Abstract

The geotechnical and compositional characteristics of mudrock deposits in the Douala Sub basin were investigated using a combination of analytical methods, including particle size distribution, Atterberg limits, mineralogical (X-ray diffraction) analyses, and whole rock geochemistry. The goal is to characterize the nature and physicochemical properties of mudrock in order to determine its applicability in industries. Grain size analysis reveals that clay-sized particles dominate the samples, with a plasticity index ranging from 6.8% to 20.67%. The mudrock materials are primarily composed of kaolinite (16.8-49.4%), quartz (15.8-68.9%), and illite (00-15.3%), which are typical of the Douala Sub- basin sedimentary environment and morphoclimatic conditions. SiO₂ (42.77-73.5%) and Al₂O₃ (13.13-29.98%) are the most abundant oxides in the samples. Iron oxide content is moderate (1.73- 17.18%). Methylene blue values range from 1.12 to 6.95, confirming the clay content of (39.43-45.43%) and also attesting that the sediments in the study area are rich in 1:1 clay. They are suitable for ceramic applications such as (refractory bricks and tiles) and pottery due to the physicochemical parameters associated with mineralogical and geochemical data

1. Introduction

The use of clay-rich materials is critical in the production of ceramics and building materials. However, clayey deposits are known to form from sedimentary, alluvial, and residual sources. Since antiquity, humans have valued clay-rich materials for the production of bricks and other articles ¹. It is necessary to have a good understanding of the occurrence, quantity, and properties of clay deposits in order to properly and efficiently exploit them ². The development of the global economy, infrastructure growth, and economic sector growth are all dependent on industrial materials ³. Clay-rich materials are used in a variety of applications, including ceramics, paper, paint, ⁴, rubber and plastics, insecticides, food additives, cosmetics, pharmaceuticals, drilling fluids, fertilizer carriers, and geochemical barriers ⁵. Due to its wide range of industrial applications, clay is now regarded as an indispensable development tool. Mineralogical and chemical composition, malleability, thermal behaviour, colour, and strength after baking are all important properties in the ceramic industry ⁶. Many authors have produced scientifically significant documents in the Douala Sub basin, particularly in the fields of stratigraphy and tectonic evolution ^{7, 8, 9, 10}. In the 1980s, the presence of clay in the Douala sedimentary basin attracted an industrial set for the production of ceramics and construction materials ¹¹. Several clay deposits have been discovered in the Douala Sub Basin, including Missole, Bomkoul, and Yansoki. The chemical and mineralogical composition of Bomkoul clay revealed its kaolinitic nature ^{12, 13, 14}. The firing properties of ceramics was depicted by ¹⁵. Preliminary studies on clayey materials from the Bomkoul area was carried out by ¹⁶ and ¹⁷ highlighted the physicochemical and mineralogical characterization of Yansoki area. Most of the research works cited above within the Douala Sub basin on fine grained sediment are limited to particular localities. Despite the impressive work done on clayey materials in the Douala Sub basin, no comparative study has been conducted to bring out the physico-chemical characteristics of mudrock found in various localities such as Kombe, Kompina, Tonde, Ngoma, Djapoma, Missole, and Loungahe. The goal of this research is to characterize mudrock samples using geotechnical, mineralogical, and chemical composition in order to assess their suitability for industrial applications. This research will add to the existing literature on the industrial application of clays in Cameroon.

2. Geological Setting and Geographical Setting

The study area includes the central and north-western parts of the Douala Sub Sedimentary basin. It is situated between the latitudes of 3°10'-4°12' N and the longitudes of 9°50'- 9°52". (Figure 1). The Douala Sub-basin was formed during the opening of the South Atlantic Ocean and is bounded to the south by the Kribi-Campo Sub-basin and to the north by the Rio Del Rey basin (Figure 1). The sedimentary formations are related to the three stages of geodynamic and sedimentary evolution that have affected the basins since their formation ^{18, 19}.

The lithostratigraphic entities of the Douala Sub basin are made up of seven formations that are linked to its geodynamic and sedimentary evolution (Figure 2). These formations are distinguished by Cretaceous, Tertiary, and Quaternary sediments that are locally covered with lateritic cuirasses ¹⁹. The Mundeck Formation, which represents the syn-rift period (Aptian-Cenomanian), is composed of continental and fluvio-deltaic deposits such as clays, coarse-grained sandstones, and conglomerates. The remaining six formations representing the post-rift period are: (1) the Logbadjeck Formation (Cenomanian-Campanian), which is discordant to the Mundeck Formation and is composed of microconglomerates, sand, sandstone, limestone, and clay; (2) the Logbaba Formation (Maastrichtian), which is composed primarily of sandstone, sand, and fossiliferous clay; and (3) the Paleocene-Eocene Nkapa Formation, rich in marl and clay with sand lenses and fine to coarse-grained sandstone; (4) the Souellaba Formation (Oligocene) lying unconformably on Nkapa deposits and characterized by marl deposits with some interstratified lenses and sand channels; (5) the Matanda Formation (Miocene), characterised by deltaic facies interstratified with volcano-clastic layers; and (6) the Wouri Formation (Plio-Pleistocene) which is composed of gravelly and sandy deposits and a clayey matrix.

