Experimental Study of Thermal Conductivity in Soil Stabilization for Sustainable Construction Applications

Abstract

Soils in Saudi Arabia are emerging as potential sustainable building materials, a notion central to this study. The research is crucial for advancing construction practices in arid areas by enhancing soil thermal properties through stabilization. Focusing on Hejaz region soils, the study evaluates the impact of stabilizers such as cement, lime, and cement kiln dust (CKD) on their thermal behavior. This investigation, using two specific soil types designated as Soil A and Soil B, varied the concentration of additives from 0% to 15% over a 12-week duration. Employing a TLS-100 for thermal measurements, it was found that Soil A, with a 12.5% cement concentration, showed a significant 164.54% increase in thermal conductivity. When treated with 2.5% lime, Soil A reached a thermal conductivity of 0.555 W/(m·K), whereas Soil B exhibited a 53.00% decrease under similar lime concentration, reflecting diverse soil responses. Notably, a 15% CKD application in Soil A led to an astounding 213.55% rise in thermal conductivity, with Soil B recording an 82.7% increase. The findings emphasize the substantial influence of soil stabilization in improving the thermal characteristics of Hejaz soils, especially with cement and CKD, and, to a varying extent. This study is pivotal in identifying precise, soil-specific stabilization methods in Saudi Arabia’s Hejaz region, essential for developing sustainable engineering applications and optimizing construction materials for better thermal efficiency.

1. Introduction

The growing environmental challenges in urban areas necessitate the integration of sustainable infrastructure solutions, underscoring the importance of sustainable urban development [1,2]. This study specifically examines the thermal properties of two soil types from the Hejaz region in Saudi Arabia, treated with different stabilizers: cement, lime, and cement kiln dust (CKD). Central to our investigation is the role of thermal conductivity in soil properties, a key indicator of a soil’s ability to transmit heat, which directly impacts its performance in construction applications. By analyzing how stabilization with cement, lime, and CKD alters the thermal conductivity of Hejaz soils, we connect this to the broader context of sustainable construction. Recent research, such as the study by Ghavami et al. (2021), highlights the environmental benefits of using CKD, particularly its role in reducing emissions and serving as a sustainable cement-free stabilizer [3]. Current research in soil stabilization highlights significant benefits but also reveals problems that require further investigation. For instance, while the addition of CKD at 10% is economical and environmentally beneficial, aligning with sustainable waste management practices [4], there is a lack of comprehensive understanding of its long-term environmental impact. The use of CKD can help reduce the environmental impact and disposal costs, benefiting various industrial applications [5,6,7,8]. Stabilizing clayey soil with 15% CKD and 10% NaCl can also reduce energy consumption, e-CO₂ emission, and costs [9]; questions remain regarding the scalability and effectiveness of this method in different soil types and environmental conditions. The variability in soil response to stabilization methods poses a significant challenge in achieving consistent and predictable improvements in soil properties. This variability is underscored by studies showing that the thermal conductivity of cement-stabilized earth blocks (CSEBs) varies with bulk density and the proportion of cement and lime added [10,11]. The benefits of lime stabilization depend on factors like lime percentage, soil pulverization level, curing duration, and stress state [12]. In response to these gaps, our study aims to provide a detailed analysis of the effects of varying stabilizer concentrations (0% to 15%) on the thermal behavior of Hejaz soil. This research focuses on two primary inquiries: the impact of stabilization concentrations on thermal behavior over time, and the differential reactions of two soil types to identical treatments. We employ TLS-100 measurements to explore the complex relationship between stabilization methods and soil properties in the Hejaz region. The goal is to contribute valuable insights to the field of sustainable construction practices, addressing some of the existing challenges in soil stabilization research.

This paper is divided into five sections. The first part explores the importance of soil thermal properties in sustainable construction and urban planning. Next, the second section reviews the literature on Geotechnical Soil Stabilization, focusing on practices in Saudi Arabia and the thermal and mechanical properties of stabilized soil. The third section describes the methodology, including soil selection, preparation, stabilization, use of the TLS-100 for thermal conductivity measurement, and data analysis using Python. The fourth section presents findings on soil characterization, thermal properties of untreated Hejaz soils, the effects of stabilization, and a comparative analysis of stabilizers. The study concludes with the fifth section, summarizing the study.

2. Literature Review

2.1. Geotechnical Soil Stabilization

Soil stabilization is essential in geotechnical engineering, enhancing soil’s physical properties for construction and engineering applications. This enhancement is achieved through the integration of various additives, which serve to bolster soil strength, diminish compressibility, and augment durability. Predominantly, Portland cement, lime, and cement kiln dust (CKD) have emerged as the preferred materials for soil stabilization. The process of cement stabilization involves the admixture of Portland cement into the soil, triggering a hydration reaction that culminates in the formation of cementitious compounds, effectively binding the soil particles together. This method has demonstrated its efficacy in fortifying the soil’s bearing capacity, thereby rendering it more resilient against external loads [13]. Concurrently, lime stabilization utilizes quicklime or hydrated lime, which interacts with the soil’s clay particles, resulting in enhanced soil strength, reduced plasticity, and improved workability, thus facilitating ease of manipulation during construction activities [14]. CKD, a byproduct of cement production, has also found its niche in soil stabilization. This approach offers a sustainable solution by repurposing industrial waste to enhance soil properties. Studies have concluded that soil stabilization is an effective and economical method for improving road pavement for engineering benefits [15]. The synergistic application of cement and lime in soil stabilization capitalizes on the immediate strength conferred by cement and the prolonged benefits afforded by lime, culminating in a comprehensive enhancement of soil properties [16]. Moreover, the environmental and economic dimensions of soil stabilization are indispensable. Utilizing industrial byproducts like CKD provides an eco-friendly and cost-efficient alternative to conventional methods, supporting sustainable construction practices [17]. In essence, the practice of soil stabilization, through the use of cement, lime, and CKD, plays a crucial role in augmenting soil properties for engineering applications, simultaneously promoting sustainable practices via the utilization of industrial byproducts.

Soil stabilization, crucial for enhancing soil properties for engineering applications, heavily relies on materials like cement, lime, and cement kiln dust (CKD), a byproduct of cement manufacturing; enhances soil workability; increases bearing capacity; and reduces clay settlement evident through decreased pH, electrical conductivity, and total dissolved solids in CKD-clay systems over time [18]. When mixed with polypropylene fibers, CKD not only strengthens soil but also enhances its ductility [19]. Additionally, its combined use with lime offers an economical solution for road pavements [15]. Blending CKD with lime kiln dust and fly ash further stabilizes soil subgrades [17]. Cement–lime stabilization, especially with Sugarcane Press Mud (SPM), significantly enhances strength, outperforming lime alone [20], with 1% SPM identified as the optimal amount for strength gain [20]. The combination of cement, lime, and bitumen has been identified as optimal for road construction [21]. Furthermore, adding silica fume to this mixture significantly reduces soil swell and swelling pressure [22]. Bagasse ash, lime, and cement together increase soil strength, lower its plasticity, and reduce swelling, with bagasse ash serving as a substitute for cement due to its high silica content [23]. Portland cement and lime effectively improve the shear strength in soils contaminated with organic compounds such as anthracene and glycerol [24]. They are also successful in stabilizing peat soil, increasing its shear strength by 14% [25]. Ground granulated blast furnace slag (GGBS) and CKD are employed to improve the physical and mechanical properties of soils. The optimal binder composition is found to be 6% of a mixture comprising 25% GGBS and 75% CKD [26].

