Geotechnical Behavior and Microstructural Analysis of Expansive Subgrade Soils Stabilized With Coffee Husk Ash

Abstract

This study examines the enhancement of engineering properties in expansive subgrade soils through stabilization with coffee husk ash (CHA), an agricultural byproduct obtained from coffee husk combustion. CHA was incorporated at 5%, 10%, 15%, 20%, and 25% by dry soil weight, and the soil–CHA mixtures were cured for 7, 14, and 28 days. Mechanical and index properties of the virgin soil, including compaction characteristics, plasticity limits, soil compressive strength (unconfined compressive strength [UCS]), and soil penetration resistance index (California bearing ratio [CBR]), were first evaluated. The effect of CHA dosage on the heavy compaction test, UCS, and CBR was then assessed, along with the influence of curing time on strength and bearing capacity. Replacing 20% soil with CHA reduced plasticity while enhancing mechanical performance. Performance declined beyond this optimum. UCS increased markedly with curing, reaching 327.5 kPa (2.72‐fold improvement) after 28 days at the optimal dosage. Two‐way ANOVA revealed that both curing time and CHA content, along with their interaction, had highly significant effects on UCS (p < 0.001), explaining nearly all of its variability (R² = 0.998). The soaked CBR at 20% CHA satisfied IRC:37 (2018) requirements for high‐volume roads. Microstructural analyses (X‐ray diffraction [XRD], scanning electron microscopy [SEM], energy dispersive X‐ray spectroscopy [EDS], and Fourier‐transform infrared [FTIR]) verified cementitious compound formation, validating pozzolanic stabilization. Pavement design using IITPAVE software, and cost analysis indicated a 21.1% lower construction cost per kilometer compared to untreated soil. The novelty of this research lies in ANOVA validation of UCS, integration of microstructural and mechanical results, and extension to pavement design with cost–benefit analysis. CHA offers an eco‐friendly and cost‐effective solution to mitigate swell–shrink behavior and improve load‐bearing capacity in sustainable road construction.

1. Introduction

Expansive clays remain a significant challenge in geotechnical engineering because of their pronounced volume changes under moisture fluctuations. Their deformations often exceed elastic limits and cannot be reliably predicted using conventional theories, leading to severe structural damage to pavements and foundations [1, 2]. Mineralogically, expansive soils typically contain kaolinite, illite, and montmorillonite, with the smectite-group mineral montmorillonite being the most problematic due to its high swelling potential [3–5]. Repeated wetting and drying cycles further cause desiccation cracking, which increases permeability, reduces strength, and accelerates deterioration [6]. The need for frequent repairs due to seasonal cycles makes traditional remediation approaches economically unsustainable [7–12].

The global economic burden of expansive soils is considerable, with repair costs estimated at several billion dollars annually [13]. For instance, damages attributed to black cotton soil amount to about $9 billion in the United States, $1 billion in China, and $0.5 billion each in the United Kingdom and India [14–17]. Adjusted for inflation, the worldwide annual loss now exceeds $15 billion. Although not as catastrophic as earthquakes or floods, expansive soils are classified as natural hazards due to the scale of infrastructure damage they cause.

Stabilization is widely used to mitigate the unfavorable properties of expansive soils. It enhances shear strength, bearing capacity, and density while reducing compressibility and accelerating consolidation [18]. Techniques range from hydraulic, mechanical, and chemical to biological methods, but many remain time-consuming or costly. The effectiveness of Stabilization depends mainly on the choice of additive, which modifies soil behavior through physical and chemical mechanisms [19–21].

Recent studies have evaluated a range of stabilizers. Lime–fly ash (FA) blends improved strength and reduced swelling, leading to a 28% reduction in pavement thickness [22]. Polypropylene fibers enhanced strength and stiffness, with unconfined compressive strength (UCS) increasing up to 5.4 times at optimal content [23]. Bioenzyme treatment showed long-term improvements in UCS and soaked California bearing ratio (CBR), supported by microstructural evidence of calcium–silicate–hydrate (C–S–H) formation [24]. Natural fibers, such as coir and banana, reduced desiccation cracking by up to 92% and improved strength at low fiber contents [6]. Waste-derived additives, such as sugarcane bagasse ash and marble dust, have also shown promise in reducing plasticity and swelling while significantly enhancing the UCS [25]. Collectively, these efforts demonstrate the growing interest in sustainable and low-carbon alternatives for expansive soil stabilization, including industrial byproducts, agricultural wastes, and polymers [12, 26–32].

Within this context, coffee husk ash (CHA) has emerged as a potential additive. Global coffee production continues to rise, generating large volumes of solid waste from processing. In 2016/2017, production reached 9.12 million tonnes, valued at $90 billion (ICO, 2017). By 2019/2020, Ethiopia ranked fifth globally (440,580 tons), with the top five producers accounting for over 7.29 million tons [33]. Consumption has also increased, from 9.85 million tons in 2019/2020 to 10.04 million tons in 2020/2021 (ICO, 2021) [34]. Projections suggest that production may reach 17.69 million tons by 2050 [35, 36]. Coffee processing wastes, particularly husks from dry processing and pulp from wet processing, account for 30%–50% of the coffee weight [37–39]. Their disposal poses serious environmental risks, including methane emissions, groundwater contamination, and soil degradation [40]. With projected growth, cumulative coffee husk and pulp waste may exceed 130 million tons by 2050, highlighting the need for sustainable valorization.

Previous research has demonstrated CHA’s potential in stabilizing expansive soils. Atahu et al. [41] reported that 20% CHA reduced swelling by 43%, while improving soaked CBR from 1% to 3.1%. Munirwan et al. [42] observed significant improvements in UCS, cohesion, and internal friction angle with 25% CHA, alongside reduced plasticity and moisture demand. Tessema et al. [43] demonstrated synergistic benefits of combining CHA with gypsum, which reduced plasticity by 81.5% and increased UCS by 171.65%, meeting subgrade requirements. These findings confirm CHA’s pozzolanic reactivity and its potential as a sustainable stabilizer.

