Sustainable Talcum Powder: A Developing Solution for Reduction the Swelling Potential of Expansive Soil

Abstract

Expansive soils are clayey soils that undergo significant volume changes due to moisture content variations which can severely affect the stability of foundations and infrastructure. This study investigates the use of talcum powder as a novel stabilizing additive to reduce the swelling potential of expansive soils with particular focus on the behavior of the treated soil under curing conditions. Talcum powder concentrations of 5%, 10%, 15%, 20% and 25% by dry weight of soil was considered. A comprehensive series of laboratory tests were conducted, including swelling pressure, Atterberg limits, modified Proctor compaction and unconfined compressive strength at 4 curing times: 0 days, 7 days, 14 days and 28 days. In addition, mineralogical and microstructural analyses were carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Experimental results revealed that incorporating talcum powder at a content of 25% by dry weight effectively reduced the swelling pressure by 37.5%. The compression index decreases with the increase in the talcum powder content. The results highlight the material’s significant capability to enhance the engineering properties of expansive soils, particularly under curing conditions and offer a cost-effective and readily available solution for soil stabilization applications.

1. Introduction

Expansive soils are ones that undergo significant changes in volume in response to variations in their moisture content. These soils are commonly found in many regions around the world [1]. The most serious issues arise in soils that contain a high percentage of montmorillonite [2]. These soils swell when they absorb water and shrink as they get dry. This phenomenon creates pressure that can crack foundations, roadways and pipelines while some damage caused by expansive soils may be limited to minor maintenance [3]. In the context of studying unsaturated soils, several experimental techniques and constitutive models have been developed to evaluate their mechanical behavior under varying suction conditions. One notable device is the UPC oedometer which allows the measurements of volume change and compressibility parameters in unsaturated soils. The concept of oedometer testing on unsaturated specimens’ dates to the works of Croney and Coleman (1953) [4] and later by Bishop and Blight (1963) [5] who emphasized the importance of suction control in characterizing unsaturated soil behavior. There are many methods of treatment such as soil replacement and chemical stabilization. These methods have proven their effectiveness in significantly reducing the swelling potential of expansive soils. Selecting the most suitable method depends on factors such as the depth of the soil to be treated, the type of structure to be built, as well as the cost and practicality of the treatment approach. Several materials have been used to stabilize expansive soils. Cement enhances soil strength and reduces swelling through hydration and pozzolanic reactions. However, its production is energy-intensive, environmentally unfriendly due to high CO₂ emissions and it can significantly increase soil brittleness and stiffness [6]. Limestone powder primarily improves the soil by acting as a filler and sometimes reacting with minerals to reduce plasticity. Yet, it often is usually limited unless combined with other reactive agents and souring high-purity limestone can be costly or geographically restricted [7]. Fly ash exhibits pozzolanic properties and reduces swelling, yet its chemical composition can vary significantly and may include hazardous components [8]. Marble dust contributes mainly by filing soil voids and increasing dry density, although souring it in consistent quality and quantity is often challenging [9]. Quarry stone powder acts as a filler that improves compaction; However, it also faces issues related to availability, particle size uniformity and processing [10]. Metakaolin improves the swelling and plasticity characteristics of expansive soil due to its fine particle size, but its relatively high temperature requirement leads to an increase in energy consumption [11]. In contrast, talcum powder stands out as a more sustainable alternative; it is a hydrated magnesium silicate with the chemical formula Mg₃Si₄O₁₀(OH)₂ [12]. It is insoluble in water and only slightly soluble in diluted mineral acids. Additionally, talc is chemically stable and does not require high temperature processing, resulting in significantly lower energy consumption and carbon emissions [13]. Its structure consists of silicate layers stacked on top of each other without strong chemical bonds and these layers are held together by weak Van der Waals forces which give talc its characteristic softness and slippery feel. Moreover, talc is environmentally friendly due to its chemical inertness, making it less likely to react with surrounding materials or create harmful substances [14]. Talc is naturally sourced by mining rocks that contain high amounts of it typically metamorphic rocks enriched with magnesium and silica such as serpentinite, dolomite and soapstone [15]. Egypt has several talc deposits where metamorphic rocks provide favorable geological conditions for talc formation. The Eastern Desert is the most significant region for talc mining in Egypt. Specifically, talc deposits are found in the Aswan and Red Sea governorates.

