Effect of Metakaolin on Strength Properties of Lateritic Soil Intended for Use as Road Construction Material

. An excellent all-weather road is essential in providing reliable transportation services that comprise social and economic development elements. However, in most cases, the road has to be constructed on a soft foundation soil where large deformations usually occur, which causes increases in maintenance costs and leads to interruption of traffic service, especially during the wet season. It is necessary to stabilize or improve the in-situ soils. This study explores the potential of using metakaolin to improve the geotechnical properties of lateritic soil for road construction materials. The soil classifies as A-6(4) and CL according to the American Association of State Highway and Transport Officials and the Unified Soil Classification System. The soil was treated with 5, 17.5 and 30 % concentrations of metakaolin by dry weight and was compacted using three compaction energies: British Standard Light (BSL), West African Standard (WAS) and British Standard Heavy (BSH). California Bearing Ratio (CBR) and Unconfined Compressive Strength (UCS) tests were carried out to evaluate the effect of metakaolin on the soil investigated. Results showed a general improvement in the engineering properties of the soil with an increase in metakaolin content, particularly when compacted at the BSH energy level. However, the results did not meet the 1500-3000 kN/m 2 7 days UCS criterion stipulated by the Nigerian General Specification for road base courses. However, 30 % lateritic soil/metakaolin blended soil compacted using WAS and BSH energy levels suffice for use as sub-base in road construction, having met the 750-1500 kN/m 2 7 days UCS criterion stipulated by the Nigerian General Specification. The Peak CBR value for the treated soil, compacted using the three energy levels of BSL, WAS, and BSH, occurred at 30 % metakaolin concentration with corresponding soaked CBR values of 17, 23 and 31 %. The Nigerian General Specification recommends a nominal strength criterion of a soaked CBR value of 30 and 80 % to be attained by material to be used as subbase and base course in road construction. Based on the above criterion, only the 30 % metakaolin treated blend compacted at the BSH energy level met the 30 % requirement for sub-base materials.


INTRODUCTION
The continued support for transport sector projects reflects the close link between development and transport by development agencies. Transport services are essential for the social and economic development of poor rural and urban populations. The World Bank recognizes the importance of providing transport services, with 23 % of its loans allocated to the transport sector. Transport is an intermediate service industry providing added value to investments in other sectors and contributing to economic growth [34]. Access to essential services by many people in developing countries is severely impeded by poor roads and the consequential poor transport services. It is estimated that some 1.2 billion people do not have access to an all-weather road and that 40-60 % are more than 8 km from a health centre. Transport is also essential in achieving the Millennium Development Goals. It is vital for inclusive, sustainable globalization to overcome poverty, promote growth, and access challenges in fragile states and for Public-Private partnerships [24]. An excellent all-weather road is essential in providing reliable transportation services required for safe access to markets, employment opportunities, education facilities, health centres, etc., comprising the elements of social and economic development. However, in most cases, the roads have to be constructed on a soft foundation soil where large deformations usually occur, which causes increases in maintenance costs and leads to interruption of traffic service, especially during the wet season.
Lateritic soil found in some locations is not usually suitable for subgrade, sub-base and base course due to several difficulties during construction, such as workability, field compaction, and insufficient strength. Furthermore, the acidic nature of the tropical soils has raised doubts about the efficiency of soil-lime reactions in a low pH environment and hence the long-term improvement [3].
It is necessary to stabilize or improve the in-situ soils with other selected soils/aggregates or with binders, to build a strong road network to support heavier vehicles or higher traffic flows and serve in all-weather conditions [2]. These binders are cement and/or lime, which bind the soil particles together through chemical reactions [21]. However, cement production has severe environmental impacts, using vast amounts of fossil fuels and being responsible for the emission of more than 5 % of all the carbon dioxide worldwide [47]. Hence the focus of this study is to provide an alternative to reduce cement usage.
Metakaolin is a dehydroxylated form of kaolinite, following the chemical removal of the bonded hydroxyl ions from the kaolinite minerals, typically heating to approximately 750 °C. As kaolin contains no carbonates, no CO 2 is released during heating, reducing embodied CO 2 in the final materials when replacing cement or lime [16]. Due to the pozzolanic properties of metakaolin, there has been growing interest in its use as a cement replacement and an additive to lime [29,49]. Thus, this study intends to determine the effect of treating lateritic soil with metakaolin during road construction.

METHODOLOGY
The study was conducted in two phases. Phase one involves the determination of engineering properties of the soil without the addition of the additive, index properties, and compaction: Brit-ish Standard Light (BSL), West African Standard (WAS) and British Standard Heavy (BSH), Unconfined Compressive Strength (UCS) and California Bearing Ratio (CBR) tests were carried out by [10]. The second phase involves adding varying proportions of metakaolin by the dry weight of the soil to determine the engineering properties when metakaolin is used as a stabilizing agent. In the case of tests on the stabilized/treated soils, 5, 17.5 and 30 % concentrations of metakaolin by dry weight of the soil were added to the soil to increase the engineering properties of the soil. Similar tests were out on the treated soil by [9].