Despite these advances, gaps remain. The influence of CHA dosage and curing duration on the strength development of highly plastic clays, particularly for pavement applications, has not been systematically evaluated. Moreover, prior studies have rarely coupled geotechnical testing with microstructural analysis or extended laboratory findings to pavement design and cost–benefit evaluation. To address this gap, the present study integrates flexible pavement design with a comprehensive evaluation of CHA-stabilized subgrades. Geotechnical tests include the modified proctor, UCS, and CBR, conducted across varying CHA dosages. The research begins with chemical and mineralogical characterization of CHA to assess its pozzolanic potential, followed by strength evaluation over curing periods of 7, 14, and 28 days. Microstructural evolution and pozzolanic reactions are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive spectroscopy, and Fourier-transform infrared (FTIR) spectroscopy. Unlike previous studies, the novelty of this research lies in validating UCS results using ANOVA, integrating microstructural and mechanical findings, and extending the outcomes to pavement design with cost–benefit analysis. Ultimately, the findings provide a sustainable and cost-effective approach for improving expansive subgrades in pavement construction.

2. Materials and Methods

The materials outlined herein were employed to investigate the performance and microstructural characteristics of expansive soil stabilized with CHA.

2.1. Materials

2.1.1. Expansive Soil

Soil specimens were collected from Shagger City Administration, Oromia Regional State, Ethiopia (8°54′23.7″N, 38°49′48.9″E). The site was selected based on visual inspection and prior research data. A disturbed sample was obtained by excavating a 1.5 m deep pit below the original ground level and stored in polyethylene bags for laboratory testing. The physical properties of soil are assessed according to the standards outlined in ASTM. Figure 1 displays the particle size distribution curve of the expansive soil. The chemical composition data, shown in Figure 2, were obtained using wavelength-dispersive X-ray fluorescence (XRF) spectrometry. The soil is rich in alumina (Al₂O₃) and silica (SiO₂) phases, a prime requirement for chemical Stabilization. Table 1 outlines the comprehensive laboratory tests conducted on the expansive soil, categorized based on the Unified Soil Classification System ASTM:D2487 [45]. The soil falls into the highly plastic clay (CH) classification and is dark gray to black.

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Table 1 The physical properties of the expansive soil.

Engineering and index characteristics	Values	Standards
Specific gravity	2.73	ASTM D 854-98
Consistency limits:
Liquid limit (LL)	86	ASTM D 4318-98
Plastic limit (PL)	53
Plastic index (PI)	33
Grain size distribution:
Gravel (>4.75 mm)	1%	ASTM D 422-98
Sand (4.75 mm (about 0.19 in)−0.075 mm)	5.44%
Silt and clay (<0.075 mm)	93.56%
Soil classification:
USCS	CH	ASTM D 2487
AASHTO	A-7-5	ASTM D 3282-93
Swelling potential:
Free swell index	113	IS 2720-PART 40
Compaction characteristics:
MDD (kN/m3)	15.3	ASTM D 1557-91
OMC (%)	21.97
Strength characteristics:
CBR soaked	1.35	ASTM D 1883-99
UCS (kPa)	120.5	ASTM D 2166-98

2.1.2. CHA

In this study, coffee husks were collected from farms and processing facilities in Yirga Chafee, Southern Ethiopia. These husks were then subjected to controlled thermal treatment to produce CHA. The husks were heated in a muffle furnace at a ramp rate of 10°C/min until 500°C and maintained for 4 h. They cooled slowly inside the furnace over approximately 12 h to minimize thermal shock and promote stable mineral phases, as recommended in earlier studies [41, 42]. The resulting ash was ground, sieved through a Number 40 (425 µm) mesh, and characterized by a specific gravity of 2.65 and a light gray color. Its chemical composition depends significantly on processing parameters, including burning method, duration, temperature, separation process, and grinding. Major and minor oxides in CHA were analyzed using wavelength-dispersive XRF spectrometry. The results indicate that 17.74% silicon dioxide (SiO₂), 3.35% aluminum oxide (Al₂O₃), 2.46% ferric oxide (Fe₂O₃), 12.76% calcium oxide (CaO), and 31.42% potassium oxide (K₂O). The combined SiO₂ + Al₂O₃ + Fe₂O₃ content (23.55%) falls below the 70% minimum required by ASTM C618 [46] for pozzolanic materials. In addition, the loss on ignition (LOI) value of 19.42% is beyond the standard maximum value of 10%. Nevertheless, high CaO and K₂O content indicates the possibility for other uses, like cementitious or alkali-activated material. Therefore, instead of functioning as a pozzolan, CHA is more appropriate as a cementing material since it contains SiO₂, Al₂O₃, Fe₂O₃, and CaO [33]. CHA, however, consists primarily of CaO and K₂O, as shown in Figure 2. Notably, the XRF results indicated a critical deficiency of CaO in the expansive soil, despite its high levels of SiO₂, Al₂O₃, and Fe₂O₃. Such a deficiency poses a significant challenge to successful soil stabilization. However, CHA had a high content of CaO, and therefore, it is a suitable stabilizing agent to address this limitation. Successful Stabilization of clayey soils depends on the ability of the stabilizer to contribute sufficient calcium (Ca), as noted by Prusinski and Bhattacharja [47]. Likewise, Miraki et al. [48] emphasized that sufficient Ca should be supplied to achieve effective soil stabilization. Significantly, the effectiveness of stabilizing agents depends on their chemical composition. SiO₂ and Al₂O₃ within soil can react with Ca hydroxide to produce cementitious phases. Such a reaction will significantly enhance soil strength and stability, as verified by Sharma and Sivapullaiah [49]. The mechanism of Stabilization using CHA can be attributed to both short-term and long-term reactions [50, 51]. In the short term, Ca ions released from CHA promote cation exchange, flocculation, and agglomeration, which enhance soil structure and reduce plasticity. Over the long term, the high-pH environment created by Ca compounds increases the solubility of SiO₂ and Al₂O₃, enabling pozzolanic reactions that form stable cementitious phases such as C–S–H and Ca–aluminate–hydrate (C–A–H). These reactions progressively improve strength, durability, and load-bearing capacity while mitigating swelling behavior. Equations (1)–(4) illustrate the chemical reactions responsible for developing cementitious compounds within the soil-binder matrix, contributing to increased strength in the stabilized soil [7, 52–55]. 1 $\begin{array}{}\text{CaO\hspace{0.17em}(s)}+{\text{H}}_2\text{O\hspace{0.17em}(l)}\to {\text{Ca(OH)}}_2\text{\hspace{0.17em}(aq)}+\text{Heat}\end{array}$ 2 $\begin{array}{}{\text{Ca(OH)}}_2\text{(aq)}+\text{Heat}\to {\text{Ca}}^{2+}\text{(aq)}+2\left({\text{OH}}^{-}\right)\text{(aq)}\end{array}$ 3 $\begin{array}{}{\text{Ca}}^{2+}\left(\text{aq}\right)+2\left({\text{OH}}^{-}\right)\text{(aq)}+\text{Soluble\hspace{0.17em}clay\hspace{0.17em}silica\hspace{0.17em}}\left({\text{SiO}}_2\left(\text{s}\right)\right)\to \text{\hspace{0.17em}C}\text{–}\text{S}\text{–}\text{H}\left(\text{gel}\right)\end{array}$ 4 $\begin{array}{}{\text{Ca}}^{2+}\left(\text{aq}\right)+2\left({\text{OH}}^{-}\right)\text{(aq)}+\text{Soluble\hspace{0.17em}clay\hspace{0.17em}alumina}\left({\text{Al}}_2{\text{O}}_3\left(\text{s}\right)\right)\to \text{\hspace{0.17em}C}\text{–}\text{A}\text{–}\text{H}\left(\text{gel}\right)\end{array}$

