the optimum moisture content to be considered is the apex of this curve, which will be projected vertically downwards on the abscissa axis.

CCEB have a variety of shapes and physical-mechanical properties depending on the constituents used, but their manufacturing stages are similar. They include extraction, preparation, mixing, pressing and curing ( Figures 2-6 ).

1) Extraction

The material must be extracted in such a way that the sample is representative of the quarry. Because of this principle, it is advisable to carry out tests to determine the characteristics of a soil, such as sight and touch tests to identify the granulometry of the soil, and smell tests to determine the presence of organic matter. For our study, the soil was extracted from various quarries in the town of Moundou in Chad (Ecole Normale, Lac Wey and Koutou).

2) Characterisation of materials

Montmorillonite clay: high plasticity, swelling, requires rigorous stabilisation.

Sand: reduces shrinkage, improves compactness.

Laterite: gives cohesion, improves compressive strength.

Cement: main stabiliser for mechanical strength.

3) Preparation

The aim of preparation is to combine the different materials used to make the blocks into a single homogeneous material. The preparation stage allows us to crush and sieve out elements larger than 5 mm. The operations involved in preparing the material determine the final quality of the blocks. The material prepared in this way must be kept free from any impurities that could cause it to become polluted, as pollution can cause the material to lose its physical and chemical characteristics. It is therefore important to dry the material before subjecting it to mixing. Preparation is an essential stage in obtaining quality products.

4) Mixing

The prepared material must be mixed homogeneously with the stabiliser (cement) and water. This mixture is initially dry (earth + cement), and the water is added to this dry mixture by spreading. The water is added gradually until a homogeneous mixture is obtained with the optimum water content of the blocks determined from the mixing of the earth fractions.

5) Pressing

This is a technique used to tighten the grains. It can be mechanical, hydraulic, frictional, impact or isostatic. Pressing densifies the material and the tightening of the solid grains gives the resulting products good compressive strength. For this study, we used a light manual press with a pressing force of 5 to 10 tonnes.

6) Curing

The curing of blocks over time plays a very important role in their resistance. The precautions taken during manufacture and storage can mitigate disorders in the behaviour of the products. It is essential to keep the blocks moist, as this allows the cement’s reagents to react to achieve maximum strength during the curing period.

In our case, the manufactured blocks are kept in a humid atmosphere at room temperature in the laboratory and are covered by a cure bag to maintain the humidity. The maximum curing time for our blocks is 28 days.

7) Quality control

Compressive strength is the most important mechanical property for the choice of construction materials [19] . The dry compressive strength test was developed from tests on cylindrical specimens. These consist of placing a block on a press fitted with a hydraulic compression ram and loading it until it breaks ( Figure 7 ). Compressive strength tests were carried out on blocks measuring 13 × 15 × 30 cm³. The maximum load reached was recorded and the compressive strength was calculated according to the following relationship:

$R_c=\frac{F}{A}$ (6)

$R_c$: Compressive strength (MPa); F: Compressive breaking load (N) and A: area of load application (mm²).

Given that buildings are frequently exposed to the risk of deterioration caused by rainwater, rising capillaries and even immersion in the event of flooding, it is essential to know how they behave when wet, in order to determine their minimum characteristics under the most unfavorable conditions. This test is identical to the dry compressive strength test, except that on this occasion, a sample of each dosage aged 28 days was soaked in water for 24 hours before being subjected to compression ( Figure 8 ) [20] .

This test is used to determine the behaviour of bricks with respect to humidity conditions. The principle of the test is to dry the blocks in an oven at a temperature of 105˚C. On leaving the oven and after cooling, they are weighed, i.e. $M_1$; after this operation they are immersed on the underside to a depth of 5 mm. Ten (10) minutes later, it is taken out of the water to be weighed, i.e. $M_2$, and the immersed surface is measured [21] . The absorption coefficient is calculated by the relationship:

$C_b=\frac{M_2-M_1}{S\times \sqrt{t}}\times 100$ (7)

C_b: absorption coefficient (g/cm²∙s^1/2); M₁ and M₂: masses (g); S: immersed surface area (cm²) and t: time in seconds.

The results of preliminary investigations on the untreated soils being studied are presented in Table 2 .

The particle size analysis showed that the three soils each had a percentage of 0.080 mm sieve passages greater than 35% ( Table 2 ). According to standard NF P 11 - 300, the soils used are fine soils. The uniformity and curvature coefficients obtained from the curves, presented in Table 2 , show that the sand has a spread and well-calibrated grain size compared to the laterite and clay. The latter are spread out and poorly graded.