MATERIALS AND METHOD
The lateritic soil was obtained using the same method of disturbed sampling from a borrow pit at Fankacen Dumi village behind an industrial estate, Bauchi State, Nigeria (latitude 10°16'46.54"N, longitude 9°51'54.25"E). The soil is reddish brown.
The raw material for the metakaolin production is kaolin clay, sourced from Alkaleri, Alkaleri Local Government Area of Bauchi State. The kaolin would be burnt at a temperature ranging from 700-800 °C in a kiln at the Department of Industrial design, Faculty of Environmental Technology, Abubakar Tafawa Balewa University, Bauchi, to obtain the metakaolin.
The water used is portable drinking water; therefore, no laboratory test was conducted.

RESULTS AND DISCUSSION
Physical and Chemical Properties of Lateritic Soil and Metakaolin. Results of the physical properties test for the untreated soil are presented in Table 1. From the results, the soil contains 52 % sand fraction, 36 % silt fraction and 16 % clay fraction. The preliminary result also showed that the soil has a moisture content of 9 % and classifies as A-6(4) by the American Association of State Highway Transportation Officials [1] soil classification system and CL by the unified soil classification system [7]. It is reddish brown with a plasticity index of 13 %. The Optimum Moisture Content & Maximum Dry Density values recorded for the three energy levels of BSL, WAS, and BSH were 18, 16.5, 15.9 % and 1.82, 1.86, 1.90 Mg/m 3 , with corresponding soaked CBR values of 11, 12 and 14 % and Unconfined Compressive Strength values of 247, 472, 630 for the energy levels. These classifications showed that the soil is a silty clay soil of low plastic. The liquid limit and plasticity index values of 37 % and 13 % confirmed that the soil is indeed low plastic [7]. Existing literature has credited that Atterberg limits results have been handy indicators of soil behaviour [22]. These classifications, coupled with the low values of Maximum Dry Density, Unconfined Compressive Strength and CBR recorded, show that the soil falls below the standard recommendation for most geotechnical construction works. Especially for sub-base or base courses in highway construction [2,11,13,33,35,39,40].
The Oxide composition of the lateritic soil was determined using XRF spectroscopy, and the result is summarized in Table 2. Index properties. The variation of index properties of lateritic soil treated with metakaolin is shown in Figure 1. The results showed a decreasing trend in the liquid limit from 37 % to 26.9 %, increasing metakaolin content from 0 to 30 %. This could be due to the porous nature of metakaolin replacing the fine soil particles. The gradual reduction in liquid limit could also be associated with the agglomeration and flocculation of Clay particles, which is a result of ion exchange at the surface of the particles [3,15,18,42,46]. Plastic limit generally decreased with higher metakaolin contents, from a value of 24.2 to 16.6 % at 30 % metakaolin content. The reduction in liquid and plastic limits resulted in a general decrease in the plasticity index value of the lateritic soil/metakaolin blend. A plasticity index value of 12.8 % recorded for the untreated soil was reduced to 10.1 % at 30 % metakaolin addition. The decrease in plasticity index is an indication of soil improvement. The decline in plasticity index is attributed to the effect of metakaolin on the affinity for H + ions of clay and silt fractions which caused the clay and silt fractions to spontaneously form flocs due to negative face charges and positive edge charges. These flocs adhere to each other, forming agglomerates [3]. This plasticity index reduction agrees with [5,8,26,42].
Compaction Characteristics. Figure 2 shows the lateritic soil's relationship between moisture content and dry density.  [15,31,37]. In general, the compaction curve trend agrees with the findings of several researchers [3,19,23,37,48,50].   lateritic soil for the three compaction efforts used is presented in Figure 3 (a, b &c). From Figure 3a,  Content of the soil-metakaolin mixtures largely depend on the soil type, the fineness of the metakaolin particles and the plastic nature of the soils. It has been well documented that soil particles are randomly oriented on the dry side, while on the wet side, the soil particle is oriented in parallel. In the parallel orientation, the extra water forms a water film surrounding the soil particles, enhancing workability and contact between the soil particles [12]. On the wet side of the Optimum Moisture Content, soil particles are arranged in parallel directions creating more connections of surface particles, resulting in easy mixing, compaction and better reaction between soil-MK mixtures. It is evident from the plot of the Maximum Dry Density and Optimum Moisture Content that the best results were achieved using the BSH compaction effort. The effects of the various replacement levels on the moisture contents showed a divergent behaviour. As the replacement levels increase, the moisture content decrease, which is an indication of better performance.

Effect of Metakaolin on Compaction Characteristics of Lateritic soil. The variation of Maximum
Dry Density of lateritic soil/metakaolin mixture for BSL, WAS, and BSH compaction effort is presented in Figure 4.

