Strength Development and Durability of Cement-Stabilized Tropical Clay–Quarry Dust Mixtures for Pavement Construction

Abstract

Road and pavement construction require huge volumes of borrowed soils in addition to the foundation soils. Unfortunately, not all soils are suitable for construction purposes. Soil stabilization is a fundamental technique used to enhance the engineering properties of weak ground/soil to meet the demands of large infrastructure projects, such as roads. It is in this regard that this study investigates the strength development, durability, and effectiveness of cement and quarry dust as stabilizers to enhance the geotechnical properties of a weak tropical clay soil. Cement was added in the range of 0% to 10% while quarry dust was used to partially replace soil in the range of 0% to 50%. The results show significant improvements in the Atterberg limits and strength properties of the tropical clay. The liquid limit reduced from 43.2% to 25.1% while the plasticity index reduced from 17.6% to 10.2% at 50% quarry dust and 10% cement content. Similarly, the maximum dry unit weight increased from 17.4 kN/m³ to 21.3 kN/m³ while the optimum moisture content decreased from 17.1% to 12.9%. The maximum soaked CBR value was 172%, representing a 1497% enhancement over untreated soil. Also, the maximum unconfined compressive strength (UCS) reached 2566 kN/m² at 28 days of curing, representing a 1793.73% increase when compared to the untreated soil. Cement content was found to be the predominant factor influencing strength development. The study shows that cement–quarry dust blends compacted at high energy can be adopted in sustainable road construction.

1. Introduction

The construction of roads requires huge volumes of soil in addition to the foundation soils (subgrade soils). Unfortunately, not all soils are suitable for construction purposes. Soil stabilization is a fundamental technique in geotechnical engineering, particularly for enhancing the engineering properties of weak soils to meet the demands of infrastructure projects such as road pavements, embankments, and foundations [1,2]. Tropical clays are residual soils whose properties are heavily influenced by the properties of the parent rock and mineralogy composition. In tropical regions, where residual soils such as black clays and lateritic soils are abundant, these materials often exhibit undesirable characteristics, including high plasticity, low shear strength, excessive swelling and shrinkage, and poor durability under varying moisture conditions. These are related to the presence of fines and water-absorbing clay minerals, such as montmorillonite. High plasticity values have traditionally been used to describe the strength and swell properties of tropical soils. Such properties can lead to the premature failure of road pavements, leading to increased maintenance costs and compromised safety [3,4,5].

The utilization of chemical stabilizers, such as cement, has been successfully adopted to improve the strength of soil, reduce permeability, and enhance its resistance to environmental degradation [4,6]. Cement has been used in the stabilization of problematic soils, such as loess [7,8], lateritic soils [9,10], gravelly–silty sand with low plasticity [11], and black cotton soils [12,13]. However, traditional cement stabilization can be economically burdensome and environmentally intensive due to high cement consumption and associated carbon emissions [5,14]. Therefore, sustainable stabilization of soils using different eco-friendly materials, such as biopolymers [15,16], agricultural waste [14,17], construction waste [18,19], and industrial waste [20,21], continues to be investigated as alternatives.

The incorporation of industrial by-products such as quarry dust, a fine aggregate waste generated from rock-crushing operations, has also been considered as a sustainable alternative for soil improvement [9,22,23,24,25]. Quarry dust is non-biodegradable and poses a significant inhalation risk to both humans and animals. Furthermore, its direct disposal is detrimental to the environment by preventing the growth of vegetation when it is dumped [24]. Therefore, utilizing quarry dust in the construction of roads not only solves the challenge of materials’ scarcity but also presents numerous advantages to humanity and the environment. As a construction material, quarry dust exhibits superior shear strength relative to sand due to its high shear strength [26,27]. Moreover, quarry dust has higher permeability due to its coarser particle composition, and its properties are not susceptible to changes in moisture content [28], making it an appropriate material for road construction sub-layers. The main chemical composition of quarry dust is Si0₂, Al₂O₃, Fe₂O₃, and CaO, while the mineralogical composition can be quartz, feldspar, muscovite, and biotite. Currently, there are limited studies on the pozzolanic effects of quarry dust on soil improvement. Recent studies on its usage in soil stabilization have focused on its ability to alter the gradation of the soil to impact its geotechnical properties. Little is known about quarry dust’s effects on the formation of cementitious contributions to the soil matrix. Moreover, quarry dust can be pozzolanic or inert in nature depending on its source and particle size [23,24]. Therefore, quarry dust materials not only reduce the reliance on virgin materials but also promote waste valorization, aligning with global sustainability goals.

The combined use of cement and quarry dust offers a synergistic and complementary strategy in soil stabilization. The incorporation of quarry dust into clayey soils modifies the soil’s index properties, thereby promoting a transition to more desirable engineering properties [28,29]. In a study by Etim et al. [9], the application of cement stabilization to soil–quarry dust mixtures increased the dry density, reduced the optimum moisture content, and increased the unconfined compressive strength. The addition of cement binds together the resultant coarser matrix and hence improves stability and durability while ensuring the optimal performance of the stabilized soil, particularly in the presence of moisture [28,30]. Fundamentally, these materials function synergistically with quarry dust, modifying the soil matrix and cement, delivering the essential cohesion and strength [31,32,33]. Also, the modification made to the natural soil by quarry dust can lead to a significant reduction in the quantity of cement required to achieve the desired engineering property of the soil. In the recent study by Niraula et al. [33], it was observed that the inclusion of up to 50% quarry dust significantly reduced soil plasticity, enhanced maximum dry density, and reduced optimum moisture content, with negligible influence observed from cement contents below 4%. Optimal stabilization was attained with cement contents ranging from 4% to 6%, providing sufficient strength for general applications, whereas a cement content of 8% was necessary to achieve the requirements for high-strength purposes. However, the effects of variable compaction energy and the durability of soil–cement–quarry dust mixtures are often overlooked.

