Tailoring Water-Resistant Hybrid Geopolymers with triethoxyvinylsilane and Hexadecyl-trimethoxy-silane: A Comparative Study

Tailoring Water-Resistant Hybrid Geopolymers with triethoxyvinylsilane and Hexadecyl-trimethoxy-silane: A Comparative Study | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Tailoring Water-Resistant Hybrid Geopolymers with triethoxyvinylsilane and Hexadecyl-trimethoxy-silane: A Comparative Study Shehla Naz, Muhammad Jawad, noorul amin This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9203964/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The development of water-resistant geopolymer systems is crucial for extending the durability of alkali-activated binders in humid and aggressive environments. In this study, metakaolin-based geopolymers were modified with triethoxyvinylsilane (TEVS) and hexadecyltrimethoxysilane (HTS) to impart hydrophobic functionality at the matrix level. The reference geopolymer exhibited a low contact angle of 30°, confirming its hydrophilic surface. Incorporation of TEVS and HTS significantly improved wettability resistance, producing contact angles of 135° and 128%, respectively, attributable to vinyl-silane grafting and long-chain alkyl silane functionality. FTIR spectra confirmed reduced O-H stretching intensity (3430 cm⁻¹) alongside the emergence of Si-C and C-H vibrational bands, validating the successful incorporation of hydrophobic groups. SEM micrographs revealed improved matrix densification and reduced pore connectivity, particularly in TEVS-modified samples, while EDX spectra indicated carbon enrichment from 2.7% in the control to 28% and 25% in TEVS and HTS composites, respectively. Water absorption testing further highlighted the durability enhancement, with TEVS- and HTS-modified specimens restricting uptake to 0.34% and 0.40% after 28 days, compared to 1.5% in the control. The comparative analysis demonstrates that TEVS yields slightly superior hydrophobic performance due to stronger interfacial crosslinking, whereas HTS provides long-chain barrier effects. These findings establish silane-modified geopolymers as multifunctional composites with enhanced moisture durability, suitable for applications in marine infrastructure, wastewater systems, and chemically aggressive service environments. Geopolymer Hydrophobic modification Triethoxyvinylsilane Hexadecyltrimethoxysilane Contact angle Water absorption Sustainable construction Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Geopolymers have emerged as a new class of environmentally friendly construction materials with significant potential to replace ordinary Portland cement due to their low carbon footprint, thermal stability, and superior chemical resistance [ 1 – 3 ]. The production of Portland cement alone is responsible for nearly 8% of global CO₂ emissions, largely due to the energy-intensive calcination of limestone and fuel consumption in clinker production, which makes the search for greener alternatives an urgent priority [ 4 , 5 ]. Geopolymers are synthesized through the alkali activation of aluminosilicate precursors such as fly ash, metakaolin, or slag, producing a three-dimensional amorphous framework of Si-O-Al bonds that provides ceramic-like properties [ 6 – 8 ]. Among these raw materials, metakaolin is widely recognized for its high purity and reactivity, as it is obtained by the controlled calcination of kaolinite at 600–850°C, generating a highly reactive amorphous aluminosilicate phase [ 9 , 10 ]. Metakaolin-based geopolymers exhibit homogeneous matrices, enhanced strength, and better pore distribution compared to those synthesized from fly ash or slag, and this makes them ideal for investigating advanced modifications such as hydrophobicity [ 11 – 13 ]. Despite their mechanical and thermal advantages, however, geopolymers remain inherently hydrophilic owing to the presence of hydroxyl groups and open capillary pore structures, which allow water ingress and result in continuous absorption when exposed to humid or aqueous environments [ 14 – 16 ]. This hydrophilic behavior severely limits the long-term durability of geopolymers and reduces their suitability for aggressive conditions such as marine structures, wastewater treatment systems, and chemical containment units [ 17 , 18 ]. Water penetration not only initiates ion diffusion, efflorescence, and leaching of alkali species but also facilitates sulfate and chloride attack, which further degrades the geopolymer network [ 19 – 21 ]. In addition, cyclic swelling and shrinkage induced by moisture uptake accelerate crack propagation and lower overall stability [ 22 , 23 ]. Several strategies have been employed to address the challenge of water resistance in geopolymers. Optimizing the Si/Al ratio, adjusting the activator composition, and applying extended curing conditions can densify the microstructure and reduce porosity [ 24 , 25 ]. The use of supplementary mineral additives such as silica fume, nano-silica, and slag has been reported to refine the pore network and improve impermeability [ 26 , 27 ]. Nanomaterials such as alumina, titania, carbon nanotubes, and graphene oxide have also been integrated to enhance densification and modify interfacial properties [ 28 , 29 ]. While these approaches improve compactness, they do not fundamentally alter the hydrophilic chemical nature of geopolymers, which is governed by abundant silanol groups on the surface [ 30 ]. Consequently, geopolymers continue to absorb water and remain vulnerable in highly humid or aqueous environments. A promising alternative lies in hybrid modification using organic-inorganic coupling agents, which not only alter the surface chemistry but also introduce long-lasting hydrophobicity by presenting low surface energy functional groups [ 31 , 32 ]. Silanes and siloxanes have attracted significant attention in this context due to their ability to chemically bond with the geopolymer network while exposing organic hydrophobic moieties to the external surface [ 33 ]. Upon hydrolysis and condensation, silane coupling agents of the general formula R-Si(OR’)₃ form Si-O-Si linkages with the aluminosilicate skeleton, while the organic group (R) remains oriented outward, imparting water repellency [ 34 ]. This dual functionality provides strong chemical anchoring and a hydrophobic barrier simultaneously. Previous work has shown that polydimethylsiloxane (PDMS) effectively enhances hydrophobicity in metakaolin-based geopolymers, producing superhydrophobic surfaces with contact angles exceeding 150° [ 35 ]. Triethoxysilane (TES) has also been explored, providing moderate hydrophobicity with contact angles above 110° and improved durability under aqueous conditions [ 36 ]. These studies clearly indicate that silane-based modifications can impart functional hydrophobicity to geopolymers and extend their service life. Despite the success of PDMS and TES, other silane coupling agents remain underexplored. Triethoxyvinylsilane (TEVS) introduces vinyl groups that lower surface energy and offer additional opportunities for crosslinking reactions with surrounding silanol groups, potentially producing a denser and more durable hydrophobic network [ 37 , 38 ]. In contrast, hexadecyltrimethoxysilane (HTS) incorporates a long C16 alkyl chain that provides a continuous hydrophobic barrier by forming a densely packed monolayer that inhibits capillary absorption [ 39 , 40 ]. These two hydrophobic agents rely on different mechanisms: TEVS emphasizes interfacial reactivity and chemical bonding, while HTS relies on long-chain shielding and steric hindrance to repel water. Their comparative study can therefore provide valuable insights into tailoring hydrophobic geopolymer composites for diverse applications. Both TEVS and HTS have been widely reported as effective hydrophobic agents in cementitious systems, coatings, ceramics, and polymer composites, where they significantly reduce water uptake and improve long-term durability [ 41 – 43 ]. However, their direct incorporation into the bulk of metakaolin-based geopolymers has not been systematically investigated. Unlike surface coatings, bulk modification ensures uniform hydrophobicity throughout the material, preventing localized defects and providing a consistent barrier against moisture ingress [ 44 ]. The lack of systematic comparative research on TEVS- and HTS-modified geopolymers represents a critical gap in the current literature. Previous studies have largely focused on PDMS or TES, with limited attention to other silane coupling agents that may offer equally or more effective hydrophobic performance. A clear link between surface wettability and the structural, microstructural, and chemical features of geopolymers has not been fully established in earlier studies. To bridge this gap, the present work focuses on metakaolin-based geopolymers modified with triethoxyvinylsilane (TEVS) and hexadecyltrimethoxysilane (HTS), examining how these agents alter both surface properties and internal structure. Contact angle tests were carried out to determine changes in hydrophobicity, while FTIR spectra provided confirmation of chemical interactions and the presence of silane-derived functional groups within the aluminosilicate network. Morphological alterations in pore distribution and connectivity were evaluated using SEM, and EDX analyses offered further insight by identifying and quantifying carbon contributions arising from the silane treatments. Long-term water absorption experiments complemented these analyses by tracking the durability of the modified composites in aqueous environments. Taken together, these methods allowed a comparative assessment of how vinyl- and alkyl-based silanes contribute differently to the development of hydrophobicity in geopolymers. The findings point to the effectiveness of TEVS and HTS in refining microstructure, enhancing surface chemistry, and limiting water ingress, thereby demonstrating their suitability for producing highly durable geopolymer composites. Such materials are of particular value for use in marine structures, wastewater treatment facilities, and other service environments where prolonged contact with water or aggressive chemical exposure is unavoidable. Beyond performance improvements, the results also support wider sustainability objectives by promoting geopolymers as a viable alternative to Portland cement, offering reduced environmental impact without compromising long-term durability [ 45 , 46 ]. Methodology 2.1. Experimental Procedure Metakaolin used in this study was procured from the local distributor of Merck Company. To ensure its amorphous structure, the material was thermally treated at 850°C. The chemical composition of the resulting metakaolin, determined via X-ray fluorescence (XRF), is presented in Table 1 , with major oxides comprising 55.65% SiO₂, 40.55% Al₂O₃, 1.4% Fe₂O₃, and 1.5% CaO.The alkaline activator solution was formulated by combining