The exploration of industrial wastes, such as cement kiln dust (CKD), lime kiln dust (LKD), and fly ash (FA), for soil stabilization contributes to improved soil stability and sustainability [1,5]. These materials not only provide an eco-friendly alternative to traditional stabilizers but also enhance the long-term stability of soil subgrades [13]. CKD and lime effectively reduced the swelling potential of the soil and improved its geotechnical properties [16]. CKD significantly enhanced the engineering properties of clayey soils, making them more suitable for use as pavement subgrades [27]. This exploration of various soil stabilization techniques and materials, especially cement, CKD, and lime, signifies significant advancements in the field. This proves that the integration of cement, lime, and cement kiln dust (CKD) in soil stabilization has become a central focus in geotechnical engineering, aiming to enhance the mechanical and physical properties of soils for a variety of construction purposes. Cement, being a well-established binding material, enhances soil strength and diminishes its compressibility. Lime, a traditional stabilizer, induces pozzolanic reactions, leading to long-term strength gain and decreased soil plasticity [15,28]. Moreover, the utilization of secondary raw materials, including waste and byproducts from industries such as cement kiln dust (CKD), has emerged as a sustainable and cost-effective alternative in soil stabilization, particularly in challenging terrains like peatlands. This approach not only offers environmental benefits but also contributes to the sustainability of the cement manufacturing process [13,17,29].

The stabilization of lateritic soil using a combination of CKD and lime has proven effective, reducing plasticity, maximizing dry density, and decreasing swell potential, thereby classifying the soil as suitable for road pavement applications [15]. This improvement is attributed to changes in soil particle interactions and enhancements in compaction and strength [15]. Similarly, fresh CKD has been shown to effectively stabilize high swelling clays, reducing their tendency to swell and improving overall soil stability [18]. The utilization of industrial wastes, such as CKD, in soil stabilization underlines a shift toward sustainable practices, emphasizing the recycling potential of these materials [13]. CKD, along with lime kiln dust (LKD) and fly ash (FA), has proven to be effective in stabilizing weak soil subgrades, highlighting CKD or FA/LKD for long-term stabilization and FA and LKD for short-term applications, thus improving the soil’s bearing capacity and stability and providing a sustainable alternative to traditional stabilizers [17]. Furthermore, the use of CKD in soil stabilization plays a critical role in addressing environmental concerns. An effective approach for mitigating arsenic contamination in soil is the application of lime dust and cement kiln dust (CKD). The effectiveness of this methodology has been demonstrated in the immobilization of arsenic, the reduction of its mobility, and the enhancement of soil safety [30]. The efficacy of CKD in the process of soil stabilization is heightened when employed in combination with additional substances, such as polypropylene fibers, which enhance the material’s ability to undergo deformation without experiencing fractures and subsequently reinforce it. Furthermore, the inclusion of glass fiber in fine-grained soil results in the improvement in its physical and mechanical characteristics, thereby increasing its stability and ability to bear loads [19,31]. The use of Cement By-Pass Dust (CBPD) in soil stabilization has shown improvements in the properties of expansive clay, showing similar results to those achieved with lime or cement [28]. In the context of expansive soils, CKD has been utilized to reduce swelling and improve geotechnical properties, with the addition of lime further enhancing these effects [16]. Enhancing the stability of clayey subgrade materials with CKD stabilization has shown significant improvements in the engineering properties of clayey soils, making them suitable as pavement subgrades [27]. This demonstrates that using cement, lime, and CKD in soil stabilization offers a comprehensive method for improving soil properties in construction applications, contributing to safer and more sustainable construction practices, and showcasing the potential of utilizing industrial byproducts in geotechnical engineering.

2.2. Soil Stabilization in Saudi Arabia

Soil stabilization is vital in geotechnical engineering, especially in Saudi Arabia. The unique geological and climatic conditions here require diverse stabilization techniques to support infrastructure such as roads, buildings, and bridges. The region’s expansive clays, sabkha soils, and oil-contaminated areas require innovative stabilization solutions to ensure construction safety and longevity. Studies on soil stabilization with cement and lime indicate that cement generally surpasses lime in enhancing soil strength and durability [32,33,34]. In Al-Hofuf’s ‘Hamrah’, a soil mixture with 15% cement attained a maximum compressive strength of 6.3 MPa, notably higher than the 1.46 MPa achieved with 10% lime [32]. This demonstrates cement’s superiority in soil stabilization, offering a more durable solution. Also, a mix of Portland cement, limestone powder, and cement kiln dust has effectively improved soil properties in oil-contaminated areas [35], showing their potential in addressing contamination and enhancing soil compaction and strength.

Cement kiln dust (CKD), a byproduct of cement manufacturing, is acknowledged as an effective soil stabilizer. It notably enhances the geotechnical properties of clay soil used in road construction, such as increasing strength and improving compaction behavior, thus providing the dual benefits of repurposing industrial byproducts and promoting sustainable construction practices [36]. Although generally less effective than cement in enhancing strength, lime stabilization has significant potential in improving soil workability and strength for certain soil types of compositions, underlining the necessity of selecting the appropriate stabilizing agent based on the soil’s unique characteristics. For low-volume roads, a composite material of cement and bitumen has shown superior performance, enhancing soil compaction, strength, and durability, thereby highlighting the effectiveness of using multiple stabilizing agents to achieve desired soil properties for specific applications [37]. In Saudi Arabia, the challenge of expansive soils, prone to significant volumetric changes with moisture variation, is effectively tackled through chemical stabilization using lime and cement. This approach results in decreased expansion tendencies and increased soil strength, crucial for preserving construction integrity and preventing structural damage [38,39]. Additionally, Sabkha soils, known for their high salt content and weak load-bearing capacity, especially in coastal regions, have shown improved load-bearing abilities after stabilization with cement and lime, rendering them more suitable for construction [40]. This underscores the vital role of tailored stabilization techniques in addressing the unique challenges posed by different soil types. Yet, the specific impact of stabilization on soil thermal behavior, especially in unique environments such as the Hejaz region, remains underexplored. This study aims to address this gap by examining the thermal properties of cement-, lime-, and cement kiln dust (CKD)-stabilized Hejaz soil. We explore how different stabilization concentrations influence soil thermal properties, a crucial aspect for the development of sustainable infrastructure solutions. The results of this study could directly contribute to enhancing stabilization practices in Saudi Arabia, exploring sustainable materials, and promoting more sustainable construction practices. This research has significant potential to contribute substantially to environmental science, sustainable materials, and geotechnical engineering. Furthermore, the applications of this research are not limited to Saudi Arabia but can be extended to other geotechnical contexts worldwide.

