Post-consumer nitrile examination gloves were collected from healthcare facilities in Nirjuli and sterilized using standard steam autoclaving at 121 °C and 15 psi for 30 min before mechanical shredding into 2–5 mm fragments (Figure 2). Autoclaving is a globally accepted and validated method for treating biomedical waste and has been proven to effectively inactivate bacteria, viruses, and spores under standard operating conditions, ensuring microbiological safety of polymer-based medical disposables. Previous studies have shown that steam sterilization at 121 °C for durations as short as 15 min achieves more than a 6-log reduction in microbial load, meeting international sterilization assurance levels for healthcare materials, and its reliability has been comprehensively documented in the literature (Rutala et al., 2023). This treatment method is also explicitly recommended by the World Health Organization and national biomedical waste management guidelines for contaminated plastic waste prior to reuse or recycling (Mcdonnell and Burke, 2011). Following sterilization, the nitrile gloves which are generally composed of an acrylonitrile-butadiene copolymer were shredded, and their inherent tensile strength and chemical resistance were expected to provide discrete reinforcement within the soil matrix, contributing to improved mechanical performance (Consoli et al., 2005).

Medical-grade calcium sulfate hemihydrate (CaSO₄·½H₂O) was obtained from discarded orthopedic casts and mechanically cleaned to remove fabric liners and other contaminants. The material was then crushed, sieved through a 150-micron mesh, and thermally dried at 150 °C for at least 1 h, a process that significantly reduces any residual biological risk. X-ray fluorescence analysis confirmed CaO and SO₃ contents of 38.2% and 46.5%, respectively, consistent with medical-grade Plaster of Paris specifications (Singh and Garg, 2000). Previous studies have shown that calcium sulfate–based biomedical wastes pose negligible microbiological hazards after proper physical separation and thermal treatment, making them suitable for secondary reuse in construction and soil stabilization applications. In particular, Navale et al. (2019) demonstrated the safe reuse of biomedical POP waste following appropriate preprocessing, while Singh and Garg (2000) reported the chemical stability and environmental safety of waste gypsum and POP derivatives for civil engineering applications.

Figure 3a illustrates the variation of MDU and OMC of silty sand soil with increasing percentages of shredded NG. The untreated soil exhibits an MDU of 17.46 kN/m³ and an OMC of 14.7%. With the addition of 0.35% NG, the MDU increases to 17.56 kN/m³, while the OMC decreases to 14.2%, indicating improved compaction efficiency. This behavior can be attributed to the flexible and thin nitrile fragments occupying interparticle voids, which enhances particle rearrangement and packing under compactive effort. At 0.5% NG content, the soil achieves a near-optimal MDU of 17.50 kN/m³ with a further reduction in OMC to 13.8%. This suggests that an appropriate quantity of nitrile fibers promotes denser soil fabric by improving mechanical interlocking and reducing water demand due to the hydrophobic nature of nitrile polymers. However, at 1% NG content, the MDU decreases to 17.30 kN/m³ despite continued reduction in OMC (13.3%). This reduction in dry unit weight is attributed to excess fiber content, which disrupts soil-to-soil contact, creates a more open structure, and increases internal voids, thereby limiting effective densification. Similar non-linear trends in compaction behavior have been reported for fiber-reinforced soils, where an optimum fiber content enhances packing efficiency, beyond which excessive fibers hinder compaction (Tang et al., 2007; Consoli et al., 2009a). These obesrvations indicate that shredded NG reduce moisture demand and marginally improve compaction characteristics at low dosages, with 0.35%–0.5% NG representing the optimum range for achieving favorable compaction behavior in silty sand soils.

The variation of MDU and OMC of silty sand soil with increasing POP content is observed in Figure 3b. With the addition of 3% POP, the MDU increases to 17.72 kN/m³, accompanied by a slight reduction in OMC to 14.5%. This improvement is primarily attributed to the filler effect of fine POP particles, which occupy voids between soil grains and enhance particle packing under compactive effort. As the POP content increases to 6%, the MDU further rises to 17.87 kN/m³, while the OMC decreases to 14.2%, indicating more efficient densification and reduced water demand. At this dosage, POP acts not only as a physical filler but also undergoes partial hydration, forming calcium sulfate dihydrate (gypsum), which improves interparticle bonding and soil fabric continuity. A further increase in POP content to 9% results in a marginal increase in MDU to 17.90 kN/m³, while the OMC decreases more noticeably to 13.7%. The continued reduction in OMC is associated with water consumption during the hydration of calcium sulfate hemihydrate and the reduced affinity of the stabilized soil matrix for free water. However, the relatively small gain in MDU beyond 6% POP suggests that excess POP may lead to the formation of bulky hydration products and local agglomeration, which limits further improvement in packing efficiency. Similar trends have been reported in gypsum- and calcium-based soil stabilization studies, where an optimum additive content enhances density and reduces moisture demand, while excessive binder content yields diminishing returns in compaction performance (Nalbantoglu and Gucbilmez, 2001; Ren et al., 2022). These results indicate that POP significantly improves compaction characteristics of silty sand by increasing dry unit weight and lowering optimum moisture content, with 6%–9% POP identified as an effective range, and 6% representing the most efficient balance between densification and material utilization.

