UNIVERSITEIT GENT
INTERUNIVERSITY PROGRAMME
MASTER OF SCIENCE IN
PHYSICAL LAND RESOURCES
Universiteit Gent
Vrije Universiteit Brussel
Belgium
Stabilization of soils from Cameroon for
construction purposes
September 2006
Promotor: Master dissertation in partial fulfilment
Prof. J. Wastiels of the requirements for the Degree of
Abstract
Local soils are the most common, easily available and cheap construction materials used for simple structures in most parts of Cameroon. Due to their poor durability, severe limitations have been unraveled while using these soils. Various techniques have been used over the years to improve on the durability of these soils. The possibilities of using the mineral polymerization (MIP) technique for the stabilization of kaolinitic soils with the aim of making them more suitable for construction purposes was investigated using four soil samples ( FET, FBM, FNB, and FNK) from Cameroon. This technique is based on the micro structural transformation of some clay minerals into solid and stable materials having hydroxysodalite, feldspatiod or zeolite characteristics under the action of alkaline reactants, at atmospheric pressure and low (quasi environmental) temperatures.
Granulometric analysis, plasticity and loss on ignition tests were used to characterize the soil samples. They all had the required amount of kaolinite; averagely 40%. Adding 12% sodium hydroxide at optimum water and sand content to sample FNK, compressive strengths which meet the requirements for construction materials precursors were obtained. These strengths were 25MPa and 10MPa under dried and immersed conditions respectively. Sample FBM also gave acceptable compressive strengths by adding 8% sodium hydroxide at optimum water and sand content to the sample which were 12MPa and 7MPa under dried and immersed conditions respectively. Despite all efforts, we were unable to obtain acceptable compressive strengths for the FET and FNB samples probably due to the presence in the samples of other kinds of clays or colloidal materials which are undesirable for the technique.
With a soil sample that meets the requirements for this technique, optimizing the quantity of various constituents results in very good mechanical and physical characteristics of the resulting construction materials. The smaller the grain sizes used, the more the properties of the materials are improved as there will be a larger surface area for reactions to occur. Ideal curing temperatures lie between 75°C and 85°C as higher temperatures only increase cost and pollution with no considerable improvement in the mechanical and physical properties of the materials.
The mineral polymerization technique can be considered as a potential technique for the improvement of some soils in Cameroon for it is cheaper, environmentally friendlier than traditional methods and produces strong and durable construction materials.
Acknowledgements
The scientific journey that has produced this thesis has seen the contribution of seasoned scientists, institutions, family and friends. Without them, I wonder whether this adventure of mine would have yielded any fruits. I am most grateful and will be eternally indebted to my thesis promoter Prof. Dr. Ir. Jan Wastiels and advisor Dr. Mazen Alshaaer. Their patience and invaluable guidance has been the main driving force behind this work. Words cannot express what I have learnt from them within and outside this research. I will also like to express my gratitude to the academic and administrative staff of the Physical Land Resources Programme in the University of Ghent and the Vrije Universitiet Brussels under the leaderships of Prof. Dr. E. Van Ranst and Prof. Dr. F. De Smedt respectively. Their guidance and assistance has been of much help and value to this work.
Special thanks go to Dr. Uphie Melo Chinje the director of the Local materials promotion authority (MIPROMALO) Cameroon who assisted me immensely in sample collection. By granting me access to the library and laboratories of her institution under the supervision of her assistants, the fieldwork and collection of samples for this work saw the light of day. The mind searching discussions and exchanges we had on the subject were also enriching. Thank you once more madam.
I acknowledge the warmth of my course mates throughout the two years. My cluster friends Lambive, Messiga, Uzoma, Prabin, Lee and Nawal have been exceptional “we remain united”. Hearty thanks go to my mum and dad Helen Nkeh and William Nkeh respectively for their support and love through out my educational ladder. I will always remain indebted to them reason why l dedicate this work to them. I am equally grateful to my sisters and brother, Relindis, Jacqueline, Loveline, Mary, Therese and Julius. I wish to thank all my friends and most especially, Eric Andangfung who inspired me into the field of engineering geology. I am particularly thankful to Ursula for her last minute support. Her support, understanding, and advice helped me to courageously surmount all difficulties provoked by my long absence from home. Finally I would like to thank all the good people at the Vlaamse Interuniversitaire Raad (VLIR) and the Flemish Government for financing my studies.
Table of Content
Abstract ... i
Acknowledgements... iii
Table of Content ... iv
List of Figures... vi
List of Tables ... viii
Chapter-1 General Introduction ...1
1.1 General Background ...1
1.2 The use of soils for construction materials in Cameroon ...1
1.3 Problem Description ...2
1.4 Objectives...3
Chapter-2 Literature Review ...4
2.1 Improvement of Soils for Construction ...4
2.1.1 Mechanical Stabilization ...4
2.1.2 Physical Techniques...5
2.1.3 Physico-chemical techniques – fired bricks ...6
2.1.4 Chemical Stabilization ...7
2.2 Clay Minerals and other silicate minerals in soils...9
2.3 Chemistry of Inorganic Polymers...15
2.4 Mineral Polymerization Technique ...16
Chapter-3 Area Description...22
3.1 Introduction ...22
3.2 Collection of Samples...22
3.3 The Yaoundé area deposits ...24
3.3.1 Nkolbison sample ...24
3.3.2 Mvog Betsi sample ...24
3.3.3 Simbock sample...25
3.3.4 Parent material for Yaoundé soils...25
3.4 Bambili Sample ...25
Chapter-4 Characteristics of Soil Samples ...27
4.1 Introduction ...27
4.2 Moisture Content ...27
4.3 Grain Size Distribution ...28
4.4 Atterberg Limits ...31
4.5 Plasticity Index (PI) ...32
4.6 Loss on Ignition...33 Chapter-5 Methodology...38 5.1 Materials...38 5.2 Fabrication of Specimens...38 5.2.1 Mixing ...39 5.2.2 Moulding ...39 5.2.3 Curing...40
5.2.4 Post curing and Pre-test Treatments ...40
5.4.2 Loss of compressive strength ...42
5.5 Water Absorption ...42
5.6 Efflorescence and pH...42
5.7 Homogeneity of the mixture ...43
5.8 Checking the effect of maximum grain size of soil samples ...43
5.9 Checking the effect of curing temperature...43
5.10 Improving specimen’s characteristics using sodium hydroxide ...43
Chapter-6 Observations, Results and Discussions...44
6.1 Mvog Betsi sample, (FET)...44
6.1.1 Fabrication of Specimens ...44
6.1.2 Physical Characteristics of Specimens...45
6.1.3 Compressive strength and Stability ...50
6.1.4 Efflorescence ...53
6.2 Bambili sample, (FBM) ...53
6.2.1 Fabrication of specimens...54
6.2.2 Physical Characteristics of Specimens...54
6.2.3 Compressive strength and stability ...59
6.2.4 Efflorescence ...62
6.2.5 NaOH as stabilizing argent...63
6.3 Simbock sample, (FNB)...64
6.3.1 Fabrication of Specimens ...64
6.3.2 Physical Characteristics of Specimens...65
6.3.3 Compressive strength and stability ...70
6.3.4 Efflorescence ...73
6.4 Nkolbison sample, (FNK) ...73
6.4.1 Fabrication of Specimens ...74
6.4.2 Physical Characteristics of Specimens...75
6.4.3 Compressive strength and stability ...79
6.4.4 Efflorescence ...82
6.5 Changing the maximum grain size of soil samples ...83
6.5.1 Physical Characteristics of Specimens...83
6.5.2 Compressive strength and Stability ...85
6.6 Influence of curing temperature ...86
6.6.1 Influence of curing temperature on density...86
6.6.2 Compressive strength and stability ...87
Chapter-7 Conclusions and Recommendations ...89
7.1 Specifications for the use of soils in construction...89
7.2 Mvog Betsi sample, (FET)...90
7.3 Bambili sample, (FBM) ...90
7.4 Simbock sample, (FNB)...91
7.5 Nkolbison sample, (FNK) ...91
7.6 Changing the maximum grain size of soil samples ...92
7.7 Influence of curing temperature ...92
7.8 Sodium hydroxide as stabilizing argent...92
7.9 Recommendations ...93
List of Figures