In an earlier study by Sitaram and Purushotham [28], it was observed that the incorporation of cement resulted in a significant enhancement of shear strength parameters of lithomargic clays. The angle of internal friction showed an improvement of about 114% with the addition of 10% cement. Similarly, the cohesion value demonstrated a substantial increase of about 217% at the same cement content. However, the effects of variable compaction energy and the durability were not addressed in the study.

Strength development in cement-stabilized clay soils is affected by factors such as cement dosage, moisture content, the nature of clay minerals, and the duration of curing [4], and it has been established in the literature that the strength of stabilized soils increases with cement content [34]. However, at low water content, which is typical for road construction, the influence of moisture content on strength development is rarely discussed, since all compactions are performed at the optimum moisture content. Also, researchers reported that curing time is an important parameter that influences strength development in cement-stabilized soils [33,35,36]. Moreover, it is also imperative to evaluate the durability of stabilized soils, even when the matrix attains satisfactory levels of strength and stiffness under dry conditions, as it may encounter challenges in both achieving and maintaining the required strength for load bearing in changing environments [37,38]. Investigations have been conducted on the durability of various types of stabilized soils, with different methods for measuring durability currently in use [39]. Certain studies have undertaken comparisons among these established durability assessment techniques [40,41].

Despite these advancements, key aspects of cement-stabilized tropical clay–quarry dust mixtures remain underexplored, particularly in terms of strength development over time, and durability under variable environmental cycles. Strength development refers to the progressive gain in mechanical properties during curing, influenced by hydration kinetics and admixture proportions. Durability assessments are very important for predicting service life in tropical climates prone to seasonal rainfall and high humidity. However, comprehensive studies integrating these factors for tropical clay–quarry dust mixtures are scarce, especially under varying cement contents, different compaction energies, and quarry dust dosages. This study aims to bridge these knowledge gaps by evaluating the effects of cement stabilization on tropical clay mixed with quarry dust. Specifically, the objectives are as follows: (1) to evaluate the effects of cement and quarry dust content on the geotechnical properties of tropical clays; (2) to investigate strength development through UCS at different curing periods; (3) to assess durability via the 7-day UCS immersion test; and (4) to develop predictive models for strength development of clay–quarry dust–cement mixtures.

2. Materials and Methods

The clay soil was collected from a construction site located at Amansea in Awka North Local Government Area, Anambra State, Nigeria (coordinates: 6°14′54″ N, 7°8′5″ E, elevation: 30 m above sea level). Quarry dust was procured from a construction materials market in Agu Awka, Anambra State, Nigeria. Portland Limestone Cement (Grade 42.5 N) was sourced from authorized local suppliers in Awka and was compliant with the requirements in [42]. The clay soil samples were partially replaced by 0%, 10%, 20%, 30%, 40%, and 50% quarry dust. These soil mixtures were then stabilized with 0%, 2%, 4%, 6%, 8%, and 10% Limestone, Portland cement, and subjected to classification, strength, and durability tests. In soil stabilization practices for road construction in Nigeria, the maximum percentage of cement typically recommended is 12% by weight of dry soil. This limit is derived from empirical studies and aligns with the Nigerian General Specifications for Roads and Bridges. For cement-stabilized subgrade (CSS) and cement-treated base (CTB) applications, the specified range is generally >5% to 10%, with 12% serving as the maximum to ensure economic viability and performance under tropical conditions prevalent in Nigeria. Quarry dust has been used in different studies for the partial replacement of soil in the dosage ranges of 0% to 50% [23,24,25,32].

The sample preparation was performed according to the procedure described in [43]. Specifically, the classification tests carried out were for the determination of natural moisture content, specific gravity, Atterberg limits, and particle size distribution according to the requirements of [44].

Accordingly, strength tests conducted on the samples were the compaction, unconfined compressive strength (UCS), and California bearing ratio (CBR) tests. All these tests were performed in compliance with the standard requirements [44,45]. Two compaction energies were employed in this study—the British Standard Light (BSL) and the British Standard Heavy (BSH). The BSL (compaction energy = 605.493 kJ/m³) is equivalent to Standard Proctor (compaction energy = 593.9 kJ/m³), while the BSH (compaction energy = 2723.526 kJ/m³) is equivalent to the Modified Proctor (compaction energy = 2671.38 kJ/m³). Also, the CBR tests on the stabilized soils followed the procedure recommended by the Nigerian Specifications for Road and Bridges [46]. These specifications involve curing the cement-stabilized soil specimens under a wax seal for 6 days, followed by a 24 h soaking period prior to testing, after which the specimens were permitted to drain for another 15 min. The UCS test specimens were prepared in triplicate within a 38 mm diameter mould and subjected to 7-, 14-, and 28-day curing periods before being tested.

The 7-day UCS immersion test described in [38,45] was employed in this study. The procedure involves curing one set of stabilized samples in an impermeable membrane for 7 days and subsequently soaking them in water for another 7 days while curing the second set of identical stabilized specimens in a similar impermeable membrane for 14 days. The two samples were subjected to an unconfined compression test according to [45], and the durability index was calculated as a ratio of the UCS of the immersed to the non-immersed samples, as shown in Equation (1):

$$\mathrm{D}\mathrm{u}\mathrm{r}\mathrm{a}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y} \mathrm{I}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}={\displaystyle \frac{\mathrm{U}\mathrm{C}\mathrm{S} \mathrm{o}\mathrm{f} \mathrm{s}\mathrm{o}\mathrm{a}\mathrm{k}\mathrm{e}\mathrm{d} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}{\mathrm{U}\mathrm{C}\mathrm{S} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{u}\mathrm{r}\mathrm{e}\mathrm{d} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}}$$

(1)

3. Results

3.1. Index Properties

The general geotechnical properties of the soil and quarry dust are shown in

Table 1. Properties of the soil and quarry dust.