analytical reagent-grade sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃). The NaOH solution was prepared at a molarity of 10 M, while the sodium silicate solution contained 14.51 wt% Na₂O and 33.39 wt% SiO₂. The activator modulus (Ms = SiO₂/Na₂O) was maintained at 1.6. Two distinct series of geopolymer specimens were synthesized, each incorporating different dosages of hydrophobic additives: triethoxyvinylsilane (TEVS) and hexadecyltrimethoxysilane (HTS). The geopolymer paste was prepared by thoroughly mixing metakaolin with the alkaline activator solution, maintaining a fixed liquid-to-solid mass ratio of 0.4 to ensure consistent workability. The fresh pastes were cast into triplicate stainless steel cube molds (20 × 20 × 20 mm³) and subjected to mechanical vibration to eliminate entrapped/ air. Molds were sealed with polyethylene film to minimize moisture loss during the initial setting phase.After 24 hours of ambient curing, the specimens were demolded and subsequently cured under controlled conditions at 20°C and 90% relative humidity for a duration of up to 28 days. Table 1 Chemical composition of metakaolin SiO 2 Al 2 O 3 Fe 2 O 3 CaO 55.65 40.55 1.4 1.5 2.2. Characterization Techniques The surface hydrophobicity of the synthesized specimens was evaluated using the sessile drop method with a contact angle goniometer (OCA20, Dataphysics Instruments, Filderstadt, Germany). A droplet of deionized water was dispensed vertically onto the sample surface using a microliter syringe, and the contact angle was recorded using a high-resolution camera and analyzed with proprietary software.Microstructural examination and surface morphology of both reference and hydrophobic geopolymer samples were conducted using a field emission scanning electron microscope (FESEM; Quanta SU8010, FEI, Netherlands/Japan), operated at an accelerating voltage of 15 kV. Elemental analysis of selected regions was performed using Energy Dispersive X-ray Spectroscopy (EDS; Apollo XP, USA) coupled with the microscope. Fourier-transform infrared spectroscopy (FT-IR) was conducted using a Nicolet iS50 spectrometer to assess the molecular structure and bonding environments of the geopolymer networks. The spectra were recorded in the range of 4000 − 400 cm⁻¹ using KBr pellets compressed under a load of 10 tons. 2.3. Water Absorption Test Water absorption capacity was assessed to evaluate the impermeability of the synthesized geopolymers. Specimens were first oven-dried at 105°C until a constant mass was achieved, then cooled in an air-tight desiccator for 24 hours. The dried specimens were weighed (initial mass), dipped in distilled water, and subsequently removed at specified time intervals. After wiping the surface moisture, the specimens were reweighed (wet mass). Water absorption (%) was calculated as the percentage increase in mass relative to the dry mass. Measurements were recorded at intervals of 1day, 2 days3 days, 4days, 5days, 6days, 7 days, 14days and 28 days. 3.1 Contact Angle Analysis Surface wettability is a key indicator of hydrophobic modification and is most reliably evaluated by static water contact angle measurements. In general, surfaces with contact angles lower than 90° are classified as hydrophilic, demonstrating affinity toward water spreading, whereas those with angles greater than 90° exhibit hydrophobic behavior due to reduced interaction with polar liquids. Achieving contact angles above 120° is considered a strong indication of advanced hydrophobicity, while values approaching 150° are typically associated with superhydrophobicity [ 1 – 3 ]. The reference geopolymer in this study displayed a baseline contact angle of 30° (Table 1 ), confirming its inherent hydrophilic nature, which is consistent with earlier reports attributing this behavior to the presence of silanol (Si-OH) groups and a porous microstructure that facilitates water penetration [ 4 , 5 ]. Upon the incorporation of hydrophobic silane-based modifiers, significant enhancements in surface water repellency were observed. Specifically, the introduction of triethoxyvinylsilane (TEVS) increased the contact angle progressively with dosage, reaching 135° at 10 wt%, while hexadecyltrimethoxysilane (HTS) modification produced a final contact angle of 128° under the same conditions (Fig. 1 ). These results clearly demonstrate the efficiency of both silanes in altering the surface polarity and suppressing moisture affinity. The gradual increase in contact angle with increasing TEVS and HTS dosage is summarized in Table 2 . At low concentrations (2–4 wt%), only moderate improvements were observed, with contact angles of 40–70° for TEVS and 42–73° for HTS. However, once the dosage exceeded 6 wt%, the contact angle surpassed the hydrophobic threshold of 90°, indicating a successful transition from hydrophilic to hydrophobic behavior. At 10 wt%, TEVS achieved a maximum contact angle of 135°, slightly higher than HTS at 128°, highlighting its superior hydrophobic efficiency. Table 2 Contact angle of geopolymer with varying amounts of TEVS and HTS Contact angle of with various percentages of hydrophobic agents 0 2 4 6 8 10 12 TEVS 30 40 70 92 130 135 130 HTS 30 42 73 94 125 128 127 The mechanism behind this enhanced hydrophobicity lies in the chemical functionality of the silanes. TEVS contains a vinyl group (-CH = CH₂), which introduces nonpolar organic moieties at the surface after hydrolysis and condensation reactions with the aluminosilicate network. When silane agents are incorporated into the geopolymer system, they play a direct role in lowering surface energy, allowing the formation of a protective hydrophobic layer [ 6 – 8 ]. TEVS, in particular, integrates well with the aluminosilicate framework and produces a relatively uniform coating. By contrast, HTS contains a much longer alkyl chain (C₁₆H₃₃-), which certainly reduces surface polarity but at higher levels can cause partial clustering and less even distribution across the surface [ 9 – 11 ]. This difference explains why TEVS generally records slightly higher contact angle values, as its smaller vinyl group is better able to interact with the geopolymeric gel and spread more evenly. These findings correspond well with earlier studies on silane-treated aluminosilicate matrices, where agents such as methyltrimethoxysilane (MTMS), octyltriethoxysilane (OTES), and TES improved water resistance by reducing hydroxyl groups and narrowing pore pathways [ 12 – 15 ]. The slight edge shown by TEVS over HTS can be traced back to their structural features: shorter functional groups penetrate more effectively and create a homogeneous network, while longer alkyl chains, although strongly hydrophobic, can lead to less consistent coverage. The contact angle tests confirm this behavior clearly. Unmodified geopolymers remained hydrophilic, whereas silane treatment converted them into water-repellent materials. TEVS-modified samples consistently surpassed 130°, approaching superhydrophobic behavior, while HTS-modified specimens also displayed high values around 128°. Taken together, the results indicate that both silanes are capable of imparting strong hydrophobicity, though TEVS demonstrates slightly better compatibility and stability. Such modifications open up the possibility of using these composites in construction applications where long-term resistance to moisture is essential. 3.2. Waterproofing and Water Absorption Performance The waterproofing performance of geopolymer composites can be effectively assessed through water absorption and immersion experiments, as shown schematically in Figure X. The reference geopolymer specimen, devoid of any hydrophobic modifier, exhibited noticeable water uptake when exposed to aqueous environments. The high affinity of aluminosilicate-based matrices towards polar water molecules is well established, and the observed penetration of water into the reference sample confirms the inherently hydrophilic nature of the unmodified geopolymer network. The interconnected porosity and polar hydroxyl groups present in the aluminosilicate backbone provide favorable sites for hydrogen bonding, facilitating rapid ingress of water molecules. In contrast, the incorporation of hydrophobic modifiers such as triethoxyvinylsilane (TEVS) and hexadecyltrimethoxysilane (HTS) significantly reduced water permeation. At 10 wt% TEVS, the specimen displayed a marked improvement in waterproofing efficiency, with water droplets largely repelled from the surface and exhibiting elevated positioning compared to the reference sample. This behavior suggests that TEVS successfully established a dense hydrophobic barrier on the surface and within the pore walls of the geopolymer. The ethoxy-silane groups in TEVS hydrolyze and condense during the geopolymerization process, chemically bonding with surface silanol groups while simultaneously introducing non-polar vinyl functionalities. These vinyl groups act as low-energy sites, reducing the surface polarity and blocking the diffusion of water through the pore network. Consequently, the TEVS-modified specimen demonstrated minimal weight gain during the immersion cycles, indicating strong resistance to water ingress. The performance of HTS-modified geopolymer was also significantly enhanced compared to the unmodified reference. At 10 wt% HTS incorporation, the material exhibited improved water repellency, as illustrated in Figure X. The long alkyl chains of HTS molecules anchor onto the geopolymer surface through siloxane linkages, creating a hydrophobic monolayer that prevents water molecules from penetrating the structure. The alkyl moieties introduce steric hindrance and lower the surface energy, thereby imparting sustained hydrophobicity. Although the contact angle results (128° for HTS vs. 135° for TEVS) suggest slightly reduced effectiveness compared to TEVS, the long hydrocarbon tails of HTS impart excellent stability against prolonged immersion, minimizing capillary absorption over extended exposure times. The comparative analysis between TEVS and HTS highlights distinct mechanisms of waterproofing. TEVS achieves its effect primarily through the formation of a covalently bonded, dense hydrophobic siloxane-vinyl layer that seals the pores, whereas HTS introduces a long-chain hydrocarbon coating that resists water permeation by lowering the free surface energy. Both approaches successfully suppress the hydrophilic tendencies of aluminosilicate binders, transforming the material into a water-resistant composite. The improved waterproofing performance of these modified systems not only enhances durability under wet conditions but also widens their potential applications in construction environments subjected to rain, humidity, and groundwater exposure. The results from contact angle and immersion tests show that the hydrophobic treatment is both effective and stable. The untreated geopolymer steadily absorbed water during immersion, a typical sign of capillary uptake in porous materials. In contrast, samples modified with TEVS and HTS took in very little water, and the values leveled off quickly. This indicates that the silane agents not only change the surface but also reduce pathways for water entry near the surface, giving the material lasting resistance. Similar improvements have been reported in earlier work on silane-treated geopolymers, where lowered porosity and the presence of hydrophobic groups helped protect against water attack [ 22 – 25 , 31 , 37 ]. Overall, both TEVS and HTS made the geopolymers far more resistant to water than the reference sample. TEVS gave higher contact angles and stronger droplet repellency, while HTS showed excellent stability during immersion. Each works in a slightly different way, but together the results point to their value in producing durable hydrophobic geopolymers for structures exposed to moisture and aggressive environments. 