This study aims to evaluate the effectiveness of using an alternative available material in the form of talcum powder in stabilizing expansive soils and enhancing their geotechnical properties. In this research, laboratory testing was conducted on soil samples treated with different percentages of talcum powder. Soil characteristics techniques such as X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze mineralogical and microstructural changes. Geotechnical tests included the free swell index, one-dimensional oedometer test, Atterberg limits, modified Proctor compaction test and unconfined compressive strength (UCS) test (these tests were studied).

2. Materials and Methods

2.1. Materials

2.1.1. Bentonite

The expansive soil used in this study is commercially known as bentonite. The soil samples used in this study were collected from the Fayoum region, Egypt, an area known for its clayey soils that exhibit expansive soil behavior due to periodic wetting and drying cycles and the presence of high-plasticity clay minerals. The chemical components of the bentonite, according to the data sheet, are mentioned in

Table 1. Chemical composition of bentonite according to manufacturer’s data sheet.

Property	Value
SiO2 (%)	55.0
Al2O3 (%)	22.4
Fe2O3 (%)	4.2
MgO (%)	2.2
CaO (%)	0.8
Na2O (%)	2.0
K2O (%)	1.3
Loss on ignition measured at 850 °C for 2 h (%)	10.0

. Mineralogically, 55% quartz, 26% kaolinite, 12% Montmorillonite and 7% other constituents. The physical and geotechnical properties are detailed in

Table 2. Physical and geotechnical properties of the used bentonite from the lab tests.

Property	Value
Specific gravity, Gs	2.63
Unit weight, $\gamma$ (kN/m3)	10.7
Water content (%)	8.69
Void ratio, e	3.01
Free swell index, FSI (%)	116.67
Liquid limit, L.L. (%)	41.57
Plastic limit, P.L. (%)	23.01
Plasticity index, P.I. (%)	18.55
Maximum dry density, $\gamma$dry max. (kN/m3)	16.52
Optimum moisture content, O.M.C. (%)	21.05

. The hydrometer test was performed on bentonite used to measure the particle size distribution of fine-grained soils, particularly silt and clay fractions. The results show that the soil contains 55% clay and 45% silt as shown in Figure 1.

2.1.2. Talcum Powder

The talcum powder used in this study was obtained from “Talc Egypt Company” located in Cairo, Egypt. Talcum powder is known for its exceptionally fine texture and softness. It is also highly absorbent, making it effective in reducing moisture levels [15]. The general physical and chemical characteristics of talcum powder are listed in

Table 3. Physical and chemical properties of the talcum powder as reported in the manufacturer’s data sheet.

Property	Value
Color	White
Specific gravity, Gs	2.78
Unit weight, $\gamma$ (kN/m3)	7.8
pH (10% solids)	7.0
SiO2 (%)	65.0
MgO (%)	32.0
Al2O3 (%)	>1.0
Fe2O3 (%)	>1.0
CaO (%)	>1.0

according to the data sheet.

2.2. Sample Preparation

Preparing the soil mixture is an important step to achieve uniform compaction for the test samples. The main goal of this step is to make a new soil mix with a certain amount of additive and then press it down in the oedometer cell to get the required dry density using a specific amount of compaction energy. As shown in Figure 2, The compaction tool which used in this study consists of a disc attached to a rod with a 136 g weight that slides along the rod and drops from a height of 150 mm to strike the disc and compact the material [16]. In this study, compaction energy of 8 J was applied, corresponding to 20 blows per layer while this compaction energy produces a maximum dry unit weight equal to 75–80% of the highest dry unit weight, which is consistent with the findings of Sakr et al. [17]. The objective was to evaluate the performance of the talc-treated expansive soil under suboptimal compaction which often represents the in-situ conditions in rural infrastructure projects. Thus, the selected compact energy, although lower than standard Proctor energy, is representative of site-specific situations and provides useful insights into the stabilization effectiveness under realistic conditions.