Figure 1, Basic structural unit of silicates [21]...10
Figure 2, Linking of (SiO4)-4 tetrahedra to form silicates...10
Figure 3, An exploded view of the aluminum octahedral unit [21]...12
Figure 4, Structure of gibbsite [21]...12
Figure 5, Structure of kaolinite...13
Figure 6: Sketch picture of kaolinite structure showing distances between atoms ...15
Figure 7, General formulas of polysiloxanes (1) and polyphosphazenes (2)...16
Figure 8, Hydrosodalite unit cell, [(Si-O-Al-O), nNa, nH2O] (Source : S. Kowalak et al, (Modified 2000))...18
Figure 9, Schema showing reaction between Kaolinite and NaOH...19
Figure 10, Map of Cameroon showing sampling locations...23
Figure 11, Grain size distribution curve and summary of other properties - FET...29
Figure 12, Grain size distribution curve and summary of other properties - FBM ...30
Figure 13, Grain size distribution curve and summary of other properties – FNB ...30
Figure 14, Grain size distribution curve and summary of other properties – FNK ...31
Figure 15, An illustration of boundaries between Atterberg’s limits ...31
Figure 16, Incremental loss on ignition at various temperatures - FET...34
Figure 17, Total loss in weight at various temperatures - FET ...34
Figure 18, Incremental loss on ignition at various temperatures – FBM...35
Figure 19, Total loss in weight at various temperatures – FBM ...35
Figure 20, Incremental loss on ignition at various temperatures – FNK ...36
Figure 21, Total loss in weight at various temperatures – FNK...36
Figure 22, Incremental loss on ignition at various temperatures – FNB ...37
Figure 23, Total loss in weight at various temperatures - FNB...37
Figure 24, Variation of density with % NaOH – FET ...47
Figure 25, Variation of density with water content – FET...47
Figure 26, Variation of density with sand content - FET...48
Figure 27, Variation of water absorption with NaOH content - FET ...49
Figure 28, Variation of water absorption with water content - FET...49
Figure 29, Variation of water absorption with sand content – FET...50
Figure 30, Variation of compressive strength with water content – FET ...51
Figure 31, Variation of compressive strength and NaOH content – FET...52
Figure 32, Variation of compressive strength with sand content – FET...53
Figure 33, Variation of density with NaOH content – FBM...55
Figure 34, Variation of density with water content- FBM...56
Figure 35, Variation of density with sand content – FBM...57
Figure 36, Variation of water absorption with NaOH content – FBM ...58
Figure 37, Variation of water absorption with water content - FBM ...58
Figure 38, Variation of water absorption with sand content - FBM...59
Figure 39, Variation of compressive strength with NaOH content - FBM ...60
Figure 40, Variation of compressive strength with water content - FBM...61
Figure 41, Variation of compressive strength with sand content – FBM ...62
Figure 45, Variation of density with sand content - FNB ...68
Figure 46, Variation of water absorption with NaOH - content...69
Figure 47, Variation of water absorption with water content - FNB ...69
Figure 48, Variation of water absorption with sand content – FNB...70
Figure 49, Variation of compressive strength with NaOH content – FNB ...71
Figure 50, Variation of compressive strength with water content – FNB...72
Figure 51, Variation of compressive strength with sand content – FNB ...72
Figure 52, Variation of density with NaOH content – FNK ...75
Figure 53, Variation of density with water content – FNK...76
Figure 54, Variation of density with sand content - FNK...77
Figure 55, Variation of water absorption with NaOH content - FNK ...78
Figure 56, Variation of water absorption with water content - FNK...78
Figure 57, Variation of water absorption with sand content – FNK...79
Figure 58, Variation of compressive strength with NaOH content – FNK...80
Figure 59, Variation of compressive strength with water content - FNK...81
Figure 60, Variation of compressive strength with sand content – FNK...82
Figure 61, Variation of density with grain size – FBM ...84
Figure 62, Variation of water absorption with grain size – FBM...85
Figure 63, Variation of compressive strength with grain size – FBM...86
Figure 64, Variation of density with curing temperature - FBM...87
List of Tables
Table 1, Main types of silicates ...11
Table 2, Geographical location of sampling sites...24
Table 3, Hygroscopic moisture content of samples ...28
Table 4, Atterberg limits and Plasticity indices for the various samples...33
Table 5, Composition of each series of specimens - FET...44
Table 6, Variation of density with NaOH content - FET...46
Table 7, Variation of density with water content - FET ...46
Table 8, Variation of density with sand content - FET...46
Table 9, Variation of water absorption with NaOH content - FET ...48
Table 10, Variation of water absorption with water content - FET...49
Table 11, Variation of water absorption with sand content - FET ...50
Table 12, Variation of compressive strength with water content - FET ...51
Table 13, Variation of compressive strength with NaOH content - FET...52
Table 14, Variation of compressive strength with sand content - FET...52
Table 15, Composition of each series of specimens - FBM...54
Table 16, Variation in density with NaOH content - FBM...55
Table 17, Variation of density with water content - FBM ...56
Table 18, Variation of density with sand content - FBM...56
Table 19, Variation of water absorption with NaOH content - FBM ...57
Table 20, Variation of water absorption with water content - FBM...58
Table 21, Variation of water absorption with sand content - FBM ...59
Table 22, Variation of Compressive strength with NaOH content - FBM ...60
Table 23, Variation of compressive strength with water content - FBM...61
Table 24, Variation of compressive strength with sand content - FBM ...61
Table 25, Effects of NaOH on compressive strength - FBM ...63
Table 26, Composition of each series of specimens - FNB ...64
Table 27, Variation of density with NaOH content - FNB ...66
Table 28, Variation of density with water content - FNB...66
Table 29, Variation of density with sand content - FNB ...67
Table 30, Variation of water absorption with NaOH content - FNB...68
Table 31, Variation of water absorption with water content - FNB ...69
Table 32, Variation of water absorption with sand content - FNB...70
Table 33, Variation of compressive strength with NaOH content - FNB...71
Table 34, Variation of compressive strength with water content - FNB ...71
Table 35, Variation of compressive strength with sand content - FNB...72
Table 36, Composition for each series of specimens - FNK...74
Table 37, Variation of density with NaOH content - FNK ...75
Table 38, Variation of density with water content - FNK...76
Table 39, Variation of density with sand content - FNK ...77
Table 40, Variation of water absorption with NaOH content - FNK...77
Table 41, Variation of water absorption with water content - FNK ...78
Table 42, Variation of water absorption with sand content - FNK...79
Table 46, Variation of density with grain size - FBM ...83
Table 47, Variation of water absorption with grain size - FBM...84
Table 48, Variation of compressive strength with grain size - FBM...85
Table 49, Variation of density with curing temperature - FBM...87
Chapter-1
General Introduction
1.1
General Background
Cameroon is highly under-developed, with poverty, poor healthcare, malnutrition, poor road infrastructure, and poor housing conditions being very prominent. The use of cheap and high quality local natural resources for construction purposes is therefore very vital for its sustainable development. With very low purchasing powers, very few Cameroonians can afford for conventional building materials which entail a lot of money to buy and transport them to construction sites. However, Cameroon is potentially provided as far as natural construction materials are concern. Their advantages are increasing from the scientific, economical and environmental points of view. Qualitative and quantitative data on these materials are not yet sufficient for their valorization. Knowledge of the benefits and usefulness of natural resources, how people used them in the past and how they are using them now for construction is very vital in order to boost development in Cameroon. To gain this knowledge, an assessment of the geotechnical properties of materials and various methods that have been used or maybe used to improve upon these properties is necessary.