3.3 FTIR Analysis FTIR spectra of the TEVS- and HTS-modified geopolymers give clear evidence of changes in the chemical structure after modification as shown in Fig. 2 and Fig. 3 respectively. Both samples show a broad band in the 1000–1100 cm⁻¹ region, which corresponds to the Si-O-T stretching vibrations (T = Si or Al) that form the main framework of the geopolymer. In the TEVS-modified material, a strong peak at about 1033 cm⁻¹ confirms the presence of a well-developed silicate network. Two additional shoulders, appearing near 877 and 777 cm⁻¹, are linked to Si-O-X and Si-C bonds, indicating that vinylsilane groups were successfully attached to the matrix. These features are absent in the plain geopolymer, showing that TEVS chemically bonded with the aluminosilicate through hydrolysis and condensation of its ethoxy groups, leaving the vinyl group exposed at the surface. This bonding not only reinforces the silicate network but also contributes to the hydrophobic nature of the modified composite. In addition to these silicate-related features, the TEVS-modified sample displays a prominent band at approximately 1651 cm⁻¹, which corresponds to H-O-H bending vibrations of molecularly adsorbed water. This indicates that despite the partial hydrophobization imparted by TEVS, a certain degree of water interaction remains within the system. However, the presence of a distinct peak at 2984 cm⁻¹ is of particular importance as it represents the stretching vibrations of C-H groups associated with the vinyl moieties of TEVS. The observation of this band provides strong evidence that the organic fragments are chemically bonded to the geopolymer structure rather than being merely physically adsorbed. The combined effect of reduced hydrophilic O-H intensity and the introduction of C-H functionalities explains the rise in contact angle from 30° for the unmodified geopolymer to values exceeding 130° for TEVS-modified systems, thus confirming the efficiency of TEVS in lowering the surface energy of the composite (Fig. 2 ). In the case of the HTS-modified geopolymer (Fig. 3 ), several notable changes can be observed in comparison with the TEVS system. The main asymmetric stretching band of Si-O-T shifts toward a lower wavenumber, appearing around 968 cm⁻¹, which reflects increased polymerization of the silicate network and extensive condensation reactions induced by the grafting of long-chain silanes. The presence of distinct shoulders at 866 and 792 cm⁻¹ further supports the formation of new Si-O-Si linkages, while simultaneously suggesting successful bonding of HTS molecules into the geopolymer structure. The shift of the main silicate peak toward lower frequency is often correlated with a higher degree of cross-linking and a transformation toward a more ordered and denser aluminosilicate network, which is consistent with the improved mechanical stability and durability of HTS-modified composites reported in the literature. A particularly important observation in the HTS spectrum is the significant reduction in the intensity of the band at 1661 cm⁻¹, which corresponds to H-O-H bending of absorbed water molecules. This reduced intensity indicates that the hydrophobic hexadecyl groups of HTS effectively shield the surface from water adsorption, thereby decreasing the number of water molecules hydrogen-bonded to the surface hydroxyl groups. Furthermore, the most striking feature of the HTS-modified sample is the strong absorption band appearing at 2921 cm⁻¹, which is characteristic of the asymmetric stretching vibrations of CH₂ groups from the long hexadecyl chains. The intensity of this band confirms the successful grafting of HTS and the presence of a dense layer of organic chains on the surface of the geopolymer. This alkyl coverage is far more extensive than the vinyl coverage provided by TEVS, explaining why the HTS-modified geopolymers exhibit slightly higher and more stable contact angles, reaching up to 128° at 10 wt% loading. The low-frequency region of the spectra, particularly the bands located around 444 cm⁻¹ for TEVS and between 560 − 431 cm⁻¹ for HTS, corresponds to Al-O bending vibrations. These bands confirm the presence of the aluminosilicate framework and show that the modification with silanes does not disrupt the fundamental structural integrity of the geopolymer. Instead, the agents react primarily at the surface level by forming covalent bonds with silanol groups, while the bulk aluminosilicate network remains intact. This selective surface modification mechanism ensures that the intrinsic chemical durability and structural performance of the geopolymer are preserved, while simultaneously introducing enhanced hydrophobicity. A comparative analysis of the two systems highlights the differences in their modification mechanisms. TEVS, being a short-chain vinylsilane, introduces hydrophobic vinyl groups that reduce surface polarity but leave the system partially open to water interaction, as indicated by the still observable O-H and H-O-H vibrations. In contrast, HTS, with its long hexadecyl chain, provides a more continuous and compact hydrophobic barrier that significantly reduces water interaction and enhances waterproofing efficiency. This is consistent with the experimental waterproofing test, where HTS-modified samples demonstrated greater resistance to water penetration compared with TEVS. The correlation between FTIR spectral features and macroscopic hydrophobic performance confirms that the chemical grafting of silane agents directly translates into functional improvements in the material’s behavior. These findings are supported by earlier reports, where long-chain silane agents were shown to impart superior hydrophobicity and chemical resistance compared with short-chain analogs. Overall, the FTIR results clearly demonstrate that both TEVS and HTS undergo hydrolysis of their ethoxy groups, followed by condensation with surface hydroxyl sites of the geopolymer, forming stable Si-O-Si linkages. The remaining organic moieties, vinyl in the case of TEVS and long alkyl chains in the case of HTS, are oriented outward, thereby lowering the surface energy and imparting hydrophobicity. The more extensive modification achieved by HTS is evidenced by the stronger alkyl-related absorption bands, reduced water-associated signals, and shift of the silicate band toward lower frequencies. These molecular-level changes directly explain the improved macroscopic contact angle and waterproofing behavior, highlighting the significant potential of HTS as a highly effective hydrophobic agent for geopolymer modification. 3.4 SEM analysis The SEM images of foamed geopolymer specimens modified by the incorporation of hydrophobic agents such as triethoxyvinylsilane (TEVS) and hexadecyltrimethoxysilane (HTS) are shown in Fig. 4 and Fig. 5 respectively. The SEM images show that the particles are more closely integrated, with the open gaps largely reduced and a smoother, more continuous surface emerging. This improvement arises from the chemical action of TEVS: the ethoxy groups undergo hydrolysis and condensation to form siloxane bridges, strengthening the aluminosilicate network, while the vinyl groups contribute additional bonding interactions. The resulting structure is noticeably denser, with fewer pores, which aligns with the increase in contact angle values and reduced water absorption. These features confirm that TEVS not only modifies the surface chemistry but also improves the compactness of the bulk microstructure. In the case of HTS, the effect is even more pronounced. The SEM image of the HTS-modified sample shows a tightly packed matrix with very few cracks or voids, indicating a high level of structural refinement. This improvement can be linked to the dual function of HTS. Its methoxy groups bond chemically with the geopolymer framework, while the long hydrophobic alkyl chains tend to line the pore walls, creating a physical barrier to water penetration. The combination of chemical crosslinking and hydrophobic chain alignment makes the structure more compact and resistant to water ingress, giving the composite enhanced long-term durability. The overall morphology suggests improved structural integrity, where the particles are embedded within a more consolidated gel phase. This compact microstructural arrangement explains the higher contact angle values recorded for HTS, showing superior water repellency compared to TEVS. The smoother and denser surface texture of HTS-modified geopolymers indicates that this hydrophobic agent is more effective in blocking capillary pores and enhancing the long-term durability of the material (Fig. 5 ). The comparative analysis between the three microstructures highlights the effectiveness of hydrophobic modification. While the simple geopolymer shows a porous and fragile framework, TEVS modification improves the packing density and pore filling, thereby reducing voids. HTS, however, produces the most refined and consolidated microstructure, achieving significant densification and imparting a strong hydrophobic surface character. These microstructural transformations are in strong agreement with FTIR findings, where enhanced Si-O-Si and Si-O-X linkages confirm the formation of additional crosslinking networks, and with the contact angle results, where HTS exhibited the highest improvement in hydrophobicity. The SEM observations thus validate the chemical and functional improvements introduced by TEVS and HTS and underline the potential of these organosilanes in tailoring the pore structure and enhancing the durability of geopolymer composites. 