2.3. Soil Stabilization: Thermal and Mechanical Insights

The exploration of thermal properties in stabilized soil is a key aspect of geotechnical engineering, focused on enhancing the performance, sustainability, and resilience of construction materials. Over the past several decades, extensive studies have been conducted to better understand the effects of various stabilizing chemicals and procedures on the thermal, mechanical, and geotechnical properties of soils. Cement kiln dust (CKD), a byproduct of the cement manufacturing process, has been extensively researched for its potential in soil stabilization, demonstrating improvements in geotechnical qualities and construction suitability [41]. When combined with glass or polypropylene fibers, CKD impacts the physicochemical properties, geomechanical strength, and ductility of fine-grained soil. This addresses the brittle nature of CKD-stabilized soils and underscores the importance of considering mechanical and thermal properties in construction applications [19,31].

The combination of traditional stabilizers like cement and lime with innovative materials such as sodium silicate, Oil Shale Ash (OSA), and Ordinary Portland Cement (OPC) has shown promising results in enhancing the mechanical properties, durability, and thermal insulation of soil [42,43]. These combinations yield a resilient and long-lasting construction material, characterized by superior compressive strength, low water absorption, and minimal shrinkage. The use of alternative materials like Pottery Burnt Hull Ash (BHA) in combination with lime has demonstrated enhanced strength and reduced heat conductivity. This finding highlights the potential of integrating agricultural waste products into sustainable construction practices [44]. Moreover, air-entraining additives (AEAs) combined with cement, hydrated lime, fly ash, and metakaolinite affect the mechanical, thermal, and physical properties of earth mortars, leading to enhanced compressive strength, reduced water absorption, and modified thermal conductivity [45].

In pavement engineering, the use of CKD and Reclaimed Asphalt Pavement (RAP) significantly enhances the mechanical properties of subgrade soils, ensuring a stable and durable pavement structure that can withstand traffic loads and environmental conditions [46]. This proves that the comprehensive analysis of various soil stabilization techniques and materials has led to significant advancements in the field, providing numerous options for enhancing soil properties and ensuring stable, durable, and sustainable construction practices, as evidenced by the many studies.

Various methodologies have been employed to investigate the thermal properties of soils without stabilization, revealing a common need for enhanced understanding and measurement techniques. Experimental studies have yielded valuable data on thermal conductivity and resistivity, crucial for predicting soil behavior under environmental stressors [47,48]. Semi-empirical models have advanced this understanding by correlating thermal properties with soil density, a critical factor in soil stabilization [49]. The impact of soil composition on thermal behavior underscores the need for more detailed research in this area, especially for applications involving stabilized soils [50]. The development of new laboratory equipment highlights the existing gap in precise measurement tools, indicating a significant area for future innovation [51]. This collective body of work points toward a substantial need for advancing knowledge on the thermal properties of soils as well as stabilized soils.

Concurrently, the development of specialized laboratory equipment such as the TLS-100 device addresses the need for precise measurement tools. This device, in particular, has been instrumental in advancing our understanding of soil thermal properties. For example, a study on the thermal conductivity of Iraqi soils using the TLS-100 demonstrated how intrinsic soil composition influences thermal conductivity, providing valuable data for civil engineering applications [52]. Similarly, research on silty clay soils has demonstrated a clear relationship between thermal conductivity and resistivity, findings that are vital for the design of structures that must withstand temperature fluctuations [47]. Moreover, the use of bio-cementation techniques has significantly enhanced soil thermal conductivity, as measured by the TLS-100, suggesting improvements in the efficiency of energy piles [53].

In light of these significant findings, the current study adopts the TLS-100 device to measure the thermal properties of Hejaz soil, following the same measurement tool used in the Iraqi soil study [52]. This study will extend the application of the TLS-100 to evaluate the effects of stabilization with cement, CKD, and lime on Hejaz soil.

3. Materials and Methods

In this study, focusing on the Hejaz region of Saudi Arabia, we adopted a comprehensive experimental approach to examine the thermal properties of soil. Soil samples, named Soil A and Soil B, were specifically sourced from the Hejaz region, as shown in Figure 1. These soils were then subjected to stabilization using a blend of lime, cement, and cement kiln dust (CKD). The samples were prepared following the standard Proctor method, then sealed, and cured for four weeks. They were then exposed to natural weathering conditions, including sunlight exposure for periods of 6, 8, and 12 weeks. Re-testing these samples for thermal conductivity provided insights into the effectiveness of the stabilization methods. A detailed visual illustration of this experimental procedure can be seen in Figure 2.

3.1. Sample Preparation and Stabilization Agent Selection

Soils, sampled from the Hejaz region, were treated with a blend of cement, lime, and CKD in varying concentrations: 0%, 2.5%, 5%, 7.5%, 10%, 12.5%, and 15%. The preparation and stabilization process adheres to rigorous ASTM D698 standards [54] for soil compaction. The widespread use of ASTM D698 standards in studies highlights their importance in ensuring the reproducibility and reliability of soil stabilization research. Established test methods like compaction, CBR, and compressive strength are crucial for validating the stabilization effectiveness of CKD and lime in lateritic soils [15], and high swelling clays [18], while also aiding in the structural analysis of soils treated with cement, lime, and bitumen [30].

3.2. Post-Stabilization and Curing Process

After stabilization, the samples were sealed in airtight plastic bags and cured for four weeks. Following this initial curing period, we measured the thermal conductivity to establish a baseline before subjecting the samples to a natural weathering process under direct sunlight. Subsequent thermal tests were conducted at six-, eight-, and twelve-week intervals, enabling us to track the temporal evolution of the soil’s thermal properties.

3.3. Application of the TLS-100 Thermal Conductivity Meter

We used the TLS-100 Thermal Conductivity Meter, equipped with a 50 mm sensor, to measure the thermal properties of the stabilized soil samples in our study. This meter adheres to ASTM D5334 and IEEE 442-2017 standards [55,56], ensuring accuracy and consistency in the measurements. Our approach mirrors the methods previously used for non-stabilized Iraqi soils [52], thereby establishing a dependable benchmark for assessing soil thermal properties. The experimental protocol is visually outlined in Figure 3, a detailed flowchart. This chart methodically depicts the step-by-step methodology used in our study, providing a clear and systematic visual guide of the entire process.

3.4. Data Visualization with Python

Our study employs Python for data visualization, capitalizing on its powerful libraries for statistical analyses and graphical representation. This approach ensures efficient handling and clear presentation of our complex data sets. This methodology aligns with contemporary research practices, enhancing the interpretability of results related to the impact of variables on thermal conductivity [57,58,59].

4. Results and Discussion

Our study investigates the thermal properties of two distinct Hejaz soils, designated as Soil A and Soil B. The study’s results and discussions are organized into six sections: Soil Characterization Insights, Chemical and Physical Properties in Soil Stabilization, the thermal properties of untreated Hejaz soils, the effects of stabilization on Soil A and Soil B’s thermal properties, and a comparative analysis of different stabilizers on these soils. This approach allows us to elucidate the most effective soil stabilization techniques for optimal thermal efficiency in sustainable construction and environmental management within the Hejaz region.

