Abstract

This work focuses on the physical, chemical, and mechanical characterization of diatomite through a rigorous study. Lake Chad contains a significant quantity of diatomite, whose use has a very minimal impact on the environment. This study involves the use of three different diatomite samples (44, D, and M) collected from the Lake Chad province, specifically in Ngouri. With the aim of promoting eco-friendly materials, the three samples (44, D, and M) were analyzed. The physical properties (bulk density, porosity, void ratio, and Atterberg limits), chemical properties (oxide content), and mechanical properties (compressive and tensile strengths) were determined. The results show that the diatomite is very fine, with more than 80% passing through the 80 µm sieve, siliceous, with high porosity (around 39%) and low density. Diatomite exhibits favorable characteristics for the production of compressed earth bricks (CEB) and other sustainable materials. Therefore, these results confirm the potential of diatomite as an eco-material for ecological housing in the Sahelian climate. The characteristics are favorable for the manufacturing of compressed earth bricks (CEB) and other durable construction materials. These findings confirm the potential of diatomite as a local material for ecological housing in Sahelian regions.

1. Introduction

Earthen construction has been used for centuries around the world, particularly in Sub-Saharan Africa ¹. Due to its availability and easy accessibility for quick use, earth remains a material of choice ².

Lake Chad experienced four major expansion periods between 39,000 BC and 300 BC, leaving thick deposits of diatomaceous earth and lacustrine sediments in its strata ³. This period is referred to as "Mega-Chad," when the lake covered a much larger area than it does today ⁴.

Human habitat construction has historically aimed to provide shelter from wind, heat, and precipitation—natural weather events that influence the quality of earth materials used ⁵. While various types of earth are available for construction, this study focuses specifically on diatomite ⁶, a material valued for its physicochemical ⁷, thermomechanical ^{8 9} properties.

Diatomite is a whitish, sometimes grayish, sedimentary rock in its natural state, as shown in Figure 1. Lake Chad's diatomite is a major deposit of diatomaceous earth, particularly in the Bodélé Depression ⁷.

The objective of this study is therefore to examine the potential use of Lake Chad diatomite as a raw material in eco-construction. To achieve this, the physicochemical and geotechnical properties were determined, including water content, density, natural water content, particle size distribution, sedimentation, porosity, void ratio, bulk and specific density, plastic and liquid limits, plasticity index, natural water content, as well as Atterberg limits.

Material and Method

Study Area Presentation

The city of Ngouri, which is the departmental capital of Waye and located in the western part of the country in the Lake Chad province, is a crossroads.

Sampling

The sample was extracted from three different quarries for earth brick production consisting essentially of diatomite, where shallow sampling was carried out randomly according to the NF ISO 18400-102 standard (NF ISO 18400-102, 2017). The collected soils were disturbed, packaged and placed in bags under the same conditions and transported from N'gouri to the ENSTP laboratory in N'Djamena during the month of July 2025. These three samples were named 44, D and M.

2. Diatomite Characterization Method

The laboratory tests carried out are:

- Dry bulk density (NF P94-053)

- Specific gravity (NF P94-054)

- Water content (NF P94-050)

- Porosity and void ratio (NF P94-050)

- Atterberg limits (NF P94-051)

- Particle size and sedimentation analysis (NF P94-056/057)

- Compression and tensile strength tests

- Chemical composition (XRF).

Dry Bulk Density (Standard: NF P94-053)

Objective:

To determine the dry bulk density of a soil, which is the mass of dry soil per unit volume, including the volume of both solids and voids.

Test Procedure:

A representative undisturbed or remoulded soil sample is collected. Its natural volume is measured either directly (for cylindrical samples) or using paraffin-coating and water displacement if irregular. The sample is then oven-dried at 105–110°C for at least 24 hours or until constant mass is achieved. The dried sample is weighed precisely. The dry bulk density is computed using the following formula:

where:

• ρd: dry bulk density (g/cm³ or kg/m³)

• Md: dry mass of soil

• V: total volume of the sample.

Specific Gravity of Soil Solids (Standard: NF P94-054)

Objective:

To determine the specific gravity of soil particles, defined as the ratio of the density of soil solids to the density of water at a specified temperature (usually 20°C).