1.2
The use of soils for construction materials in Cameroon
One of the demands of rising populations and rising standards of living is the increasing use of resources for construction. As a result, some construction resources such as soils, which rarely possess the characteristics of volume stability, strength and durability required in construction, have to be used. In order to use these materials and come out with good results, there is the need for their improvement. The improvement of these materials is termed stabilization. Soil stabilization can be described as the modification of soils to meet specific engineering requirements. Soil stabilization is commonly used to describe any physical, chemical or biological method or combination of such methods used to improve certain properties of a natural soil for intended purpose. The use of conventional construction materials requires a high level of technical know how. In Cameroon, well trained technicians who can easily apply these methods are rare and expensive to pay for by an average citizen. Cheap and easy to use naturally
occurring local materials which require simple and cheap techniques to improve upon their properties will therefore be of great necessity and advantage.
It is rather unfortunate that in Cameroon a greater majority of the population uses basic earth materials without any modification on their properties for construction. Most buildings built with only basic earth materials suffer relatively rapid deterioration due to low dry strengths which decrease to zero under wet conditions and also have high porosities and water absorption. They also show the development of shrinkage cracks under dry conditions, swelling under wet conditions and a generally high susceptibility to damage due to periodic wetting and drying.
1.3
Problem Description
Modern construction materials are accompanied by a lot of energy consumption and environmental degradation. The high cost of energy and transportation has made the prices of cement and its related products beyond the rich of most Cameroonians. The first stage of all industrialization, and increasing standards of living, requires concrete for building infrastructures. The manufacture of traditional Portland cement requires calcining calcium carbonate. This yields calcium oxide and carbon dioxide gas. The emission of carbon dioxide is therefore, becoming a growing concern; it is an important contributor to the green house effect. As regards development statistics, the worldwide level of cement production is expected to be 3.5 billion tons by the year 2015. This would put the share in the global pollution (all human activities combined) at 18%[1].
Most specifications used in tropical countries were developed to meet the needs of the temperate climatic conditions of Europe and North America and do not seem to recognize the special characteristics of tropical soils. In Cameroon, much still has to be done to adapt these specifications to local realities, especially as the country has a varied surface geology. Most of the specifications in use are those borrowed from France. Modifications to existing specifications, taking into account the peculiarities of local soils are indeed necessary if they are to be used effectively. An elaborate study of the properties of the natural construction materials and ways of improving on their properties in all parts of Cameroon so as to come out with specifications that will reflect local climatic and environmental conditions is of great necessity.
This study is going to evaluate the potentials of the Mineral polymerization (MIP) Technique which has been well studied in the Mechanics of materials and construction laboratory of the Vrije Universiteit Brussel at solving one of these problems. This technique of soil stabilization is not well known in Cameroon though soils suitable for the technique are likely quite abundant given the local climatic conditions and geology. With this technique, stone-like materials are produced from kaolinitic soils at atmospheric pressures and low temperatures. This requires less equipment and is less expensive to produce and also more environmentally friendly since rapid deforestation due to utilization of wood for energy and the emission of large quantities of carbon dioxide to the atmosphere will be greatly reduced. The products of this technique can be used as structural materials with attractive properties in construction and other applications.
1.4
Objectives
Based on the problems outlined above, four soil samples were collected from Cameroon, whose quality and suitability for construction purposes will be evaluated using the Mineral Polymerization (MIP) technique. We aim at producing high quality and durable construction materials using an environmentally friendly and easy to apply technique at low cost.
Secondly, different relationships between materials compositions and manufacturing processes, physical and mechanical properties and durability of the materials produced will be assessed.
Chapter-2
Literature Review
2.1
Improvement of Soils for Construction
The most common soils which are used for construction in Cameroon are lateritic soils. In regions where it is available and cheap, wood is commonly used but its greatest short coming is its low durability since it is hardly seasoned and treated to fight against insect attacks and humidity. Lateritic soils are the products of the intensive weathering that occur as a result of the tropical and sub-tropical climatic conditions in this country. These soils are generally rich in secondary oxides and sesquioxides of iron and/or aluminum. They are nearly devoid of bases and primary silicates but may contain large amounts of secondary quartz and kaolinite.[2]
The most common and simplest process for the manufacturing of bricks from lateritic soils in most countries in Africa consists in taking these soils and drying them in open-air[3].Given the nature of the type of soils found in this region, it is rather difficult to manufacture bricks by the traditional process of firing at high temperature around 900°C to 1100°C. Bricks manufactured simply by mixing the soils with water and drying in open-air have not been able to give good results in terms of dimensional stability, strength, stiffness, permeability and durability. With these problems encountered, there is thus a need to look for means of improving these soils. Efforts have been made of recent towards this and results have shown that blocks and bricks made from lateritic soils can be improved to produce masonry units with strengths high enough to meet building standards [4].
2.1.1 Mechanical Stabilization
Mechanical stabilization is a very commonly used technique in Cameroon especially in road construction, where lateritic soils are used for sub-base and base course. This involves decreasing of the soil voids by mechanical means so as to increase the density and strength, and to achieve a decrease in compressibility, permeability and porosity [5]. Laboratory analysis on the soils is carried out in order to determine optimum conditions before this method is applied. The parameters mostly analyzed are the grain size range and optimum moisture content that will
give high density values after compaction. These laboratory analyses already constitute a limitation to average Cameroonians.
Even with a proper application of this method, long-term problems still occur. The improved physical contact between the soil grains leads to an increase in strength and a reduction in porosity, which in turn leads to some reduction in water absorption and migration. Even though initial strength may be high, long-term stability cannot be assured. The improvement in physical strength alone is not enough to ensure that the ingress of water is significantly reduced on a long-term or permanent basis [5]. This short coming has been observed in many roads in Cameroon which gave good results at the time they were constructed.