To prepare the bentonite-talcum powder mixture, the samples were first oven-dried at 105 °C [18], then the specimens were compacted manually inside a metallic oedometer ring which also served as the mold ensuring consistent dimensions throughout the testing process. The bentonite-talc mixtures were prepared by adding water at an estimated initial moisture content of 35–40% by dry weight to ensure proper workability and simulate field-like moisture conditions. This procedure was supported by Zheng and Yuan (2025) [19], who highlighted the significant influence of initial water content on the density and cohesion of benitoite, as well as by Song et al. (2021) [20], who reported a clear relationship between moisture content and the swelling behavior of expansive soils. The prepared mixture was then compacted manually in an oedometer ring in two layers with each layer receiving 20 blows using a sliding hammer apparatus. This compaction method, though lower in energy than standard procedures, was selected to simulate suboptimal field conditions. The precision of specimen dimensions was maintained to ensure consistency and minimize experimental error. The height and diameter of the specimens were carefully controlled within a tolerance range of ±0.5 mm using a digital caliper before testing. Any sample that did not fall within the standard oedometer ring dimensions was discarded. This control ensured uniform stress distribution and reliable swelling measurements.

3. Experimental Program

This study experimentally investigates the impact of adding 5%, 10%, 15%, 20% and 25% of talcum powder to expansive soil on X-ray diffraction, scanning electron microscopy, and various geotechnical and engineering properties. A total of 61 tests were conducted on the expansive soil both before and after treatment with different talcum powder percentages. The experimental test program is outlined in

Table 4. Overview of the experimental test series.

Experimental Series	Type of Test	Number of Tests	Test Details
S1	XRD test	4	Untreated soil, talcum powder, and soil treated with 5% and 10% talc.
S2	SEM test	3	Natural soil samples treated with 5% and 10% talc at 10 µm and 40 µm particle sizes
S3	Free swell index	7	1 with kerosene, 1 with natural soil, and with different talcum powder contents (5–25%).
S4	Oedometer test	6	Natural soil and treated sample at 5 different talc percentages
S5	Atterberg limits	12	Liquid limit and plastic limit for untreated and 5 treated soil samples with talc.
S6	Modified Proctor test	6	Natural soil and soil with 5 incremental talc percentages (5% to 25%).
S7	Unconfined compressive strength	24	Talc contents * 4 curing periods (0, 7, 14, and 28 days)
Total		61

.

3.1. X-Ray Diffraction (XRD)

X-ray diffraction is an analytical technique used to identify and characterize the mineralogical composition and crystalline structure of materials. In geotechnical engineering, XRD is commonly employed to examine soil and clay minerals, providing insights into changes in mineral phases due to chemical stabilization. The technique involves directing X-rays at a powdered sample and measuring the intensity and angles of the diffracted beams. Each mineral produces a unique diffraction pattern, allowing accurate identification and quantification of its presence in the sample. In this study, XRD analysis was performed to observe the mineralogical changes in expansive soil before and after the addition of talcum powder helping to understand the effects of the additive on the soil’s structural properties. The XRD analysis was carried out using CuKα radiation with a wavelength of 1.54060 A°. The data were collected at ambient temperature (229 K). The diffractometer used was equipped with a Cu target and Ni filter to eliminate Kβ radiation. The scan was conducted over a 2ϴ range suitable for identifying the crystalline phases present in the specimens.

3.2. Scanning Electron Microscope (SEM)

Scanning electron microscopy is a powerful imaging technique used to analyze the surface morphology and microstructural features of materials at high magnifications. In geotechnical engineering, SEM is widely applied to study the texture, particle shape, arrangement, and bonding characteristics of soil samples. The method involves scanning a focused beam of electrons over the sample surface which generates detailed images based on the interaction between the electrons and the atoms in the material. These images reveal fine-scale features that are not visible with traditional optical microscopy. In this study, SEM analysis was conducted to examine the microstructural changes in expansive soil before and after the addition of talcum powder, providing valuable information on how the treatment alters the soil fabric.