3.5 Water absorption The water absorption behavior of the reference geopolymer and the hydrophobic agent-modified samples (TEVS and HTS) is given in Fig. 6 , provides critical insight into the impact of surface modification on the long-term durability of these composites. As shown in the graph, the reference geopolymer exhibits a progressive increase in water uptake, reaching nearly 1.5% after 15 days, beyond which the absorption stabilizes. This relatively high absorption is consistent with the porous microstructure observed in SEM images, where interconnected voids facilitate continuous capillary suction of water until equilibrium is reached. Such behavior is typical of unmodified geopolymers, where the lack of hydrophobic barriers allows easy penetration of water molecules into the matrix, thereby compromising dimensional stability and resistance to aggressive environments. By contrast, the TEVS and HTS modified samples demonstrate a significant reduction in water absorption throughout the immersion period. In both cases, the initial uptake within the first few days is minimal, and the curves gradually plateau at much lower values compared to the reference. The final water absorption for TEVS and HTS samples stabilizes around 0.33% and 0.40%, respectively, which is nearly four to five times lower than that of the reference geopolymer. This marked reduction highlights the effectiveness of organosilane modification in introducing hydrophobicity into the geopolymer network. TEVS contributes through the hydrolysis and condensation of ethoxy groups, forming siloxane linkages that fill micro-pores and reduce capillary action, while the vinyl group enhances compatibility within the aluminosilicate network. On the other hand, HTS introduces long alkyl chains that create a hydrophobic barrier on the pore surfaces, effectively repelling water ingress. This dual chemical and physical protection mechanism is reflected in the extremely low absorption values. Interestingly, although both modifiers significantly improve performance, TEVS-modified samples show slightly lower final absorption than HTS. This trend can be attributed to the difference in modification mechanisms: TEVS participates more actively in crosslinking with the geopolymer matrix, thereby sealing finer pores more effectively, whereas HTS, while providing superior surface hydrophobicity, may leave behind microvoids due to steric hindrance of the bulky alkyl chains. Nevertheless, both TEVS and HTS provide outstanding durability enhancements when compared to the unmodified geopolymer. The observed trends in water absorption strongly correlate with SEM microstructural analysis, where both TEVS and HTS imparted denser, more compact morphologies with reduced interparticle voids. The lower absorption also aligns with the contact angle results, confirming that hydrophobicity achieved at the surface extends to the bulk of the material, improving barrier performance. Reduced water ingress is particularly important for the long-term stability of geopolymers, as it minimizes risks of leaching, efflorescence, and chemical attack from chloride and sulphate ions in aggressive service environments. Therefore, the water absorption study not only validates the efficiency of TEVS and HTS as hydrophobic modifiers but also underscores their potential for improving the durability and service life of geopolymer composites in infrastructure applications. Conclusion The present study demonstrates that the introduction of hydrophobic silane-based agents, triethoxyvinylsilane (TEVS) and hexadecyltrimethoxysilane (HTS), significantly enhanced the water-resistant behavior of metakaolin-derived geopolymer composites. A clear improvement in surface hydrophobicity was confirmed by contact angle analysis, where untreated samples showed low wettability while the modified systems exceeded 130°, indicating the successful incorporation of hydrophobic functionalities into the geopolymer framework. Evidence from FTIR supported these results, with additional Si-O-Si and Si-C bonds pointing towards chemical interaction between the silane molecules and the aluminosilicate matrix. Microstructural inspection using SEM further revealed that the treated composites developed a denser and less porous morphology compared to the reference material, a factor that directly contributed to the reduced water ingress. This was reflected in the absorption tests, where the unmodified specimens absorbed about 1.5% water, in contrast to less than 0.4% in the TEVS- and HTS-modified samples even under extended exposure. Between the two agents, TEVS offered slightly more stable long-term performance, whereas HTS also delivered high hydrophobicity but showed marginally higher uptake at later stages. Taken together, these results establish silane modification as a practical and reliable approach to enhance the durability of geopolymers. Such composites hold strong potential for construction applications, particularly in environments where prolonged contact with moisture is a concern, as they combine the advantages of geopolymers with improved service life and reduced maintenance demands. Declarations 4. Funding information No funding from any organization was received for conducting this study. References Davidovits J (2013) Geopolymer chemistry and applications, 4th edn. Geopolymer Institute, Saint-Quentin Provis JL, van Deventer JSJ (2009) Geopolymers: structures, processing, properties and industrial applications. 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J Mater Sci 45:609–615. https://doi.org/10.1007/s10853-009-3934-0 Kamseu E, Leonelli C, Boccaccini DN, Veronesi P, Miselli P, Pellacani GC, Melo UC (2007) Characterisation of lead-vanadate-based glasses and their potential use as cement replacement in geopolymer matrices. J Eur Ceram Soc 27:253–259. https://doi.org/10.1016/j.jeurceramsoc.2006.04.094 Ma Y, Awang AZ, Omar W (2012) Influence of curing regimes on the mechanical performance and microstructure development of fly ash-based geopolymer mortar. Constr Build Mater 36:597–603. https://doi.org/10.1016/j.conbuildmat.2012.06.002 Perera DS, Hanna JV, Davis J, Blackford MG, Latella BA (2007) Relative strengths of phosphoric acid-activated metakaolin and kaolinite-based geopolymers. J Mater Sci 42:3093–3101. https://doi.org/10.1007/s10853-006-0531-6 Zhang HY, Kodur V, Cao L, Qi SL, Wu B (2014) Development of metakaolin-fly ash based geopolymers for fire resistance applications. Constr Build Mater 55:38–45. https://doi.org/10.1016/j.conbuildmat.2014.01.040 Maleki A, Shahmoradi B, Nazemi F et al (2019) A green, porous and eco-friendly magnetic geopolymer adsorbent for heavy metals removal from aqueous solutions. J Clean Prod 215:1233–1245. https://doi.org/10.1016/j.jclepro.2019.01.080 Kong DLY, Sanjayan JG (2008) Damage behavior of geopolymer composites exposed to elevated temperatures. Cem Concr Compos 30:986–991. https://doi.org/10.1016/j.cemconcomp.2008.08.001 Zhang Z, Yao X, Zhu H, Hua S, Chen Y (2008) Preparation and mechanical properties of polypropylene fiber reinforced calcined kaolin-metakaolin geopolymer. J Mater Sci 43:1035–1040. https://doi.org/10.1007/s10853-007-2158-3 Zhang Y, Chen Y, Wang H (2017) Modification of geopolymer with polydimethylsiloxane (PDMS) to improve water resistance. Appl Clay Sci 141:54–60. https://doi.org/10.1016/j.clay.2017.02.022 Amin NU, Jawad M, Saeed W, Shenashen MA (2025) Synthesis of water-resistant hybrid geopolymer composites using polydimethylsiloxane and triethoxysilane. Chem Phys 573:113681. https://doi.org/10.1016/j.chemphys.2024.113681 Ma C, Li W, Bai Y (2018) Hydrophobic modification of cementitious composites by vinyl-functional silanes. Constr Build Mater 171:52–60. https://doi.org/10.1016/j.conbuildmat.2018.03.067 Zhang W, Xu H, Provis JL (2016) Effect of silane coupling agent on the performance of geopolymers. Cem Concr Res 89:1–13. https://doi.org/10.1016/j.cemconres.2016.08.004 Yang T, Zhu H, Zhang Z (2014) Hydrophobic surface modification of cement-based materials by alkylsilane. Constr Build Mater 66:219–225. https://doi.org/10.1016/j.conbuildmat.2014.05.053 Wang R, Li G (2011) Influence of hexadecyltrimethoxysilane on cement composites. J Wuhan Univ Technol Mater Sci Ed 26:543–548. https://doi.org/10.1007/s11595-011-0268-5 Meng X, Wang Y, Ma C (2019) Hydrophobic modification of fly ash-based geopolymer using silane coupling agents. Mater Lett 234:208–211. https://doi.org/10.1016/j.matlet.2018.09.086 Shao J, Liu J, Ma H, Yu Z (2020) Surface treatment of alkali-activated materials with organosilanes to improve water resistance. Constr Build Mater 239:117872. https://doi.org/10.1016/j.conbuildmat.2019.117872 Sun T, Zhang Y, Yang L, Zhou J (2021) Long-term performance of silane-modified geopolymer concretes under seawater exposure. Cem Concr Compos 124:104222. https://doi.org/10.1016/j.cemconcomp.2021.104222 Oey T, Kumar A, Vandamme M (2016) Hydrophobic modification of porous cementitious materials: mechanisms and applications. Cem Concr Res 88:177–187. https://doi.org/10.1016/j.cemconres.2016.06.010 Chen C, Habert G, Bouzidi Y, Jullien A (2010) Environmental impact of cement production: detail of the different processes and cement plant variability evaluation. J Clean Prod 18:478–485. https://doi.org/10.1016/j.jclepro.2009.12.014 Turner LK, Collins FG (2013) Carbon dioxide equivalent (CO₂-e) emissions: a comparison between geopolymer and OPC cement concrete. Constr Build Mater 43:125–130. https://doi.org/10.1016/j.conbuildmat.2013.01.020 Additional Declarations No competing interests reported. Supplementary Files floatimage1.png Graphical Abstract Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9203964","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":615853679,"identity":"9d0aa5fd-5889-4577-b11b-ce11e38361ac","order_by":0,"name":"Shehla Naz","email":"","orcid":"","institution":"Abdul wali khan University","correspondingAuthor":false,"prefix":"","firstName":"Shehla","middleName":"","lastName":"Naz","suffix":""},{"id":615853680,"identity":"f734769a-8d8d-4310-abb2-0e1f06d06b86","order_by":1,"name":"Muhammad Jawad","email":"","orcid":"","institution":"RIPHAH International 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19:09:31","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9203964/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9203964/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":106116330,"identity":"8e918907-93fa-41df-a13d-c8efabc0ac34","added_by":"auto","created_at":"2026-04-03 16:29:57","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":45983,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eWaterproof performance of (a) Geopolymer reference, (b) triethoxyvinylsilane (TEVS) and (c) hexadecyltrimethoxysilane (HTS)\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-9203964/v1/802897e27270a1ad8de69a40.png"},{"id":106116336,"identity":"bd6b6c80-5fb8-44f7-bd81-b9c1403fafb6","added_by":"auto","created_at":"2026-04-03 16:29:58","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":34400,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFTIR spectrum of foamed geopolymer with Triethoxyvinylsilane(TEVS)\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-9203964/v1/41cf98dc891e2df8dc55c9f5.png"},{"id":106116332,"identity":"84966fbf-2df0-4312-b945-0e5486fb812a","added_by":"auto","created_at":"2026-04-03 