2.1.2 Physical Techniques
Additional fines or aggregates maybe blended into a material to adjust its granulometric composition before compacting. This will result in a uniform, well graded, dense soil-aggregate mixture after compaction [6]. Various means of defining the assortment of sizes required to achieve the maximum density have been devised, of which the most common is to regard the material as a selection of spheres of decreasing size such that [7]:
n
D d P
p= ………. (1)
Where p is the proportion of spheres smaller than d in diameter, P is the proportion of spheres smaller than D in diameter and n is the grading coefficient (D>d). For maximum density, n ranges from 0.33 to 0.5
This is a very cheap and easy to apply method, e.g. the use of sand in stabilizing fine grained soils rich in clays will lead to a decrease in their degree of plasticity and swelling. Using this method, lateritic soils in Kumasi Ghana were blended with alluvial gravel and this resulted in a more remarkable improvement in the particle size distribution with the resulting mixture producing strength gain and markedly enhanced material quality [8]. Four day soaked CBR values of at least 80% could be achieved in the laboratory for some of the blended specimens compared to about 46% for the unblended specimens. These blending techniques are very
is the Yaoundé-Nsimalen-Mbalmayo road which was constructed in 2003 with the base course made of lateritic soils (75%) blended with crushed rocks (25%) of diameter 0-25mm. Based on the specifications in use, very good results were obtained (Razel Cameroun,2003).
Soil reinforcement by incorporating components such as fibers, electrical or electro-osmosis treatment and the use of sand drains are other physical techniques which can be used to improve upon the properties of soils [5]. These methods require a lot of technical know how and money and are not commonly used in Cameroon.
2.1.3 Physico-chemical techniques – fired bricks
These techniques use a combination of both physical and chemical methods. A typical example is firing of bricks. When bricks are fired, the actual reactions which enable a suitable product to be formed are chemical while the heating itself is a physical process. Some people refer to this as thermal stabilization.
Though sun-dried bricks are the most popular, heat stabilized bricks are also widely used in Cameroon. The technique of making fired bricks requires a lot of experience and knowledge. The problem of cost of energy and controlling baking temperatures has lead to a lot of research. This motivated Mbumbia et al [9] who worked on lateritic soils from Cameroon and found out that it is possible to produce strong bricks at lower temperatures without additives, simply by crushing the raw materials to obtain medium to fine particles. However the means to crush is a limitation to the average Cameroonian. Chinje and Monget [10] also tried to solve this problem of cost of energy. Theyconstructed a traditional down-draught wood fired kiln for firing tests of clay bricks through which they discovered that massive dry wood gave better results than wet wood which is mostly used by local producers. They also discovered that adding saw dust of <12% to body composition reduced drying times and improved the mechanical properties of the final products but raised porosity.
In an effort to lower production cost, the effects of fluxes on the temperature at which fired clay bricks mature have also been studied [11]. It has also been found that just the energy needed accounts for about 30% of the cost of production [9],and this has resulted inthe firing technique not widely used in Cameroon.
2.1.4 Chemical Stabilization
Chemical stabilization techniques rely on the use of admixtures to alter the chemical properties of the soil in order to achieve desired effects [6]. Chemical stabilization includes the use of chemicals and emulsions as compaction aids to soils, as binders and water repellants and as a means of modifying the behavior of clays [12]. Physical stabilization techniques are the cheaper and easier to use techniques but when they do not offer good results, geotextiles or chemical admixture stabilization is the final solution. To carry out any chemical stabilization, the raw material properties have to be determined and suitable chemicals selected based on the available financial resources.
2.1.4.1 Lime stabilization
There is evidence that lime (Ca(OH)) was used in Roman times for the stabilization of roads in areas with smectitic soils. Lime produces a dramatic decrease in the plasticity index (Liquid limit – Plastic limit); first by increasing the plastic limit and secondly by producing hydrated Ca-Al silicate cementing agents which bind the clay particles together [13]. When lime is added to a clay soil, it has an immediate effect on the properties of the soil as cation exchange begins to take place between the metallic ions associated with surfaces of the clay particles and the calcium ions of the lime. Clay particles are surrounded by a diffuse hydrous double layer which is modified by the ion exchange of calcium. This alters the density of the electrical charge around the clay particles which leads to them being attracted closer to each other to form flocs, the process being termed flocculation. It is this process which is primarily responsible for the modification of the engineering properties of clay soils when they are treated with lime [14]. Many of the engineering properties of clay soils are enhanced by the addition of lime. These properties vary and depend upon the character of the clay soil, the type and length of curing and the method and quality of construction [15]. The largest increases in plastic limits are observed in montmorillonitic clays, while for kaolinitic clays the change is small. The liquid limit for kaolinitic clays is increased while for montmorillonitic clays it is reduced. Therefore montmorillonitic clays respond more rapidly to lime stabilization and so exhibit earlier gains in
by montmorillonitic clays is not as high as that achieved by kaolinitic clays [15]. However, the optimum addition of lime needed for maximum modification of the soil is normally between 1% and 3% lime by weight and further additions of lime do not bring changes in the plastic limit but increase the strength. A reduction of the swell percent and swell pressure of clay soils to zero with additions of 6% lime has also been reported [16]. Hence the quantity of lime to be added for a specific soil should be determined from laboratory test and analysis on samples.
The most important beneficial effects of lime stabilization on soils include; the broadening of the range of moisture tolerance of the soil, the reduction of the soil plasticity, the increase in the strength and fair improvements of volume stability by reductions in shrinkage and swelling [5].
2.1.4.2 Cement stabilization
Cement can be used as an effective stabilizer for a wide range of materials. Portland cement can be used both to modify and improve the quality of a soil or to transform the soil into a cemented mass which significantly increases its strength and durability, with the amount of cement additive depending on whether the soil is to be modified or stabilized [6]. The amount should be determined in the laboratory by varying the proportions of cement-soil mixtures and testing their properties and the proportion which offers the properties which meet specifications selected. Unlike in lime stabilization, the chemical reaction takes place in the cement rather than the soil. Hydration reactions in cement lead to the formation of calcium silicate hydrates which bind soil grains together [17]. Clay masses containing more than 5% montmorillonite are hardly stabilized with economically justifiable amounts of cement. This can be explained by the fact that the Ca2+ ions of the cement are adsorbed by montmorillonite preventing the hydration of cement and also montmorillonite is very susceptible in contact with water because of its expansive property. Comparing the performances of lime and cement as modifiers for laterites with immediate and time effects taken into consideration, results show marginally better performances for lime with respect to both effects [18]. Cement stabilization is quite popular in the road construction sector in Cameroon. Lateritic soils are stabilized with cement and used for the base course of paved roads. The results obtained so far have been quite good based on the specifications in use and their durability. With the high cost of cement and the technical know how required, this method has not been very popular in Cameroon for building construction.
2.1.4.3 Stabilization using organic compounds
Large organic cations have been found to produce decreases in the expansive properties of soils. Bitumen, asphalt and tars are the most common organic soil stabilizers. Cut backs and emulsions are commonly used though emulsions are preferred over cutbacks because of energy constraints and pollution control effects. In the case of cut backs (Kerosene + tar), kerosene evaporates when the cut back is spread on the soil while for emulsions (water + tar), water evaporates during curing. The tar left on the soil forms a water resistant binder between its grains. Water absorption is greatly limited as the tar acts as a waterproof. This leads to improvements in strength and volume stability (reduced expansion) of the soils.