3.3. Free Swell Index

To investigate the swelling potential of the expansive soil, the free swell index tests were performed according to Holtz and Gibbs (1956) [21]. First, the soil samples were oven-dried at 105 °C and then sieved through a No. 40 sieve. Next, a specified amount of kerosene was poured into a 100 mL graduated cylinder, followed by the gradual addition of 10 g of the pre-sieved dry soil in small increments. The volume was recorded after the soil settled in kerosene. In a separate 100 mL graduated cylinder, 30 mL of distilled water was added, and the same 10 g of soil was progressively mixed in 0.50 g increments. After all the soil was added, distilled water was poured to fill the cylinder to the 100 mL mark to remove any remaining unmixed particles. The soil sample was allowed to swell undisturbed for 24 h. The final volume of the expanded soil was then measured. The free swell index (FSI) was calculated as:

$$FSI={\displaystyle \frac{V_f-V_i}{V_i}}*100$$

(1)

where FSI is the free swell index (%), V_f is the final volume of the expanded soil (cm³) and V_i is the initial volume of the natural expansive soil (cm³). It is important to note that bentonite does not react with kerosene; therefore, the volume measured in kerosene represents the un-swollen (reference) volume. The inclusion of this kerosene-based measurement allows for a more accurate determination of the swelling potential by isolating the effect of water-indued expansion. This also explains the presence of an additional entry in Table 4 which was used for comparison.

3.4. Oedometer Tests

To evaluate the swelling pressure of the treated expansive soil, oedometer tests were performed in accordance with standard procedures outlined in ASTM D4546 [22]. Initially, undisturbed or reconstituted soil specimens were prepared and placed inside a fixed ring oedometer cell with a porous stone at the base and top to facilitate drainage. The sample was first subjected to a small seating pressure (typically 5–10 kPa) to ensure proper contact. After that, the specimen was submerged in water to allow full saturation and initiate swelling. The soil was allowed to swell under zero additional vertical stress until it reached equilibrium indicated by a discontinuation of vertical pressure. Once swelling stabilized, incremental vertical loads were gradually applied to compress the specimen and counteract the swelling deformation. The vertical pressure required to bring the specimen back to its original height (before swelling) was recorded as the swelling pressure. This value reflects the internal pressure the soil exerts when allowed to absorb moisture under confined conditions. The compression index (C_c) is calculated from the results of a one-dimensional oedometer test. It is obtained as the slope of the straight-line portion of the void ratio and logarithm of effective stress curve relationship. It is essential for predicating primary swelling pressure in geotechnical design.

3.5. Atterberg Limits

To assess the consistency and plasticity characteristics of the treated expansive soil. Atterberg limits tests were conducted in accordance with ASTM D4318 [23]. These tests determine the moisture content ranges at which fine-grained soils transition between different states. The liquid limit was determined using the Casagrande cup method where soil paste was placed in a brass cup, grooved and repeatedly dropped from a standard height until the groove closed over a specified distance after a certain number of blows. The plastic limit was obtained by rolling the soil into threads until they crumbled at a diameter of 3 mm. The plasticity index was then calculated as the difference between the liquid and plastic limits.

3.6. Modified Proctor Test

To determine the compaction characteristics of the treated expansive soil, modified Proctor compaction tests were conducted in accordance with ASTM D1557 [24]. The objective of the test is to establish the relationship between the moisture content and the dry density of the soil. In this procedure, the soil was oven-dried and sieved through a No. 4 sieve before being mixed with varying amounts of water to prepare a series of samples with different moisture contents. Each sample was compacted in a standard mold in 5 equal layers with each layer receiving 25 blows from a 4.50 kg rammer dropped from a height of 457 mm delivering total compaction energy. After compaction, the wet mass and volume of the soil were measured to calculate the dry density. The process was repeated for different moisture contents to plot a compaction curve.

3.7. Unconfined Compressive Strength

To examine the strength of the treated expansive soil over time, unconfined compressive strength (UCS) tests were carried out in accordance with ASTM D2166 [25]. At curing intervals of 0, 7, 14 and 28 days. The unconfined compressive strength (UCS) test was performed on cylindrical specimens with a diameter of 38 mm and a height of 76 mm maintaining a height-to-diameter 2:1. The degree of saturation can influence drainage behavior during confined compressive strength (UCS) testing. Then, they compacted at their respective optimum moisture content and maximum dry density obtained from the modified Proctor test. The samples were extruded from the molds, where some were tested immediately, while others were stored in sealed conditions at room temperature (20 °C) and tested after curing periods of 7, 14, and 28 days. During the test, axial load was applied to the soil specimens without lateral confinement at a constant strain rate until failure occurred. The UCS value was calculated by dividing the maximum axial load at failure by the cross-sectional area of the specimen. Results from curing periods were used to assess the long-term stabilization performance.