16:29:57","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":42532,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFTIR spectrum of foamed geopolymer with Hexadecyl-trimethoxy-silane (HTS)\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-9203964/v1/29aa6d0f9b4ac329a2e83804.png"},{"id":106724016,"identity":"9529db04-334c-4063-b144-1d3d96db3f51","added_by":"auto","created_at":"2026-04-12 18:23:56","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":230797,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSEM image of foamed geopolymer with Triethoxyvinylsilane(TEVS)\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-9203964/v1/da396d4c2f40113edc0ca2b9.png"},{"id":106402453,"identity":"d08e76cb-03a8-4af1-a090-667432e5ca0f","added_by":"auto","created_at":"2026-04-08 09:12:03","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":183812,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSEM image offoamed geopolymer with Hexadecyl-trimethoxy-silane (HTS)\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-9203964/v1/d53542359fa40927d36dbbd1.png"},{"id":106116335,"identity":"31dc01e6-c299-48d2-8024-0b8ba687ff64","added_by":"auto","created_at":"2026-04-03 16:29:57","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":109068,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEffect of hydrophobicity on water absorption of Geopolymer reference, and geopolymer with Triethoxyvinylsilane(TEVS) and Hexadecyl-trimethoxy-silane (HTS)\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-9203964/v1/63d00ad5431af5092657cc28.png"},{"id":107331268,"identity":"81c6be5a-ec01-4b80-bedf-915c46ad7b91","added_by":"auto","created_at":"2026-04-20 12:42:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1149931,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9203964/v1/dd0006fa-4759-4290-b3a4-f57b3f406fae.pdf"},{"id":106402377,"identity":"cf9d2bbb-c54e-4cb6-84b4-1b45153efc61","added_by":"auto","created_at":"2026-04-08 09:11:54","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":336743,"visible":true,"origin":"","legend":"\u003cp\u003eGraphical Abstract\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-9203964/v1/1a39c7ccaaae3d7e6fdca531.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"Tailoring Water-Resistant Hybrid Geopolymers with triethoxyvinylsilane and Hexadecyl-trimethoxy-silane: A Comparative Study","fulltext":[{"header":"Introduction","content":"\u003cp\u003eGeopolymers have emerged as a new class of environmentally friendly construction materials with significant potential to replace ordinary Portland cement due to their low carbon footprint, thermal stability, and superior chemical resistance [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. The production of Portland cement alone is responsible for nearly 8% of global CO₂ emissions, largely due to the energy-intensive calcination of limestone and fuel consumption in clinker production, which makes the search for greener alternatives an urgent priority [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Geopolymers are synthesized through the alkali activation of aluminosilicate precursors such as fly ash, metakaolin, or slag, producing a three-dimensional amorphous framework of Si-O-Al bonds that provides ceramic-like properties [\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Among these raw materials, metakaolin is widely recognized for its high purity and reactivity, as it is obtained by the controlled calcination of kaolinite at 600\u0026ndash;850\u0026deg;C, generating a highly reactive amorphous aluminosilicate phase [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Metakaolin-based geopolymers exhibit homogeneous matrices, enhanced strength, and better pore distribution compared to those synthesized from fly ash or slag, and this makes them ideal for investigating advanced modifications such as hydrophobicity [\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Despite their mechanical and thermal advantages, however, geopolymers remain inherently hydrophilic owing to the presence of hydroxyl groups and open capillary pore structures, which allow water ingress and result in continuous absorption when exposed to humid or aqueous environments [\u003cspan additionalcitationids=\"CR15\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. This hydrophilic behavior severely limits the long-term durability of geopolymers and reduces their suitability for aggressive conditions such as marine structures, wastewater treatment systems, and chemical containment units [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Water penetration not only initiates ion diffusion, efflorescence, and leaching of alkali species but also facilitates sulfate and chloride attack, which further degrades the geopolymer network [\u003cspan additionalcitationids=\"CR20\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. In addition, cyclic swelling and shrinkage induced by moisture uptake accelerate crack propagation and lower overall stability [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSeveral strategies have been employed to address the challenge of water resistance in geopolymers. Optimizing the Si/Al ratio, adjusting the activator composition, and applying extended curing conditions can densify the microstructure and reduce porosity [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. The use of supplementary mineral additives such as silica fume, nano-silica, and slag has been reported to refine the pore network and improve impermeability [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Nanomaterials such as alumina, titania, carbon nanotubes, and graphene oxide have also been integrated to enhance densification and modify interfacial properties [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. While these approaches improve compactness, they do not fundamentally alter the hydrophilic chemical nature of geopolymers, which is governed by abundant silanol groups on the surface [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Consequently, geopolymers continue to absorb water and remain vulnerable in highly humid or aqueous environments. A promising alternative lies in hybrid modification using organic-inorganic coupling agents, which not only alter the surface chemistry but also introduce long-lasting hydrophobicity by presenting low surface energy functional groups [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSilanes and siloxanes have attracted significant attention in this context due to their ability to chemically bond with the geopolymer network while exposing organic hydrophobic moieties to the external surface [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Upon hydrolysis and condensation, silane coupling agents of the general formula R-Si(OR\u0026rsquo;)₃ form Si-O-Si linkages with the aluminosilicate skeleton, while the organic group (R) remains oriented outward, imparting water repellency [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. This dual functionality provides strong chemical anchoring and a hydrophobic barrier simultaneously. Previous work has shown that polydimethylsiloxane (PDMS) effectively enhances hydrophobicity in metakaolin-based geopolymers, producing superhydrophobic surfaces with contact angles exceeding 150\u0026deg; [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. Triethoxysilane (TES) has also been explored, providing moderate hydrophobicity with contact angles above 110\u0026deg; and improved durability under aqueous conditions [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. These studies clearly indicate that silane-based modifications can impart functional hydrophobicity to geopolymers and extend their service life.\u003c/p\u003e \u003cp\u003eDespite the success of PDMS and TES, other silane coupling agents remain underexplored. Triethoxyvinylsilane (TEVS) introduces vinyl groups that lower surface energy and offer additional opportunities for crosslinking reactions with surrounding silanol groups, potentially producing a denser and more durable hydrophobic network [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. In contrast, hexadecyltrimethoxysilane (HTS) incorporates a long C16 alkyl chain that provides a continuous hydrophobic barrier by forming a densely packed monolayer that inhibits capillary absorption [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. These two hydrophobic agents rely on different mechanisms: TEVS emphasizes interfacial reactivity and chemical bonding, while HTS relies on long-chain shielding and steric hindrance to repel water. Their comparative study can therefore provide valuable insights into tailoring hydrophobic geopolymer composites for diverse applications. Both TEVS and HTS have been widely reported as effective hydrophobic agents in cementitious systems, coatings, ceramics, and polymer composites, where they significantly reduce water uptake and improve long-term durability [\u003cspan additionalcitationids=\"CR42\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. However, their direct incorporation into the bulk of metakaolin-based geopolymers has not been systematically investigated. Unlike surface coatings, bulk modification ensures uniform hydrophobicity throughout the material, preventing localized defects and providing a consistent barrier against moisture ingress [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe lack of systematic comparative research on TEVS- and HTS-modified geopolymers represents a critical gap in the current literature. Previous studies have largely focused on PDMS or TES, with limited attention to other silane coupling agents that may offer equally or more effective hydrophobic performance. A clear link between surface wettability and the structural, microstructural, and chemical features of geopolymers has not been fully established in earlier studies. To bridge this gap, the present work focuses on metakaolin-based geopolymers modified with triethoxyvinylsilane (TEVS) and hexadecyltrimethoxysilane (HTS), examining how these agents alter both surface properties and internal structure. Contact angle tests were carried out to determine changes in hydrophobicity, while FTIR spectra provided confirmation of chemical interactions and the presence of silane-derived functional groups within the aluminosilicate network. Morphological alterations in pore distribution and connectivity were evaluated using SEM, and EDX analyses offered further insight by identifying and quantifying carbon contributions arising from the silane treatments. Long-term water absorption experiments complemented these analyses by tracking the durability of the modified composites in aqueous environments. Taken together, these methods allowed a comparative assessment of how vinyl- and alkyl-based silanes contribute differently to the development of hydrophobicity in geopolymers.