Test Procedure (Pycnometer Method):

The soil is oven-dried and cooled in a desiccator. Approximately 50 g of soil is weighed and introduced into a clean, dry pycnometer. Distilled, de-aired water is added, and air bubbles are carefully removed by agitation or vacuum. The pycnometer is filled completely, and the total mass is recorded. After cleaning, the pycnometer is filled with only distilled water and reweighed. The specific gravity is calculated using:

where:

• Ms: mass of dry soil

• Mw: mass of pycnometer + water

• Msw: mass of pycnometer + soil + water

• Gs: specific gravity of soil solids (typically 2.6–2.8 for most soils).

Water Content Determination (Standard: NF P94-050)

Objective:

To determine the natural moisture content of a soil sample, which is crucial for classification and strength analysis.

Test Procedure:

A small sample of moist soil is placed in a pre-weighed container. The container and soil are weighed again, then dried in an oven at 105–110°C for 24 hours. Once cooled in a desiccator, the final mass is recorded. The water content (W) is calculated as:

where:

• Mwet: mass of wet soil

• Mdry: mass of soil after drying

• W: water content in %.

Porosity and Void Ratio (Standard: NF P94-050)

Objective:

To determine the volume of voids in a soil relative to its total volume (porosity) and relative to the volume of solids (void ratio).

Test Procedure:

After determining the dry bulk density and the specific gravity of the soil solids, the void ratio (e) and porosity (n) are calculated using the following relationships:

where:

• ρd: dry bulk density

• ρw: density of water (typically 1.0 g/cm³)

• Gs: specific gravity

• e: void ratio (dimensionless)

• n: porosity (%).

Atterberg Limits (Standard: NF P94-051)

Objective:

To classify fine-grained soils based on their consistency as water content changes. The Liquid Limit (LL), Plastic Limit (PL), and Plasticity Index (PI) are determined.

Test Procedure:

Liquid Limit (Casagrande Method): A paste of soil is placed in the Casagrande cup and a standard groove is cut through the center. The cup is repeatedly dropped at a standard rate (2 drops/sec) until the groove closes over 12 mm length. The moisture content is determined for each trial, and a flow curve is plotted (log of number of blows vs. moisture content). The LL is defined at 25 blows.

Plastic Limit:

Soil is rolled into threads on a glass plate until it crumbles at a diameter of 3 mm. The moisture content at this point is recorded as the PL.

Plasticity Index:

PI=LL−PLPI = LL - PLPI=LL−PL

It reflects the range of moisture content where the soil remains plastic.

Particle Size Distribution (Standard: NF P94-056/057)

Objective:

To determine the gradation of soil particles and classify soil as gravel, sand, silt, or clay.

Test Procedure:

Sieve Analysis (for particles > 80 µm):

Dry soil is passed through a series of standard sieves (e.g., 4 mm to 80 µm). The mass retained on each sieve is measured, and the percentage passing is calculated. A semi-log plot of particle size versus % passing gives the grain-size distribution curve.

Hydrometer Analysis (for particles < 80 µm):

Soil is dispersed in a deflocculating solution (e.g., sodium hexametaphosphate) and placed in a sedimentation cylinder. A hydrometer measures the density of the suspension over time. Readings are taken at specific intervals, and particle diameters are determined using Stokes' law:

where D: particle diameter,

L: depth,

η: viscosity,

t: time.

Compression and Tensile Strength Tests

Objective:

The measurements of the mechanical resistances are carried out thanks to the hydraulic press to determine the resistance in traction and in compression

To assess the mechanical strength of soils or rocks under axial loading conditions.

Tensile test

The tensile strength of clay specimens is generally low, as the structures are designed to work solely under compression. However, if necessary, tensile strength can be measured using indirect tests, such as three-point bending. The characterisation device consists of an upper piston with a force sensor and a displacement comparator attached to it, and a lower piston on which the test specimen is placed. The lower piston rises with the test specimen until it comes into contact with the upper piston. From this point onwards, the force sensor records the forces exerted on the test specimen. For tensile strength, the measurement is given below. The test device is illustrated in Figure 2.