4.1. Soil Characterization Insights

The Characterization Properties of Soils unveil significant contrasts between Soil A and Soil B as illustrated in

Table 1. Comprehensive Characterization Properties of Soils.

Property	Soil B	Soil A
Fine Content (%)	92.5	95.3
Sand Content (%)	7.4	4.9
Unified Soil Classification	ML (clayey silt)	CL (silty clay)
Optimum Moisture Content (%)	23	18.5
Maximum Dry Density (KN/m3)	15.89	20.69
Liquid Limit (%)	41	41
Plastic Limit (%)	26	18
Plasticity Index (%)	17	23
pH (With Water)	8.66	7.59
pH (With Calcium)	8.13	4.57
Conductivity (mS/cm)	7.911	16.96

. Soil A, with a fine content of 95.3% and a silty clay composition (CL), suggests a denser structure with higher water retention but lower permeability, as opposed to Soil B’s 92.5% fine content and clayey silt composition (ML), indicating a balance between moisture retention and drainage. The contrast extends to their moisture and density properties, where Soil A exhibits lower optimum moisture content (18.5%) and higher maximum dry density (20.69 KN/m³) compared to Soil B’s 23% and 15.89 KN/m³, respectively, impacting soil compaction and strength. Moreover, the plasticity properties, with both soils sharing the same liquid limit (41%) but differing in the plastic limit and plasticity index, highlight Soil A’s higher shrink–swell potential and Soil B’s relative stability.

pH variations further delineate these soils; Soil A shows significant reactivity to acidic conditions, dropping from a pH of 7.59 (with water) to 4.57 (with calcium), while Soil B maintains a more stable alkaline nature (8.66 with water, 8.13 with calcium). These pH changes are vital for nutrient availability and soil fertility. Additionally, Soil A’s higher conductivity (16.96 mS/cm) indicates a greater salt concentration, potentially impacting plant growth and soil health, compared to Soil B’s lower conductivity (7.911 mS/cm).

While this study primarily focuses on thermal characteristics in soil stabilization, understanding these soil properties is indispensable for generalizing our findings to broader construction practices. The distinctions between Soil A and B in terms of physical composition, moisture content, density, plasticity, pH response, and conductivity offer valuable insights. Soil A, suited for scenarios requiring higher water retention, demands careful management due to its acidification potential and higher plasticity. In contrast, Soil B, with its higher sand content and stable pH, emerges as a more favorable candidate for construction and less intensive agricultural applications.

4.2. Chemical and Physical Properties in Soil Stabilization

Chemical Composition: The chemical analysis reveals distinct characteristics for each stabilizer. Lime is exceptionally high in calcium (Ca) content at 98.09%, significantly higher than cement and CKD, suggesting its strong potential for altering soil pH and enhancing binding properties. In contrast, cement, with a Si content of 10.52% and an Fe content of 5.45%, indicates a likelihood of improved strength and durability in stabilized soils. CKD, with a balanced composition of 79.33% Ca, 3.53% Si, and 3.86% Fe, presents as a versatile stabilizer, potentially imparting both strength and binding properties to the soils as shown in

Table 2. Chemical Composition of CKD, Cement, and Lime.

Material	Ca (%)	Si (%)	Al (%)	Mg (%)	S (%)	Fe (%)
CKD	79.33	3.53	1.78	0.4	2.05	3.86
Cement	75.93	10.52	2.79	0.57	2.67	5.45
Lime	98.09	0.33	0.41	0.23	0.16	0.25

.

Physical Properties: Analyzing the physical properties, as presented in Figure 4A–C for OMC, and Figure 4D–F for MDD, distinct trends emerge. For OMC, there is a general decrease with increasing stabilizer concentration in Soil A for both cement and CKD, as seen in Figure 4A,C. This reduction suggests a denser soil structure with less water-holding capacity. Conversely, lime leads to an increase in OMC in both soils Figure 4B, indicative of enhanced moisture retention. MDD trends, as illustrated in Figure 4D–F, show that cement and CKD contribute to increased soil density, with Soil A reaching a peak MDD of 23 KN/m3 at 15% cement concentration. However, lime displays a decreasing trend in MDD with increasing concentration in Soil A, hinting at a potential weakening effect at higher concentrations.

The study highlights the critical role of stabilizers’ chemical composition in influencing their interaction with soils. While CKD and cement generally enhance soil density and reduce moisture content, lime shows an increase in moisture retention, affecting soil compaction differently. Selecting the appropriate stabilizer and concentration is crucial, depending on the desired soil property modifications for specific sustainable construction applications.

4.3. Thermal Properties of Untreated Hejaz Soils

In the study of Hejaz soils, Soil A and Soil B were evaluated to determine their baseline thermal conductivity, a vital parameter for understanding soil behavior under different environmental conditions. Initially, in airtight conditions at week four, Soil A’s thermal conductivity was 0.251 W/m·K, and Soil B’s was slightly higher at 0.271 W/m·K. These initial data, critical for benchmarking subsequent changes, were recorded in

Table 3. Baseline Thermal Properties of Untreated Hejaz Soils.

Soil Type	Condition	Week	Thermal Conductivity (K) (W/m·K)
Soil A	Airtight Container	Week 4	0.251
	Sun Exposure	Week 6	0.198
		Week 8	0.175
		Week 12	0.154
Soil B	Airtight Container	Week 4	0.271
	Sun Exposure	Week 6	0.231
		Week 8	0.198
		Week 12	0.175

.

As the study progressed, the soil was exposed to the sun, and notable changes in their thermal conductivity were observed. By the six-week mark, Soil A’s conductivity decreased by 21.1% to 0.198 W/m·K, and further decreased by 30.3% to 0.175 W/m·K at eight weeks, and eventually by 38.6% to 0.154 W/m·K at twelve weeks. In a similar pattern, Soil B’s thermal conductivity reduced by 14.8% to 0.231 W/m·K at six weeks, by 27% to 0.198 W/m·K at eight weeks, and finally by 35.4% to 0.175 W/m·K at twelve weeks. This consistent decrease in thermal conductivity over time for both soil types suggests a reduction in their heat conduction capability, likely due to structural alterations from sustained sun exposure. These findings, as illustrated in Figure 5, underscore the dynamic nature of soil properties in response to environmental factors.

The decision to conclude this measurement by the end of week 12 was taken balancing the need to observe significant changes in thermal properties against the practicalities of maintaining a controlled study environment. It had a balance that was adequate to capture the impact of environmental exposure on the soil’s thermal conductivity while minimizing the introduction of external variables. The findings from this time-bound study provide a solid foundation for future research into soil stabilization and its impact on thermal properties. Furthermore, these results underscore the potential of Hejaz soils as sustainable building materials. The observed reduction in thermal conductivity, particularly under sun exposure, points to their applicability in roles that benefit from low heat transfer, such as thermal insulation. With proper treatment and stabilization, these soils could be optimized for specific construction needs, enhancing their suitability as sustainable building materials in arid regions like the Hejaz. This study not only contributes to the understanding of soil thermal dynamics but also opens new possibilities for the use of local soils in energy-efficient and eco-friendly construction practices.

