Evaluating the Use of Rice Husk Ash for Soil Stabilisation to Enhance Sustainable Rural Transport Systems in Low-Income Countries

Abstract

Rural roads are critical for connecting isolated communities to essential services such as education and health and administrative services, as well as production and market opportunities in low-income countries. More than 70% of movements of people and goods in Sub-Saharan Africa are heavily reliant on rural transport systems, using both motorised but mainly alternative means of transport. However, rural roads often suffer from poor construction due to the use of low-strength, in situ soils and limited financial resources, leading to premature failures and subsequent traffic disruptions with significant economic losses. This study investigates the use of rice husk ash (RHA), a waste byproduct from rice production, as a sustainable supplement to Ordinary Portland Cement (OPC) for soil stabilisation in order to increase durability and sustainability of rural roads, hence limit recurrent maintenance needs and associated transport costs and challenges. To conduct this study, soil samples collected from Mulungushi, Zambia, were treated with combinations of 6–10% OPC and 10–15% RHA by weight. Laboratory tests measured maximum dry density (MDD), optimum moisture content (OMC), and California Bearing Ratio (CBR) values; the main parameters assessed to ensure the quality of road construction soils. Results showed that while the MDD did not change significantly and varied between 1505 kg/m³ and 1519 kg/m³, the OMC increased hugely from 19.6% to as high as 26.2% after treatment with RHA. The CBR value improved significantly, with the 8% OPC + 10% RHA mixture achieving the highest resistance to deformation. These results suggest that RHA can enhance the durability and sustainability of rural roads and hence improve transport systems and subsequently improve socioeconomic factors in rural areas.

1. Introduction

Developing countries rely on road networks as the principal means of transportation, providing access to education, health and other essential services, amenities, and economic opportunities. Rural roads play a key role in connecting rural communities and rural to urban regions, as well as increasing accessibility and socio-economic activities [1]. It is these roads that, when in good condition, allow smooth transport of people and goods, improve economy and community cohesiveness while promoting social equity and social accessibility across the entire country [2,3]. Improving the quality of rural roads can help improve accessibility to critical services and workplaces, using both motorised (MT) and non-motorised transport (NMT) means of transport [4]. This can also significantly help vulnerable groups such as school children, disabled people, the elderly and pregnant women who are usually the heavy burden bearers of poor rural transport systems [4] and henceforth help achieve the sustainable development goals especially in their transportation agenda of leave no one behind. Despite the obvious significance, rural roads and the entire rural transport infrastructure are often constructed with little engineering input to improve their resiliency against both environmental and functional stresses. The United Nations reports that approximately 1 billion people globally still do not have access to all-weather roads, with the major fraction of these people located in Sub-Saharan Africa due to budget restraints [5,6]. Moreover, previous research reported that improving rural access in developing countries increased the annual household income, increased agriculture production and lowered transport costs for market outputs [7]. In Zambia, for example, rural and feeder roads make up close to 45% of the total classified road network and 41% of the Core Road Network (CRN) while certainly making up almost all of the unclassified roads [8]. The CRN comprises interconnected trunk, main, urban, district and feeder (primary, secondary and tertiary) roads and is defined as the barest minimum network which, when improved, will spur economic development and contribute to poverty reduction [9]. Rural roads are often built using gravel soils or weak in situ soils with a minimum of skills or engineering input hence their low bearing capacity leading to consistently poor conditions [7,10]. The use of substandard materials along with less engineered methods can lead to premature road failures and both recurrent and increased maintenance costs with higher traffic disruptions and travel times as well as vehicle operating expenses [11].