2.2
Clay Minerals and other silicate minerals in soils
To better understand the engineering properties of soils, a profound knowledge of the minerals that make up these soils is important. Amongst the minerals which make up soils, clay minerals are the most abundant. The physical and chemical characteristics of soil minerals are very important in considerations in planning, constructing and maintaining buildings, roads and airports.
Clays are naturally occurring materials formed by the weathering and decomposition of igneous rocks [19]. These layered materials fall under the group of phyllosilicates that are themselves part of the silicate mineral class. Clays are materials based on a two-dimensional stack of inorganic layers. The layers are made of either tetrahedral sheets of SiO2 motifs or octahedral
sheets of metal oxide and hydroxide (where the metal can be Al, Fe, or Mg) or both. Cohesion between the layers is maintained by weak electrostatic and Van der Waals interactions mediated by interlayer cations and water molecules [20].
Like in all silicate minerals, in phyllosilicates, the (SiO4)-4 tetrahedron is the most basic structural
unit. The SiO44- tetrahedral structure is shown in figure 1 below. The shape of a silicate anion is a
tetrahedron, not a sphere like a simple anion. The Si-O bond is approximately 50% covalent and 50% ionic, that is, although the bond arises in part from the attraction of oppositely charged ions,
total bonding energy of Si+4 is distributed equally among 4 O-2 ions; hence, the strength of any Si-O bond is equal to 2 the total bonding energy in the O-2 ion. Therefore each O-2 has the potential of bonding to another (SiO4)-4 group and entering into another tetrahedral grouping.
The uniting of the tetrahedral groups leads to sharing or bridging O-2 ions, and hence, polymerization. However, you never get more than one O-2 being shared between two silica tetrahedra. . Figure 2, illustrates how different tetrahedral are linked to each other to produce phyllosilicates.
Figure 1, Basic structural unit of silicates [21]
Figure 2, Linking of (SiO4)-4 tetrahedra to form silicates
Silicate minerals are classified on the basis of the degree of polymerization of the (SiO4)-4
tetrahedra and this results in 6 main types of silicates. The main types of silicates are grouped in table 1 based on the number of bridging oxygens (#BO) per (SiO4)-4 tetrahedron
Table 1, Main types of silicates
The Si - O combination has a radius ratio of 0.30, which means that the silicon ion fits nicely into a tetrahedral polyhedron. This gives the orthosilicate anion, which could at least theoretically be neutralized by four protons (hydrogen ions). This anion tends to react readily with alkali and alkali earth ions. If each of the four oxygen ions bond with two silicon ions the result is a quartz crystal. In phyllosilicates, only one plane of oxygen ions bond with two silicon ions. This bonding is extended in two directions to form a sheet of silicon tetrahedrons [21]. The second basic building block of the phyllosilicates is an aluminum octahedral unit. The aluminum/oxygen radius ratio is 0.41, which falls at the maximum ratio for tetrahedral coordination and minimum ratio for octahedral coordination. Depending on conditions, aluminum can coordinate with either four or six oxygen ions. As it turns out, however, within the phyllosilicate mineral structure the aluminum ion is "more comfortable" in an octahedral coordination. An exploded view of the aluminum octahedral is shown in figure 3.
be balanced and the charge can at least be partially balance if each oxygen ion is bonded with two aluminum ions. Once again, this could theoretically happen by the three face oxygen ions, the two edge oxygen ions, or the single corner oxygen ion bonding with two Al ions. In this case aluminum is slightly less electropositive than is silicon and is able to approach close enough that corner oxygen ions can be shared. In a matrix of these octahedral units each oxygen will be bonded to two aluminum ions, leaving it with a remaining -1 charge. The charge can be satisfied by attaching a proton (hydrogen ion) as shown in figure 4. When this type of structure is continued in three dimensions we have the mineral GIBBSITE [21].
Figure 3, An exploded view of the aluminum octahedral unit [21]
Figure 4, Structure of gibbsite [21]
We have another option for balancing the remaining -1 charge on the oxygen ions. In the sheet of silicon tetrahedral units, apical oxygen ions still have an unbalanced charge. The two sheets can be brought together with the apical oxygen ions of the tetrahedral layer also being in the octahedral layer. As a result, the charge on these oxygen ions is balanced by bonding to one silicon ion and two aluminum ions. This is the basic structure of our first phyllosilicate mineral,
KAOLINITE. Its structure is illustrated in figure 5. It is about 0.7 to 0.74 nm thick (from the bottom oxygen to the top oxygen) and extends 10 nm and more in the other two directions [21].
Figure 5, Structure of kaolinite
We can speak of this three dimensional structure as a clay micelle. The kaolinite mineral is actually made up of many micelles piled one atop the other. Since the surface on one micelle contains hydrogen ions and the other surface only oxygen ions there is a tendency for hydrogen bonds to form between micelles. While individual hydrogen bonds are very low energy, the bonding energy is additive and the sum of the many hydrogen bonds between micelles results in the micelles being very strongly bonded together and nearly impossible to separate. Thus, we speak of kaolinite as being a nonexpanding phyllosilicate. Since each micelle is constructed of a layer of silicon tetrahedral units and a layer of octahedral units, kaolinite is called a 1:1 clay mineral. Thus, kaolinite is a 1:1 nonexpanding clay mineral. The composition of kaolinite is very constant, without any isomorphic substitution either in the tetrahedral or the octahedral sheet. This absence of isomorphic substitution is the cause that no charge is present on the lattice [13]. Since we replaced the hydrogen ion on one layer of octahedral oxygen ions by a silicon ion it is only logical that the remainder of the hydrogen ions can be similarly replaced. This results in another class of clay minerals the 2:1 clay minerals. These minerals consist of two silicon tetrahedral layers and one aluminum octahedral layer e.g. smectites and micas [21]. In this work,
minerals, it is characterized by a good dimensional stability, low plasticity and doesn’t swell when saturated with water. It retains its white color when fired.
In general Kaolinite is an inorganic polymer, with its backbone chain made up of silicon and aluminum atoms. It is generally formed in soils as a result of the chemical weathering of feldspars (equation 2) and other clay minerals like illite (equation 3) and smectites.
2KAlSi3O8 + 9H2O + 2H+ Al2Si2O5(OH)4 + 2K+ + 4H2 SiO4 --- (2)
(K-feldspar) (Kaolinite)
2KAl2(AlSi3)O10(OH)2 + 5H2O 3Al2Si2O5(OH)4 + 2KOH --- (3)
(Illite) (Kaolinite)
When kaolinite is heated to temperatures of about 500 to 600°C, the water that was chemically bound to it is lost leading to a highly disordered structure (metakaolinite) as shown in the figure 6 below. This loss on ignition could be related to the amount of kaolinite present in a soil sample. For it has been shown that normally about 13.95% the initial weight of kaolinite is lost when it is heated to temperatures between 450 and 550°C which corresponds to the weight of water lost. This is the technique we have used in this work to determine the quantity of kaolinite in the soil samples. The dehydroxylation process of kaolinite leading to the formation of metakaolinite at 500-600°C (depending on the degree of structural disorder) can be expressed as in equation 4. Al2(OH)4Si2O5 Al2Si2O7+ 2H2O ………. (4)
The rupture of the kaolinite framework and transformation into metakaolinite induces a sharp modification of the intra-cell geometry with an abrupt modification of the Si-O, Al-O, Al-Si distances (distances labeled as 1, 2, and 3 in the sketch picture of kaolinite in Figure 6). When the dehydroxylation reaction is completed metakaolinite is reorganized and shows a larger degree of long-range order compared to the intermediate reaction products[22].