2.2. Experimental Methodology

2.2.1. Geotechnical Characterization

This study investigated the influence of CHA on the geotechnical properties of expansive clay soil by partially replacing the native soil with CHA at 5%, 10%, 15%, 20%, and 25% by dry weight. The dosage level (5%–25%) was selected from preliminary experimental results and a thorough review of the existing literature on CHA use for soil stabilization [42, 43]. This literature also repeatedly shows that the most effective dosage for increasing strength is usually in the range of 10%–25%. In addition, our initial tests proved that dosages less than 5% produced very minor improvements, but dosages higher than 25% led to reduced UCS and CBR.

The soil was oven-dried and sieved through a Number 4 (4.75 mm) sieve following ASTM:D422 [56] to ensure uniform particle size distribution. Subsequently, a compaction test was carried out using the modified proctor method ASTM:D1557 [57], in which mixtures were compacted in five layers with a 4.5 kg rammer dropped from a height of 457 mm. The resulting maximum dry density (MDD) and optimum moisture content (OMC) were used to prepare test specimens. Following compaction, the samples were sealed in polyethene bags and cured at 21 ± 1°C under room temperature for 7, 14, and 28 days [58]. Additionally, the Atterberg limits of untreated and CHA-stabilized soils were determined following ASTM:D4318 [59], using soil fractions passing the Number 40 (425 µm) sieve.

UCS testing was carried out on cylindrical specimens (76 mm height × 38 mm diameter) prepared at their respective MDD and OMC values. Following curing, specimens were subjected to axial compression at a strain rate of 1.25 mm/min, ASTM:D2166 (2016) [60], using a Zhejiang TuGong Instrument Co., Ltd compression machine. The peak compressive stress was recorded and averaged across triplicate samples. To assess bearing capacity improvement, soaked CBR tests were performed as per ASTM:D1883 (2016) [61]. Compacted specimens, cured under identical conditions, were immersed in water for 96 h under a 4.54 kg surcharge. Penetration resistance was measured at a rate of 1.27 mm/min using a 50 mm plunger, with load values recorded at penetrations of 2.54 mm and 5.08 mm. Triplicate testing ensured data reliability.

2.2.2. Microstructural Analysis

The microstructure of clay soil, comprising particle size, bonding characteristics, and mineral composition, is responsible for determining primary geotechnical properties, such as bearing capacity, which directly affect its suitability for construction [62]. To investigate the development of cementitious phases in untreated as well as CHA-treated soil mixtures, extensive microstructural studies were performed. The microstructural analysis included XRD, SEM, energy dispersive X-ray spectroscopy (EDS), and FTIR spectroscopy. The XRD (SHIMADZU XRD-7000) using CuKα radiation at 30 mA and 40 kV scanned samples in 10°–80° 2θ at 0.02° intervals and 4°/min to identify changes in minerals. SEM (JEOL JSM-6390LV) at 10 kV was used to scan the surface morphology following sputter coating with a 10 nm gold layer, with images taken at up to 10,000x magnification. SEM was coupled with EDS (Oxford Instruments X-act) to identify elemental composition in a probe current of 65.4–67.0 µA and a working distance of 8–15 mm [63]. FTIR (Shimadzu IRSpirit-X) in transmittance mode captured spectra between 4000 and 400 cm⁻¹ at a resolution of 4 cm⁻¹, averaging 16 scans for each sample, to analyze molecular structures and bonding changes. These enabled phase composition and morphological changes to be quantified and gave a precise insight into CHA–clay interactions, crack formation, stiffness evolution, mineralogical changes, and reaction product formation [21, 64].

3. Results and Discussion

3.1. Geotechnical Characterization

3.1.1. Atterberg Limit Test

Figure 3 demonstrates the impact of varying CHA content on the liquid limit (LL) and plasticity index (PI) of an expansive soil. The untreated soil exhibits high plasticity, with an LL of 86%, a plastic limit (PL) of 53%, and a PI of 32%. As the proportion of CHA increases from 0% to 25%, a significant reduction in both LL and PI is observed. This decrease in plasticity, particularly the PI dropping from 32 to 27, indicates that incorporating CHA mitigates the swelling tendencies of the expansive soil [42]. The observed trends align with established principles of soil stabilization using pozzolanic additives, where decreased plasticity correlates with improved volume stability.

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3.1.2. Modified Proctor Compaction Test

The compaction behavior of expansive soil treated with varying percentages of CHA is presented in Figure 4 in terms of MDD and OMC, respectively. As depicted in Figure 4, the MDD initially increases from 15.3 kN/m³ for untreated soil to 15.6 kN/m³ at 5% CHA content. This improvement can be attributed to the partial filling of voids within the clay matrix by finer CHA particles, leading to a denser soil structure. However, with further increases in CHA content, the MDD gradually declines, reaching 15.5 kN/m³ at 10%, 15.2 kN/m³ at 15%, and 14.6 kN/m³ at 25%. This reduction is most likely caused by the inferior density of CHA in contrast to natural soil constituents, forming a lighter composite matrix.