While gravel roads are often the go-to choice for rural roads in developing countries due to the high cost of stabilisation in clay and silt soil-dominated regions, as well as the limited affordability of paving the roads to a bituminous or a concrete surface layer [12]. However, these roads are also vulnerable to excessive erosion when exposed to extreme climatic conditions, making them only usable during dry seasons and even so with unwanted air-polluting dust while they seem to be practically impassable during rainy seasons [13,14]. Nevertheless, there is also a challenge of the scarcity of quality aggregates which not only can increase the cost of road construction and maintenance [15] but also is not environmentally sustainable due to the continuous destruction of natural environments searching for appropriate aggregate quarries. Traditional soil stabilisers such as cement and lime are expensive and associated with environmental concerns such as carbon emissions. High embodied carbon emissions that go with the production of Ordinary Portland Cement (OPC) account for 6–7% of the global embodied carbon emissions [16] and research indicates that nearly 900 kg of CO₂ is emitted per ton of OPC produced [17]. This highlights that the need for sustainable and cost-effective soil stabilisation techniques for the construction of climate-resilient roads is imperative and requires urgent engineering responses [18]. Due to both the high level of poverty and low level of industrialisation, the Sub-Saharan African (SSA) countries are currently among the regions with the lowest per capita carbon emissions compared to more developed nations. As of 2023, approximately only 1.0 kt per capita CO₂ emissions were recorded from SSA [19]; well below the 2.7 kt per capita CO₂ emissions threshold prescribed at the Paris Agreement, required to keep the global temperature increase until 2050 below 2 °C [20]. The inclusion of low-cost waste materials from agricultural productions can not only reduce the upfront cost of construction but also reduce the quantity of emitted carbon emissions through improving the road’s durability and resilience, hence minimising recurrent maintenance activities [21].

One of the sectors that hugely benefits from the good condition of rural roads is agriculture and it plays a significant role in the economies of developing countries. In Zambia, for example, the agricultural sector is second to mining and agricultural production occurs mainly in rural areas, with the resulting commodities being transported to urban centres for further processing and distribution. Subsequently, the variety of agricultural practices results in the generation of substantial volumes of agricultural waste byproducts, raising environmental concerns regarding waste disposal. Luckily, research has shown that the waste generated from agricultural production such as rice husks, cassava starch, sugarcane bagasse, maize cobs and groundnut shells demonstrates pozzolanic properties and could be suitable as supplementary cementitious materials needed in road construction [22,23,24,25,26]. However, some studies reported that the quantities of agricultural waste generated in Africa are still insufficient to motivate the cement production and road construction industries to incorporate them into their long-term and sustainable business models [22]. Regardless of existing knowledge that the use of such agricultural byproducts to some extent may be useful in low-volume road constructions as a partial replacement for cement, the cement content required for stabilisation is lower than in urban highway construction. This paper presents the findings of testing the suitability and application of rice husk ash (RHA) as a supplementary cementitious material (SCM) in soil stabilisation for rural road constructions in Zambia and potentially in the SSA region.

2. Rice Husk Ash as a Supplementary Cementitious Material

Rice husk is an agricultural byproduct of rice paddy farming. When rice husks are combusted, approximately 25% of their weight is converted into rice husk ash (RHA) [27]. When rice husks are combusted to be converted into RHA, through a process called calcination, they form an amorphous silica content of up to 85–95% of the initial weight [28]. The literature shows that the calcination is optimally performed at a temperature ranging from 600 to 800 °C, where the ash consists predominantly of amorphous silica content [29,30]. The use of RHA for soil stabilisation purposes presents a two-fold advantage by addressing environmental concerns related to disposing of agricultural waste while providing a cost-effective and sustainable alternative to conventional soil stabilisation techniques. Also, it aligns with sustainable development goals by promoting the use of waste materials, reducing reliance on conventional resources, and further reducing the effects of climate change [31,32,33]. RHA is rich in silica content and therefore has the potential to improve mechanical properties and durability of cementitious systems, leading to potential creation of composite materials with enhanced density, strength and stiffness [33].