When heating is continued to about 900°C, metakaolinite transforms into a short-range ordered material with a mullite-like structure, which is completely anhydrous. This is made up of 3 phases; mullite, -alumina and silica, according to the reaction shown in equation 5.
Figure 6: Sketch picture of kaolinite structure showing distances between atoms
2.3
Chemistry of Inorganic Polymers
Inorganic polymers are a group of compounds containing non-carbon elements as the principal backbone, where the variation in the side groups attached to the inorganic backbones yields polymers with a great range of properties. Broadly, the inorganic polymers we will deal with in this work may be considered as three-dimensional framework aluminosilicates consisting of corner-sharing silicate and aluminate tetrahedra, containing metal cations which balance the negative charge of the aluminate groups. They are typically made with compositions (in terms of oxides) of approximately M2O·3SiO2·Al2O3·12H2O, where M is an alkali metal cation (usually
Na or K)[23].
These polymers can be tailored for a variety of applications, including ceramic precursors, electronic materials, nonlinear optical materials, and biomedical materials [24]. Inorganic polymers set and harden in a physically similar process to Portland cement and have potentially similar applications. Inorganic polymers may also have applications in toxic waste stabilization, advance composites and as cement additives [25, 26, and 27]. Despite having cost, environmental and performance advantages over current binder technologies, the great potentials of inorganic polymers are only now being realized. This has been mainly attributed to the fact
formulations of inorganic polymers and their chemistry, structure and performance characteristics [23]. The two largest classes of polymers with inorganic backbones are the polysiloxanes (silicones) and the polyphosphazenes, whose general formulas are shown in figure 7 below.
Figure 7, General formulas of polysiloxanes (1) and polyphosphazenes (2)
Polysiloxanes were first prepared on a laboratory scale by Kipping in England in the early 1930s, and were developed as useful polymers in the late 1930s and 1940s in the United States by Hyde at the Corning Company, by Rochow, Pattnode and Gilliam at the General Electric Company, McGregor at the Mellon Institute, and by scientists at Union Carbide. Similar developments were also reported in Russia [28]. The bond between silicon and oxygen is very strong, but very flexible. So silicones can stand high temperatures without decomposing, but they have very low glass transition temperatures. Like the polysiloxanes, polyphosphazenes are made of alternating atoms; in this case, the chain is made up of alternating phosphorus and nitrogen atoms.
Compared with most organic polymers, inorganic polymers are generally stronger, harder, more brittle and usually insoluble. They do not burn (with few exceptions) and only soften or melt at very high temperatures.
2.4
Mineral Polymerization Technique
Mineral polymerization (MIP) has been defined as the micro structural transformation of clay minerals into a tectosilicate structure under the action of alkaline reactants, at atmospheric pressure and low (quasi environmental) temperatures [29].
When added to an appropriate soil, sodium hydroxide reacts with kaolinite which has a sheet-like (2 dimensional) structure and transforms it to a more stable 3 dimensional (tectosilicate) zeolite or feldspathoid structure at temperatures below 100°C. Stone like shaped objects can thus be
made which are stable in water and are high temperature resistant. If there is the need, fine and clean sands are also added to the mixture to improve upon the granulometry of the resulting material and thus improve on the strength and durability.
Mineral polymers have also been widely referred to as geopolymers [30]. Over the years there has been great improvement over the development of new materials based on mineral (geo) polymerization. Davidovits [30]describes the reactions which lead to the formation of these new materials as reactions which involve the chemical reaction of alumino-silicate oxides (AL3+in four fold coordination) with alkali polysilicates yielding polymeric Si-O-Al bonds; with the amorphous to semi-crystalline three dimensional silico-aluminate structures being of the poly(sialate) type (-Si-O-Al-O-), the poly(sialate-siloxo) type (-Si-O-Al-O-Si-O-) and the poly(sialate-disiloxo) type (-Si-O-Al-O-Si-O-Si-O-).
Some early workers discovered that when sodium hydroxide is added to kaolinite, it attacks the clay mineral lattice and this result in the formation of sodium silicate and sodium aluminate, with the latter proceeding to precipitate insoluble aluminum oxide hydrates, which give the soil considerable durability [7, 31]. As we saw before, Kaolinite is an inorganic polymer, with its backbone chain made up of silicon and aluminum atoms. It is generally formed in soils as a result of the chemical weathering of feldspars (equation 2) and other clay minerals like illite (equation 3) and smectites. The composition of kaolinite is very constant without any isomorphic substitution either in the tetrahedral sheet or in the octahedral sheet. Hence no permanent charge is present on the lattice. Charges can thus only arise from broken bonds on the edges of the mineral. Exchange of H+ ions at the edges of kaolinite clay mineral for Na+ ions seem to be the most likely initial reaction when sodium hydroxide is added to kaolinite [32]. This leads to dispersion of clay particles because of the repulsion between the Na+ ions which have large radii. This results in sodium silicate and sodium aluminate and possibly other sodium compounds. By adding sodium hydroxide to Kaolinite, we thus in principle reverse equation (2) whereby the 2-dimensional network polymer is transformed into a 3-dimensional frame-work polymer which is harder and more stable (equation 6).
Si2O5Al2(OH)4 + 2NaOH Na2Si2Al2O8.3H2O ---(6)
(Kaolinite) (Hydrosodalite)
The new mineral formed from this reaction has a zeolitic structure as illustrated in figure 8.
Figure 8, Hydrosodalite unit cell, [(Si-O-Al-O), nNa, nH2O] (Source : S. Kowalak et al, (Modified 2000))
Not all soils are suitable for this technique. The soils most contain a reasonable amount of kaolinite which is the main precursor mineral for the important chemical reactions, with the other clay minerals present only in small amounts or absent [29]. For this reason, sedimentary clays which contain swelling clays should be avoided as these clays do not take part in the reaction and so retain their normal undesirable characteristics in the resulting product. The presence of non clay minerals is acceptable. Laboratory analysis most therefore be carried out to determine the mineralogical composition and the properties of the soil sample before they can be used.
Wastiels (2001) came out with the requirements for the mineralogical composition and physical properties of soils, which are summarized below.
1.) Mineralogical composition
- Kaolinite must be present; (a minimum of 20% of total soil mass) although lower amounts maybe acceptable in some cases.
- Other clay minerals than kaolinite should be avoided as much as possible, especially the swelling clays like smectites, only a few percent are tolerable.
- Non-clay minerals can be present e.g. quartz and feldspar. Most crystalline alluminosilicates are acceptable and even useful. Carbonates and iron oxides are acceptable.