Property	Value (Clay)	Value (Quarry Dust)
Natural moisture content (%)	34.1	---
Percentage passing BS No 200 sieve (%)	54.9	22
Liquid Limit LL (%)	43.2	Non-plastic
Plastic Limit PL (%)	25.6	Non-plastic
Plasticity Index PI (%)	17.6	Non-plastic
Specific gravity	2.46	2.71
AASHTO classification	A-7-6 (clayey soil)	A-1-b (Stone fragments, gravel, sands)
USCS classification	CL (Inorganic low-plastic clay)	GP (Poorly graded gravels and gravel-sand mixtures)
MDUW (BSL) (kN/m3)	15.89	---
MDUW (BSH) (kN/m3)	17.49	---
OMC (BSL) (%)	19.4	---
OMC (BSH) (%)	17.1	---
UCS (BSL) (kN/m2)	135.5	---
UCS (BSH) (kN/m2)	163.9	---
CBR (BSL) (%)	6.4	---
CBR (BSH) (%)	8.4	---
Colour	Grey	Grey
Subgrade rating	Poor	Good to excellent

, while the particle size distribution curves are shown in Figure 1.

According to the Nigerian General Specification for Roads and Bridges [46], materials passing through a 425-micron sieve are required to have a liquid limit not exceeding 35% and a plasticity index not exceeding 12%. This upper boundary of plasticity index for soil materials used in road construction was also recommended in [47,48], especially where these materials are exposed to rainfall energy, such as in unpaved roads. In this study, the clay possesses a plasticity index of 17.6% and a liquid limit of 43.2%, thereby rendering it unsuitable for use as a subbase or base course material for pavement construction. In a previous study, it was found that roadways constructed with soil that has liquid limits in the range of 43% to 60.50% have been prone to premature failures [3].

The clay soil was classified as A-7-6 according to the AASHTO classification system [49], and as inorganic clay with low plasticity (CL) according to the Unified Soil Classification System [50]. From the sieve analysis results, 54.9% of the soil sample passed through sieve No. 200 (75 μm), and the index properties suggest that it is unsuitable for base/sub-base construction in its untreated state, indicating the need for soil stabilization. The quarry dust was non-plastic, and 22% of the sample passed through the No. 200 sieve, thereby placing it in the category of granular materials based on AASHTO specifications.

In

Table 2. Chemical oxide composition of the samples used in the study.

Chemical Oxide Composition	Amansea Clay	Quarry Dust	Portland Limestone Cement (42.5 N)
Silica oxide (SiO2)	88.031	72.396	11.256
Magnesium oxide (MgO)	0.00	0.00	0.00
Alumina (Al2O3)	4.653	11.510	3.291
Phosphorus Penta oxide (P2O5)	0.000	0.132	0.000
Potassium oxide (K2O)	0.142	6.698	0.737
Lime (CaO)	0.244	3.805	79.684
Tin Oxide (TiO2)	3.152	1.078	0.156
Iron Oxide (Fe2O3)	2.221	1.582	1.724

, the chemical oxide compositions of the clay soil, quarry dust, and Portland limestone cement are presented. The quarry dust and Amansea clay are rich in silica oxide (SiO₂) at 88.03% and 72.34%, respectively, while the Portland limestone cement is rich in Calcium oxide (CaO) (79.68%) and deficient in SiO₂ (11.26%). Furthermore, the silica sesquioxide ratio of the clay showed that it is a tropically weathered non-lateritic soil, as shown in the relationship below:

$${\displaystyle \frac{{\mathrm{S}\mathrm{i}\mathrm{O}}_2}{{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3+{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3}}={\displaystyle \frac{88.031}{4.653+2.221}}=12.8>2.0\left(\mathrm{n}\mathrm{o}\mathrm{n}-\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{c} \mathrm{s}\mathrm{o}\mathrm{i}\mathrm{l}\right)$$

(2)

Generally, the chemical oxide composition of the quarry dust showed that there was no significant concentration of heavy metals or alkaline minerals in the quarry dust. Therefore, there is no potential leaching or contamination of toxic materials into the soil and groundwater.

3.2. Atterberg Limits

The untreated clay has a plasticity index of 17.6% and a liquid limit of 43.2%, thereby making it unsuitable for use as a subbase or base material. Figure 2, Figure 3 and Figure 4 show that the Atterberg limits properties of the soil reduced as the quantity of quarry dust and cement increased. As the quarry dust content increased from 0% to 50%, the Atterberg limits reduced. Specifically, the liquid limit decreased from 43.2% to 31.8%, the plastic limit from 25.6% to 18.0%, and the plasticity index from 17.6% to 13.8%. These results indicate that neither quarry dust nor cement alone was sufficient to modify the Atterberg limits of the soil to comply with the specifications outlined by the Federal Ministry of Works and Housing [46]. However, the combined application of cement and quarry dust effectively enhanced the consistency limits of the clay soil, commencing at 30% quarry dust and 2% cement content. This result showed that repurposing industrial waste, such as quarry dust, into soil stabilization significantly reduced the demand for cement in improving the plasticity properties of weak clays. Reusing industrial waste is seen as one of the major pillars of sustainability and economic circularity. At a 30% quarry dust and 2% cement composition, the liquid limit was observed to be 31%, with a plasticity index of 11.1%. The most significant reductions in liquid limit and plasticity index were observed at 50% quarry dust and 10% cement contents, with values of 25.1% and 10.2%, respectively. At soil mixtures of 30%QD + 2%Cc and above, the plasticity requirements for soils suitable for base/sub-base construction were met. Therefore, the carbon footprint of cement in highway construction for the modification of plasticity properties can be significantly reduced by blending the soil with quarry dust, leading to the sustainable construction of roads and highways.