Due to high silica content, the RHA is highly pozzolanic, and its reactivity is enhanced when used as an SCM [34]. Previous research showed that using RHA of up to 15% of cement replacement can notably boost concrete’s strength properties, including compressive, splitting tensile, and flexural strength [35]. This highlights the potential of RHA to enhance construction materials’ performance, resilience, and sustainability. The RHA’s high silica content also contributes to the formation of additional calcium silicate hydrate (C-S-H) gel, which densifies the concrete microstructure and enhances its overall performance [36]. The ability of RHA to strengthen and densify materials is what makes it effective in the stabilisation of granular, soft, and low-bearing capacity soils. The pozzolanic reactivity of RHA occurs when it is mixed with cement in the presence of water, releasing calcium ions (Ca²⁺) and reacting with silicates and aluminates to form calcium silicate hydrate [37]. Besides the cement in stabilised soils, RHA also reacts with the soil to produce minerals other than the C-S-H gel. However, this depends on the soil type and its mineralogy.

The amorphous nature of the silica in RHA and its large surface area are key parameters in driving the pozzolanic reaction. The fundamental pozzolanic mechanism relies on the cement hydration, where the silica reacts with calcium hydroxide [38].

Structurally, the particle sizes of RHA range from approximately 0.001–0.1 mm, and the material is highly porous, with a honeycomb structure that likely increases the soil’s cohesive nature [39]. The fine particle sizes formed in RHA and its particle fineness give it the desirable pozzolanic properties [40]. In granular soils, this honeycomb structure helps to improve the cohesion and, by extension, binding property of the single-grained soil structures; thus, increasing its shear strength and ability to bind to cement when stabilised. To understand the physicochemical properties of RHA and the bonding mechanism between RHA, cement, and soil in stabilised soils, X-ray diffraction and Scanning Electron Microscope (SEM) tests are used. Due to limitations with access to this testing equipment, the chemical composition of RHA from related studies is presented in Table 1. The literature reviewed in this paper indicates that the physicochemical properties of RHA fall within similar ranges; the authors believe that the RHA utilised in this study, therefore, possesses comparable properties.

Several studies have shown the positive effect of RHA inclusion on various soil parameters in soil stabilisation for road construction. Pushpakumara and Mendis [41] reported that RHA and lime increased the unconfined compressive strength by 54% and the internal friction angle by 60% of a clayey soil sample. Increases in the soil’s CBR values and plasticity due to adding RHA were also reported in previous studies [42]. Yan et al. [43] argued about improved performance of the road with cement stabilised base material and recycled aggregates while noting that flexural tensile strength, dry shrinkage and temperature shrinkage coefficients increased after adding RHA. However, Hao et al. [30] noted that despite the advantages of RHA in engineering applications, its use could be improved by formulating relevant standards, and the present study is a step moving in that direction.

Table 1. Summary of the chemical composition of RHA from the literature.

Composition	Content (%) Arabani & Tahami (2017) [44]	Content (%) Zareei et al. (2017) [28]	Content (%) Anjum et al. (2025) [45]	Content (%) Valenzuela et al. (2025) [46]	Content (%) Shehata et al. (2024) [47]
SiO2	91.42	86.73	86.80	76.23	91.10
CaO	1.03	0.39	1.87	2.20	0.57
K2O	2.596	0.01	-	2.25	2.40
Na2O	1.12	9.76	-	0.67	-
Al2O3	0.114	0.04	0.23	9.89	0.03
MgO	0.821	0.08	0.30	0.77	-
SO3	0.572	1.32	-	0.27	-
Fe2O3	0.197	0.61	0.10	-	0.05
Others	0.021	-	4.47	1.22	5.85
Loss of ignition (LOI)	2.109	0.54	-	-	4–6

Table 1. Summary of the chemical composition of RHA from the literature.