- Organic compounds are to be avoided.
- Components not stable in strongly alkaline conditions should be avoided e.g. amorphous silica, microcrystalline silicates, sulphates etc.
2.) Physical Properties
- Particles with a grain size larger than 5mm have to be sieved out.
- Minimum clay content should be 10%, with ideal clay contents being 20–30%
- The ideal overall granulometric curve is broad and regular, between clay and coarse sand size, with clay size fraction consisting of kaolinite.
- Following the standard Atterberg limit test, a classification as CL or ML of the fraction <450 microns is advisable.
It was shown that with the precursor mineral kaolinite, adding sodium hydroxide to it will yield sodalite based Na-poly(sialate) (Si2O4,Al2O4,2Na),3H2O within 20 seconds time at temperatures
of 150°C and pressures of 5-10MPa [30].This is illustrated in the schema in figure 9 below.
Figure 9, Schema showing reaction between Kaolinite and NaOH
The inorganic polymerization technique therefore results in the formation of amorphous or micro-crystalline equivalents of some synthetic zeolites. These are more stable and have higher strengths than the precursor clay minerals and so will result in far better construction materials.
It is based on the above reactions that new materials have been produced which have replaced some traditional materials that were organic in origin and had a lot of short comings. These materials have lead to applications in the construction industry (bricks, tiles, fire resistant chip-board panels, cement), used as binders, fire resistant and heat resistant fiber composites, sealants for industry, tooling for aeronautics, radioactive and toxic waste encapsulation, etc.[33].
Instead of using kaolinite as the precursor, metakaolin has also been widely used with very encouraging results obtained [23]. However the recent trend has been to use fly ash as the precursor for the reactions [34]. Fly ash being an industrial waste has an advantage over metakaolin. It is a waste resource produced in huge quantities by coal-fired power stations, making it an ideal environmentally friendly feedstock. It consists of fine, amorphous and reactive aluminosilicate particles. Class F fly ash, which is high in aluminosilicate and low in calcium, is the preferred grade for inorganic polymerization. Fly ash is far less pure than metakaolin, but produce more complex inorganic polymer structures [34].
Effects of other types of alkali apart from sodium hydroxide and their concentrations have also been investigated, with the most popular being potassium hydroxide [23, 34]. These works showed that metakaolin or fly ash is more soluble in sodium hydroxide than in potassium hydroxide, because products using the latter contained more un-reacted particles than those using the former. The sodium based inorganic polymer was also found to be stronger than its potassium based equivalent. So sodium hydroxide is preferred over potassium hydroxide for the making of construction materials.
The stabilization of kaolinitic clays from Suriname using the MIP technique resulted in a remarkable improvement of the strength in both dried and immersed specimens. For the dried specimens, the improvement in strength was 8 to 53 times that of the unstabilized soil [35]. Brownish-red portions of soils from the Al-Mudawwara deposit in Southern Jordan have also been stabilized using this technique and it was discovered that low priced, stable, reliable and high quality construction materials with maximum compressive strengths of 22MPa and 50MPa tested under wet and dried conditions respectively by using 6% sodium hydroxide were obtained [36]. Testing Kaolinitic clays from Vietnam stabilized using this technique, it was concluded that clay from the Huu Khanh kaolin mine satisfies the criteria to be used as precursors of low priced,
stable, reliable and high quality construction materials, with maximum compressive strengths of 12.78MPa and 34.00MPa tested under wet and dried conditions respectively [37].
The stabilizing effects of alkaline additives have also been investigated using latosols from Ghana and very god results obtained [5]. These results attest to the fact that with the MIP technique, it is possible to produce building materials whose properties can be compared to those of more traditional materials like baked bricks or concrete blocks. The amount of sodium hydroxide added depending on the soil, i.e. not fixed. These materials are cheaper and environmentally friendlier than their traditional equivalents due to the lower amount of energy they need for their processing.
Chapter-3
Area Description
3.1
Introduction
Cameroon is located in West – Central Africa. It has a total area of about 475440km2 comprising 469440km2 land and 6000km2 water. It has boundaries with The Central African Republic, Chad, The Republic of Congo, Equatorial Guinea, Gabon, and Nigeria. The country has a coastline along the Atlantic of about 402km.The Climate varies with terrain from tropical along the coastal area, to semi arid in the north. The topography is varied with plains in the east, south and southwest, dissected plateaus in the centre, mountains in the west, southwest and northwest and plains in the north. Altitudes range from zero meters along the coast to about 4095 m above sea level on mount Cameroon which is the highest mountain in West Africa and an active volcano. The country has a population of about 17 million, with a growth rate of about 1.97% (2005 est.).
3.2
Collection of Samples
With the help of the Director and staff of the Local Materials Promotion Authority (MIPROMALO) in Cameroon, we used the guide on suitable samples for the MIP technique (Wastiels (2001) [29]) to help us in sample collection. MIPROMALO is a research institute under the ministry of scientific research in Cameroon which has as one of its main functions, the identification of potential local construction materials. This was the main reason why l solicited their help since obviously they possess data that could serve as a guide in selecting potential sampling sites. Like in most under-developed countries, this research institute lacks modern equipment and has very few researchers. The data available there was not quite sufficient to help us determine with certainty what will eventually give the expected results. There were some documents (unpublished) containing data about the mineralogical and chemical composition of soils around Yaoundé in particular and the country in general. Most of the data was too general and incomplete. There was also no possibility of carrying out some tests in order to help us determine the mineralogical and chemical composition of samples. Three sampling sites were selected around Yaoundé which is the capital of Cameroon and another site selected in Bambili, which is a small village about 15km from Bamenda the capital of the North West province. Figure 10 shows the location of the sampling sites in Cameroon.
Figure 10, Map of Cameroon showing sampling locations
YAOUNDE SHOWING SAMPLING LOCATIONS Location of Bambili
3.3
The Yaoundé area deposits
Three sampling sites were chosen around Yaoundé. This was based on data obtained from MIPROMALO and works by Liboum et al [11].The sites chosen were at Nkolbison, Mvog Betsi and Simbock. Table 2 below presents the location of all four sampling sites obtained using a GPS.
Table 2, Geographical location of sampling sites
Site Symbol Geographic Coordinates (DMS) Elevation above sea level
Nkolbison FNK E011°25.197 N03°52.597 2401ft (732m)
Mvog Betsi FET E011°29.085 N03°52.147 2428ft (740.54m)
Simbock FNB E011°28.176 N03°48.986 2297ft (700.58m)
Bambili FBM E010°15.011 N05°58.600 5872ft (1791m)
3.3.1 Nkolbison sample
The sample was collected from a clay deposit which had just been discovered by MIPROMALO some few months before. Soils collected from this deposit were being tested for the eventual manufacture of baked bricks. While data on its mineralogical and chemical composition was not available, the results of the L.O.I test proved that the clay deposit was rich in kaolinite. The soils have cream colored portions mixed with reddish portions (pepper and salt).
The deposit covers a vast area of about 3 hectares and has a depth of about 6m. The topography is rugged, and where the sample was collected at a depth of 1.5m, there is a slope of about 2%.