4. Results and Discussion

This section presents the results obtained from the experimental testing program conducted on expansive soil mixed with talcum powder. The results are divided into two main sections: the first explains the swelling behavior and geotechnical properties of both the treated and untreated expansive soil samples and the second looks at the characteristics of the materials.

4.1. Swelling Behavior and Geotechnical Properties of Expansive Soil Before and After Talcum Powder Treatment

4.1.1. Free Swell Index

The obtained values of the free swell index (FSI) of expansive soil with increasing talcum powder content while added in 5% increments up to 25% are illustrated in Figure 3. The results clearly indicate a continuous reduction in the free swell index as the addition of talcum powder increases. The initial FSI of approximately 116% at 0% additive content decreases significantly to around 47% at 25% talcum powder.

The free swell index was calculated using the standard Equation (1). This progressive decline suggests that talcum powder effectively reduces the swelling potential of the expansive soil. This behavior can be attributed to the role of talcum powder: its filler effect which occupies the voids between soil particles and its ability to physically reduce the swelling mechanism by limiting the water interaction with expansive clay minerals.

4.1.2. Swelling Pressure

Figure 4 presents the results of one-dimensional consolidation tests (oedometer tests) performed on expansive soil treated with different percentages of talcum powder ranging from 0% to 25% increments. The graph describes the variation of void ratio (e) with applied vertical pressure (p) showing six distinct curves corresponding to each additive level. The untreated expansive soil exhibits the highest initial void ratio and the most significant reduction in void ratio with increasing pressure indicating a highly compressible structure. In contact, the treated samples show a marked improvement in compressibility behavior with a lower initial void ratio and more gradual reductions underloading. As the percentage of talcum powder increases, the soil matrix becomes more stable and the compressibility decreases. This behavior is primarily attributed to the filler effect of talcum powder, which consists of fine platy particles that occupy void spaces between soil grains. The presence of talcum powder improves particle rearrangement and densification, which leads to a more compact soil structure. The curves for 20% and 25% talcum powder show the lowest void ratios throughout the pressure range up to 1600 kN/m² indicating enhanced resistance to deformation. Overall, the results confirm that talcum powder significantly improves the compressibility characteristics of expansive soil.

Figure 5 displays the variations in the compressibility index (C_c) of expansive soil treated with different percentages of talcum powder ranging from 0% to 25% in increments of 5%. The results show a clearly decreasing trend in compressibility index values with increasing talcum powder content. The untreated soil exhibits the highest compressibility index of about 0.57, indicating significant compressibility under loading conditions. As the percentages of talcum powder increase, the compressibility index value steadily decreases reaching approximately 0.28 at 25% talcum powder which contributes to a denser and more stable soil structure by occupying void spaces and limiting particle rearrangement during the consolidation. This reduction in compressibility highlights the efficiency of talcum powder as a stabilizing agent, improving the mechanical behavior of expansive soils. The fine and plate-like particles of talc fill micro-pores, thus reducing the soil’s compressibility potential. These outcomes demonstrate that talcum powder addition not only decreases compressibility but also enhances long term stability of expansive soil.

4.1.3. Atterberg Limits

Figure 6 investigates the influence of incremental addition of talcum powder on the liquid limit (LL), plastic limit (PL) and plasticity index (PI) of expansive soil. The results clearly demonstrate a progressive reduction in all three Atterberg limits with increasing talcum powder content. Initially, the liquid limit of the untreated expansive soil was approximately 42%. With each 5% increase in talcum powder the liquid limit showed a noticeable decline reaching around 17% at 25% talcum powder content. The plastic limit followed a similar decreasing trend, falling from about 23% to approximately 15%. The most significant reduction was observed in the plasticity index, which dropped sharply from 19% to about 2% over the same range of talcum powder additions. This behavior is likely due to the inert and non-plastic nature of talcum powder which dilutes the active clay minerals responsible for high plasticity.