4.4. Impact of Stabilization on Soil Thermal Properties

In this section, we focus on the thermal effects of different stabilizers on Hejaz soils, as detailed in

Table 4. Cement, Lime, and CKD Influence on Thermal Conductivity at Different Time Intervals of Stabilized soil.

Week		% of Added Stabilizers	Thermal Conductivity (K)
			Soil A Cement	Soil B Cement	Soil A Lime	Soil B Lime	Soil A CKD	Soil B CKD
Plastic Wrap	Week 4	0%	0.251	0.271	0.251	0.271	0.251	0.271
		2.50%	0.528	0.552	0.555	0.463	0.427	0.526
		5%	0.596	0.467	0.543	0.466	0.468	0.576
		7.5%	0.506	0.452	0.52	0.459	0.507	0.474
		10%	0.435	0.432	0.512	0.416	0.517	0.438
		12.5%	0.664	0.42	0.511	0.431	0.553	0.397
		15%	0.447	0.404	0.53	0.441	0.788	0.495
Sun Exposure	Week 6	0%	0.198	0.231	0.198	0.231	0.198	0.231
		2.50%	0.443	0.334	0.525	0.306	0.257	0.373
		5%	0.413	0.31	0.485	0.241	0.358	0.33
		7.5%	0.305	0.228	0.457	0.311	0.377	0.272
		10%	0.29	0.308	0.431	0.221	0.382	0.239
		12.5%	0.361	0.332	0.416	0.333	0.364	0.221
		15%	0.318	0.309	0.386	0.288	0.425	0.258
	Week 8	0%	0.175	0.198	0.175	0.198	0.175	0.198
		2.50%	0.43	0.327	0.52	0.312	0.253	0.348
		5%	0.415	0.322	0.488	0.23	0.368	0.334
		7.5%	0.274	0.234	0.434	0.296	0.353	0.266
		10%	0.302	0.299	0.466	0.193	0.372	0.247
		12.5%	0.364	0.304	0.353	0.307	0.41	0.219
		15%	0.262	0.3	0.397	0.307	0.363	0.248
	Week 12	0%	0.154	0.175	0.154	0.175	0.154	0.175
		2.50%	0.49	0.342	0.52	0.307	0.243	0.375
		5%	0.396	0.309	0.465	0.219	0.343	0.343
		7.5%	0.267	0.227	0.464	0.225	0.362	0.258
		10%	0.291	0.319	0.446	0.214	0.376	0.275
		12.5%	0.361	0.302	0.413	0.309	0.419	0.229
		15%	0.33	0.308	0.379	0.271	0.356	0.25

. Our analysis encompasses the impact of cement, lime, and cement kiln dust (CKD) on the thermal properties of two distinct soil types, Soil A and Soil B. We specifically discuss the following aspects: cement’s effect on Hejaz soil from a thermal perspective for both Soil A and Soil B, lime’s influence on the thermal properties of Hejaz soil, and the CKD effect on the thermal conductivity of Soil A. Additionally, a comparative analysis of stabilizer agents’ thermal effects on Hejaz soils is presented, providing a comprehensive understanding of how each stabilizer modifies the soil’s thermal behavior, crucial for sustainable building practices in arid environments.

4.4.1. Cement’s Effect on Hejaz Soil: A Thermal Perspective

In evaluating the thermal conductivity properties of Hejaz soil modified with varying percentages of cement, an in-depth examination of the data from weeks 4 to 12, as depicted in Figure 6A–D, and Table 4, offers significant insights.

Initially, in week 4 (Figure 6A), under airtight conditions, the untreated Hejaz soil exhibited a baseline thermal conductivity (K) of 0.251 W/(m·K). The introduction of cement notably enhanced this thermal conductivity. For instance, a 2.5% addition of cement resulted in a 110.36% increase in K, elevating it to 0.528 W/(m·K). This increase can be attributed to the cement hydration process, which over time leads to a more compact cement paste cohesion structure [60], thereby enhancing thermal conductivity. Furthermore, the calcium silicate hydrates (C–S–H) formed in the hydrated Portland cement contribute to this behavior, exhibiting a dynamic mechanical behavior [61], which may enhance soil’s thermal properties.

A further analysis reveals that the influence of sunlight exposure on the soil’s thermal conductivity is significant. In week 6 (Figure 6B), soils with cement content, particularly at lower concentrations such as 2.5% and 5%, experienced a decrease in K from their peak values in week 5. The 2.5% cement sample saw a 16.1% reduction in K, emphasizing the vulnerability of thermal conductivity to environmental conditions. This decrease under sunlight exposure can be attributed to processes such as elastic mismatch, particle–particle friction, and micro-cracking, which are likely to occur in the soil [62], leading to a reduction in thermal conductivity.

By week 12, the thermal conductivity of the soil with a 15% cement concentration was reduced by 26.18%, a lesser decrease compared to the untreated soil, which experienced a 38.65% reduction in K. This lesser decrease in thermal conductivity in cement-treated soil over time can be attributed to the thermal degradation of hydration products like C–S–H and calcium hydroxide (CH) in the cement paste. Such degradation can induce micro-cracking and pore-coarsening, yet it imparts a degree of resilience to the soil structure, maintaining its integrity against environmental stresses [63].

The data collectively indicate that the addition of cement to Hejaz soil significantly enhances its thermal conductivity, particularly at higher concentrations. However, this enhancement is subject to environmental factors, such as sunlight exposure, which can inversely affect the thermal conductivity over time. The results suggest that while cement stabilization can improve the thermal performance of Hejaz soil, the long-term stability of these improvements under varying environmental conditions remains a critical consideration. These findings have significant implications for the application of cement-stabilized Hejaz soil in construction and thermal management systems, particularly in contexts where environmental exposure is a critical factor.

4.4.2. Lime’s Effect on Hejaz Soil A (Thermal)

The experimental investigation into the thermal conductivity (K) of Hejaz soil, modified with varying concentrations of lime and subjected to environmental conditions over a 12-week period, presents intriguing insights into the material’s thermal behavior. This study’s comprehensive data, depicted in Table 4 and Figure 6A–D, not only illuminate the immediate impacts of lime addition but also reveal the longer-term thermal dynamics under sun exposure.

Initially, in week 4 (Figure 6A), the introduction of lime into the soil exhibited a marked influence on its thermal properties. The baseline K for untreated soil stood at 0.251 W/(m·K). The addition of 2.5% lime elevated K significantly to 0.555 W/(m·K), registering a substantial increase of 121.12%. This pronounced enhancement can be attributed to the changes in the microstructure and water retention properties of the soil [64], as lime treatment leads to a denser soil matrix with reduced porosity, thereby improving thermal conductivity. Incremental increases in lime content to 5% and 7.5% resulted in K values of 0.543 and 0.52 W/(m·K), respectively. These figures, while still representing considerable improvements from the baseline, suggest a diminishing return with higher lime concentrations. Further increases to 10%, 12.5%, and 15% lime yielded K values of 0.512, 0.511, and 0.53 W/(m·K), respectively, indicating a leveling off in the effectiveness of lime as a thermal conductivity enhancer at these higher concentrations.