3.3.2 Mvog Betsi sample
Soils from this deposit are being used by MIPROMALO for the making of baked bricks and cement stabilized bricks. MIPROMALO carried out a geotechnical identification of the soils. Analysis carried out by Liboum et al [11]could justify our sampling from this area. He reported the presence of a reasonable amount of kaolinite and a deficiency in the swelling clays. The deposit covers a very wide area of about 20 hectares and the sample was collected at a depth of 4m. The site has a rugged topography, and is covered by short trees and grass. The soils are reddish in color, indicating they are rich in iron oxides and sesquioxides.
3.3.3 Simbock sample
We collected samples from Simbock also based on the findings of Liboum et al [11]. The deposit is being exploited for road construction in Yaoundé. The area is covered by forest and grass. The sample was collected at a depth of 2m. These soils are yellowish in color.
3.3.4 Parent material for Yaoundé soils
The rocks which underlie the soils in Yaoundé are commonly referred to as the Neoproterozoic Yaoundé “series”. The Yaoundé “series” is an association of metasedimentary (garnet-kyanite, garnet-plagioclase and amphibole-biotite gneisses and micaschists) and metaplutonic (mostly garnet-pyroxene metadiorites) rocks that underwent a regional metamorphism culminating in the granulite facies. The deformation is polyphase, the latest stage corresponding to the southward thrusting of the unit onto the Congo craton. Except for the metamorphism which is well constrained at ca. 620 Ma, the deposition age of the package is still uncertain, but is assumed to be younger than 1.0 Ga. TDM ages of most of the rocks indicate a mixed source between
Paleoproterozoic and Neoproterozoic components. The question still debated is the tectonic position of the “series” (autochthonous or allochthonous on the craton) and the tectonic significance of granulitic rocks (root of a collision zone or intracontinental).
These rocks are naturally very rich in feldspars and quartz. Given that this area is found in the tropical rain forest of Africa, climatic conditions greatly favor intense weathering. The intense weathering is responsible for the accumulation of thick layers of soils on these rocks. From equation (2), chemical weathering of feldspars will result in the production of kaolinite, reason why the soils are very rich in kaolinite. Given the nature of the terrain at the locations where we collected the samples, the swelling clays which are mostly sedimentary in origin should be very minute in the samples.
3.4
Bambili Sample
Bambili is a small village about 15km from Bamenda the North West provincial Capital. It is located in the West Cameroon Highlands between the Bamenda Plateau and the Bambutos Mountains. Average air temperatures are about 21–24 °C throughout the year. Most precipitation
the Adamawa and Bamenda Plateau regions. Highland vegetation includes cloud forest, riparian forest, and extensive montane grasslands, such as the Bamenda Grass fields. The Bamenda Highlands are predominantly grass-covered, with gallery forest persisting in steep valley sides or gullies. Local geology is dominated by granitic and volcanic rocks of probable Tertiary age and numerous maars and crater lakes of indeterminate age mark the landscape. The sample was collected not far from the Bamenda-kumbo highway, where there is a huge kaolin-rich soil deposit. It has a cream-white color and looks hard but easily crumbles when squeezed with the fingers. The deposit is being tested for the manufacturing of roofing tiles.
The parent rocks are rhyolites of Tertiary age. These rhylolites mostly occur in the form of tors, commonly referred to as the Bambili tors. They are acid volcanic rocks, thus are very rich in quartz and feldspars. Weathering of these feldspars over the years has given rise to soils very rich in kaolinite according to equation 2. Swelling clays are almost absent given the nature of the terrain from where the sample was collected. The terrain is very highly rugged with steep slopes giving no room for sedimentary clays to be formed.
Chapter-4
Characteristics of Soil Samples
4.1
Introduction
In order to carry out any successful stabilization on a particular soil, we should first of all identify and classify the type of soil we are dealing with. This is because not all soils are suitable for all techniques. For this work, the guide for the general characteristics of suitable soils for the MIP technique mentioned in the previous chapter was used to decide on what to sample [29]. The geotechnical properties of the samples were determined in the laboratory to further confirm our choices. An understanding of these geotechnical properties was also necessary in order to optimize the soil/sand/water/NaOH ratios. The following aspects were determined in the Laboratory; liquid limit, plastic limit, plasticity index, moisture content, particle size distribution, loss on ignition and organic matter content. The moisture contents and plastic limits served as a guide for us to determine the optimum water contents that will result in the best densities and strengths for the materials after stabilization The particle size distribution is important when we have to optimize soil/sand ratio in order to achieve good physical stability. The loss on ignition
helped us to estimate the amount of kaolinite present in each soil sample.
4.2
Moisture Content
The moisture content, which is attributed to the unbonded water in the soil samples, was determined. Moisture content is the ratio of the weight of water to the weight of solids in a given volume of soil. The moisture content of a sample is an indication of how stable the soil is with respect to strength and compressibility. Samples with very high moisture contents may undergo large volume changes in response to applied loads. Also, soils with high moisture contents tend to be quite weak. The consistency of soils may be very soft or very hard depending upon their mineralogical and water contents. Between these extremes, the soil may be molded and formed without cracking or rupturing the soil mass. The hygroscopic moisture content was that which was determined. To determine the hygroscopic moisture content of a soil, a weighed amount of the soil which was first dried at room temperature was put in an oven set at a temperature of 105°C. After drying, the sample was removed and weighed. The difference between the initial
The natural moisture content is given by the ratio of the weight of water and that of the dry soil. This is expressed as a percentage, see table 3 below for the hygroscopic moisture contents of the various samples.
Table 3, Hygroscopic moisture content of samples
Soil Sample Hygroscopic water content (%)
Bambili (FBM) 4.98
Mvog Betsi (FET) 2.86
Simbock (FNB) 5.57
Nkolbison (FNK) 2.62
4.3
Grain Size Distribution
The grain-size distribution expresses the percent of the sediment mass that is finer than a given grain size. This was determined for each of the samples by carrying out a sieve analysis using standardized sieves and sedimentation using a hydrometer. The results were then represented on a chart from which we could determine if the sample is well graded (size of the particles being distributed over a wide range of sizes), uniformly graded (the size of particles being distributed over a narrow range of sizes) or gap graded (several distinct size ranges occurring within a sample). In order to reduce cost of transportation, the soil samples from the Yaoundé area were dried in the sun and sieved with a sieve having openings of 5mm. Only the portions <5mm in diameter was brought to Belgium since for the work, large grain sizes are undesirable.
In the Mechanics of Materials and Construction (MEMC) laboratory of VUB, a weighed amount of each sample was used for grain size analysis using sieves with standardized openings. The portion for each sample that was below 2mm was then separated. On this portion, wet sieving was carried out using a 75 m sieve. The fraction >75 m was then dried in an oven set at 105°C and after drying it was sieved using sieves with their openings ranging from 850 m to 75 m. For the fraction <75 m, a sedimentation test was performed by means of a hydrometer using sodium hexametaphosphate as dispersing argent. The hydrometer method is based on Stokes' Law which indicates that a larger grain size will result in a larger terminal velocity when dropping through a fluid (i.e. the larger size reaches the bottom quicker, assuming uniform density). The percent by weight of soil passing each opening was then plotted as a function of the grain diameter