3. Materials and Methods

The raw materials for this study came from Kombe, Kompina, Tonde, Ngoma, Djapoma, Missole, and Loungahe (Figure 3), and samples were collected along road cuts, valleys, hills, and river channels. To avoid contamination, 13 representative samples were assembled and stored in sample bags. Physical and chemical analyses were performed on the samples. The samples were analyzed using XRD. Prior to laboratory analysis, the samples were air dried and gently crushed to increase surface area. The liquid limit, plastic limit, and plasticity index, particle size, and methylene value were then determined. The physicochemical properties mentioned above were determined at the Geotechnical laboratory of Fotso Victor University Institute of Technology in Bandjoun. The laboratory experiments performed on each sample, as well as the methods employed, are briefly described below.

The Atterberg limit was used to determine the plastic behaviour of materials in accordance with the French standard NFP 94- 051 ²². The Cassagrande apparatus was used to calculate the liquid limit (LL) and plastic limit (PL) (PL). The plasticity index was calculated by subtracting LL from PL.

Sieve analysis was performed on a 500 g sample using ASTM C 136 as a reference, and the soil was washed with an 800 µm sieve. The sediment which remains on the sieve relates to sand and pebble group, while the part that goes through accounts for clay-silt fraction. After 24 hours in the oven, the material that accumulated on the 800 µm sieve was used for sieve analysis, while the fraction that passed through was used for hydrometer analysis. The clay particle size distribution was determined using wet sieving in accordance with the NFP 94-093 French standard ²² and a dispersive agent of sodium hexametaphosphate ((NaPO₃)₆, 5wt%).

The methylene blue test was performed using a methylene blue standard solution made by combining 10 g ± 0.1 g of the powder with distilled water and stirring for 45 minutes at room temperature. A sample that has been baked for 24 hours at 105°C is weighed and gently crushed with a spatula before being placed in a beaker. Before adding 500 ml distilled water, the sediment is stirred for 5 minutes with a speed adjustable blender at 600 ± 60 rpm (revolutions per minute). Following the above procedure, 5 ml of solution is added to the mixture and blended for 1 minute at a rate of 400 ± 40 rpm and a drop of the suspension is taken out. The first drop will typically have a dark blue stain and a water circle surrounding it. The procedure is repeated, adding solution until a light-blue circle (halo) forms around the central blue stain. In the first 5 minutes, another 5ml of solution is added. After 5 minutes, the solution volume is reduced to 2ml and stirred for 1 minute while a drop of the suspension is extracted using a glass rod and dropped on filter paper to observe the circle formation. When a light blue circle forms around the central blue stain, the experiment is considered complete. The volume of methylene blue (VBS) is obtained through; (used methylene blue/ dry mass of sample *100).

The mineralogical and bulk chemical analysis was performed in the clay, geochemistry and sedimentary environments (AGEs) laboratory of the University of Liège in Belgium. In this study, the advanced Burker D8 diffractometer (copper radiation K1, = 1.5418X, V = 40 kV, I = 30 mA) was used in accordance with the methodology proposed by ²³. The diffractograms are obtained from disoriented powder total fractions with measurements in 2 ranging from 2° to 45°, a scanning pitch size of 0.02°, and a time per step of 2s).

4. Results and Discussion

Grain size distribution and plasticity index are two important factors that influence the suitability of fine-grained materials for various industrial applications, and special consideration should be given to the finer (< 2 um) fraction for ceramic ^{24, 25, 26}. The particle size distribution result is shown in (Table 1). The particle distribution of the studied fine-grained materials varied from facies to facies, with proportions ranging from (39.43-49.33) for clay, (29.23-35.57) for silt, and (15.10-26.98) for sand.

The results of the samples show that the clay size particle (0.002 mm) is the most dominant, accounting for more than 39% (Table 1) of all samples. The high amounts of fine particles (silt + clay) and fine clay minerals in the samples support the plasticity index (6.8% to 20.67%). As shown in (Table 2), ²⁷, the amount of clay fraction is used to determine the manufactured products. However, particle size and plasticity index are two of the most important geotechnical characteristics used to select a suitable construction material.