The dynamics shifted notably in week 6 (Figure 6B) under sun exposure. The non-stabilized soil’s K decreased to 0.198 W/(m·K), a 21.12% reduction from the baseline. In contrast, lime-treated soils demonstrated varying degrees of resilience. The 2.5% lime mixture maintained a high K of 0.525 W/(m·K), a 109.16% increase from the baseline, affirming the stabilizing effect of lime at this concentration. However, as lime content rose, the rate of K reduction became more pronounced. This trend suggests that while lime is effective in enhancing soil’s thermal conductivity, its protective capacity against environmental degradation is not absolute and varies with concentration. The durability of lime-stabilized soils and the pozzolanic reaction’s impact on soil properties play a crucial role in these observations [65].

By week 8 (Figure 6C), the untreated soil experienced a further decrease in K to 0.175 W/(m·K), a 30.28% reduction from the initial baseline. Lime-stabilized soils, particularly at higher concentrations, continued to display a decrease in K but remained notably above the untreated baseline. This trend is consistent with the geotechnical behavior and physicochemical changes observed in lime-treated soils [66], where the addition of lime alters the soil’s properties in a way that can affect its thermal conductivity. The final observations at week 12 (Figure 6D) solidified these findings, with lime-stabilized soils exhibiting consistently higher K values, highlighting the nuanced and concentration-dependent effectiveness of lime in improving and sustaining K under prolonged sun exposure.

These observations indicate that the addition of lime to Hejaz soil significantly enhances its thermal conductivity, particularly at lower concentrations like 2.5%, which maintained the most considerable improvements throughout the study. However, the effectiveness of lime in improving and sustaining K under prolonged sun exposure is nuanced and concentration-dependent, highlighting the need for a balanced approach in the application of lime for soil stabilization in thermally challenging environments.

4.4.3. CKD Effect on Hejaz Soil A (Thermal)

The experimental investigation into the thermal conductivity of Hejaz soil amended with varying concentrations of cement kiln dust (CKD) yielded significant insights, as detailed in the data and corresponding figures. In the initial phase of the study (week 4, Figure 6A), the introduction of CKD to the soil exhibited a pronounced effect on its thermal properties. For instance, a mere 2.5% addition of CKD elevated the thermal conductivity (K) value from 0.251 to 0.427 W/(m·K), representing a substantial increase of 70.12%. This enhancement in thermal conductivity is attributed to the pozzolanic reaction of CKD in the soil, which leads to increased density and improved soil structure, as evidenced by the significant increase in density, CBR, and UCS observed in CKD-stabilized soils [67]. This pozzolanic reaction, crucial for soil stabilization, results in the bonding of dispersed soil particles, enhancing the soil’s thermal properties [68].

As the CKD content increased to higher percentages, the K values continued to rise, reaching a peak at 15% CKD with a 213.55% increase. This exponential growth in thermal conductivity is consistent with the findings on the strength improvement in CKD-stabilized expansive clayey soil, highlighting CKD’s role in enhancing soil stabilization [69]. The study on analytical tests evaluating pozzolanic reactions in lime-stabilized soils, although focused on lime, provides further insights into the quantification of pozzolanic products [70], which are relevant for understanding CKD’s effects in soil [4].

Under sun exposure in week 6 (Figure 6B), all samples, including those treated with CKD, experienced a decrease in thermal conductivity. However, the CKD-treated soils still demonstrated superior performance compared to untreated soil, indicating CKD’s effectiveness in enhancing and maintaining thermal conductivity under varying environmental stressors. This sustained performance, despite environmental challenges, aligns with the findings from the study on the sustainable utilization of CKD for improving collapsible soil [71], which sheds light on the mechanisms by which CKD enhances soil properties.

These findings imply that adding CKD to Hejaz soil greatly increases its thermal conductivity, with the most significant improvements correlated with higher CKD percentages. Additionally, CKD-treated soils exhibit a degree of thermal resilience against external variables, especially under prolonged sun exposure. The study underscores CKD’s potential as a long-term soil stabilizer that can enhance thermal characteristics and provide a reliable solution for engineering applications sensitive to environmental conditions.

4.4.4. Cement’s Effect on Hejaz Soil B (Thermal)

The experimental investigation into the thermal conductivity of Hejaz Soil B, stabilized with varying percentages of cement, reveals significant insights into soil–cement interactions. Initially, at week 4 (Figure 7), the addition of 2.5% cement resulted in a dramatic increase in thermal conductivity, registering a 103.7% surge to 0.552 W/m·K. This marked elevation indicates an enhanced thermal pathway formation, likely due to the filler effect of cement particles within the soil matrix. Research on the thermal conductivity of cement paste containing various fillers, such as waste glass powder, metakaolin, and limestone [72], provides insights into how the filler effect of cement influences thermal properties, supporting our observations.

Further cement incorporation to 5% continues this upward trend, albeit at a reduced rate, with a 72.3% increase in conductivity to 0.467 W/m·K. However, as the cement content escalates to 7.5% and beyond, the incremental gains in thermal conductivity diminish. For instance, at 10% cement, the conductivity peaks at 0.432 W/m·K, followed by a gradual decline to 0.404 W/m·K at 15% cement content. This pattern suggests a saturation point beyond which additional cement does not proportionally enhance the soil’s thermal transfer capabilities.

The study’s progression into weeks 6 and 8, under sun exposure conditions, introduces another layer of complexity (as seen in Figure 7B,C). In week 6, the control soil (0% cement) exhibited a decrease in thermal conductivity by 14.8% to 0.231 W/m·K, highlighting the soil’s inherent vulnerability to external thermal influences. Conversely, soil samples with a 5% cement admixture displayed a notable increase in thermal conductivity by 32.5% compared to the baseline, indicating a bolstered thermal resilience attributable to cement addition. Studies on the thermal conductivity of thermal insulation cement in geothermal wells offer relevant information on cement’s properties [73], including its role affecting thermal conductivity, further elucidating the resilience observed in our cement-treated samples.

By week 8, while overall thermal conductivity decreases under sustained sun exposure, soils with higher cement content (e.g., 10% and above) show a relatively smaller decrement in conductivity. This observation suggests that while cement enhances initial thermal conductivity, its long-term effectiveness in maintaining these properties under continuous thermal stress varies depending on the concentration.

By week 12 (Figure 7D), a clear trend emerges, with a consistent decrease in thermal conductivity across all cement percentages from their respective peaks in earlier weeks. For instance, at 7.5% cement, conductivity reduces to 0.227 W/m·K, further decreasing to 0.308 W/m·K at 15% cement. This trend indicates a dynamic interplay between cement content, soil composition, and thermal properties over time. The data, as systematically presented in Table 4, suggest an optimal range of cement content that maximizes thermal conductivity. This finding is crucial for determining the most effective cement content for soil stabilization in terms of thermal management.