3.3. Compaction Properties

The natural and stabilized clay samples were subjected to compaction tests using the British Standard Light (BSL) and British Standard Heavy (BSH) compaction efforts. For the natural soil, the maximum dry unit weight (MDUW) and optimum moisture content (OMC) were 15.9 kN/m³ and 19.4%, respectively, at BSL and 17.4 kN/m³ and 17.1% at BSH compaction efforts.

The effects of quarry dust and cement content variations on the compaction parameters are shown in Figure 5 and Figure 6. Generally, the MDUW was observed to increase with increasing cement and quarry dust content. Similarly, the OMC reduced with increasing cement and quarry dust contents. The combination of 50% QD and 10% cement content produced the highest increase in the maximum dry unit weight, with a value of 19.4 kN/m³, representing a 22.01% increase from the MDUW of the untreated soil. At the BSL compaction effort, the OMC reduced from 19.4% to 14.5%, a decrease of about 25.6%. With the BSH compaction effort, the OMC reduced from 17.1% to 12.9%, a decrease of about 32.4% at 50%QD + 10%Cc. A reduction in the OMC and a continuous increase in MDUW by increasing the cement and quarry dust content means the cement absorbs most of the water for hydration reaction [51].

The synergistic effects of cement-stabilized clay–quarry dust are, therefore, obvious from the results of the compaction parameters. The enhanced MDUW and a reduction in the OMC allow locally available clay soil to be used as a sub-base or base material for pavement construction. In many tropical regions of the world, this can reduce the need to extract and transport virgin aggregates for highway pavement foundation. This decreases fuel consumption, emissions, and additional environmental disruption from aggregate mining or quarrying sustainably.

3.4. California Bearing Ratio (CBR)

Figure 7 and Figure 8 show the variation in CBR values obtained for the untreated clay and the soil mixtures with QD and cement when subjected to BSH and BSL compaction efforts, respectively. In its natural state, a CBR value of 8% and 6% was observed for the clay soil at BSH and BSL compaction efforts, respectively. The soaked CBR of clay increased continuously with quarry dust content up to 50% addition. When the soil–quarry dust mixture was stabilized with cement, a progressive improvement in strength was observed. The CBR of the soil peaked at 50% QD and 10% cement content, with a value of 173% at BSH compaction energy.

The soaked CBR values exceeding 100% at different cement–quarry dust mixtures and compaction energies suggest the potential achievement of a stable material that can resist deformation under traffic load and moisture. Achieving stability for base or sub-base materials reduces the need for frequent repairs or replacements, thereby conserving resources and minimizing destruction to the environment and overuse of natural resources over time. With cement alone, the maximum CBR observed at BSH compaction energy was 79%, but with the addition of 20% quarry dust, the value increased to 122%. Therefore, the need for additional cement for the improvement of highway foundation materials can be curbed sustainably through the introduction of industrial waste such as quarry dust.

3.5. Unconfined Compressive Strength

3.5.1. BSL Compaction Effort

Figure 9, Figure 10 and Figure 11 show the results of UCS of cement-stabilized clay–quarry dust mixtures for the curing ages of 7, 14, and 28 days and for the BSL compaction efforts. The UCS of the natural clay was observed to be 135.5 kN/m² when subjected to BSL compaction energy. The results show that the UCS continuously increased with increasing quarry dust contents. On stabilizing with cement contents ranging from 2% to 10%, Calcium Silicate Hydrate (CSH) and Calcium Aluminate Hydrate (CAH) were produced by the hydration reaction of the cement, which increased the strength of the soil after curing. The standard requires that cement-stabilized soils should meet a minimum UCS value of 1725 kN/m² after 7 days of curing [46].

According to the results in Figure 9, the 7-day UCS requirement was not met when the clay was stabilized with quarry dust and cement within the ranges considered in this study. However, a peak UCS value of 1697 kN/m² was observed at 50% quarry dust and 10% cement content, which reflects a 1085.98% increase in strength. Therefore, further considerations should be made when a low compaction effort is to be used on-site for the stabilization of clay materials, such as in the construction of rural or low-volume traffic roads. An increase in strength was observed after the samples were cured for 14 days, as shown in Figure 10. The UCS increased from 588 kN/m² to 700 kN/m² with 2% cement and 50% quarry dust content, representing an increase of about 19% from the results observed from the 7-day curing period. Moreover, at 40% and 50% quarry dust contents with 10% cement stabilization, the 14-day UCS exceeded the threshold value of 1725 kN/m² with a value of 2003 kN/m².

At 28 days of curing, an increase of 15.97% (700 kN/m² to 811.8 kN/m²) was observed in the UCS value from the 14-day UCS at 2% cement and 50% quarry dust content, as shown in Figure 11. Furthermore, at 50% quarry dust and 8% cement content, a UCS value of 1795 kN/m² was observed, an increase of 38.61% from the 7-day UCS value. The strength of the stabilized soil mixture peaked at 10% cement and 50% quarry dust content, with a value of 2344 kN/m². The increase in UCS value is connected to the increase in strength brought on by the cement’s continuous hydration during the curing period. The presence of moisture in the mixture causes the hydration reaction to continue, increasing the mixture’s strength over time [35].

Also, the stress–strain plot of the natural soil (without quarry dust) stabilized with cement and cured for 7 days is shown in Figure 12. After 7 days of curing, the peak axial strain dropped from 2.5% for the untreated soil to 2.26%, 1.9%, 1.62%, and 1.19% for the treated soil using cement contents of 2%, 4%, 6%, 8%, and 10%. However, higher brittleness for high cement content and higher curing time were reported [52], which is consistent with the results obtained for the ductility change with increasing cement and curing time in this research.

Generally, there is no explicit minimum percentage of axial strain recommended for cement-stabilized pavement bases in standards, including those applied in Nigeria. However, practical and research-based evidence suggests that axial strain at failure should generally be at least 0.5% to avoid excessive brittleness, which could lead to cracking under traffic or environmental stresses. The strain values obtained in this study are within the range of the strain values observed in other studies utilizing cement as a binder [9,10].