Figure 3c illustrates the variation of MDU and OMC of silty sand soil stabilized with a constant 0.5% shredded NG and varying POP contents. The soil amended with 0.5% NG alone exhibits an MDU of 17.50 kN/m³ and an OMC of 13.80%, indicating that nitrile fibers marginally improve particle packing while reducing moisture demand due to their hydrophobic polymeric nature. With the addition of 3% POP, the MDU increases to 17.80 kN/m³ and the OMC decreases to 13.67%, reflecting the combined effect of POP acting as a fine filler material and the flexible NG fragments promoting better particle rearrangement under compaction. As the POP content is increased to 6%, the MDU further rises to 17.85 kN/m³, while the OMC reduces to 13.60%, suggesting enhanced densification resulting from void filling by POP particles and partial hydration of calcium sulfate hemihydrate, which improves interparticle bonding and soil fabric continuity. At 9% POP, the MDU reaches its maximum value of 17.95 kN/m³ with a corresponding minimum OMC of 13.50%. The continuous reduction in OMC with increasing POP content is attributed to water consumption during the hydration of POP to gypsum (CaSO₄·2H₂O) and the formation of a denser, less water-absorptive soil matrix. The progressive increase in MDU demonstrates a synergistic stabilization mechanism, where NG provides discrete reinforcement and flexibility, while POP contributes to binding and pore refinement. However, the relatively smaller increment in MDU beyond 6% POP suggests diminishing compaction efficiency gains, likely due to the formation of bulky hydration products that limit further improvement in packing density. Similar synergistic trends between fibers and calcium-based binders have been reported in stabilized soils, where an optimal combination enhances compaction and reduces moisture sensitivity more effectively than individual additives (Consoli et al., 2009a; Nalbantoglu and Gucbilmez, 2001; Ren et al., 2022). These results indicate that the combined use of 0.5% NG with 6%–9% POP significantly improves compaction characteristics of silty sand, with 0.5% NG + 6% POP representing an efficient and practical optimum when both densification and material economy are considered.

Figures 4a–c shows the UCS test specimens for soil, Soil with nitrile gloves, Soil with nitrile gloves and POP. The stress-strain curve of plain soil is shown in Figure 5a. Plain soil exhibited minimal strength gain with curing, increasing from 70 kPa (0 days) to 93.6 kPa (28 days), reflecting limited cementation in natural silty sand. Nitrile gloves addition significantly enhanced both initial and long-term strength as shown in Figures 5b–d. For a NG content of 0.5%, the UCS after 28 days of curing reached 241.9 kPa (Figure 5c), indicating a substantial enhancement of 158% relative to the untreated soil.

The strengthening mechanism involves discrete reinforcement through stress transfer from soil matrix to tensile-resistant nitrile fragments, which bridge potential failure planes and provide post-peak ductility (Yetimoglu et al., 2005). However, at 1% NG, 28-day UCS decreased to 99 kPa (Figure 5d), indicating that excessive fiber content creates weak zones and reduces effective stress transmission (Michalowski and Čermák, 2003).

Figure 6a shows the UCS values of soil with various percentages of nitrile gloves at different curing periods. A substantial increase in the UCS values was observed at 0.5% NG content after 28 days of curing which can be attributed to the attainment of an optimum fiber dosage combined with the beneficial effects of extended curing duration. At this fiber content, nitrile gloves (NG) fibers are likely to be uniformly dispersed within the soil matrix, enabling effective stress transfer and improved soil-fiber interaction. Previous studies on fiber-reinforced soils have reported that an optimum fiber content enhances strength primarily through mechanical interlocking, frictional resistance, and the development of a reinforcing network within the soil structure (Tang et al., 2007; Consoli et al., 2002).