The results of the samples show that clay size particle of (<0.002 mm) is the most dominant, constituting greater than 39% (Table 1) in all the samples.

In this case, considering the amount of sand (22, 19 to 26.89 wt. %), fine particles (40. 07 to 48.58 wt. %), and plasticity index, samples MT, KC, NO, LR3, TD1, and KR1 are suitable for manufacturing bricks, tiles, and sandstones ²⁸.

Methylene blue values range from 1.12 to 6.95, confirming that the sediments in the study area are rich in 1:1 clay (39.43-45.43%). (Table 1).

The Belgian textural classification diagram ²⁹ allows for the identification of specific fields of application for ceramic clay materials. Particle size plotting in this textural classification diagram of clay materials reveals that the samples fall into the clay domains (Figure 4). Grain size data were also plotted on ternary diagram proposed by ³⁰ to demonstrate clay material suitability for ceramics (Figure 5). The plots show that the samples can be used to make tiles and masonry blocks.

Plasticity is an important parameter to consider when manufacturing ceramics products ²⁴ because it allows easier shaping and cohesion of manufactured products. The liquidity limit (LL), plasticity limit (PL), and plasticity index (PI) results are reported in (Table 3). The plasticity index ranges between 5.5 and 24.The plasticity index threshold limit at which a raw material is considered good for the ceramics industry is 10% ³². Clay materials with a plasticity index of <10% can cause cracks in manufactured products, particularly during extrusion, and are thus unsuitable for building construction due to the significant variation in water quantity ³³. Nonetheless, samples with IP values of <10% could be used in raw structural ceramics. According to the findings of this study, the majority of samples have a plasticity index > 10%, validating their use as a good building material (Table 3). The clay plasticity diagram proposed by ³¹ shows that the majority of the samples fall in the medium compressibility domain, while samples KC, NO, BG1 and MZ fall in the high compressibility domain of organic clays and mineral silts (Figure 6).

Plasticity and particle size of sediment are two most important and related parameters. Particle size distribution and mineralogical composition influence the plasticity of clayey materials ³⁴. Similarly, the dominance of kaolin is attributed to the plastic nature of clays ³⁵. The diagram proposed by ³⁶ was used to determine the ceramic properties of the investigated sediments (Figure 7), and it revealed that the materials are suitable for ceramics with the exception of samples MZ1 and TD1, which are suitable for pottery. Nonetheless, the addition of degreasers is required to reduce plasticity and improve material cohesion, making them suitable for ceramics.

The mineralogical analyses of the studied materials are presented in (Figure 8) and (Table 3). The mineralogical composition of mudrock from the study reveals several mineral phases comprising of kaolinite, illite, quartz, K-feldspar, plagioclase, anatase, gibbisite, actinolite and goethite.

Clay mineral represents the bulk of the mineralogical assemblage in most of the samples (Table 4). Amongst these mineral variation, clay minerals constitute the greatest proportion with respective percentages being; 10.2% (BG1) - 49.4% (KB). Aside clay minerals, a considerable proportion of quartz is equally noticed in all the samples with percentage varying from 15.8 % (MT) to 68.9% (DA1).

The percentage of goethite ranges from 2.2% (DA1) to 20.4%. (MT). It is high in samples MT, NO, and BG1 (12.2-20.4), but low in the other samples (2-6%). Gibbsite content ranges from 9.8% (JA2) to 15.3%. (BG1). Orthoclase has a relatively higher percentage in all samples, ranging from 7.2% (BE1) to 22.7%. (KR1). Actinolite content ranges from 2% (KC) to 10.2%. (MT). Anatase accounts for a smaller proportion (1%-3%). The clay fraction is primarily composed of kaolinite and illite. The most abundant mineral in the clay fraction is kaolinite. Its percentage ranges between 10 (BG1) and 49.47%. (KB).

The mineralogical composition of clay material influences its industrial valorisation ³⁷. The majority of the peaks in the studied material are quartz (Figure 7), implying a high silica content ³⁷, which is attributed to its resistance to weathering. The presence of quartz is critical, as it can significantly improve soil geotechnical properties such as compaction, dry density, stiffness, and shear resistance, as well as reduce the shrinkage behaviour of sintered ceramic products ³⁸. The presence of kaolinite in the studied sediments demonstrates that monosiallitisation is the dominant process in the study area ³⁷. The availability of quartz and kaolinite in a material facilitates the shaping and drying of the ceramic products ³⁹. An elevated quantity of clay material increases plasticity and vitrification of ceramic products ³⁹. Based on the refractory properties of ceramics, samples KC and KB, which are plastic and rich in kaolinite, are good. The presence of Illite is detected in some samples, albeit at a low concentration (Table 4). This mineral improves clay plasticity by promoting the location of glazed phases during course firing ³⁷.