3.5.2. BS Heavy Compaction Effort

Figure 13, Figure 14 and Figure 15 show the results of the UCS of cement-stabilized clay–quarry dust mixtures for curing ages of 7, 14, and 28 days for BSH compaction efforts. The UCS of the natural clay was 163.9 kN/m² when subjected to BSH compaction energy, which was an increase of 20.95% from the value using BSL compaction energy. For the BSL compaction energy, a steady increase in the UCS values was observed as both cement and quarry dust contents increased. Moreover, it was observed that the soil could not be effectively stabilized with cement alone. The 7-day UCS value at 10% cement content was 798 kN/m², which was below the required value of 1725 kN/m² (see Figure 13). However, it still represented a 386% increase over the UCS of the natural soil. However, at 40% quarry dust content, the 7-day UCS value increased to 1821 kN/m². This shows that quarry dust can enhance the strength of cement-stabilized clays to meet design standards without the need for an increase in cement content. Cement production emits approximately 0.8–1 ton of CO₂ per ton, and this partial replacement lowers the carbon footprint of the stabilization process.

The sample compacted using BSH compaction effort and cured for 14 days showed an increment in strength (see Figure 14). At 2% cement and 50% quarry dust content, the UCS increased from 715 kN/m² to 810 kN/m² (an increase of 13.28%) when the curing period was increased from 7 days to 14 days. After 28 days of curing, another increase of 13.7% was observed in the UCS value from the 14 days, as shown in Figure 15. Moreover, at 50% quarry dust, 6% cement content, and 28 days curing, a UCS value of 1727.5 kN/m² was observed, a 41.94% increase from the 7-day curing value. The strength of the stabilized soil mixture peaked at 10% cement and 50% quarry dust content, with a value of 2566 kN/m². This agreed with [53], who reported 2670 kN/m² of UCS using 8% cement content to treat low-plasticity clay.

The stress–strain results of the natural soil (without quarry dust), stabilized with cement and cured for a 7-day period, are shown in Figure 16. The axial strain decreased for cement contents of 2%, 4%, 6%, 8%, and 10% from 2.3% for untreated soil to about 2.08%, 1.75%, 1.44%, 1.01%, and 0.89%, respectively. The observed decrease in axial strain at failure from 2.3% for untreated soil to values ranging from 2.08% (at 2% cement) down to 0.9% (at 10% cement) after a 7-day curing period is a characteristic response in cement-stabilized soils during unconfined compressive strength (UCS) testing. This trend reflects an increase in material stiffness and brittleness as cement content rises, which is well-documented in the geotechnical literature [53]. The reduction in axial strain at failure primarily stems from the microstructural changes induced by cement hydration, which transform the soil from a relatively ductile, plastic material to a more rigid, cemented composite.

3.6. Durability

The results of strength indices and durability tests carried out on the clay soil stabilized with cement and quarry dust are presented in Figure 17 and Figure 18 for BSL and BSH compaction efforts, respectively. The use of only cement or quarry dust did not satisfy the minimum resistance to the loss of the strength value of 20% specified in [54] at both BSL and BSH compaction efforts. The use of cement–quarry dust mixture improved the resistance to the loss of the strength of the clay soil, with the attainment of maximum resistance to the loss of the strength value of 89.53% at a combination of 50%QD + 10%Cc (BSH compaction effort).

At the BSL compaction effort, a maximum resistance to the loss in strength of 82.5% was obtained at 50%QD + 10%Cc. By implication, most of the sample mixes considered in the study can maintain the required durability when subjected to alternate wetting and drying, indicating high service life. This durability reduces the frequency of road repairs or replacements, conserves resources, and minimizes environmental impacts on the pavement’s lifecycle.

In BS 1924 [45], no specific value is prescribed for resistance to the strength loss as determined by immersion tests. However, Ola [54] proposed a maximum permissible strength loss of 20%, corresponding to an 80% retention of strength. This has been widely adopted in Nigeria as a criterion for pavement durability assessment and could potentially be relevant in all tropical climate countries.

4. Discussion

4.1. Plasticity

The addition of quarry dust to clay soil reduces the fraction of clay particles, thereby lowering its plasticity. Similarly, when cement is added, it absorbs water during the hydration reaction, hence leaving less free water in the mixture to keep the soil plastic. As quarry dust content increased from 0% to 50%, the liquid limit reduced from 43.2% to 31.8%, the plastic limit reduced from 25.6% to 18.0%, and the plasticity index decreased from 17.6% to 13.8%. When cement was added from 2% to 10%, the liquid limit further reduced from 43.2% to 33.6%, the plastic limit reduced from 25.6% to 19.8%, and the plasticity index decreased from 17.6% to 13.8%. From the results, it was observed that using either quarry dust or cement alone was not sufficient to meet the required standards set by the Federal Ministry of Works and Housing [46]. However, combining both materials was effective and helped the mixtures meet the requirements. Significant improvements in the soil’s strength began at 30% quarry dust with 2% cement, where the liquid limit reached 31% and the plasticity index reduced to 11.1%. The best results, with the lowest consistency limit values, occurred at 50% quarry dust plus 10% cement, with a liquid limit of 25.1% and a plasticity index (PI) of 10.2%.

There are environmental, economic, and social sustainability gains from these findings. The use of quarry dust (a byproduct of stone crushing) in soil stabilization repurposes a material that would otherwise be discarded in landfills or dumped on storage sites into very useful materials. By incorporating quarry dust into soil–cement stabilization, the process reduces waste accumulation, minimizes land degradation from quarry waste disposal, and aligns with circular economy principles. Stabilizing weak clay soil with quarry dust and minimal cement (2%) significantly alters the plasticity and transforms it into a viable subbase or base material, and reduces the need for costly imported materials. This is particularly beneficial in developing regions or rural areas where budgets for infrastructure are limited.