4.1.4. Modified Proctor Test

The modified Proctor test was conducted to evaluate the compaction characteristics of expansive soil treated with varying percentages of talcum powder. The test results, illustrated in Figure 7, show that both the maximum dry density (MDD) and the optimum moisture content (OMC) were significantly affected by the addition of talcum powder. Initially, the untreated soil exhibited a maximum dry density of approximately 16.52 kN/m³ at an optimum moisture content of about 21%. As talcum powder was gradually introduced, there was a general increase in maximum dry density, reaching a peak value of about 17.53% kN/m³ at 25% talcum content. Simultaneously, the optimum moisture content shifted slightly across the different mixtures with the most compacted sample (25% talcum powder) achieving its peak dry density at a lower moisture content compared to the untreated soil. This improvement in compaction behavior is likely due to the fine and inert nature of talcum powder which enhances particle packing and reduces void spaces within the soil matrix. The influence of talcum powder on the compaction behavior of expansive soil was further assessed by examining the trends in maximum dry density (MDD) and optimum moisture content (OMC) as a function of additive content as shown in Figure 8. The observed patterns show a continuous enhancement in soil densification with increasing talcum percentages coupled with a gradual reduction in the moisture levels required to achieve this state. These trends reflect a beneficial modification in the soil structure where the talcum powder likely acts as a fine filler improving the graduation and interparticle interaction of the mix. As a result, the treated soil becomes more efficient with less water input. This shift in behavior suggests that talcum powder not only improves the mechanical response of the soil during compaction but also contributes to greater consistency and predictability in field performance. By reducing moisture sensitivity, the treated soil becomes less prone to volumetric changes that typically challenge expansive soils under fluctuating environmental conditions.

To clarify the composition of the tested mixes,

Table 5. Compaction characteristics of different percentages of talcum powder with bentonite.

(%) of Talcum Powder	Maximum Dry Density (kN/m3)	Optimum Moisture Content (%)
0.0	16.52	21.05
5	16.69	20.64
10	16.85	19.83
15	17.03	18.97
20	17.22	17.83
25	17.53	17.30

presents the mix design details, including the percentages of talcum powder added to the expansive soil, maximum dry density of each sample and the moisture content used for sample preparation. All specimens were compacted at their respective optimum moisture content obtained from the modified Proctor compaction test.

4.1.5. Unconfined Compressive Strength (UCS)

Figure 9 presents the stress-strain relationship behavior of expansive soil specimens treated with varying percentages of talcum powder (0%, 5%, 10%, 15%, 20%, and 25%) and subjected to different curing durations: (a) 0 days, (b) 7 days, (c) 14 days and (d) 28 days. The unconfined compressive strength (UCS) tests were performed using a constant axial strain rate of 1 mm/min. At the initial testing stage (uncured samples), the UCS values show limited improvement with talcum powder addition as shown in Figure 9a. The untreated soil reached a UCS of 56.75 kN/m². The maximum UCS (62 kN/m²) was achieved with the sample that was treated with 15% talcum powder content indicating a slight early-age strength gain due to filler effects and possible particle rearrangement. However, the strength at this stage remains marginal, and curves show a quick rise followed by brittle behavior with limited ductility.

Curing noticeably improves the UCS. The control sample’s UCS increased to about 103 kN/m² while the treated samples exhibited significantly higher values as shown in Figure 9b. The peak strength occurred at 15% talcum powder (111 kN/m²), suggesting that short-term cueing of physical bonding mechanics. The overall shape of the curves becomes smoother and more gradual, indicating a transition toward more ductile behavior, especially in specimens with higher talcum contents.

At 14 days of curing, UCS values continued to increase across all mixed proportions. The highest UCS (154.25 kN/m²) was achieved with 15% talcum powder showing that the strength gained with talcum powder becomes more pronounced with time as shown in Figure 9c. The curves exhibit a pronounced peak followed by a slight post-peak drop characteristic of strain-hardening behavior. The untreated soil lagged significantly behind (132.5 kN/m²) confirming the effectiveness of talcum powder in enhancing mechanical strength over time.