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Additionally, higher ash content promotes the flocculation and aggregation of clay particles, forming a more open fabric structure that further reduces dry density. The reduced compaction resistance observed aligns with established principles for high-plasticity soils [65], suggesting practical advantages in field compaction energy requirements. Similar trends have been documented in studies involving other stabilizers, where the treated soil exhibited a significantly lower specific gravity than untreated expansive soil [66].

Conversely, Figure 4 illustrates a progressive increase in OMC with higher CHA content rising from 21.97% for untreated soil to 33.25% at 25% CHA. This trend is primarily attributed to the porous and absorptive nature of CHA, which increases the soil’s water demand to achieve optimum compaction. Furthermore, the cementitious properties of CHA, rich in reactive siliceous and aluminous compounds, trigger chemical reactions that consume additional water for reaction kinetics and particle lubrication during compaction. Similar mechanisms have been observed in alkaline and silicate-based soil treatments, where pozzolanic reactions reduce the available mixing water, thereby increasing OMC [67]. These findings oppose results reported by Munirwan et al. [42] and Tessema et al. [43] potentially attributable to fundamental differences in (1) clay mineralogy, (2) CHA chemical composition, or (3) particle size distribution characteristics.

3.1.3. UCS

The compressive strength of soil provides direct evidence of its suitability for geotechnical applications. The influence of varying dosages of CHA at 5%, 10%, 15%, 20%, and 25% by weight of dry soil on the UCS of highly plastic clay was investigated. Specimens were compacted at their respective OMC using the modified proctor compaction method and subsequently cured for 0, 7, 14, and 28 days under controlled conditions. These curing intervals were selected to assess the combined effects of CHA content and curing duration on strength development, shear resistance, and volumetric stability in treated clay soils. The variation of UCS values (averaged from three identical samples for each condition) as a function of curing time and CHA content is illustrated in Figure 5. The UCS of the untreated expansive soil was found to be 120.5 kPa. Immediate post-compaction testing (0 days) showed minor strength gains, with UCS values increasing from the control (120.5 kPa) to 123.8 kPa (5% CHA), 127.4 kPa (10% CHA), 142.7 kPa (15% CHA), and 149.6 kPa (20% CHA), before slightly declining to 145.2 kPa at 25% CHA. These early gains are attributed primarily to physical stabilization effects, namely, improved particle packing and moisture absorption, rather than chemical reactions, due to the low initial reactivity of SiO₂ and Al₂O₃ in CHA. Significant strength enhancements were observed during later curing periods. At 7 days, UCS values increased to 135.0 kPa (5% CHA), 153.0 kPa (10% CHA), 246.8 kPa (15% CHA), and 302.2 kPa (20% CHA), followed by a reduction to 267.2 kPa at 25% CHA. The continued increase in UCS with extended curing indicates progressive pozzolanic activity. Shear strength development was further observed at 14 days (maximum UCS: 313.3 kPa at 20% CHA) and 28 days (maximum UCS: 327.5 kPa at 20% CHA), reflecting a 171% increase compared to the untreated soil.

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The strength enhancement mechanism is attributed to progressive cementitious bonding within clay–CHA matrix, resulting in sustained strength development up to 28 days, with only marginal gains beyond 14 days [68, 69]. This improvement is likely due to the dissolution of pozzolanic particles, which facilitates the formation of a denser microstructure [20]. This phenomenon is further examined through SEM micrographs.

The negligible strength improvement observed after 28 days implies that the reaction mechanism in CHA-treated soil is nearly complete. This trend aligns with prior studies highlighting early curing as critical for strength development. Karimiazar et al. [68] reported substantial strength gains in nano-SiO₂–lime–treated expansive soil within 28 days, with minimal subsequent enhancement. Similarly, Kang et al. [70] observed that expansive clay stabilized with Class C FA and lime kiln dust (20 wt%) achieved an increase in UCS from 181.2 to 497.2 kPa after 28 days. Comparable trends were noted by Dhar and Hussain [71] (188–571 kPa with 3%–8% lime) and Jebo et al. [44] (120.5 to 493 kPa with 25% scoria). Saride and Dutta [14] further demonstrated that FA-stabilized soils primarily gain strength and stiffness during early curing, reflected in CBR, dynamic shear modulus, and resilient modulus improvements. While UCS may continue to increase with prolonged curing, the rate of increase diminishes, plateauing after 28 days, regardless of the FA content [72].

The 8.5% strength decline at 25% CHA indicates an excess of unreacted ash particles. These fine, loose particles can interfere with the interlocking of larger cemented aggregates, acting like roller bearings that create localized zones of weakness and reduce overall frictional resistance. Consequently, beyond this point, excess ash behaves as an inert filler rather than a beneficial pozzolanic material that enhances strength [49, 73, 74]. The 20% CHA mixture consistently exhibited superior performance across all curing periods (302.2 kPa at 7 days, 313.3 kPa at 14 days, and 327.5 kPa at 28 days), confirming its effectiveness as the optimal stabilization dosage. Strength improvement with CHA stabilization is mainly due to soil structural changes, as shown in the microstructural analysis in the subsequent section.

3.1.4. Statistical Significance of UCS Improvements

While the UCS results demonstrated substantial improvements with CHA addition (Section 3.1.3), it was necessary to confirm that these gains were statistically significant and not the result of experimental variability across triplicate tests. To this end, a two-way ANOVA was conducted (Table 2), with curing time and CHA content as independent variables. The analysis revealed highly significant main effects of curing time, F(3, 48) = 2754.00, p < 0.001, partial η² = 0.994, and CHA content, F(5, 48) = 2424.92, p < 0.001, partial η² = 0.996. The analysis indicates that both factors independently exerted a strong influence on UCS. In addition, the curing time × CHA interaction effect was also significant, F(15, 48) = 321.08, p < 0.001, partial η² = 0.990, suggesting that the degree of strength enhancement provided by CHA varied depending on the curing period. The overall model was robust, F(23, 48) = 1095.77, p < 0.001, with R² = 0.998, accounting for nearly all of the variance in UCS values. The error bars in Figure 6, representing 95% confidence intervals, are consistently narrow, indicating reliable triplicate UCS values with minimal within-group variability.

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Table 2 ANOVA results for the effect of curing period and CHA content on UCS.