These results were consistent with previous studies [9,24]. More importantly, previous research reported that, with PI closer to 10% but less than about 12%, the granular soil matrix is suitable for road construction [47]. When such material is used in unpaved roads, where it is consistently exposed to traffic and environmental stress, there is a good balance resisting soil detachment due to these stresses by both cohesion and gravity [48]. According to [9], the overall decrease in the liquid limit (LL) and plasticity index (PI) can also be linked to a cation exchange process involving calcium oxide (CaO) from the cement and water in the cement–quarry dust-treated soil. The calcium ions (Ca²⁺) released by the cement are taken up by certain clay particles in the soil, displacing lower-valence sodium ions (Na⁺), as shown in the next Equations (3)–(5) below. This leads to a compression of the soil’s diffuse double layer, resulting in soil flocculation and improved workability. Soils with a flocculated structure typically show low plasticity and high shear strength, and the improvement of soil properties is mainly due to the physicochemical interactions between soil particles and additives [9]:

CaO_(s) + H₂O_(l) → Ca(OH)_2(aq)

(3)

Ca(OH)_2(aq) → Ca²⁺_(aq) + 2OH⁻_(aq)

(4)

Clay − Na_(s) + Ca²⁺_(aq) → Clay₂ − Ca_(s) + 2Na⁺_(aq)

(5)

4.2. Compaction

The results from the compaction tests indicated a general increase in the dry density, accompanied by a general decrease in the optimum moisture content for the clay soil stabilized with cement and quarry dust. Quarry dust is a byproduct of stone crushing and primarily consists of rock particles (like quartz, granite, and feldspar). These particles are known to have a higher specific gravity than the individual clay minerals they replace in the soil matrix. Cement also has a higher specific gravity than most clay minerals. Specifically, the composite in this study comprises clay soil, quarry dust, and cement with respective specific gravities of 2.46, 2.75, and 3.15, yielding an overall higher specific gravity for the mixture and thereby reinforcing the explanation for the observed increase in MDUW.

Cement requires water for its chemical hydration reactions to form the binding cementitious products. This water is chemically consumed and becomes part of the solidified matrix. Therefore, a portion of the total water added to the soil–cement–quarry dust mixture is used up by the cement, making it unavailable as free water that contributes to lubrication for compaction [55]. This reduces the amount of free water required for the mix to achieve its maximum density and explains the reason for the reduction in the optimum moisture content. The cation exchange and flocculation process caused by cement significantly reduce the plasticity of the clay. Highly plastic clays require more water to achieve a workable consistency and allow the particles to slide past each other for compaction [23,32,56].

The improvements in compaction parameters when clay soil is stabilized with cement and quarry dust have important implications for sustainability in construction and geotechnical applications. A densification level that can be achieved using 10% cement can also be achieved by adding 50% quarry dust to 4% cement. In this way, greenhouse gas effects are reduced, and waste is also valorised. Furthermore, by enabling the use of local tropical clay soil, this stabilization method supports affordable infrastructure development in rural regions, where high-quality geomaterials are often unavailable or expensive to import. This promotes inclusive growth and improves living standards. Higher MDUW and lower OMC result in denser, more stable soil that is less prone to deformation under load or moisture changes. This improved densification leads to enhanced performance of the highway under traffic load and moisture ingress, hence leading to reduced maintenance needs and reduced institutional costs over the pavement’s lifecycle.

4.3. California Bearing Ratio

According to the FMWH [46] and TRRL [57], the California bearing ratio (CBR) of geomaterials should have a minimum value of 30% under soaked conditions and 80% under unsoaked conditions. Also, for soils stabilized with cement, the minimum soaked CBR value should reach 180% following a 6-day curing period. While this may seem very restrictive, modern pavement design codes have granted engineers enhanced flexibility in the selection of pavement materials and methodologies. This is called performance-based design and can deliver economic and environmental advantages through the integration of innovative solutions and materials. CD 225 [58] establishes thresholds for vertical strain in the subgrade and maximum surface deflection applicable to each foundation class. While performance assurance is normally achieved via on-site testing, direct measurement of surface modulus is rarely conducted, and engineers still rely on CBR values to estimate the modulus. In this study, the authors are of the opinion that a cement-stabilized material with a soaked CBR value of 100% and above is considered to have excellent strength and stiffness. This means that the stabilized soil, even after being subjected to a worst-case scenario (being soaked for six days to simulate a saturated condition), is as strong or stronger than an ideal granular base course material.

Observations from this study indicate that at a cement content of 10%, none of the mixtures complied with Nigeria’s Federal Ministry of Works and Housing standard for cement-stabilized base and sub-base materials by reaching the threshold of 180% [46]. However, this finding does not negate the significant gain in strength achieved through the stabilization process. For example, the CBR values increased significantly with both cement and QD percentages for both BSL and BSH compaction efforts. This increase is roughly linear and accelerates at higher cement levels, which is a positive interaction effect.

The results show that at 0% cement, the CBR increased from 8.4% (with 0%QD) to 49.7% (with 50%QD). At 10% cement, the CBR increased from 78.9% (with 0%QD) to 172.5% (with 50%QD). The highest CBR value (172.5%) occurred at 10% cement and 50%QD, suggesting maximum stabilization.

Arshad [59] proposed a model for predicting the resilient modulus of cement-stabilized soils as follows:

M_R = 49.37(CBR)^0.59

(6)

where

• M_R = Resilient modulus (MPa);
• CBR = California bearing ratio (%).

The main consideration from Equation (6) is that any stabilized material with CBR exceeding 100% can be used as pavement foundation class 3 and class 4 [58].