Composition	Content (%) Arabani & Tahami (2017) [44]	Content (%) Zareei et al. (2017) [28]	Content (%) Anjum et al. (2025) [45]	Content (%) Valenzuela et al. (2025) [46]	Content (%) Shehata et al. (2024) [47]
SiO2	91.42	86.73	86.80	76.23	91.10
CaO	1.03	0.39	1.87	2.20	0.57
K2O	2.596	0.01	-	2.25	2.40
Na2O	1.12	9.76	-	0.67	-
Al2O3	0.114	0.04	0.23	9.89	0.03
MgO	0.821	0.08	0.30	0.77	-
SO3	0.572	1.32	-	0.27	-
Fe2O3	0.197	0.61	0.10	-	0.05
Others	0.021	-	4.47	1.22	5.85
Loss of ignition (LOI)	2.109	0.54	-	-	4–6

RHA (Rice Husk Ash) can be used to stabilise expansive soils by reducing their swelling potential and increasing their strength. Ma et al. [48] investigated the use of RHA to improve the properties of expansive soil, and the results showed that adding RHA decreased swelling potential and increased the strength of the soil. Another study evaluated the use of RHA as a partial replacement for cement in soil stabilisation [49]. The authors found that RHA could effectively substitute part of the cement, which led to cost savings and improved soil properties. Rahmat [50] examined RHA’s impact on the geotechnical properties of soft clay, demonstrating that RHA increased the undrained shear strength and reduced the compressibility of the clay. The literature clearly indicates that RHA is a beneficial additive for improving soil stability and strength.

3. Materials and Methods

Soil samples were collected from the Mulungushi district in Zambia to investigate the influence of stabilising in situ soil with cement and RHA ratios. Mulungushi is a rural area in Central Province, Zambia, where the agricultural sector is the primary economic activity. It is only sparsely connected to nearby towns by gravel feeder roads. These feeder roads ultimately link to the Great North Road, a major trunk road that connects Zambia with its neighbouring countries. Escarpments characterise the regional topography; Mulungushi lies within the valley and is susceptible to significant erosions during the rainy season. The region is known for recurrent road failures associated with the stability and strength of local soil. Moreover, Mulungushi’s geology is characterised by metasedimentary rocks in the Lufubu Metamorphic Complex, which forms part of the Katanga Supergroup, with various schists, gneisses and granites making up the underlying rock bed [51].

3.1. Sieve Analysis

Soil samples were collected from test pits of average depth 1 m in Mulungushi, using a shovel after removing at least 300 mm of topsoil. The soil was air-dried in the laboratory for at least 24 h, and then large clumps of soil were gently broken apart with a pick. To facilitate classification of the soil samples, the sieve analysis test was conducted following BS 1377-2: 1990 [52]. British standard sieves were stacked in ascending order, with the largest aperture sieve size placed at the top and fixed in the mechanical sieve shaker. The soil retained on each sieve was weighed and used to plot the particle size distribution chart.

Table 2. Particle size range of the untreated soil.

BS test sieve (mm)	14–20	2–5	0.0006–1.18	0.00015–0.0003	<0.000075
Cumulative percentage passing	96.4	61.4	26.4	2.4	1.4

shows the range of particle sizes present in the soil sample, while Figure 1 shows its particle size distribution curve, which indicates a well-graded, slightly silty, gravelly sand with a coefficient of uniformity (Cu) of 30. The soil was classified according to BS 5930 [53]. Its gradation and consistency limits suggest that it can reach sufficient compaction levels and hence is deemed appropriate for road construction purposes. The colour of the soil was reddish-brown. A smooth curve was obtained by multiplying a correction factor, as outlined in BS 1377-2: 1990, by the actual mass retained on each sieve to determine the corrected mass m. The correction factor is given by:

$$Correctionfactor={\displaystyle \frac{m_2}{m_3}}$$

where $m_2$—is the portion of dried soil passing the 20 mm sieve; and $m_3$—is the portion of dried soil after riffling the soil passing the 20 mm sieve. This procedure is performed for subsequent sieves between 20 mm and 6.3 mm.

3.2. RHA Preparation

The rice husks were obtained from a few rice mills and farms in Mongu, Western Province, in Zambia. They were cleaned by washing to remove any impurities and thereafter dried to eliminate moisture and aid in ash production. It was important to the authors that the findings of this research have a direct application to rural road construction in low-income countries. Thus, the calcination process was simplified and made as close as possible to a rural setup, where they are likely to use kilns. The rice husks were then carefully placed in a make-shift kiln and fired up to convert to rice husk ash (RHA). The rice husks were burnt by keeping a constant fire for 7 h, during which time the husks changed from whitish to black ash colour at the end of the 7 h. The RHA were then collected and stored in a dry and covered area to prevent moisture absorption. Figure 2 summarises this process of RHA processing. The rice husks were a brownish colour before the firing process and changed to black after calcination.