Source	Type III SS	df	MS	F	p-Value	Partial η2
Corrected model	464,445.81	23	20,193.30	1095.77	<0.001	0.998
Intercept	2,863,583.46	1	2,863,583.46	155,390.26	<0.001	1
Curing time	152,255.01	3	50,751.67	2754.00	<0.001	0.994
CHA	223,435.81	5	44,687.16	2424.92	<0.001	0.996
Curing time × CHA	88,754.99	15	5917.00	321.082	<0.001	0.99
Error	884.56	48	18.428	—	—	—
Total	3,328,913.83	72	—	—	—	—
Corrected total	465,330.37	71	—	—	—	—

3.1.5. CBR

In road construction, the CBR serves as the key metric for evaluating the strength and suitability of subgrade soils and construction materials. The influence of varying dosages of CHA at 5%, 10%, 15%, 20%, and 25% by dry weight of soil, on the CBR of highly plastic clay was investigated. Specimens were cured under controlled moisture conditions for periods of 0, 7, 14, and 28 days. The untreated expansive soil demonstrated a soaked CBR value of 1.35%, indicating inferior subgrade performance. Figure 7 illustrates the Soaked CBR variations of untreated and CHA-treated soils at different dosages and curing periods. An improvement in the CBR was observed with increasing percentages of CHA in the soil. At 0 days of curing, the CBR of untreated soil was 1.35%, which improved to 2.96%, 4.71%, 7.79%, 8.20%, and 6.63% with 5%, 10%, 15%, 20%, and 25% CHA content, respectively. This immediate gain in CBR is primarily attributed to the filler effect and initial pozzolanic reaction between CHA and soil particles, which enhances particle bonding and reduces voids. With a 7-day curing period, further strength development was observed, as the CBR increased to 3.55%, 5.15%, 8.47%, 8.75%, and 7.11%, respectively, for the same CHA replacement levels. Similarly, at 14 days, CBR enhancements of 3.85%, 5.65%, 9.09%, 9.37%, and 7.53% were recorded. Continued improvement was noted with longer curing durations; after 28 days of curing, the CBR values rose to 4.14%, 5.97%, 9.52%, 9.76%, and 7.84%, respectively. The findings indicate that CHA-stabilized expansive clay performs more effectively as a subgrade material, resulting in a pavement thickness of 570 mm compared to 745 mm for untreated clay soil. The thickness difference represents a reduction in thickness of approximately 23.5% and a corresponding construction cost saving of about 21.1% per kilometer, as presented in Section 3.3.2 (Table 3).

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Table 3 Comparative cost analysis of flexible pavement on untreated vs. CHA-stabilized subgrade.

Layer	Thickness (mm)	Volume (m3/km)	Rate (₹/m3)	Layer cost (₹)	Thickness (mm)	Volume (m3/km)	Layer cost (₹)
CBR = 1.35%	CBR = 8.2%
GSB	350	2625	2808.55	7,850,947.50	200	1500	3,931,970.00
WMM	250	1875	2914.30	5,100,025.00	250	1875	5,100,025.00
DBM	105	787.5	11,129.55	8,180,219.25	90	675	7,011,616.50
BC	40	300	12,126.20	3,395,336.00	30	225	2,546,502.00
Total	745	5587.50	—	23,556,527.75	570	4275	18,590,113.00

Consistent with UCS values, CBR test results in Figure 7 show a gradual increase in penetration resistance up to 28 days, with minimal strength gain beyond 14 days. The observations confirm that the clay–CHA blend demonstrates no marked enhancement after 28 days, hence, it can be inferred that the chemical interactions reach completion within this timeframe. The enhanced CBR of CHA-treated soil under soaked conditions arises from pozzolanic reactions and cation exchange [17, 75]. CaO in CHA induces clay flocculation, while reactive SiO₂ and Al₂O₃ form cementitious gels, as confirmed by SEM/EDS analysis [41]. These gels reduce interparticle porosity and increase shear strength by densifying the soil matrix [76]. Consequently, CHA-treated soil achieves higher CBR values than untreated soil, with peak CBR (28-day curing with 20% of CHA attributed to physicochemical modifications [1, 77].

3.2. Chemical Characterization

3.2.1. XRD

XRD characterization was carried out on the samples collected from the failure planes of specimens after UCS testing to identify the mineral phases in untreated and CHA-treated soils. The diffraction patterns (Figure 8) correspond to powdered specimens of untreated (control) soil and soils treated with 10%, 15%, 20%, and 25% CHA after a 28-day curing period, enabling assessment of mineralogical changes induced by CHA stabilization. The untreated soil exhibited distinct peaks characteristic of clay minerals, notably kaolinite at 2θ = 19.73° (184.13 cps) and quartz at 2θ = 26.63° (730.90 cps), confirming the crystalline nature of these phases. These observations align with earlier studies [49, 64].

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With progressive CHA incorporation, both quartz and kaolinite peaks decreased in intensity. Quartz declined from 730.90 cps (0% CHA) to 723.00, 610.70, 478.00, and 418.00 cps at 10%, 15%, 20%, and 25% CHA, respectively (Table 4). Similarly, kaolinite intensity fell from 184.13 to 142.35, 118.99, 100.64, and 86.09 cps. The 10% CHA treatment produced a 22.71% reduction in kaolinite peak intensity (2θ = 20.92°) and introduced new crystalline phases, including anorthite at 2θ = 27.85° (106.54 cps) and calcite at 2θ = 29.55° (113.12 cps). At 15% CHA, kaolinite dissolution continued (118.99 cps at 2θ = 20.84°), accompanied by a 16.44% reduction in quartz intensity (620.70 cps at 2θ = 26.70°), indicating partial dissolution of quartz and formation of SiO₂ gels [77]. Treatments above 15% CHA showed the continuous reduction of kaolinite (100.64 cps at 2θ = 20.68°) and several weak, poorly crystalline, or amorphous peaks. In the 20%–25% CHA range, a broad hump and new peaks were detected between 2θ = 27°–41°, attributed to the formation of C–S–H gels [78, 79]. Additional peaks at 2θ = 28°–30° and 20–35° indicate the formation of C–(A)–S–H and N–A–S–H gels, respectively [80]. These phases arise from pozzolanic reactions between reactive oxides in CHA and soil minerals, producing cementitious compounds that enhance soil stiffness and bonding. Similar trends have been documented in prior studies[67, 81].