In terms of load-bearing mechanism, the coarser fractions in QD increase inter-particle friction, reducing shear deformation underload and thereby increasing the CBR values. Moreover, during hydration, cement produces calcium-silicate-hydrates (CSH) and calcium-aluminate-hydrates (CAH), which form insoluble gels that crystallize into an interlocking matrix, bonding soil and QD particles [34,56]. This increases cohesion and strength, with CBR improvements explained by the bonds created between the soil and the quarry dust particles by cement.

The statistical evaluation of the CBR test results for all the compaction efforts is presented in Equation (7):

CBR = 1.457QD + 8.23Cc+ 29.92LogE − 100.36
(R² = 0.946; Adjusted R² = 0.943)

(7)

4.4. Unconfined Compressive Strength

According to the specifications of Thailand’s Department of Highways, the unconfined compressive strength of cement-stabilized soil and cement-stabilized crushed rock must achieve a minimum of 1.7 MPa (1700 kN/m²) and 2.4 MPa (2400 kN/m²), respectively, following a 7-day curing period [34]. These criteria are consistent with some other established international benchmarks, and comparable requirements are documented in reliable sources [46,57], which stipulate that the 7-day UCS for cement-stabilized soils shall not fall below 1720 kN/m².

Under the BSL compaction effort, the 7-day unconfined compressive strength criterion was not met as the clay was stabilized with quarry dust and cement. Nevertheless, a maximum UCS value of 1697 kN/m² was achieved at 50% quarry dust and 10% cement content, signifying a huge 1085.98% enhancement in strength. Under the BSH compaction energy, it was also observed that the soil could not be effectively stabilized with cement alone. The 7-day UCS value at 10% cement content without quarry dust was 798 kN/m², which was below the required value of 1725 kN/m². However, this still represented a significant 386% increase over the UCS of the natural soil. When quarry dust was added, the UCS requirement was met at 40% quarry dust. The improved strength of the stabilized soil reduces construction and maintenance costs, thereby making it viable for rural and developing regions. This approach facilitates affordable and reliable pavement infrastructure, improves access to essential services, and fosters community resilience.

4.5. Strength Development

Generally, the findings of the study showed that there is a significant increase in strength as the curing duration increases. At BSL compaction energy, an enhancement in strength was observed as the curing period increased from 7 days to 14 days. The unconfined compressive strength of samples stabilized with 2% cement and 50% quarry dust increased from 588 kN/m² to 700 kN/m², reflecting a 19% improvement in strength between 7 and 14 days of curing. Moreover, at QD contents of 40% and 50% combined with 10% cement stabilization, the 14-day UCS surpassed the minimum threshold of 1725 kN/m². Analysis of the unconfined compressive strength at 28 days of curing revealed notable strength development in the stabilized soil mixtures. Specifically, for mixtures containing 2% cement and 50% quarry dust, a 15.97% increase in UCS was observed, with the value rising from 700 kN/m² after 14 days to 811.8 kN/m² after 28 days. Furthermore, with 50% quarry dust and 8% cement, the UCS value reached 1795 kN/m² after 28 days, representing a 38.61% increase from the 7-day UCS value. The maximum observed strength for the stabilized soil mixture occurred with 10% cement and 50% quarry dust, and 28 days of curing, achieving a peak UCS value of 2344 kN/m² as shown in Figure 19.

Similarly, using the BSH compaction energy, the seven-day unconfined compressive strength (UCS) at 10% cement content was recorded as 798 kN/m², falling short of the stipulated threshold of 1725 kN/m². Nonetheless, this value signified a 386% increase relative to the UCS of the untreated soil. Upon curing the sample for 14 days, an augmentation in strength was noted. Specifically, at 2% cement and 50% quarry dust content, the UCS increased from 715 kN/m² to 810 kN/m², denoting a 13.28% improvement from the seven-day to the 14-day curing period. Following 28 days of curing, a 13.7% increment was observed in the UCS compared to the 14-day value at 2% cement and 50% quarry dust content. Additionally, at 50% quarry dust and 6% cement content, a UCS of 1727.5 kN/m² was attained, reflecting a 41.94% increase from the seven-day UCS. The maximum strength of the stabilized soil mixture was achieved at 10% cement and 50% quarry dust content and 28 days of curing, reaching 2566 kN/m².

The ability of the stabilized materials to achieve significant strength gains with lower cement content (e.g., 6%Cc vs. 10%Cc) and extended curing periods (e.g., 13.7% UCS increase from 14 to 28 days) means a resourceful optimization of material usage and enhancement of long-term performance. This leads to economical designs and infrastructure longevity.

Moreover, a statistical model of strength development was provided in Equations (8)–(11).

Table 3. Regression parameters of the UCS model.

	Coefficients	Standard Error	t Stat	p-Value
Intercept	−1136.18	121.9047	−9.32021	1.6 × 10−17
QD	16.03079	0.715937	22.39135	1.16 × 10−57
CC	126.336	3.579685	35.29248	1.79 × 10−90
Log E	235.1945	37.44282	6.281432	1.89 × 10−9
Time	11.5449	1.400482	8.243519	1.77 × 10−14

shows significant statistical parameters at a 95% confidence level for Equation (11).

Table 4. Correlation analysis (BSH).

	QD	Cc	LL	PL	PI	OMC	MDUW	CBR	UCS
QD	1.000
Cc	0.000	1.000
LL	−0.797	−0.533	1.000
PL	−0.774	−0.612	0.954	1.000
PI	−0.757	−0.402	0.955	0.830	1.000
OMC	−0.966	−0.088	0.803	0.792	0.754	1.000
MDUW	0.756	0.617	−0.959	−0.976	−0.861	−0.754	1.000
CBR	0.648	0.735	−0.931	−0.940	−0.839	−0.702	0.935	1.000
UCS	0.528	0.798	−0.815	−0.875	−0.684	−0.618	0.849	0.952	1.000

The green color refers to the positive correlation while the red color refers to the negative correlation.

shows the correlation analysis for some selected parameters for the clay soil with BSH compaction effort.