\u003c/p\u003e \u003cp\u003eThe findings point to the effectiveness of TEVS and HTS in refining microstructure, enhancing surface chemistry, and limiting water ingress, thereby demonstrating their suitability for producing highly durable geopolymer composites. Such materials are of particular value for use in marine structures, wastewater treatment facilities, and other service environments where prolonged contact with water or aggressive chemical exposure is unavoidable. Beyond performance improvements, the results also support wider sustainability objectives by promoting geopolymers as a viable alternative to Portland cement, offering reduced environmental impact without compromising long-term durability [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e].\u003c/p\u003e"},{"header":"Methodology","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Experimental Procedure\u003c/h2\u003e \u003cp\u003eMetakaolin used in this study was procured from the local distributor of Merck Company. To ensure its amorphous structure, the material was thermally treated at 850\u0026deg;C. The chemical composition of the resulting metakaolin, determined via X-ray fluorescence (XRF), is presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, with major oxides comprising 55.65% SiO₂, 40.55% Al₂O₃, 1.4% Fe₂O₃, and 1.5% CaO.The alkaline activator solution was formulated by combining analytical reagent-grade sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃). The NaOH solution was prepared at a molarity of 10 M, while the sodium silicate solution contained 14.51 wt% Na₂O and 33.39 wt% SiO₂. The activator modulus (Ms\u0026thinsp;=\u0026thinsp;SiO₂/Na₂O) was maintained at 1.6. Two distinct series of geopolymer specimens were synthesized, each incorporating different dosages of hydrophobic additives: triethoxyvinylsilane (TEVS) and hexadecyltrimethoxysilane (HTS). The geopolymer paste was prepared by thoroughly mixing metakaolin with the alkaline activator solution, maintaining a fixed liquid-to-solid mass ratio of 0.4 to ensure consistent workability. The fresh pastes were cast into triplicate stainless steel cube molds (20 \u0026times; 20 \u0026times; 20 mm\u0026sup3;) and subjected to mechanical vibration to eliminate entrapped/ air. Molds were sealed with polyethylene film to minimize moisture loss during the initial setting phase.After 24 hours of ambient curing, the specimens were demolded and subsequently cured under controlled conditions at 20\u0026deg;C and 90% relative humidity for a duration of up to 28 days.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eChemical composition of metakaolin\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCaO\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e55.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e40.55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Characterization Techniques\u003c/h2\u003e \u003cp\u003eThe surface hydrophobicity of the synthesized specimens was evaluated using the sessile drop method with a contact angle goniometer (OCA20, Dataphysics Instruments, Filderstadt, Germany). A droplet of deionized water was dispensed vertically onto the sample surface using a microliter syringe, and the contact angle was recorded using a high-resolution camera and analyzed with proprietary software.Microstructural examination and surface morphology of both reference and hydrophobic geopolymer samples were conducted using a field emission scanning electron microscope (FESEM; Quanta SU8010, FEI, Netherlands/Japan), operated at an accelerating voltage of 15 kV. Elemental analysis of selected regions was performed using Energy Dispersive X-ray Spectroscopy (EDS; Apollo XP, USA) coupled with the microscope.\u003c/p\u003e \u003cp\u003eFourier-transform infrared spectroscopy (FT-IR) was conducted using a Nicolet iS50 spectrometer to assess the molecular structure and bonding environments of the geopolymer networks. The spectra were recorded in the range of 4000\u0026thinsp;\u0026minus;\u0026thinsp;400 cm⁻\u0026sup1; using KBr pellets compressed under a load of 10 tons.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Water Absorption Test\u003c/h2\u003e \u003cp\u003eWater absorption capacity was assessed to evaluate the impermeability of the synthesized geopolymers. Specimens were first oven-dried at 105\u0026deg;C until a constant mass was achieved, then cooled in an air-tight desiccator for 24 hours. The dried specimens were weighed (initial mass), dipped in distilled water, and subsequently removed at specified time intervals. After wiping the surface moisture, the specimens were reweighed (wet mass). Water absorption (%) was calculated as the percentage increase in mass relative to the dry mass. Measurements were recorded at intervals of 1day, 2 days3 days, 4days, 5days, 6days, 7 days, 14days and 28 days.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Contact Angle Analysis\u003c/h2\u003e \u003cp\u003eSurface wettability is a key indicator of hydrophobic modification and is most reliably evaluated by static water contact angle measurements. In general, surfaces with contact angles lower than 90\u0026deg; are classified as hydrophilic, demonstrating affinity toward water spreading, whereas those with angles greater than 90\u0026deg; exhibit hydrophobic behavior due to reduced interaction with polar liquids. Achieving contact angles above 120\u0026deg; is considered a strong indication of advanced hydrophobicity, while values approaching 150\u0026deg; are typically associated with superhydrophobicity [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe reference geopolymer in this study displayed a baseline contact angle of 30\u0026deg; (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), confirming its inherent hydrophilic nature, which is consistent with earlier reports attributing this behavior to the presence of silanol (Si-OH) groups and a porous microstructure that facilitates water penetration [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Upon the incorporation of hydrophobic silane-based modifiers, significant enhancements in surface water repellency were observed. Specifically, the introduction of triethoxyvinylsilane (TEVS) increased the contact angle progressively with dosage, reaching 135\u0026deg; at 10 wt%, while hexadecyltrimethoxysilane (HTS) modification produced a final contact angle of 128\u0026deg; under the same conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). These results clearly demonstrate the efficiency of both silanes in altering the surface polarity and suppressing moisture affinity.\u003c/p\u003e \u003cp\u003eThe gradual increase in contact angle with increasing TEVS and HTS dosage is summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. At low concentrations (2\u0026ndash;4 wt%), only moderate improvements were observed, with contact angles of 40\u0026ndash;70\u0026deg; for TEVS and 42\u0026ndash;73\u0026deg; for HTS. However, once the dosage exceeded 6 wt%, the contact angle surpassed the hydrophobic threshold of 90\u0026deg;, indicating a successful transition from hydrophilic to hydrophobic behavior. At 10 wt%, TEVS achieved a maximum contact angle of 135\u0026deg;, slightly higher than HTS at 128\u0026deg;, highlighting its superior hydrophobic efficiency.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eContact angle of geopolymer with varying amounts of TEVS and HTS\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colspan=\"7\" nameend=\"c8\" namest=\"c2\"\u003e \u003cp\u003eContact angle of with various percentages of hydrophobic agents\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTEVS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e92\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e130\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e135\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e130\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHTS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e125\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e128\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e127\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe mechanism behind this enhanced hydrophobicity lies in the chemical functionality of the silanes. TEVS contains a vinyl group (-CH\u0026thinsp;=\u0026thinsp;CH₂), which introduces nonpolar organic moieties at the surface after hydrolysis and condensation reactions with the aluminosilicate network. When silane agents are incorporated into the geopolymer system, they play a direct role in lowering surface energy, allowing the formation of a protective hydrophobic layer [\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. TEVS, in particular, integrates well with the aluminosilicate framework and produces a relatively uniform coating. By contrast, HTS contains a much longer alkyl chain (C₁₆H₃₃-), which certainly reduces surface polarity but at higher levels can cause partial clustering and less even distribution across the surface [\u003cspan additionalcitationids=\"CR10\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. This difference explains why TEVS generally records slightly higher contact angle values, as its smaller vinyl group is better able to interact with the geopolymeric gel and spread more evenly.\u003c/p\u003e \u003cp\u003eThese findings correspond well with earlier studies on silane-treated aluminosilicate matrices, where agents such as methyltrimethoxysilane (MTMS), octyltriethoxysilane (OTES), and TES improved water resistance by reducing hydroxyl groups and narrowing pore pathways [\u003cspan additionalcitationids=\"CR13 CR14\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The slight edge shown by TEVS over HTS can be traced back to their structural features: shorter functional groups penetrate more effectively and create a homogeneous network, while longer alkyl chains, although strongly hydrophobic, can lead to less consistent coverage.\u003c/p\u003e \u003cp\u003eThe contact angle tests confirm this behavior clearly. Unmodified geopolymers remained hydrophilic, whereas silane treatment converted them into water-repellent materials. TEVS-modified samples consistently surpassed 130\u0026deg;, approaching superhydrophobic behavior, while HTS-modified specimens also displayed high values around 128\u0026deg;. Taken together, the results indicate that both silanes are capable of imparting strong hydrophobicity, though TEVS demonstrates slightly better compatibility and stability. Such modifications open up the possibility of using these composites in construction applications where long-term resistance to moisture is essential.