The most substantial UCS gains were recorded in 28 days as shown in Figure 9d. The untreated sample reached 161.25 kN/m² while talcum-treated soils achieved UCS values as high as 184.75 kN/m² particularly at 15% treatment with talcum powder. The strength begins to stabilize at higher talcum contents indicating a threshold beyond which additional talcum powder does not significantly enhance UCS. The curves also become steeper initially and more gradual near failure suggesting enhanced ductility and energy absorption due to internal structure development with curing.

Figure 10 illustrates the variation in unconfined shear strength (C_u) of expansive soil treated with different percentages of talcum powder from 0% to 25% at curing periods of 0, 7, 14 and 28 days. The results show that C_u increases with both curing time and talcum powder content up to an optimal range after which the strength stabilizes and slightly decreases. At 0 days, the C_u remains nearly constant at about 30 kN/m² across all talcum percentages indicating minimal immediate effect. After 7 days, a moderate strength gain is observed peaking at around 10–15% talcum powder content at about 51.5 kN/m². At 14 days, the C_u continues to rise, reaching a maximum of about 77.5 kN/m² at 15% talcum content, then slightly decreasing at higher contents. The greatest at higher contents. The greatest improvement occurs in 28 days with a peak C_u of 92.5 kN/m² at 15% talcum powder. These findings suggest that talcum powder effectively enhances soil strength over time with 10–15% being the optimal range. Beyond this range, additional talcum may act as an inert filler, offering limited contribution to strength. Curing plays a critical role in activating the stabilizing effects, emphasizing the time-dependent nature of talcum-soil interaction.

4.2. Morphology Analysis

4.2.1. XRD Analysis

Figure 11 shows the XRD pattern of the untreated expansive soil which is rich in quartz and montmorillonite with moderate amounts of kaolinite. Quartz is the dominant mineral appearing at a strong peak around 2ϴ = 26.6°. Montmorillonite, the primary swelling clay, is indicated by a key peak at 2ϴ = 5.9° confirming its significant presence and its critical role in the soil’s expansive behavior. Kaolinite presents in moderate amounts identified by peaks at 2ϴ = 12.36° and 24.9° contributing to the soil’s plasticity but not to swelling. This mineralogical composition aligns with the typical behavior of expansive soils which undergo volume changes due to moisture variations.

Figure 12 presents the XRD pattern of the talcum powder and reveals that the material is predominantly composed of talc (Mg₃Si₄O₁₀(OH)₂), as evidenced by several strong and sharp diffraction peaks characteristic of a well-crystallized structure. The most intense reflection is observed at 2ϴ = 28.8°. Additional prominent peaks appear at 2ϴ = 9.36° and 19.25°, which further confirms the presence of talc’s layered silicate structure.

Figure 13 assesses the XRD pattern of the expansive soil treated with talcum powder indicating notable changes in the mineralogical composition compared to the untreated sample. Most significantly the intensity of the characteristic peak for montmorillonite, typically observed at 2ϴ = 5.9°, has decreased substantially suggesting a reduction in the amount of swelling clay mineral. The decline in the intensity of the montmorillonite peak may indicate a partial alteration of its structure, potentially due to physical effects or mirror chemical interaction, although no clear hydrate phases were detected in the XRD patterns. Concurrently, peaks associated with talcum powder, particularly at 2ϴ = 9.36° and 28.8° are clearly visible confirming the incorporation of talcum powder into the soil matrix. These results indicate that the addition of talcum powder not only introduces new stable phases into the soil but also reduces the presence of expansive minerals contributing to improved dimensional stability and reduced swell potential.

4.2.2. SEM Analysis

Figure 14a shows that the bentonite particles are unevenly spread out with some flat clusters and small bits mixed in. The particles appear largely irregular with sub-angular to sub-rounded shapes. These clusters likely correspond to montmorillonite platelets bound loosely through van der Waals forces forming flocculated aggregates typical of expansive clay minerals. The visible porosity between particles indicates potential for high water adsorption aligning with the known swelling behavior of bentonite.

Figure 14b presents the surface morphology of individual particles as more distinct, revealing a layered texture with evidence of sheet-like stacking and slight edge curling. This lamellar structure is characteristic of smectite clays and supports the presence of expansive behavior due to interlayer water uptake. The rough and flaky surfaces suggest a high specific surface area which is a critical parameter in cation exchange and swelling potential. Moreover, the presence of smaller particles attached to larger aggregates may indicate particle disintegration under mechanical or environmental stresses which could influence the material’s plasticity.