This finding is pivotal for applications where temperature control is essential. This study underscores the necessity for a nuanced approach in soil–cement mixture design, taking into consideration the long-term thermal behavior and environmental impacts to achieve sustainable and efficient construction materials.

4.4.5. Lime’s Effect on Hejaz Soil B (Thermal)

In the realm of soil stabilization, the addition of lime to Hejaz Soil B presents a complex interaction between the stabilizer concentration and the soil’s thermal properties. The initial phase of this study, as depicted in Figure 7A, focuses on the enhancement of thermal conductivity (K) with varying lime concentrations. At week 4, the addition of lime resulted in a significant increase in K across all concentrations. Notably, a 5% lime addition resulted in a high thermal conductivity increase of a 71.96% increase in K. This significant increase in thermal conductivity can be attributed to the clay–lime physicochemical reactions, which lead to a denser soil structure and reduced porosity, enhancing heat transfer. These reactions, crucial for understanding the increase in thermal conductivity due to lime addition, have been examined in studies focusing on the geotechnical improvement in clay soils using lime [74].

As we move into the exposure phase, we can see that the thermal conductivity trend reverses with sun exposure in Figure 7B (week 6) and Figure 7C (week 8). In week 6, the K value of the control sample (0% lime) decreased by 14.76%, whereas the K values of the lime-treated samples decreased at all concentrations. For example, K dropped by 48.28% at 5% lime and by 46.88% and 34.69% at 10% and 15% lime, respectively. This phenomenon suggests that the initial positive effects of lime stabilization may be diminished with prolonged exposure to solar radiation. Research on the short- and long-term effects of lime and gypsum applications on acid soils in a water-limited environment has shown that lime with gypsum can reduce soil Al more deeply than lime alone over extended periods [75]. This reduction in Al levels, indicative of changes in soil chemical properties, could be linked to the observed decrease in thermal conductivity under sun exposure. By week 12 (Figure 7D), a clear trend emerges, with a consistent decrease in thermal conductivity across all lime percentages from their respective peaks in earlier weeks. For instance, at 7.5% lime, conductivity reduces to 0.227 W/m·K, further decreasing to 0.308 W/m·K at 15% lime. This phase of the study illustrates the temporal variability and the influence of environmental conditions on the thermal properties of lime-treated soil. It emphasizes the need for a meticulous balance between lime concentration and environmental exposure to harness the full potential of lime as a sustainable stabilizing agent in soil engineering. This study, through its comprehensive analysis of lime–soil interactions over time, provides a foundational understanding for optimizing lime stabilization methods for desired thermal performance in geotechnical applications.

4.4.6. CKD’s Effect on Hejaz Soil B (Thermal)

The exploration into the thermal properties of Soil B modified with varying percentages of cement kiln dust (CKD) provides a comprehensive understanding of the material’s thermal conductivity behavior under different conditions. Analyzing the data from Table 4 and correlating them with the observations from Figure 7A–D, a nuanced picture emerges, highlighting the significant role of CKD concentration in altering the thermal dynamics of the soil.

Initially, Figure 7A (week 4) establishes a baseline for the thermal conductivity of Soil B without any CKD, at 0.271 W/m·K. The introduction of 2.5% CKD leads to a notable increase in thermal conductivity to 0.526 W/m·K, illustrating an approximately 94% enhancement. This trend continues with 5% CKD, where conductivity reaches 0.576 W/m·K, a 112.5% increase from the baseline. However, at higher CKD concentrations, the rate of increase in thermal conductivity begins to decline; with 7.5% CKD, conductivity is 0.474 W/m·K (a 74.9% increase), and with 10% and 12.5% CKD, it is 0.438 W/m·K and 0.397 W/m·K, reflecting increases of 61.6% and 46.5%, respectively. Remarkably, at 15% CKD, a reversal of this trend is observed, with conductivity increasing to 0.495 W/m·K, indicating an 82.7% increase from the baseline. This increase in thermal conductivity can be related to the effects of CKD, a pozzolanic material similar to fly ash, as explored in studies on the thermal conductivity of soil mixtures with fly ash. These materials, due to their pozzolanic nature, likely reduce soil porosity and enhance particle packing, leading to improved heat transfer [76].

The introduction of solar exposure leads to a shift in this behavior. Figure 7B (week 6) presents a marked decrease in thermal conductivity across all CKD percentages. For instance, at 15% CKD, conductivity falls to 0.258 W/m·K, a 47.9% reduction from the week 4 value. This decreasing trend in conductivity with increased solar exposure continues through weeks 8 and 12, as evidenced in Figure 7C,D. By week 12, for the 15% CKD mix, conductivity is reduced to 0.25 W/m·K, nearly halving its initial week 4 value. This trend is indicative of potential progressive alterations in the soil-CKD matrix under sustained solar influence, potentially due to structural or compositional changes that adversely affect thermal properties.

In synthesizing these observations, it becomes evident that the integration of CKD into Soil B significantly modifies its thermal conductivity. This alteration is initially positive, with an increase in conductivity observed up to a certain CKD concentration. Beyond this concentration, the rate of enhancement diminishes, and with prolonged solar exposure, a consistent decrease in thermal conductivity is observed across all CKD percentages. These findings underscore the dynamic nature of the thermal properties of soil-CKD composites and suggest potential implications for their use in applications where environmental exposure and material longevity are critical considerations. The observed trends provide a foundation for further research into the sustainability and practicality of using CKD in soil modification for engineering applications.

4.5. Comparative Analysis of Stabilizer Agents’ Thermal Effects on Hejaz Soils

4.5.1. Cement Integration: Thermal Impacts on Hejaz Soils (A and B)

Comparing the thermal conductivity of Hejaz soils, A and B, with incremental cement additions reveals intricate patterns in their thermal response. For Soil A, at week 4 under plastic wrap conditions, the baseline thermal conductivity with 0% added cement was 0.251 K. When 2.5% cement was introduced, there was a significant jump in conductivity to 0.528 K, marking a 110% increase. This trend of increasing conductivity with higher cement content continued, with 5% cement leading to 0.596 K (a 137.45% increase), 7.5% to 0.506 K (101.59% increase), and 10% to 0.435 K (73.31% increase). Notably, at 12.5% cement, Soil A reached its peak conductivity of 0.664 K, a striking 164.54% increase from the baseline. However, further addition of cement to 15% caused a reduction in conductivity to 0.447 K, still a 78.09% increase from the baseline but indicating an optimal cement content threshold. In contrast, Soil B showed a different thermal behavior. Starting from a baseline conductivity of 0.271 K at 0% cement, the addition of 2.5% cement resulted in a conductivity of 0.552 K (a 103.7% increase). However, unlike Soil A, Soil B’s conductivity decreased progressively with more cement: 0.467 K at 5% (72.32% increase), 0.452 K at 7.5% (66.79% increase), and 0.432 K at 10% (59.41% increase), culminating in 0.404 K at 15% cement (49.08% increase).