The chemical composition of the materials under consideration is shown in (Table 5). The materials are distinguished by a relatively high SiO₂ and Al₂O₃ content. Fe₂O₃ concentrations are moderate, while alkali and alkaline earth oxide concentrations are low. The high concentration of SiO₂ in most samples supports the DRX results that show quartz peaks in materials. The content of aluminum oxide (Al₂O₃) is relatively high, especially in samples KC and LR3, confirming the presence of kaolinite in all samples as seen on X-ray diffractogram peaks. The high concentration of kaolinite in the study area is linked to the morphology and climatic characteristics ^{3, 40}, with monosiallitization being the primary process. This process is usually accompanied by abundant rainfall and good drainage.

This may clarify why materials of the Douala sedimentary sub-basin have relatively low to high proportion of kaolinite (10.2-49.4), greater than those of illite whose formation is related to different climatic conditions such as poor drainage, less precipitation, low gradient slopes and high evaporation ⁴¹.

This explicate the low concentration and complete absence of illite in some samples from the study area. The ternary diagram proposed by ⁴² was used to decipher the degree chemical weathering in the studied material (Figure 9). The projection of samples on the ternary diagram ⁴² indicates a high degree of alteration, confirming the morphology and climatic conditions of the basin. The presence of goethite ⁴³ accounts for the low to moderate concentration of iron oxide (1.73-17.17). The very low alkali (Na₂O and K₂O) contents of the mudrock can be attributed to the low amount of feldspar ⁴⁴. The degree of chemical alteration of the studied materials was demonstrated using a ternary diagram of the evolution of the chemical index. The placement of the samples in the diagram proposed by ⁴² indicates a significant change (Figure 9). This is consistent with the morphology and climatic conditions of the sub region. The SiO₂/Al₂O₃ ratio indicates the amount of quartz and kaolinite present in clay materials. This ratio ranges from 1.64 to 6.64 (Table 5), confirming the DRX results that showed the predominance of kaolinite associated with quartz in the samples studied.

The geotechnical and mechanical characteristics of clayey materials are essential depending on their chemical and mineralogical compositions, as well as the mineral distribution ²⁵. The samples in this study are primarily composed of poorly crystallized kaolinite (10.2 to 49.4 wt. %), which is consistent with the loss on ignition (3.57% to 18.82%), illite (0,00 to 15.3 wt.%) and quartz (15.8 to 68.9 wt.%), are suitable for the composition of ceramic pastry ³⁰ and for the manufacturing of bricks and tiles ¹⁴. Also suitable for the manufacture of tiles and bricks are samples JA2, MZ1, KW, DA1, KB, TDI BE1 with silica percentage (SiO₂> 60 wt. %), alumina (Al₂O₃) (35 wt. %), and iron (10 wt. %) ⁴⁵. These materials are suitable for the production of tiles (LR3, BE1, and KR1) and stoneware (NO), according to plots on the ternary diagram proposed by ⁴⁶, (Figure 10).

5. Conclusion

Thirteen mudrock samples collected from various localities within the Douala Sub basin were subjected to mineralogical and physicochemical analysis. This research yielded the following conclusions:

The mudrock materials are primarily composed of sand (23 to 45 % wt), silts (17 to 33 weight percent), and clays (34 to 45 % wt), with a plasticity index ranging from 13.8% to 21.6%. Methylene values range from 1.12 to 3.25.

The dominant clay mineral in the mudrock samples is kaolinite while quartz, goethite, orthoclase, and illite are also present. The samples are siliceous and aluminous, and most clayey samples contain a significant amount of iron (Fe₂O₃ less than 10% wt). The materials studied are suitable for the production of ceramic and pottery based on the geotechnical, mineralogical, and chemical results. This research will add to the existing literature on the nature and properties of fine-grained materials, as well as project them in various fields of industrial application.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgements

This paper is part of the PhD thesis of Ndengwe Alexander Tangwa at the University of Bamenda. The authors express their profound gratitude to the geochemistry and sedimentary environments (AGEs) laboratory of the University of Liège in Belgium for geochemical analyses and DRX.

Funding Information

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sector

Author Contributions

Ndengwe Alexander Tangwa took part in the project design, did the field work, sample collection, sample processing, data interpretation and compilation, conceived and wrote the manuscript draft.

Njoh Oliver Anoh conceived the project, developed the field work strategy, analytical techniques used and data interpretation. He edited, read and amended the manuscript.

Nowel Yinkfu Njamnsi assisted during field work and equally read through the manuscript

References