Table 4 d-Spacing and peak intensity of kaolinite and quartz in CHA-treated expansive soil samples.

CHA content (%)	Kaolinite d-spacing (Å)	Kaolinite peak intensity (cps)	Quartz d-spacing (Å)	Quartz peak intensity (cps)
0	4.49	184.13	3.34	730
10	4.24	142.35	3.33	723
15	4.26	118.99	3.34	610
20	4.50	100.64	3.34	478
25	4.49	86.09	3.35	418

The mineralogical evolution observed in the XRD data aligns with mechanical performance trends. The reduction in hydrophilic primary minerals, coupled with the generation of cementitious gels, yields a denser soil matrix with improved particle interlocking, reduced porosity, and improved UCS and CBR at the optimum CHA dosage. As shown in Table 4, quartz d-spacing remained stable (3.34–3.35 Å), whereas kaolinite exhibited slight variation (4.24–4.50 Å). Larger d-spacing values indicate expanded lattice structures, which are typically less dense and mechanically weaker [77]. The expanding lattice aligns with the UCS results, where the 25% CHA treatment showed an approximate 8.5% reduction in strength compared with the optimum dosage.

3.2.2. SEM

SEM analysis was conducted on samples collected from the failure planes of specimens after UCS testing. Figure 9A–F presents SEM images that distinctly illustrate the contrasting microstructures between control (untreated) and clay samples amended with varying percentages (5%,10%, 15%, 20%, and 25%) of CHA following a 28-day curing period. The images reveal the soil fabric before and after treatment, and the formation of interparticle bonds is evident upon examination.

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The SEM image of the untreated expansive clay (Figure 9A) reveals numerous large voids and a loosely packed matrix, characteristic of montmorillonite-rich soils with high swell potential and low structural cohesion [82, 83]. Upon treatment with 5% CHA (Figure 9B), the soil structure exhibits reduced pore size and improved compaction, with CHA particles partially filling the voids [73]. At 10%–15% CHA content (Figure 9C,D), clustered particle formations become prominent, suggesting enhanced aggregation driven by chemical bonding, which contributes to reduced compressibility and improved mechanical interlocking [81]. These aggregates are critical in improving interparticle friction and cohesion, directly correlating with enhanced UCS and CBR performance [68, 84].

At 20% CHA, the soil microstructure (Figure 9E) exhibits well-developed cementitious crystalline structures in reticulated and flocculated forms, enhancing interparticle bonding and enabling the soil to sustain higher loads [85, 86]. However, increasing the CHA content to 25% alters the mineralogical arrangement, as evidenced by the XRD results (Table 4). While quartz d-spacing remains stable (3.34–3.35 Å), kaolinite displays a slight expansion in lattice spacing (4.24–4.50 Å). Such increased d-spacing reflects lattice expansion, which is associated with reduced packing density and diminished mechanical integrity [77]. This structural loosening disrupts the continuity of cementitious bonds, as excess CHA particles act primarily as inert fillers rather than reactive pozzolanic agents. Consequently, the microstructure becomes less cohesive, leading to an approximate 8.5% reduction in UCS compared to the optimum 20% dosage. These results confirm that optimal strength gains occur at 15%–20% CHA, where the development of C–S–H and C–A–H gels occur most effectively without causing oversaturation.

3.2.3. EDS

The incorporation of CHA into expansive clay soil resulted in significant reductions in Atterberg limits and swelling potential, indicating enhanced volumetric stability. These macroscopic enhancements are indicative of microstructural alterations related to pozzolanic and hydration reactions during curing. XRF analysis of the parent materials revealed that the untreated clay soil is primarily composed of SiO₂ and Al₂O₃. Concurrently, CHA comprised a considerable amount of K₂O and CaO (Figure 2), in agreement with previous research on CHA stabilization [41].

EDS of the raw soil (Figure 10A) revealed silicon (Si), aluminum (Al), and Ca as the major constituents, with oxygen (O) and carbon (C) present in minor amounts, in agreement with the typical expansive clay composition [72]. On the addition of CHA, EDS profiles (Figure 10B–E and Table 5) showed progressive declines in Si, Al, and Ca contents, coupled with corresponding reductions in quartz peak intensities in XRD patterns (Section 3.2.1). These concurrent trends corroborate the consumption of reactive SiO₂ and Al₂O₃ through pozzolanic reactions with Ca²⁺ ions from CHA to form C–S–H and C–A–H phases. These cementitious materials enhance particle bonding, fill pore volumes, and account for the observed increases in strength [49].

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Table 5 EDS elemental composition of untreated and CHA-stabilized expansive clay soil.

Elements/samples	Untreated clay soil	5% CHA-treated clay soil	10% CHA-treated clay soil	15% CHA-treated clay soil	20% CHA-treated clay soil	25% CHA-treated clay soil
Weight (%)	Atomic (%)	Weight (%)	Atomic (%)	Weight (%)	Atomic (%)	Weight (%)	Atomic (%)	Weight (%)	Atomic (%)	Weight (%)	Atomic (%)
O	52.09	63.46	61.37	65.33	63.29	65.55	65.29	65.56	55.21	64.24	70.83	66.35
C	6.96	11.29	15.71	22.28	17.45	24.08	20.29	27.14	8.97	13.9	25.74	32.11
Na	0.18	0.16	0.05	0.03	0.16	0.11	0.24	0.17	0.49	0.40	0.10	0.06
Mg	0.85	0.68	0.51	0.36	0.54	0.37	0.72	0.48	1.19	0.91	0.18	0.11
Al	7.25	5.24	4.23	2.67	3.68	2.26	2.4	1.43	6.54	4.52	0.48	0.27
Si	19.07	13.24	11.62	7.05	10.18	6.0	5.96	3.41	19.15	12.69	0.92	0.49
K	0.83	0.42	0.66	0.29	0.68	0.29	0.52	1.02	1.34	0.64	0.17	0.07
Ca	7.68	3.73	1.57	0.67	1.11	0.46	1.11	0.04	2.4	1.11	1.11	0.42
Ti	0.17	0.07	0.46	0.16	0.29	0.10	0.11	0.04	0.35	0.13	0.06	0.02
Fe	4.93	1.72	3.82	1.17	2.63	0.78	1.9	0.55	4.35	1.45	0.41	0.11

Results of EDS also indicated rising oxygen and carbon contents with increased CHA content. The increased carbon might be due to two reasons: (i) residual leftover carbon from ash, and (ii) secondary carbonation, where Ca(OH)₂ reacts with CO₂ in the atmosphere to give rise to CaCO₃. The carbonation process is also confirmed by FTIR (Section 3.2.4). The bands with peaks at 1445 and 1638 cm⁻¹ were affected by the addition of 5%–25% CHA, corresponding to O─C─O stretching vibrations. Dang et al. [87] associated these bands with carbonation reactions, while Diouri et al. [88] noted that they may also partially arise from residual lignin derivatives in the ash, suggesting contributions from both inorganic and organic carbon species to the FTIR signals.