Figure 15a,b show the scanning micrographs of bentonite treated with 5% talcum powder at magnifications of 10,000× and 2500×, respectively. The microstructure presents a tightly packed and uneven shape where the typical flaky and plate-like bentonite particles can be seen, but they look like they are partly covered or mixed with smaller talcum particles. The presence of talcum powder appears to reduce interparticle voids, resulting in a denser and more cohesive structure. Additionally, the talcum particles contribute to a smoother surface texture and enhanced particle interlocking.

Figure 16a,b show the micrograph of bentonite treated with 10% talcum powder at the same previous magnifications. The images reveal a relatively compact and well-agglomerated structure with individual particles consolidated into larger aggregates compared to lower treatment levels. The surface texture is notably smoother, and talcum particles are more uniformly distributed across the bentonite matrix indicating effective physical integration. The reduction in visible interparticle voids suggests a decrease in overall porosity, potentially leading to enhanced dimensional stability. Additionally, the layered morphology of bentonite appears partially masked by the presence of talcum implying a modification in surface characteristics. This structural densification and surface coverage may contribute to altered physiochemical behavior such as reduced swelling capacity and changes in cation exchange efficiency. Overall, the microstructural changes confirm that increasing talcum content further modifies the bentonite fabric, enhancing particle interaction and aggregation.

It is also found that this additive can reduce interlayer water. This reduction in the double-layer attraction allows clay particles to come closer, leading to flocculation and potentially agglomeration. Flocculation reduces the effective surface area available for water adsorption and restricts the movement of water into the interlayer spaces, thereby significantly reducing the swelling capacity. Moreover, the presence of talcum powder particles between clay sheets may act as a physical barrier limiting the expansion of clay minerals upon wetting. This not only enhances the structural stability of the treated soil but also contributes to improving resistance against volume change over time.

5. Conclusions

• Talcum powder improves expansive soil properties by reducing the free swell index and swelling pressure. At 25% content, it lowers FSI from 116% to 47% due to its filler effect and structural modification. The oedometer tests confirm enhanced compressibility and strength, validating talcum powder as an effective soil stabilizer.
• The Atterberg limits of the expansive soil decreased with increasing talcum powder content with the plasticity index dropping from 16.93% to 2.64% with 25% talcum powder addition. These reductions show that talcum powder does not react with the soil and reduces the effects of active clay minerals. As a result, the soil exhibits lower plasticity and improved workability, confirming the suitability of talcum powder for stabilizing expansive soils with high plastic behavior.
• The addition of talcum powder was also found to affect the compaction characteristics of the expansive soil. The optimum moisture content decreased from 21.05% to 17.3% while the maximum dry density increased from 16.52 kN/m³ to 17.53 kN/m³ as the talcum powder content increased to 25%. This is due to the fine, inert nature of talcum particles which enhances particle packing and reduces voids. The resulting denser and more stable soil structure improves mechanical performance and reduces moisture sensitivity.
• Talcum powder significantly enhances the strength properties of expansive soil with unconfined compressive strength (UCS) reaching 184.75 kN/m² at 15% content after 28 days of curing higher than the untreated value of 163.5 kN/m². Undrained shear strength (C_u) also increases, peaking at 92.5 kN/m² with 15% talcum content. Strength gains decline slightly beyond 15%, indicating an optimal range of 10–15% for effective stabilization.
• XRD analysis shows a reduction in montmorillonite, which is the key swelling mineral, and the formation of stable talcum powder phases. SEM images reveal that talcum powder, especially at 5% and 10%, reduces porosity, fills interparticle voids and creates a denser, more cohesive soil structure. These changes diminish the flaky bentonite texture enhance particle bonding and improve overall dimensional stability. Thus, talcum powder functions as both a filler and stabilizer, making it a promising additive for reducing swelling and enhancing the engineering properties of expansive soils.

6. Future Work

Future studies are recommended to investigate the pozzolanic activity and chemical reactivity of talcum powder, especially when combined with activators like MgO or lime.