At 7 days:

UCS = 13.677QD + 103.59Cc + 200.369LogE − 789.818
(R² = 0.909; Adjusted R² = 0.905)

(8)

At 14 days:

UCS = 15.876QD + 125.59Cc + 237.729LogE − 951.98
(R² = 0.913; Adjusted R² = 0.909)

(9)

At 28 days:

UCS = 18.538QD + 149.82Cc + 267.485LogE − 1101.03
(R² = 0.914; Adjusted R² = 0.911)

(10)

At all curing durations:

UCS = 16.03QD + 126.336Cc + 235.1945LogE + 11.54T − 1136.18
(R² = 0.897; Adjusted R² = 0.895)

(11)

where

• UCS = Unconfined compressive strength (kN/m²);
• QD = Quarry dust content (%);
• Cc = Cement content (%);
• LogE = Log of compaction energy (kNm/m³);
• T = Curing time (days).

From the results of the correlation analysis performed on the soil, as shown in Table 4, it was observed that the liquid limit, plastic limit, plasticity index, and optimum moisture content had a strong negative correlation with the quarry dust content for both BSL and BSH compaction energy. Both the CBR and the UCS were found to have a strong positive correlation with the cement content, and the same was found to be true for the CBR and the cement content. Both the CBR and the UCS exhibited a negative correlation with the optimum moisture content. Additionally, a strong positive correlation was observed between the CBR and the 7-day UCS, as shown in Figure 20.

4.6. Durability

The attempt to stabilize the soil using different cement alone did not meet the required durability standards using BSL and BSH compaction efforts. The resistance to loss in strength for these mixtures was below 80%, which is the specified threshold for a 20% loss of strength. However, when cement was combined with quarry dust to stabilize the laterite, an increase in resistance to strength loss was observed with higher cement content and increased quarry dust content at specified cement levels. This finding suggests that higher amounts of quarry dust can achieve resistance to strength loss at lower cement content if a higher compaction effort is used. When higher compaction efforts are applied, they generally result in increased density and better compaction of the soil–cement mixture. This leads to improved strength and durability of the stabilized soil. By applying greater compaction effort, the cement particles are better distributed and packed within the soil matrix. This enhances the bonding between the cement and soil particles, resulting in a more stable and durable matrix. The increased density also reduces the presence of voids and air pockets, minimizing the potential for moisture ingress and improving resistance against environmental factors such as water penetration, wet-dry cycles, and chemical degradation. This durability reduces the need for frequent repairs and maintenance, conserves depleting resources, and minimizes environmental disruption.

5. Conclusions

This study considered the influence of cement and quarry dust on the geotechnical behaviours of a tropical clay for pavement applications. Cement content varied in the range of 0%, 2%, 4%, 6%, 8%, and 10%, while quarry dust content ranged from 0%, 10%, 20%, 30%, 40%, to 50%. The following findings were recorded from the study:

• The soil used in this study was a non-lateritic clay that is unsuitable for application as pavement base or sub-base material.
• Cement and quarry dust were not sufficient as stand-alone stabilizers to improve the properties of the non-lateritic soil to meet the requirements of Nigeria’s highway construction standard. This highlighted the complementary nature of cement and quarry dust industrial waste in the stabilization of tropical clays.
• The incorporation of cement and quarry dust enhanced the geotechnical properties of the soil, including consistency limits and California bearing ratio. Overall, the strength properties of the soil increased with increasing curing time.
• When the proper mixes are selected, cement-stabilized soil–quarry dust mixtures can be durable and can resist the detrimental effects of water on the durability of pavements. This is especially true when the sample is compacted at higher energy levels. The results of this study suggest that cement-stabilized soil–quarry dust can be adopted in the construction of unpaved roads and base layers of paved roads.

Overall, the stabilization of clay soil with cement and quarry dust offers a sustainable solution for improving the geotechnical properties of weak soils and enables their use in construction and infrastructure development. This stabilization method minimizes environmental impacts, landfill waste, CO₂ emissions from cement production, and the need for overusing virgin aggregate extraction by repurposing quarry dust and reducing reliance on cement. These enhancements align with circular economy principles, reduce the carbon footprint, and promote resource efficiency by utilizing locally available clay soil, thereby decreasing transportation-related emissions and environmental disruption.

Finally, economically and socially, this stabilization technique supports sustainable development by enabling cost-effective, durable infrastructure. Overall, stabilizing clay soil with cement and quarry dust provides an environmentally friendly, economically viable, and socially beneficial solution for sustainable construction practices.

6. Limitations and Future Studies

The current study provides valuable insights into the strength development and durability of cement-stabilized tropical clay mixed with quarry dust for pavement construction, utilizing tests such as California bearing ratio (CBR), compaction, Atterberg limits, unconfined compressive strength (UCS), and durability assessments. However, a few limitations should be acknowledged. The absence of microstructural analyses, such as X-ray diffraction (XRD) or scanning electron microscopy (SEM), limits the understanding of the pozzolanic reactions and cementitious bonding mechanisms at the particle level, which are critical for explaining the observed strength and durability enhancements, particularly with varying quarry-dust properties. Also, the study did not include shear strength tests (e.g., triaxial or direct shear tests), which are essential for evaluating the material’s resistance to shear failure under traffic-induced stresses, a key consideration for pavement stability. Moreover, the resilient modulus, a fundamental parameter in mechanistic–empirical pavement design for assessing elastic response under cyclic loading, was not investigated. This omission limits the applicability of the findings in modern pavement design frameworks, such as those outlined in the AASHTO Mechanistic–Empirical Pavement Design Guide (MEPDG). Finally, the study focused on specific tropical clay and quarry dust compositions, which may not fully represent the variability of soil types and quarry dust characteristics across different regions, potentially limiting the generalizability of the results. These limitations may be the focus of future studies.