3.2.4. FTIR Spectroscopy

The FTIR spectra of the 28-day cured untreated and CHA-treated expansive clay soil are shown in Figure 11. These spectra provide valuable information on the chemical and structural changes in the soil matrix due to CHA addition. The FTIR spectrum of the untreated expansive clay soil exhibits several characteristic absorption bands indicating its mineralogical composition. There is a dominant peak at 3623 cm⁻¹, which can be assigned to the structural hydroxyl group O─H stretching vibrations that occur in the octahedral sheets of the clay mineral structure. It is a characteristic peak of montmorillonite, which is a smectite-group mineral and the leading cause of the high swell-shrink character of clay soil [89, 90]. Another strong broad band at 3419 cm⁻¹ is due to the O─H stretching of physically adsorbed and interlayer water molecules. This is supplemented by the band at 1641 cm⁻¹ due to the H─O─H bending vibration of the adsorbed water. The strong and prominent band seen around 998 cm⁻¹ is attributed to the Si─O stretching vibrations in the silicate framework of the clay platelets [91].

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With the addition of CHA in concentrations from 5% to 25%, significant changes in the FTIR spectra become apparent. The peak intensities of hydroxyl groups and water (3623, 3419, and 1641 cm⁻¹) decrease. This decrease indicates a significant reduction in the water adsorption capacity of the soil, and this can be attributed to two simultaneous mechanisms. On the one hand, cation exchange reactions take place, by which divalent Ca²⁺ ions of the CHA replace natural monovalent sodium (Na⁺) and potassium (K⁺) ions in the clay interlayers, thus, decreasing the water affinity of the clay. On the other hand, pozzolanic reactions use up structural hydroxyl groups as new cementitious compounds are formed [41].

Further evidence of pozzolanic activity can be observed in the region of the main silicate vibration. The most prominent Si─O stretching band is moved from 998 cm⁻¹ in the untreated soil to 995–1004 cm⁻¹ for the CHA-treated samples. This indicates a change in the silicate framework reorganization, triggered by the dissolution of amorphous SiO₂ from the clay, which subsequently reacts with CHA-derived Ca, resulting in the generation of C–S–H gels [91]. These gels play a central role in soil stabilization. The development of such cementitious gels is also supported by the occurrence of new, narrow peaks in the region of 668–678 cm⁻¹ for soils with 5%–25% CHA. These peaks are indicative of Si─O─Ca or Al─O─Si bond vibrations, which support the creation of a pozzolanic gel network that solidifies soil particles [92].

The collective spectral changes correlate directly with the modified physical and mechanical behavior of the soil. The pronounced attenuation of water-related peaks (3419 and 1641 cm⁻¹) is consistent with the reduced swell–shrink potential, as the newly formed pozzolanic products and exchanged cations stabilize the clay platelets and fill interlayer spaces [93]. The optimal stabilization range of 10%–15% CHA is characterized by a well-developed Si─O network, indicated by stable peaks around 1001–1004 cm⁻¹, and significantly minimized water adsorption bands. This optimal chemical state coincides with the peak gains in UCS [89]. However, at CHA concentrations exceeding 20%, an 8.5% strength reduction is observed. The lowering of the strength can be attributed to the disruptive effects of excessive, noncohesive carbonate and organic residues, which can displace clay particles and weaken the cohesion of the stabilized matrix. This observation aligns with reports indicating that an overabundance of specific components can lead to structural disruption in stabilized clays, as reflected by changes in silicate bands below 1200 cm⁻¹ [90].

3.3. Flexible Pavement Design and Cost Analysis

3.3.1. Flexible Pavement Design

According to the IRC:37 (2018) [94] provisions, pavement thickness is primarily governed by two factors: the soaked CBR of the subgrade soil and the projected traffic loading, expressed in terms of cumulative million standard axles (msa) that the pavement is expected to endure throughout its design life. Traffic loading is evaluated based on fatigue failure, represented by the maximum tensile strain at the bottom of the bituminous layer, and rutting, represented by the maximum vertical strain at the top of the subgrade layer. The permissible tensile and compressive strains at the critical points of the pavement are determined using the empirical equations specified in IRC:37 (2018) [94]. These values are subsequently compared with the corresponding strains obtained through analysis using the IITPAVE software. For design, an 80% reliability level is adopted to determine allowable tensile strain in the bituminous layer and compressive strain in the subgrade. As an illustration, a 20-year design for a two-lane single carriageway carrying 20 msa traffic comprises a layered system of bituminous concrete (BC) wearing course, dense bituminous macadam (DBM-II/I) binder courses, wet mix macadam (WMM) base (250 mm), and granular subbase (GSB) (200–350 mm, depending on subgrade improvement), over a 500 mm compacted CHA-stabilized subgrade in accordance with IRC provisions. Strain responses are analyzed in IITPAVE under a 20 kN wheel load (0.56 MPa contact pressure, 310 mm spacing), assuming an elastic modulus of 3000 MPa for BC/DBM (VG40 bitumen at 35°C) and a Poisson’s ratio of 0.35 for all layers (Table 6).

Table 6 The resilient modulus of soil-subgrade for the pavement design as per IRC:37 (2018).