Figure 4 -Variation of Maximum Dry Density of Lateritic Soil with Metakaolin
The  Figure 5.  [45,20,27] and [42,44,19,3] reported a similar trend of increasing maximum dry densities in their respective research. The increase in Maximum Dry Density recorded for the compaction efforts may be due to flocculation and agglomeration of the clay particles, primarily due to cation exchange and the particles filling the voids within the soil matrix [14,15,41,50]. The increase could also be due to metakaolin replacing the soil particles, thus resulting in the formation of a mixture with higher Maximum Dry Density, as reported by [17,42,50]. It could also be due to an increase in the surface area of particles at a higher dosage of metakaolin.
The general trend observed in Figure 5 is that of decreasing Optimum Moisture Content with the increase in the percentage of the additive. This indicates that the additives require little water for pozzolanic reaction with the silt and clay fractions of the soils. The presence of SiO 2 , Fe 2 O 3 and TiO 2 in the additives may, in part, be responsible for the enhancement of the mechanical properties of the soil specimens. A similar assertion was made by [3]. Furthermore, the soil specimens produce heavier agglomerate particles with an attendant rise in the density of the soil. The result is consistent with those reported by [31] for peat soil modified with kaolin and heated kaolin and [3]. There for, using metakaolin treatment material for the soil is beneficial in improving the mechanical properties of the soil-additive mixtures. Also, as previously stated, on the wet side of the Optimum Moisture Content, soil particles are arranged in a parallel direction creating more contact with surface particles, resulting in easy mixing, compaction and better reaction between soil-MK mixtures.
Strength Characteristics. Over the years, the Unconfined Compressive Strength test has been the most common and suitable method for evaluating stabilized soil strength.  A general improvement in the compressive strength was observed with the age of curing, metakaolin concentration and compaction energy. The results are similar to a study on expansive soil treated with up to 10 % metakaolin conducted by [4]. The increase in Unconfined Compressive Strength is attributed to hydration reactions of the soil-metakaolin mixtures induced by the high pH of the mixture caused by the metakaolin content and also due to improvement in the grain packing of the specimens by reduction of pores by the metakaolin, and thus given rise to a dense and strong structure. Authors [22,36,28,50,2] provide assertion to this belief. Furthermore, the reactive silica present in metakaolin, which reacts and produce cementitious materials and bind the soil particle together, causes a strength gain [30,32].
It was observed that the Unconfined Compressive Strength value increased during the sevenday curing period. However, the peak seven days Unconfined Compressive Strength value of 604 kN/m 2 was recorded at a 30 % lateritic soil/metakaolin blend. This observed trend of BSL compaction energy was similar to that of WAS and BSH compaction energy levels. The peak Unconfined Compressive Strength values at these energy levels are higher than those in the BSL compaction energy, as shown in Figure 7.  This corresponds to an increase of 170 % compared to the result obtained for the natural soil compacted at the same energy level and cured for the same period. These results agree with various researches [2,18,38,40,42,51].
Comparison of (UCS) results with the recommended standard. The Nigerian General Specifications [33]

CONCLUSIONS
This study explores the potential of using metakaolin to improve the geotechnical properties of lateritic soil obtained from Bauchi State, intended for use as road construction material. Based on the results obtained in this study, the following conclusions were drawn: 1. The lateritic soil classifies as A-6(4) by the American Association of State Highway Transportation Officials [1] soil classification system, and based on the unified soil classification system [7], the soil classifies as CL. This implies that the soil has an appreciable quantity of clay and falls below the standard recommended for subbase or base courses in highway construction.
2. The oxide composition of the soil and metakaolin determined using X-ray fluorescence (XRF) spectroscopy revealed that the Silica Sesquioxide Molar Ratio of Iron and Aluminium for the soil is 1.98. The results also indicate that lateritic soil possesses a silica-alumina ratio of 3:1 with a requisite amount of Alumina and silica. In the case of metakaolin, the results revealed the presence of an appreciable amount of Al 2 O 3 , S i O 2 and Fe 2 O 3 required for materials to qualify as a class N pozzolana.
3. The geotechnical properties determined for the soil fall below the standard recommended for use as sub-base or base course materials in highway construction. With the addition of me-takaolin, there is a substantial reduction in plasticity index when compared to untreated soil.
4. Combining the soil with metakaolin improved the soil's dry density with an attendant moisture content decrease. BSH compaction effort yielded higher Maximum Dry Density due to the more incredible energy supplied. In terms of performance and workability, the treated soil would perform better as a construction material.

A remarkable improvement in Unconfined
Compressive Strength was observed at 30 % metakaolin concentration, with an average UCS value of 604, 864 and 913 kN/m 2 at seven days of curing BSL, WAS, and BSH compaction efforts.
6. In general, the seven days cured UCS value of 30 % metakaolin/lateritic soil blend compacted using WAS and BSH effort falls within the range of 750-1500 kN/m 2 UCS value specified by the Nigerian General Specification for sub-base materials.
7. Higher UCS values were recorded for all the compaction efforts at 28 days of curing. This is attributed to the pozzolanic reaction of metakaolin, except 30 % metakaolin/lateritic soil blend compacted at BSH compaction effort, which met the 30 % Nigerian General Specifications nominal strength criteria of a soaked CBR value for sub-base materials. All other combinations fail to meet the requirements.