3.3. Sample Preparation

The suitability of RHA application for strength improvements on this soil was studied by mixing a range of RHA percentages and OPC with untreated soil and performing both compaction and CBR tests to record changes in dry density and CBR values. Both tests were performed following BS 1377-4: 1990 [54]. The force applied to the plunger, during the CBR test, corresponding with the 2.5 mm and 5 mm penetration readings is recorded and used to calculate the CBR value. The CBR value is obtained by dividing the force recorded at 2.5 mm and 5 mm readings by the standard force required to achieve a 100% CBR value. The equations used to calculate the CBR values at 2.5 mm and 5 mm penetration readings are shown below:

$$\begin{gathered} CBRvalueat2.5mmpenetration={\displaystyle \frac{W}{13.2}}\ast 100\% \\ CBRvalueat5mmpenetration={\displaystyle \frac{W}{20}}\ast 100\% \end{gathered}$$

where $W$—force recorded at 2.5 mm and 5 mm penetrations in kN, given by

$$W=(Provingringreading\ast F\ast g)/1000$$

where $F$—the standard load applied to move the proving ring; and g—gravity taken as 9.81 m/s.

The untreated soil from the test pits was air-dried in the laboratory for at least 24 h, and a pick was used to gently break apart large soil clumps. The Ordinary Portland Cement (OPC) used in the experiments had a 32.5 N grade, while the RHA content was added in incremental proportions ranging from 10 to 15% by weight of the dry untreated soil. Moreover, 10% OPC content, measured by weight of the soil, was added to the untreated soil sample as a control sample (CS) by which to compare the strength results of the RHA + OPC-treated soil.

Table 3. Experimental mix proportions of soil, OPC and RHA.

Sample No.	Soil%	OPC%	RHA%
1	100	-	-
2	90	10
3	84	6	10
4	79	6	15
5	82	8	10
6	77	8	15

indicates the mix proportions of untreated soil, RHA and OPC used in the experiments.

3.4. Experimental Procedure

The index properties of the untreated soil were tested according to BS 1377-2: 1990 and the soil was classified as a well-graded, slightly silty, gravelly sand with low plasticity. The index properties of the untreated soil are summarised in

Table 4. Properties of untreated soil.

Soil Property	Value
Gravel (%)	30
Sand (%)	50
Silt (%)	20
Liquid limit (%)	30.3
Plastic limit (%)	17.72
Plasticity Index (%)	12.58
Linear Shrinkage (%)	4.29
Specific Gravity	4.9
BS Classification	Well-graded, slightly silty, gravelly sand with low plasticity
Colour	Reddish-brown
Maximum Dry Density (kg/m3)	1511.5
Optimum Moisture Content (%)	19.6
Unsoaked CBR (%)	7.43

. The authors recognise the limitations in experimental procedures due to insufficient laboratory equipment. However, all the tests conducted were performed according to standard procedures as mentioned in each section.

Similarly to untreated soil samples, the treated soil samples were also tested for maximum dry density and CBR values to study the effect of RHA additions. Moisture was gradually added to the OPC + RHA soil mixture while thoroughly mixing until the soil reached a consistent mixture. This was performed carefully to avoid over-saturation that may lead to excessive shrinkage or weakening of the treated soil. The treated mixture was then left to cure for at least 1 h before compaction. This curing period allowed for the pozzolanic reaction between the RHA and soil particles to occur, hence creating a stable matrix. The standard Proctor test was conducted by adding the untreated soil and OPC + RHA soil mixtures in layers to a 1 L corrosion-resistant mould that was attached to a baseplate and extension. For each layer, 27 blows were dropped from about 300 mm using a 2.5 kg rammer. Thereafter, the corresponding maximum dry density and optimum moisture content are determined by plotting the dry density against the moisture content curve for each sample. The optimum moisture content (OMC) for each sample corresponds with the highest dry density reading. The CBR test was conducted on untreated and OPC + RHA soil mixtures that were cured for 24 h, and thereafter, the samples were soaked for 5 days. The soil mixtures were placed in 152 mm diameter by 127 mm high mould fixed with a 152 mm diameter by 50 mm high extension. The results for 2.5 mm and 5 mm penetrations were then recorded.