The results of the Atterberg Limits obtained ( Table 2 ) show that the plasticity index (I_p) is equal to 31.41% and 16.45% for the clay and laterite, respectively. These values show that the clay and laterite used have a very plastic behaviour.

We note that for sand E_S = 90, which allows us to give the following assessment according to standard NF EN 933-8: very clean sand containing no fine clay.

The particle size distribution curves shown in Figure 9 are those for clay, laterite and sand obtained from particle size analysis tests, as well as those for the standard particle size distribution curves for materials used in the manufacture of compressed earth bricks (CEB) and adobes [22] [23] . According to the recommendations of standard NF XP P13-901 [24] , the particle size curves of the materials used to manufacture (CEB) must not deviate from these reference particle size distributions [25] . However, it is generally accepted that many soils that do not fall within the recommended range can in practice give acceptable results [26] . In the case of our study, the particle size distribution of the materials is well within the range (over 70%).

The incorporation of montmorillonite into stabilised earth brick compositions requires special care because of its high plasticity, its ability to retain water and its swelling characteristics. Nevertheless, it is expected to have good dry strength and more stable behaviour depending on the percentage of cement used. In addition, thanks to laterite and stabilisation, it is less sensitive to water, and together with sand, shrinks less during drying.

Mixing the materials with a quantity of water close to the optimum moisture content of 16.9 ( Table 1 ) enabled us to make a few bricks (around 4). The characteristics of each brick are summarized in Table 3 .

Based on this graph, the optimum water content for the clay-laterite-fine sand material is 18% ( Figure 10 ).

Table 4 and Figure 11 show the results of the dry compression tests. These results are based on at least 18 brick samples, including: 6 aged 3 days and dosed with 4%, 6% and 8% cement, 6 aged 7 days and dosed with 4%, 6% and 8% cement and 6 aged 28 days and dosed with 4%, 6% and 8% cement. They are determined by applying formula (6). These data show that compressive strength increases with curing age and cement content. Indeed Montmorillonite, being a swelling clay, can reduce the compressive strength of CCEB if present in large quantities. However, the appropriate use of 8% cement mitigated this impact and improved the compressive strength to 6.08 MPa. Comparable research indicated that CCEB stabilised with 5% cement and containing 20% sand had ideal physico-mechanical characteristics [27] . In addition, the incorporation of laterite (16%, 14%, 12%) and sand (20%) also improved the compactness and strength of CCEB. [28] have obtained similar results.

With regard to compressive strength in a wet environment, there was a decrease depending on the quantity of cement used, ranging from 19.64% to 22.52%. Despite this reduction, the strengths remain tolerable (above 4 MPa) ( Table 5 ). This reduction is attributed to the presence of montmorillonite, a type of clay that readily absorbs water and can lead to an increase in the moisture content of bricks, reducing their compressive strength. By incorporating cement, we are able to increase water resistance and stabilise compressive strength. Introducing sand into the clay contributes to granularity by reducing its plasticity and shrinkage during drying. Laterite, which contains a high concentration of iron and alumina, provides strength and resistance to erosion. It also helps to reduce water sensitivity. These results are similar to those of other researchers [29] - [31] .

According to standards NF XP P13-901, there are two categories of compressed earth bricks: those with compressive strengths greater than 4 MPa for dry $R_c$ and 2 MPa for wet $R_c$, intended for load-bearing walls, and those with compressive strengths less than 4 and 2 MPa, used for non-load-bearing walls. Based on both dry and wet compressive strengths, cement-stabilized and compressed earth bricks (CCEB) with at least 8% cement are suitable for load-bearing walls, while (CCEB) with 6% cement can be used for non-load-bearing walls.

The durability of bricks is strongly influenced by their water absorption capacity [32] . Bricks with a low absorption capacity are more durable and resistant to weathering [33] . The results of the absorption test presented in Table 6 show water absorption of between 4.21 and 3.71 g/cm²∙s^1/2. They are determined by applying formula (7). All these measurements remain below the threshold imposed by standard NF P 554, which is set at 20 g/cm²∙s^1/2 for blocks with low capillarity.

Normally, the presence of montmorillonite can increase water absorption capacity due to its porous structure. However, the addition of cement, laterite and sand has reduced this capacity by filling the pores, thereby reducing water absorption. Research carried out by [34] [35] led to the same results.