The significant increase in UCS at 28 days compared to the corresponding 14-day curing value for 0.5% NG suggests that curing time plays a critical role in mobilizing the reinforcing mechanism of fibers. Prolonged curing facilitates particle rearrangement, densification of the soil matrix, and improved interfacial bonding between fibers and soil particles, resulting in higher resistance to axial loading (Yetimoglu and Salbas, 2003; Estabragh et al., 2011). Similar strength gains have been reported in polymer and fiber-modified soils, where the reinforcing effect becomes more pronounced with time due to gradual enhancement of soil-fiber contact and frictional behavior (Correia et al., 2015). However, at a lower NG content of 0.35%, the fiber quantity may be insufficient to form a continuous reinforcing network, thereby limiting the contribution of fibers to strength development even at extended curing periods. Conversely, the reduction in UCS observed at higher NG content (1.0%) may be associated with fiber agglomeration and non-uniform distribution, leading to stress concentration and the formation of weak zones within the soil matrix, as widely reported in earlier studies (Consoli et al., 2009b). Therefore, the increase in UCS at 0.5% NG for 28-day curing represents an optimal balance between fiber content and curing duration, resulting in a substantial improvement in unconfined compressive strength.

POP addition produced significant strength improvements as shown in Figure 6b. At 6% POP, UCS increased from 156 kPa (0 days) to 352 kPa (28 days), representing a 109% gain over control soil. The increase in UCS values observed with the addition of Plaster of Paris (POP) can be attributed to its rapid hydration and subsequent formation of interlocking crystalline structures within the soil matrix. Upon mixing with water, POP (calcium sulfate hemihydrate) hydrates to form calcium sulfate dihydrate (gypsum), which acts as a binding agent between soil particles, leading to improved particle interlock and reduced pore spaces. This cementitious action enhances the stiffness and load-bearing capacity of the treated soil, resulting in higher UCS values with increasing POP content and curing time. Similar strength improvements due to gypsum-based and calcium-rich additives have been reported in stabilized soils, where the formation of cementitious bonds and crystalline networks significantly contributes to strength gain (Mahedi et al., 2020; Barman and Dash, 2022).

The progressive increase in UCS with curing duration indicates the time-dependent nature of hydration and crystallization processes, which promote densification and improved bonding at the soil–binder interface. The peak strength observed at 6% POP suggests an optimum binder content, beyond which further addition (9% POP) resulting in 251 kPa gave a comparatively lower strength gain. This reduction may be associated with excess POP leading to incomplete bonding, increased brittleness, or the formation of weak planes due to unreacted material, as reported in earlier studies on chemically stabilized soils (Khemissa and Mahamedi, 2014; Seco et al., 2011; Sharma and Sivapullaiah, 2016). Overall, the observed UCS enhancement confirms the effectiveness of POP as a soil-stabilizing agent when used at an optimum proportion and adequate curing duration.

Figures 7a–c demonstrates the non-linear relationship between POP content and strength, with 6% representing the inflection point. As reported by (Shen et al., 2019) the hydration of calcium sulfate hemihydrate produces interlocking gypsum crystals that bind soil particles through crystallization pressure and chemical bonding. Microstructural evidence presented in Section 3.4 confirms this mechanism.

Figure 8 illustrates the variation in unconfined compressive strength (UCS) of soil stabilized with combined additions of nitrile gloves (NG) fibers and Plaster of Paris (POP) at different curing periods. For soil containing 0.35% NG (Figure 8a), UCS increases progressively with increasing POP content and curing time, indicating that the cementitious hydration of POP, coupled with fiber-induced reinforcement, enhances particle bonding and stress transfer. A similar trend is observed for soil with 0.5% NG (Figure 8b), where a pronounced improvement in UCS is recorded with increasing POP percentage, particularly at longer curing durations. This behavior suggests a synergistic effect between NG fibers and POP, wherein fibers provide tensile resistance and crack-bridging action, while POP contributes to strength gain through hydration and formation of calcium sulfate dihydrate crystals that bind soil particles. In the case of soil containing 1% NG (Figure 8c), although UCS improves with POP addition and curing, the rate of strength gain is comparatively lower, likely due to fiber agglomeration and non-uniform distribution at higher fiber contents, which may induce weak zones within the soil matrix. The overall results demonstrate that UCS enhancement is governed by an optimum combination of NG content, POP dosage, and curing duration, consistent with findings reported for fiber and binder-stabilized soils in previous studies (Yetimoglu and Salbas, 2003; Tang et al., 2007; Consoli et al., 2002). The observed time-dependent strength gain further confirms the role of curing in facilitating hydration, densification, and improved soil–binder–fiber interaction.