Conditions	Soaked CBR (%)	Subgrade layer modulus (MPa)	Granular layer modulus (MPa)	Bituminous layer modulus (MPa)
Untreated	1.35	13.5	200	3000
CHA-treated	8.2	67.7	211.5	3000

Mechanistic analysis confirms that both untreated (CBR 1.35) and CHA-stabilized (CBR 8.2) subgrades meet the IRC:37 (2018) design criteria (Table 7). In the bituminous layer, the horizontal tensile strain (ε_t) is 191.5 με for untreated soil and 221.5 με for stabilized soil. Both values are lower than the allowable 257.1 με, giving safety margins of 25.5% and 13.9%. This shows that fatigue resistance is achieved under the design traffic. At the subgrade level, the vertical compressive strain (ε_v) is 424.5 με for untreated and 326.6 με for stabilized soil. These are within the allowable limit of 577.7 με, with safety margins of 26.5% and 43.5%. CHA stabilization reduces ε_v by 23.1% compared to untreated soil, showing clear improvement in subgrade performance. Overall, the pavement design is structurally adequate and ensures long-term durability in both subgrade conditions.

Table 7 Pavement design thickness as per IRC:37 (2018).

Stabilized soil type	Design composition (mm)	Horizontal tensile strain (εt) ×10−4	Vertical compressive strain (εv) ×10−4
GSB	WMM	DBM	BC	Allowable strain (με)	Actual strain (με)	Allowable strain (με)	Actual strain (με)
Untreated CBR 1.35	350	250	105	40	257.1	191.5	577.7	424.5
CHA-treated CBR 8.2	200	250	90	30	257.1	221.5	577.7	326.6

3.3.2. Cost Analysis

This cost analysis covers the construction of a 1 km two-lane highway with a 7 m carriageway. The estimates are based on the Central Public Works Department (CPWD) Analysis of Rates for Delhi (2023, Vol. 2) [95]. The scope includes material costs, such as BC with VG-40 bitumen (5.5%) and lime filler (3%) for the 40–50 mm layer, dense graded bituminous macadam with VG-40 (5%) and lime filler (2%) for the 50–100 mm layer, GSB materials, and WMM aggregates. Labor costs account for both skilled and unskilled workers across all construction phases. Equipment and machinery expenses cover batch mix plant operations, paver finishers with electronic sensors, vibratory rollers, and transportation tippers, including fuel and maintenance. Transportation costs for material delivery are also considered. Miscellaneous expenditures include site preparation, safety measures, quality control testing, and a contingency allowance of 5%–10%. All quantities, specifications, and rates follow CPWD 2023 standards and MoRTH/IRC guidelines, ensuring compliance with current construction norms while allowing for price fluctuations and site-specific conditions.

A comparative cost analysis reveals that subgrade stabilization markedly enhances the economic efficiency of flexible pavement construction. As illustrated in Table 3, for an untreated weak subgrade with a CBR of 1.35%, the required pavement thickness is 745 mm, resulting in a construction cost of ₹23.56 million per kilometer. However, stabilizing the subgrade with CHA increases the CBR to 8.2%, enabling a reduction in pavement thickness to 570 mm, with a corresponding cost of ₹18.59 million per kilometer (Table 2). The thickness improvements represent a 21.1% reduction in construction costs, equivalent to ₹4.97 million per kilometer in savings, while maintaining structural integrity. These findings underscore CHA’s potential as a sustainable and cost-efficient material for pavement engineering.

4. Conclusions and Recommendations

In this study, the impact of CHA on the geotechnical and microstructural properties of expansive soil was assessed. The following conclusions can be made from the experimental data and thorough analysis:

• •

  The inclusion of CHA influenced the compaction properties of highly plastic clay soil. As the CHA admixtures increased, the MDD decreased with a corresponding increase in OMC.

• •

  The result of the strength tests, including UCS and CBR, shows the significant influence of the blend on the soil. A maximum UCS of 327.5 kPa was achieved at 20% CHA after 28 days of curing, marking a 2.72-fold increase compared to virgin clay soil. Similarly, Soaked CBR values increased from 1.35% (untreated) to 9.76% (20% CHA) after 28 days of curing, a 7.23-fold enhancement that exceeds the IRC:37 (2018) minimum requirement of 5%–6% for subgrade soils.

• •

  The UCS and soaked CBR value increases with curing time across all CHA optimum dosages, with more pronounced gains between 14 and 28 days at lower CHA levels (5% and 10%). Higher CHA contents (15%–25%) achieve Stabilization earlier, with most strength gains occurring within the first 7 days. Beyond 14 days, the rate of improvement slows.

• •

  XRD analysis demonstrated that increasing CHA content progressively reduced the intensity of primary mineral peaks. The quartz peak at 2θ = 26.62° declined from 730.9 cps in untreated soil to 418 cps at the 25% CHA dosage, representing a 42.8% reduction. Similarly, the kaolinite peak at 2θ = 19.8° decreased from 184.13 to 86.09 cps, representing a 53.2% reduction, which indicates the breakdown of the original clay’s crystalline framework. This partial dissolution of quartz and kaolinite facilitated the release of reactive SiO₂ and Al₂O₃, promoting the formation of SiO₂ gels.

• •

  A two-way ANOVA examined the effects of curing time and CHA on UCS. The model was highly significant, F(23, 48) = 1095.77, p  < 0.001, explaining 99.8% of variance (adjusted R² = 0.997).

• •

  Upon CHA addition, EDS analysis exhibited progressive decreases in Si, Al, and Ca contents, accompanied by corresponding reductions in quartz peak intensities observed in XRD patterns. These parallel trends confirm the consumption of reactive SiO₂ and Al₂O₃ in pozzolanic reactions with Ca²⁺ ions from CHA, resulting in the formation of C–S–H and C–A–H phases.

• •

  Microstructural studies (XRD, SEM, EDS, and FTIR) confirmed that mineral transformations, formation of cementitious compounds, improved gradation, and stronger particle bonding are responsible for the increased strength and reduced swelling of soil–CHA mixes.

• •

  Pavement design using IITPAVE software and cost analysis indicated a 21.1% lower construction cost per kilometer compared to untreated soil.

Although CHA shows potential as a sustainable stabilizer for expansive soils, large-scale use may be limited by availability and variability in quality, making standardization and reliable supply strategies essential. Future research should focus on long-term durability under repeated loading, wetting–drying, and freeze–thaw cycles, supported by advanced triaxial shear testing under varied drainage conditions. Evaluating resilient modulus, permanent deformation, and pH evolution over time will provide critical insights into the durability and structural integrity of CHA-stabilized soils.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

Funding

This research received no funding from the public or commercial sector.

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