4. Results

4.1. Maximum Dry Density

The untreated soil was tested for strength properties and compared to soil sample mixtures of varying RHA and OPC content, as shown in Table 3. The maximum dry density (MDD) of the untreated soil and soil mixtures was recorded, and Figure 3 illustrates the effect of RHA inclusion on the soil’s MDD. As can be seen, the MDD of the untreated soil (with 0% RHA) was found to be 1511 kg/m³. When the soil samples with 6% OPC were tested, it was observed that the MDD significantly reduced with the addition of 10% RHA and slightly increased when 15% RHA was added. On the other hand, when samples with 8% OPC were tested, there was a notable increase in MDD at 10% RHA (1519 kg/m³) which decreased at 15% RHA inclusion (1505 kg/m³). The soil sample with 10% OPC + 0% RHA recorded MDD of 1505 kg/m³. Therefore, it is important to note that the MDD results indicated that OPC alone may not be a suitable stabiliser for this untreated soil. The samples mixed with 10% OPC + 0% RHA show that OPC does not improve the MDD of the untreated soil. However, the difference in results for the 10% RHA additions with different OPC quantities illustrates a strong correlation between OPC quantity when mixed with RHA. A higher OPC content and a 10% RHA leads to significant strength increases in the soil. This strength increase is what engineers are after in order to improve the durability and longevity of the constructed road, which in turn, justifies improved resilience to climate change effects and reduced maintenance costs, and ensures the sustainability of rural transport systems. Sustainability is further justified by the potential to reduce the use of OPC and partially reduce it by RHA.

4.2. Optimum Moisture Content

The optimum moisture content (OMC) results were obtained from each sample’s MDD test results. The untreated soil had an OMC of 19.6% and this varied with different RHA + OPC additions, as shown in Figure 4. With 6% OPC + 10% RHA, the OMC recorded was 24.8% and increased to 25.2% when the RHA was increased to 15%. Similarly, with 8% OPC + 10% RHA, the OMC recorded was 25% and increased to 26.2% when the RHA was increased to 15%. This demonstrates that with lower OPC content into the soil mixture, the RHA percentage increment does not significantly affect the OPC. However, with 8% OPC, the RHA percentage increment results in OMC increase and this increased OMC can lead to increased resilience of rural and low-volume roads to increased precipitations.

4.3. Soil Plasticity

Figure 5 shows the variation in the plasticity of the soil samples after RHA and OPC were added. While the untreated soil was classified as having low plasticity, it was observed that adding RHA caused a general decrease in the soil’s plasticity and that the higher the percentage of RHA, the more the decrease in plasticity for both 6% and 8% OPC content in the soil. With the 6% OPC, the soil plasticity index reduced from 12.58% to 1.08% when 10% RHA was added, and then marginally increased to 2.8% when 15% RHA was added. Similarly, with the 8% OPC, the soil plasticity index reduced from 12.58% to 2.08% when 10% RHA was added and increased slightly to 2.8% when 15% RHA was added. This demonstrates that the addition of RHA generally causes a reduction in the soil’s plasticity although the specific rate of reduction depends on the soil type and its structure. Granular soils, like the one used in this study, are less likely to have their particles altered by the addition of RHA. However, the lower plasticity noted in the samples reflects the previously noted densifying effect of RHA. Also, the effect of RHA in reducing the soil plasticity could be applied to clayey expansive soils with high plasticity.