Uniaxial compression tests

This test consists of placing the 4 cm × 4 cm test piece under the device shown in Figure 2 and then crushing it.

The average of the results of the mechanical resistances obtained are low compared to a material which must be used as a load carrier. at least it can be used as wall filling material

Minéralogie

The samples taken are rocks of low density, presenting a certain powdery aspect. They do not react with dilute hydrochloric acid, thus excluding carbonate rocks of the “travertine” type. We focused on more or less pure diatomites.

SEM (Scanning Electron Microscope) observation (secondary electrons) very clearly confirms this track and clearly shows a multitude of diatom tests (essentially cylindrical shape) as well as their fragments. Analyzes by X-ray Fluorescence (FRX), expressed in masses of oxides, logically indicate the predominance of silica which corresponds to the very nature of the tests for diatoms but which also comes from the presence of Quartz and clays, detected in X-ray diffraction (XRD). The diatom test consists of an amorphous silica which does not diffract and could on the contrary correspond to the bulging of the base line of the 3 XRD images that can be seen between the angles 2θ = 15° to 30° (anticathode at copper λ = 1.54060 Angstrom). The sample contains significantly more quartz, which is detected by a significantly higher intensity of the quartz lines in XRD but which also results in a higher silica content (86%).

Chemical Composition by X-Ray Fluorescence (XRF)

Objective:

To determine the elemental and oxide composition of soil or rock samples using non-destructive spectrometry.

Test Procedure:

Soil samples are oven-dried and ground to fine powder (typically <75 µm). The powder is either pressed into pellets using a binder or fused into glass discs for major element analysis. The sample is exposed to high-energy X-rays in an XRF spectrometer. The emitted secondary X-

The particle size distribution curve (NF P 94-056) and sedimentation test (NF P 94-057) for each diatomite sample reveal that the diatomite is very fine: more than 80% passes through the 80-micron sieve. This fineness promotes high permeability. The results are comparable to those obtained by Abakar ¹⁰.

The following table presents the sedimentometric characteristics of the diatomite samples.

Diatomites are porous materials with low density that do not react with diluted hydrochloric acid, indicating the absence of carbonate rocks. They are mainly composed of silica, with a content exceeding 80% according to several XRF analyses ¹¹.

The diatomite samples were subjected to strength, density, and moisture content tests, as presented in the table above.

Results of the mechanical tests

3. Results and Discussion

The dry densities range between 409 and 414 kg/m³, and the specific gravities between 671 and 679 kg/m³. The water contents are very high (96% to 101%), indicating a strong capacity for absorption. The average porosity is around 39%, with a void ratio of up to 66% (for Sample 44). The Atterberg limits indicate good plasticity (plasticity index Ip = 11 to 18%).

The analyses show that over 90% of the particles pass through a 0.08 mm sieve, confirming the fineness of the diatomite. This characteristic promotes good cohesion in brick making but may require additives to minimize shrinkage.

Diatomite is rich in silica (83.17%), with low concentrations of Al₂O₃ (5.53%), Fe₂O₃ (1.60%), and CaO (2.06%), making it particularly interesting for its thermal properties and resistance to chemical agents (Fraine & Seladji, 2019).

The compressive strength values range from 2.08 to 2.21 MPa. The tensile strength remains low, between 0.0098 and 0.013 MPa.

4. Conclusion and Perspectives

The diatomite from Lake Chad shows promising characteristics for use in eco-construction: low density, fine particle size, high silica content, and good plasticity. Its use in the production of compressed earth blocks (CEB) or insulating coatings appears to be very promising.

Future Work:

• Conduct thermal performance tests (conductivity, thermal inertia)

• Add natural binders (e.g., gum arabic, lime)

• Perform durability tests

• Design and test a pilot housing prototype in N’gouri

ACKNOWLEDGEMENTS

We would like to express our sincere thanks to all those responsible at the Ecole Nationale Supérieure des Travaux Publics in general, and to Mr WArimi Egrey Ali Mahamat Adoum in particular for their personal and individual contributions. Special thanks go to Zenab Ali Abakar.

Declaration Statement

References