Figures 9a–e illustrates the variation of California Bearing Ratio (CBR) values for untreated and treated soils under soaked and unsoaked conditions with varying contents of nitrile gloves (NG) fibers and Plaster of Paris (POP). As observed in Figure 9a, the CBR values initially increase with NG content and attain a peak at an intermediate dosage, beyond which a reduction is noted, indicating the presence of an optimum fiber content. This behavior is commonly attributed to enhanced interparticle friction, tensile resistance, and crack-bridging effects provided by randomly distributed fibers at lower dosages, while excessive fiber content leads to poor dispersion, fiber clustering, and reduced stress transfer efficiency (Yetimoglu and Salbas, 2003; Tang et al., 2007). Figure 9b shows that POP addition alone results in a moderate improvement in unsoaked CBR due to the formation of cementitious bonds arising from hydration and crystallization processes, however, soaked CBR values remain comparatively lower because moisture reduces matric suction and weakens interparticle bonding (Seco et al., 2011). Figures 9c–e demonstrate a significant enhancement in CBR when NG and POP are used in combination, particularly at intermediate POP contents, indicating a synergistic effect between fiber reinforcement and binder-induced cementation. The highest CBR values were observed for soil containing 0.5% NG with 6% POP, suggesting an optimum balance between tensile reinforcement from fibers and stiffness gain from POP hydration products. Although soaked CBR values are consistently lower than unsoaked values, the treated soils retain substantially higher bearing capacity compared to untreated soil, confirming the effectiveness of combined fiber-binder stabilization under both moisture conditions. Similar synergistic improvements in CBR have been widely reported for fiber and chemically stabilized soils in pavement applications (Estabragh et al., 2012; Sadek et al., 2010). The optimal combination (0.5% NG + 6% POP) achieved unsoaked CBR of 29% and soaked CBR of 6.8%, representing 429% and 228% improvements over control soil, respectively. According to (IRC, 2018), these values qualify the stabilized soil for heavily trafficked road subgrades which requires a minimum soaked CBR of 5%.

Scanning Electron Microscope analysis were performed on the plain soil, soil with 0.5% NG addition, soil with 6% POP addition and soil with NG and POP addition. For plain soil, SEM imaging of untreated soil (Figure 10a) revealed a loosely packed structure with angular to sub-angular particles, abundant macropores (10–50 μm), and minimal inter-particle bonding. The open fabric explains the low strength and high compressibility of natural silty sand. The SEM test of Soil +0.5% NG indicated that the addition of NG (Figure 10b) produced a more compact fabric with visible polymer fragments bridging soil particles. The flexible nitrile material conforms to particle surfaces, creating mechanical interlocking and reducing large voids. This bridging effect distributes applied stresses and inhibits crack propagation, consistent with observed strength improvements (Consoli et al., 1998). The SEM analysis of Soil +6% POP indicated that the POP-stabilized soil (Figure 10c) exhibited significant microstructural changes, including needle-like gypsum crystals (5–15 μm length) growing between soil particles, forming a three-dimensional network. Particle surfaces were coated with fine crystalline material, indicating chemical bonding. Porosity was visibly reduced through crystal growth filling voids. The interlocking gypsum structure provides mechanical strength and water resistance (Miller and Azad, 2000). The SEM imaging of Soil +0.35% NG + 9% POP showed that the combined stabilization (Figure 10d) demonstrated complementary mechanisms in which the nitrile fragments provide flexible reinforcement while gypsum crystals create rigid bonds. The resulting composite structure exhibits both strength and toughness, explaining superior performance in UCS and CBR tests.

Energy-Dispersive X-ray (EDX) analysis of the plain soil, soil with 0.5% NG addition, soil with 6% POP addition and soil with NG and POP combination were performed to determine the elemental composition and chemical characteristics of the samples. For the plain soil (Figure 11a), elemental composition showed Si (32.1%), Al (8.7%), Fe (4.2%), Ca (2.1%), and O (48.3%), consistent with silicate mineralogy typical of Himalayan sediments (Garzanti et al., 2005). For Soil +0.5% NG (Figure 11b) the presence of N (2.8%) and elevated C (8.4%) confirmed nitrile incorporation. The acrylonitrile-butadiene structure contributes to tensile strength and chemical stability. For Soil +6% POP (Figure 11c) substantial increase in Ca (12.3%) and S (8.6%) verified gypsum formation. The Ca/S molar ratio of approximately 1.0 confirms CaSO₄·2H₂O stoichiometry. Reduction in porosity-related O (42.1%) indicates void filling by hydration products. For Soil +0.35% NG + 9% POP (Figure 11d) the combined spectra showed elevated Ca (14.8%), S (10.2%), N (1.9%), and C (6.7%), confirming presence of both stabilizers. The high Ca and S content correlates with superior strength and CBR values, validating the synergistic mechanism.