4.4. California Bearing Ratio

The results of the CBR test are shown in Figure 6 and Figure 7. The CBR values are an indication of the soil’s resistance to deformation under the effects of applied loads. In this study, the results show that there were no notable differences between the CBR values of the untreated soil and the soil with 10% OPC + 0% RHA. However, the addition of OPC + RHA to the untreated soil showed improvements of the CBR values. Notably, the treatment of the soil samples using 6% OPC and 8% OPC, with the addition of 10% RHA performed better in terms of CBR values. Conversely, at both 2.5 mm and 5 mm penetration, the highest CBR value recorded was for the soil treated with 8% OPC + 10% RHA. With the CBR being probably the most used parameter when assessing the bearing capacity of soils to be used in road construction, it could be said that a mixture of untreated soil + 8% OPC + 10% RHA would be the most optimised one in the study area. This could potentially apply in most of the SSA countries, given that the climate and environment seem to be approximately the same and less variable in this region.

5. Discussion

The results from this study showed that when lower percentages of OPC by weight are added to the soil samples, the MDD of the latter may not necessarily improve. However, adding both OPC and RHA together was found to generally improve soil MDD. This was noted when the soil samples were mixed with 6% OPC and the soil density decreased while it slightly increased with RHA inclusion. Remarkably, the addition of 8% OPC to the soil showed a positive effect on the MDD when 10% of RHA was added. However, this increase in the MDD was again reduced when 8% OPC + 15% RHA fractions were added to the soil. This was because there was an insufficient amount of OPC added to allow for the adequate formation of calcium hydrosilicates in the treated soil.

Similarly, the soil samples mixed with 10% OPC + 0% RHA show that OPC alone does not improve the MDD of the untreated soil. However, the difference observed in the results following additions of 10% RHA to the soil samples with different OPC quantities illustrates a strong correlation between OPC and RHA quantities. The higher the OPC content with a 10% RHA leads to significant strength increases in the soil. The literature shows that the addition of RHA generally leads to lower soil porosity [41], a property that needs to be optimally reduced or completely removed to achieve desirable MDD results. Similarly to the results in this study, Verma et al. [55] tested the stabilisation of a colluvial soil using RHA and micro silica powder and showed that when about 15% RHA is added the MDD could decrease. Because strength properties are a vital component in assessing the suitability of stabilising materials, it is suggested that more tests be conducted on varying soil types to fully understand the effect of RHA on each soil type’s MDD. However, in the context of the study area and the SSA region, this study’s results suggest that an optimum quantity of about 10% RHA could be appropriate when engineers want to reduce the quantity of OPC for soil stabilisation in road construction.

Moreover, the RHA inclusion affected the soil’s OMC by slightly increasing it for all treated soil samples. This was expected since RHA requires more water to activate its pozzolanic reaction in the soil and it may suggest that soil treated with RHA can have an added resilience to increase precipitations. When the soil samples were treated using 6% OPC, the OMC only slightly increased and did not show any notable changes after adding 10% and 15% RHA fractions. However, when the same soil samples were treated using 8% OPC, the OMC continued to increase with the increase in the RHA additions. This agrees with the study by Pushpakumara [41] who reported similar results of increasing soil’s OMC with incremental amounts of RHA. Practically, this shows that higher amounts of RHA in the soil will lead to requirements of more compaction water to reach the OMC and, subsequently the MDD; the two engineering properties that can define the suitability of soils for road construction purposes. Practically, it is better to achieve a higher MDD with a lower OMC as this may also mean energy saving and reduced compaction time. However, this is more important in the construction of high-volume roads which require higher strengths of soil to withstand excessive loading from the traffic. Therefore, where strength requirements are limited to a certain range due to expected low levels of loading, such as in the construction of rural and low-volume roads, it is recommended to maintain lower percentages of OPC in the soil in order to achieve a lower OMC.