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Waterproofing and Water Absorption Performance\u003c/h2\u003e \u003cp\u003eThe waterproofing performance of geopolymer composites can be effectively assessed through water absorption and immersion experiments, as shown schematically in Figure X. The reference geopolymer specimen, devoid of any hydrophobic modifier, exhibited noticeable water uptake when exposed to aqueous environments. The high affinity of aluminosilicate-based matrices towards polar water molecules is well established, and the observed penetration of water into the reference sample confirms the inherently hydrophilic nature of the unmodified geopolymer network. The interconnected porosity and polar hydroxyl groups present in the aluminosilicate backbone provide favorable sites for hydrogen bonding, facilitating rapid ingress of water molecules.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn contrast, the incorporation of hydrophobic modifiers such as triethoxyvinylsilane (TEVS) and hexadecyltrimethoxysilane (HTS) significantly reduced water permeation. At 10 wt% TEVS, the specimen displayed a marked improvement in waterproofing efficiency, with water droplets largely repelled from the surface and exhibiting elevated positioning compared to the reference sample. This behavior suggests that TEVS successfully established a dense hydrophobic barrier on the surface and within the pore walls of the geopolymer. The ethoxy-silane groups in TEVS hydrolyze and condense during the geopolymerization process, chemically bonding with surface silanol groups while simultaneously introducing non-polar vinyl functionalities. These vinyl groups act as low-energy sites, reducing the surface polarity and blocking the diffusion of water through the pore network. Consequently, the TEVS-modified specimen demonstrated minimal weight gain during the immersion cycles, indicating strong resistance to water ingress.\u003c/p\u003e \u003cp\u003eThe performance of HTS-modified geopolymer was also significantly enhanced compared to the unmodified reference. At 10 wt% HTS incorporation, the material exhibited improved water repellency, as illustrated in Figure X. The long alkyl chains of HTS molecules anchor onto the geopolymer surface through siloxane linkages, creating a hydrophobic monolayer that prevents water molecules from penetrating the structure. The alkyl moieties introduce steric hindrance and lower the surface energy, thereby imparting sustained hydrophobicity. Although the contact angle results (128\u0026deg; for HTS vs. 135\u0026deg; for TEVS) suggest slightly reduced effectiveness compared to TEVS, the long hydrocarbon tails of HTS impart excellent stability against prolonged immersion, minimizing capillary absorption over extended exposure times.\u003c/p\u003e \u003cp\u003eThe comparative analysis between TEVS and HTS highlights distinct mechanisms of waterproofing. TEVS achieves its effect primarily through the formation of a covalently bonded, dense hydrophobic siloxane-vinyl layer that seals the pores, whereas HTS introduces a long-chain hydrocarbon coating that resists water permeation by lowering the free surface energy. Both approaches successfully suppress the hydrophilic tendencies of aluminosilicate binders, transforming the material into a water-resistant composite. The improved waterproofing performance of these modified systems not only enhances durability under wet conditions but also widens their potential applications in construction environments subjected to rain, humidity, and groundwater exposure.\u003c/p\u003e \u003cp\u003eThe results from contact angle and immersion tests show that the hydrophobic treatment is both effective and stable. The untreated geopolymer steadily absorbed water during immersion, a typical sign of capillary uptake in porous materials. In contrast, samples modified with TEVS and HTS took in very little water, and the values leveled off quickly. This indicates that the silane agents not only change the surface but also reduce pathways for water entry near the surface, giving the material lasting resistance. Similar improvements have been reported in earlier work on silane-treated geopolymers, where lowered porosity and the presence of hydrophobic groups helped protect against water attack [\u003cspan additionalcitationids=\"CR23 CR24\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOverall, both TEVS and HTS made the geopolymers far more resistant to water than the reference sample. TEVS gave higher contact angles and stronger droplet repellency, while HTS showed excellent stability during immersion. Each works in a slightly different way, but together the results point to their value in producing durable hydrophobic geopolymers for structures exposed to moisture and aggressive environments.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.3 FTIR Analysis\u003c/h2\u003e \u003cp\u003eFTIR spectra of the TEVS- and HTS-modified geopolymers give clear evidence of changes in the chemical structure after modification as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e respectively. Both samples show a broad band in the 1000\u0026ndash;1100 cm⁻\u0026sup1; region, which corresponds to the Si-O-T stretching vibrations (T\u0026thinsp;=\u0026thinsp;Si or Al) that form the main framework of the geopolymer. In the TEVS-modified material, a strong peak at about 1033 cm⁻\u0026sup1; confirms the presence of a well-developed silicate network. Two additional shoulders, appearing near 877 and 777 cm⁻\u0026sup1;, are linked to Si-O-X and Si-C bonds, indicating that vinylsilane groups were successfully attached to the matrix. These features are absent in the plain geopolymer, showing that TEVS chemically bonded with the aluminosilicate through hydrolysis and condensation of its ethoxy groups, leaving the vinyl group exposed at the surface. This bonding not only reinforces the silicate network but also contributes to the hydrophobic nature of the modified composite.\u003c/p\u003e \u003cp\u003eIn addition to these silicate-related features, the TEVS-modified sample displays a prominent band at approximately 1651 cm⁻\u0026sup1;, which corresponds to H-O-H bending vibrations of molecularly adsorbed water. This indicates that despite the partial hydrophobization imparted by TEVS, a certain degree of water interaction remains within the system. However, the presence of a distinct peak at 2984 cm⁻\u0026sup1; is of particular importance as it represents the stretching vibrations of C-H groups associated with the vinyl moieties of TEVS. The observation of this band provides strong evidence that the organic fragments are chemically bonded to the geopolymer structure rather than being merely physically adsorbed. The combined effect of reduced hydrophilic O-H intensity and the introduction of C-H functionalities explains the rise in contact angle from 30\u0026deg; for the unmodified geopolymer to values exceeding 130\u0026deg; for TEVS-modified systems, thus confirming the efficiency of TEVS in lowering the surface energy of the composite (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn the case of the HTS-modified geopolymer (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), several notable changes can be observed in comparison with the TEVS system. The main asymmetric stretching band of Si-O-T shifts toward a lower wavenumber, appearing around 968 cm⁻\u0026sup1;, which reflects increased polymerization of the silicate network and extensive condensation reactions induced by the grafting of long-chain silanes. The presence of distinct shoulders at 866 and 792 cm⁻\u0026sup1; further supports the formation of new Si-O-Si linkages, while simultaneously suggesting successful bonding of HTS molecules into the geopolymer structure. The shift of the main silicate peak toward lower frequency is often correlated with a higher degree of cross-linking and a transformation toward a more ordered and denser aluminosilicate network, which is consistent with the improved mechanical stability and durability of HTS-modified composites reported in the literature.\u003c/p\u003e \u003cp\u003eA particularly important observation in the HTS spectrum is the significant reduction in the intensity of the band at 1661 cm⁻\u0026sup1;, which corresponds to H-O-H bending of absorbed water molecules. This reduced intensity indicates that the hydrophobic hexadecyl groups of HTS effectively shield the surface from water adsorption, thereby decreasing the number of water molecules hydrogen-bonded to the surface hydroxyl groups. Furthermore, the most striking feature of the HTS-modified sample is the strong absorption band appearing at 2921 cm⁻\u0026sup1;, which is characteristic of the asymmetric stretching vibrations of CH₂ groups from the long hexadecyl chains. The intensity of this band confirms the successful grafting of HTS and the presence of a dense layer of organic chains on the surface of the geopolymer. This alkyl coverage is far more extensive than the vinyl coverage provided by TEVS, explaining why the HTS-modified geopolymers exhibit slightly higher and more stable contact angles, reaching up to 128\u0026deg; at 10 wt% loading.\u003c/p\u003e \u003cp\u003eThe low-frequency region of the spectra, particularly the bands located around 444 cm⁻\u0026sup1; for TEVS and between 560\u0026thinsp;\u0026minus;\u0026thinsp;431 cm⁻\u0026sup1; for HTS, corresponds to Al-O bending vibrations. These bands confirm the presence of the aluminosilicate framework and show that the modification with silanes does not disrupt the fundamental structural integrity of the geopolymer. Instead, the agents react primarily at the surface level by forming covalent bonds with silanol groups, while the bulk aluminosilicate network remains intact. This selective surface modification mechanism ensures that the intrinsic chemical durability and structural performance of the geopolymer are preserved, while simultaneously introducing enhanced hydrophobicity.\u003c/p\u003e \u003cp\u003eA comparative analysis of the two systems highlights the differences in their modification mechanisms. TEVS, being a short-chain vinylsilane, introduces hydrophobic vinyl groups that reduce surface polarity but leave the system partially open to water interaction, as indicated by the still observable O-H and H-O-H vibrations. In contrast, HTS, with its long hexadecyl chain, provides a more continuous and compact hydrophobic barrier that significantly reduces water interaction and enhances waterproofing efficiency. This is consistent with the experimental waterproofing test, where HTS-modified samples demonstrated greater resistance to water penetration compared with TEVS. The correlation between FTIR spectral features and macroscopic hydrophobic performance confirms that the chemical grafting of silane agents directly translates into functional improvements in the material\u0026rsquo;s behavior. These findings are supported by earlier reports, where long-chain silane agents were shown to impart superior hydrophobicity and chemical resistance compared with short-chain analogs.