The effect of sun exposure over a 12-week period on these soils’ thermal conductivities further demonstrates their varied response to environmental factors. For Soil A, under sun exposure, the conductivity at 2.5% cement decreased to 0.443 K at week 6 (a 76.49% increase from the baseline) and 0.43 K at week 8 (71.31% increase), and further down to 0.49 K at week 12 (95.22% increase). At the optimal 12.5% cement content, conductivity reduced from 0.664 K at week 4 to 0.361 K at week 12, marking a 45.6% decrease from its peak but still a 43.82% increase from the baseline. Soil B, under similar sun-exposed conditions, showed a consistent decrease in conductivity with increased cement content over time: from 0.552 K at 2.5% cement (week 4) to 0.342 K at week 12 (26.20% increase from the baseline), and from 0.42 K at 12.5% cement (week 4) to 0.302 K at week 12 (11.44% increase from the baseline). These observations underscore the dynamic interaction between cement content, soil type, and environmental exposure in determining thermal conductivity, crucial for optimizing the use of these soils in sustainable construction.

4.5.2. Lime Integration: Thermal Impacts on Hejaz Soils (A and B)

In a detailed analysis of the thermal conductivity of Hejaz Soil A and Soil B with varying lime additions, distinct patterns emerge when compared to their baseline values at 0% lime addition in week 4. Initially, Soil A and Soil B presented thermal conductivities of 0.251 W/(m·K) and 0.271 W/(m·K), respectively, under plastic wrap. With a 2.5% lime addition, Soil A’s thermal conductivity increased significantly to 0.555 W/(m·K), marking a 121.12% enhancement, while Soil B recorded a 70.85% increase, reaching 0.463 W/(m·K). This upward trend in conductivity with increased lime percentages was consistent across both soils. For instance, at 5% lime, Soil A and Soil B achieved conductivity values of 0.543 W/(m·K) and 0.466 W/(m·K), representing 116.33% and 72.14% increases over their respective baselines. The peak conductivity for Soil A was observed at 15% lime (0.53 W/(m·K), a 111.16% increase), whereas Soil B peaked at 7.5% lime (0.459 W/(m·K), a 69.37% increase).

Under sun exposure, the thermal conductivities of both soils exhibited reductions from their peak values. By week 12, Soil A with 2.5% lime maintained a conductivity of 0.52 W/(m·K), representing a 6.31% decrease from its peak under plastic wrap but still a 107.17% increase from its week 4 baseline. Soil B, at the same lime concentration and time frame, showed a thermal conductivity of 0.307 W/(m·K), a 53.00% reduction from its peak and a 13.28% increase over its baseline. The differential response of the soils to sunlight exposure was evident across all percentages of lime. For example, at 15% lime, Soil A’s conductivity decreased to 0.379 W/(m·K) by week 12, a 28.49% decrease from its peak but still a 51.00% increase from the baseline. In contrast, Soil B at the same lime percentage and time frame saw its conductivity decrease to 0.271 W/(m·K), a 38.32% reduction from its peak and a mere 0.00% increase from its baseline. This analysis highlights the significant impact of lime stabilization on the thermal conductivity of soils and the varying degrees of resilience to environmental conditions, underscoring the importance of tailored soil treatment strategies.

4.5.3. CKD Integration: Thermal Impacts on Hejaz Soils (A and B)

The detailed analysis of the thermal conductivity of Hejaz Soils A and B, amended with varying concentrations of cement kiln dust (CKD), reveals significant insights into the interaction between soil properties and CKD under different environmental conditions. In Figure 6A, corresponding to week 4 under plastic wrap, Soil A demonstrates a remarkable increase in thermal conductivity with the addition of CKD. At a baseline conductivity of 0.251 W/(m·K) with 0% CKD, the addition of 2.5% CKD elevates conductivity to 0.427 W/(m·K), marking a 70.12% increase. This upward trend continues with increasing CKD concentrations, culminating in a dramatic 213.55% increase to 0.788 W/(m·K) at 15% CKD. Soil B, starting from a baseline of 0.271 W/(m·K), also shows an increase in thermal conductivity, reaching a peak of 112.5% (0.576 W/(m·K)) at 5% CKD. However, unlike Soil A, Soil B’s conductivity gains diminish at higher CKD concentrations, indicating a less linear response to CKD addition.

Upon transitioning to sun-exposed conditions, as detailed in Figure 6B–D, a notable decline in thermal conductivity is observed for both soils. For Soil A, the thermal conductivity at 15% CKD decreases from its peak of 0.788 W/(m·K) at week 4 to 0.356 W/(m·K) by week 12 Figure 6D, representing a significant drop of 54.85% from its peak performance under plastic wrap. This reduction, however, still reflects a 41.83% increase from its baseline of 0.251 W/(m·K) with 0% CKD, underscoring a certain level of resilience in CKD-treated Soil A. In contrast, Soil B experiences a more drastic decrease. The 15% CKD mixture in Soil B, which initially peaked at 0.576 W/(m·K) under plastic wrap conditions, plummets to 0.25 W/(m·K) by week 12 Figure 7D, a reduction of 56.59% from its peak and slightly below its original baseline, indicating a 7.75% decrease from the initial 0.271 W/(m·K). These observations highlight the differential impact of environmental exposure on CKD-amended soils, with Soil A displaying a degree of thermal conductivity retention under solar influence, while Soil B demonstrates a marked vulnerability. The results emphasize the importance of considering both soil type and environmental factors in assessing the long-term efficacy of CKD as a soil amendment for thermal conductivity enhancement.

5. Conclusions

This research has extensively explored the impact of different stabilizers—cement, lime, and cement kiln dust (CKD)—on the thermal conductivity of Hejaz soil. Our comprehensive analysis, based on the data from two soil types (Soil A and Soil B) under various conditions, reveals that each stabilizer uniquely enhances the soil’s thermal conductivity.

Key Findings:

CKD’s Remarkable Impact: A significant increase in thermal conductivity was observed, especially in Soil A (214% increase) with 15% CKD under plastic wrap conditions.

Cement and Lime Effectiveness: Both stabilizers effectively increased thermal conductivity, with variations depending on concentration and soil type.

This study’s conclusions have a big impact on the sustainable construction industry.

Implications:

Sustainable Construction: The possibility of using stabilized soil for constructing environmentally friendly and energy-efficient infrastructure.

Effectiveness of CKD: CKD is highlighted as an extremely effective stabilizer, especially at higher concentrations. While cement and lime work well enough, CKD turns out to be an extremely effective stabilizer, especially at higher concentrations. In order to meet the growing demand for sustainable building practices, these insights are essential for developing construction materials that are both thermally efficient and structurally sound.

Limitations and Future Directions:

Scope of Study: This study primarily focused on specific soil types and stabilizers.

Further Research Needed: Investigating a broader range of soil compositions, environmental conditions, and long-term effects on the environment and stability.

This research contributes significantly to the advancement of sustainable construction practices. By demonstrating the potential of various soil stabilizers to enhance the thermal properties of Hejaz soil, it opens new avenues for the development of more efficient and environmentally conscious construction materials.