As for the effect of RHA on the plasticity of the soil samples, it was further noted that the RHA decreased the soil’s plasticity when 10% RHA was added. However, the plasticity slightly increased when RHA increased to 15%. This was consistent for the soil mixtures containing both 6% and 8% OPC contents and demonstrates that the addition of RHA generally causes a reduction in the soil’s plasticity although the specific rate of reduction may depend on the soil type and its structure. Granular soils, like the one used in this study, have a single grain structure and are not generally assessed by levels of plasticity even though there is a requirement for small levels of plasticity (about 10% in gravel roads) to bind granular particles together. Therefore, while RHA tends to increase the soil porosity, the binder replaces the few clay minerals in this granular soil, further reducing its plasticity [56]. However, the lower plasticity noted in the samples reflects the previously noted densifying effect of RHA. Also, the effect of RHA in reducing the soil plasticity could be applied to clayey expansive soils with high plasticity.

Finally, the effect of RHA on CBR was that it generally increased the CBR value of the treated samples. For the soil samples treated using 10% OPC + 0% RHA, the CBR values were notably lower than all the treated samples to indicate that OPC alone at such a reduced fraction may not improve the bearing capacity of the soil. The highest CBR values for both 2.5 mm and 5 mm penetration were recorded when 8% OPC + 10% RHA were combined to treat the soils. A greater resistance to penetration in the CBR indicates better soil strength characteristics, and therefore better quality for the soils to resist traffic loadings. The higher CBR values for the treated soil could be an indication of the soil solidification and densification that occurs after sufficient curing of the soil. Although the MDD results did not seem consistent and even showed a decline in density in some cases, it is important to test more than one strength parameter because soils often exhibit different characteristics.

While RHA stabilisation shows incredible promise in road construction potential cost reductions, it is worth noting that there would need to be a bridging in the practical knowledge gap on its application and on-site use. It is recommended that for ease of use, the RHA should be mixed with the cement of a weight ratio basis prior to application on site. The RHA-cement mix can then be applied to the soil and treated on site. Future research could focus on determining the optimal mix-ratios for the best results.

This study encountered several challenges, including limited access to advanced equipment that would have enabled detailed analysis of the materials. It is recommended that future research investigate the impact of calcination temperature on rice husk ash (RHA) and the use of rice husks that have been ground versus milled. Furthermore, future studies could evaluate the effects of RHA on different soil types and investigate the bonding mechanisms between RHA, cement, and soil.

6. Conclusions

Rural infrastructure in developing countries faces persistent challenges, particularly due to limited access to durable and cost-effective road construction materials. This compromises the efficiency and effectiveness of rural transport systems, leading to shortcomings in the effort to leave none behind in the journey to achieving sustainable development goals. This study evaluated the suitability of using rice husk ash (RHA) as a partial cement replacement in stabilising weak subgrade soils. The purpose was to improve road construction materials in order to achieve durable, cost-effective roads with limited transport disruptions due to recurrent maintenance needs. The following main conclusions were drawn from this study:

• The untreated soil was a well-graded, slightly silty, gravelly sand with a maximum dry density of 1511.5 kg/m³ and an optimum moisture content of 19.6%. When treated with 8% OPC and 10% RHA, the MDD slightly increased to 1519 kg/m³, while the OMC rose to 25%, indicating the higher water demand of the porous RHA particles and potential to contain more infiltrated water before the failure of road foundations and embankments.
• The addition of OPC and RHA reduced the soil plasticity significantly, demonstrating that higher percentages of RHA tend to reduce the soil’s plasticity index. The reduction in the soil’s plasticity with increased RHA content implies that the treatment could be effective in reducing the cohesion between soil particles and hence improving drainage properties of the soils, which is an important parameter for the soils in road construction.
• The California bearing ratio (CBR) value, which was 7.43% for untreated soil, improved markedly in treated samples, with the 8% OPC + 10% RHA mixture demonstrating the highest resistance at both 2.5 mm and 5 mm penetration tests. This positive improvement in CBR values could be the most important effect of the RHA to the soils used in road construction.

These improvements highlight the RHA’s potential as a sustainable stabiliser that can replace OPC for the stabilisation of road construction materials for low-volume and rural roads. The method can lead to durable and sustainable rural transport infrastructure, therefore improved transportation systems in rural areas of the developing countries.