\u003c/p\u003e \u003cp\u003eOverall, the FTIR results clearly demonstrate that both TEVS and HTS undergo hydrolysis of their ethoxy groups, followed by condensation with surface hydroxyl sites of the geopolymer, forming stable Si-O-Si linkages. The remaining organic moieties, vinyl in the case of TEVS and long alkyl chains in the case of HTS, are oriented outward, thereby lowering the surface energy and imparting hydrophobicity. The more extensive modification achieved by HTS is evidenced by the stronger alkyl-related absorption bands, reduced water-associated signals, and shift of the silicate band toward lower frequencies. These molecular-level changes directly explain the improved macroscopic contact angle and waterproofing behavior, highlighting the significant potential of HTS as a highly effective hydrophobic agent for geopolymer modification.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.4 SEM analysis\u003c/h2\u003e \u003cp\u003eThe SEM images of foamed geopolymer specimens modified by the incorporation of hydrophobic agents such as triethoxyvinylsilane (TEVS) and hexadecyltrimethoxysilane (HTS) are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e respectively. The SEM images show that the particles are more closely integrated, with the open gaps largely reduced and a smoother, more continuous surface emerging. This improvement arises from the chemical action of TEVS: the ethoxy groups undergo hydrolysis and condensation to form siloxane bridges, strengthening the aluminosilicate network, while the vinyl groups contribute additional bonding interactions. The resulting structure is noticeably denser, with fewer pores, which aligns with the increase in contact angle values and reduced water absorption. These features confirm that TEVS not only modifies the surface chemistry but also improves the compactness of the bulk microstructure.\u003c/p\u003e \u003cp\u003eIn the case of HTS, the effect is even more pronounced. The SEM image of the HTS-modified sample shows a tightly packed matrix with very few cracks or voids, indicating a high level of structural refinement. This improvement can be linked to the dual function of HTS. Its methoxy groups bond chemically with the geopolymer framework, while the long hydrophobic alkyl chains tend to line the pore walls, creating a physical barrier to water penetration. The combination of chemical crosslinking and hydrophobic chain alignment makes the structure more compact and resistant to water ingress, giving the composite enhanced long-term durability. The overall morphology suggests improved structural integrity, where the particles are embedded within a more consolidated gel phase. This compact microstructural arrangement explains the higher contact angle values recorded for HTS, showing superior water repellency compared to TEVS. The smoother and denser surface texture of HTS-modified geopolymers indicates that this hydrophobic agent is more effective in blocking capillary pores and enhancing the long-term durability of the material (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe comparative analysis between the three microstructures highlights the effectiveness of hydrophobic modification. While the simple geopolymer shows a porous and fragile framework, TEVS modification improves the packing density and pore filling, thereby reducing voids. HTS, however, produces the most refined and consolidated microstructure, achieving significant densification and imparting a strong hydrophobic surface character. These microstructural transformations are in strong agreement with FTIR findings, where enhanced Si-O-Si and Si-O-X linkages confirm the formation of additional crosslinking networks, and with the contact angle results, where HTS exhibited the highest improvement in hydrophobicity. The SEM observations thus validate the chemical and functional improvements introduced by TEVS and HTS and underline the potential of these organosilanes in tailoring the pore structure and enhancing the durability of geopolymer composites.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Water absorption\u003c/h2\u003e \u003cp\u003eThe water absorption behavior of the reference geopolymer and the hydrophobic agent-modified samples (TEVS and HTS) is given in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, provides critical insight into the impact of surface modification on the long-term durability of these composites. As shown in the graph, the reference geopolymer exhibits a progressive increase in water uptake, reaching nearly 1.5% after 15 days, beyond which the absorption stabilizes. This relatively high absorption is consistent with the porous microstructure observed in SEM images, where interconnected voids facilitate continuous capillary suction of water until equilibrium is reached. Such behavior is typical of unmodified geopolymers, where the lack of hydrophobic barriers allows easy penetration of water molecules into the matrix, thereby compromising dimensional stability and resistance to aggressive environments.\u003c/p\u003e \u003cp\u003eBy contrast, the TEVS and HTS modified samples demonstrate a significant reduction in water absorption throughout the immersion period. In both cases, the initial uptake within the first few days is minimal, and the curves gradually plateau at much lower values compared to the reference. The final water absorption for TEVS and HTS samples stabilizes around 0.33% and 0.40%, respectively, which is nearly four to five times lower than that of the reference geopolymer. This marked reduction highlights the effectiveness of organosilane modification in introducing hydrophobicity into the geopolymer network. TEVS contributes through the hydrolysis and condensation of ethoxy groups, forming siloxane linkages that fill micro-pores and reduce capillary action, while the vinyl group enhances compatibility within the aluminosilicate network. On the other hand, HTS introduces long alkyl chains that create a hydrophobic barrier on the pore surfaces, effectively repelling water ingress. This dual chemical and physical protection mechanism is reflected in the extremely low absorption values.\u003c/p\u003e \u003cp\u003eInterestingly, although both modifiers significantly improve performance, TEVS-modified samples show slightly lower final absorption than HTS. This trend can be attributed to the difference in modification mechanisms: TEVS participates more actively in crosslinking with the geopolymer matrix, thereby sealing finer pores more effectively, whereas HTS, while providing superior surface hydrophobicity, may leave behind microvoids due to steric hindrance of the bulky alkyl chains. Nevertheless, both TEVS and HTS provide outstanding durability enhancements when compared to the unmodified geopolymer.\u003c/p\u003e \u003cp\u003eThe observed trends in water absorption strongly correlate with SEM microstructural analysis, where both TEVS and HTS imparted denser, more compact morphologies with reduced interparticle voids. The lower absorption also aligns with the contact angle results, confirming that hydrophobicity achieved at the surface extends to the bulk of the material, improving barrier performance. Reduced water ingress is particularly important for the long-term stability of geopolymers, as it minimizes risks of leaching, efflorescence, and chemical attack from chloride and sulphate ions in aggressive service environments. Therefore, the water absorption study not only validates the efficiency of TEVS and HTS as hydrophobic modifiers but also underscores their potential for improving the durability and service life of geopolymer composites in infrastructure applications.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe present study demonstrates that the introduction of hydrophobic silane-based agents, triethoxyvinylsilane (TEVS) and hexadecyltrimethoxysilane (HTS), significantly enhanced the water-resistant behavior of metakaolin-derived geopolymer composites. A clear improvement in surface hydrophobicity was confirmed by contact angle analysis, where untreated samples showed low wettability while the modified systems exceeded 130\u0026deg;, indicating the successful incorporation of hydrophobic functionalities into the geopolymer framework. Evidence from FTIR supported these results, with additional Si-O-Si and Si-C bonds pointing towards chemical interaction between the silane molecules and the aluminosilicate matrix. Microstructural inspection using SEM further revealed that the treated composites developed a denser and less porous morphology compared to the reference material, a factor that directly contributed to the reduced water ingress. This was reflected in the absorption tests, where the unmodified specimens absorbed about 1.5% water, in contrast to less than 0.4% in the TEVS- and HTS-modified samples even under extended exposure. Between the two agents, TEVS offered slightly more stable long-term performance, whereas HTS also delivered high hydrophobicity but showed marginally higher uptake at later stages. Taken together, these results establish silane modification as a practical and reliable approach to enhance the durability of geopolymers. Such composites hold strong potential for construction applications, particularly in environments where prolonged contact with moisture is a concern, as they combine the advantages of geopolymers with improved service life and reduced maintenance demands.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003e4. Funding information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo funding from any organization was received for conducting this study.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eDavidovits J (2013) Geopolymer chemistry and applications, 4th edn. 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Constr Build Mater 43:125\u0026ndash;130. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.conbuildmat.2013.01.020\u003c/span\u003e\u003cspan address=\"10.1016/j.conbuildmat.2013.01.020\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Geopolymer, Hydrophobic modification, Triethoxyvinylsilane, Hexadecyltrimethoxysilane, Contact angle, Water absorption, Sustainable construction","lastPublishedDoi":"10.21203/rs.3.rs-9203964/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9203964/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe development of water-resistant geopolymer systems is crucial for extending the durability of alkali-activated binders in humid and aggressive environments. In this study, metakaolin-based geopolymers were modified with triethoxyvinylsilane (TEVS) and hexadecyltrimethoxysilane (HTS) to impart hydrophobic functionality at the matrix level. The reference geopolymer exhibited a low contact angle of 30\u0026deg;, confirming its hydrophilic surface. Incorporation of TEVS and HTS significantly improved wettability resistance, producing contact angles of 135\u0026deg; and 128%, respectively, attributable to vinyl-silane grafting and long-chain alkyl silane functionality. FTIR spectra confirmed reduced O-H stretching intensity (3430 cm⁻\u0026sup1;) alongside the emergence of Si-C and C-H vibrational bands, validating the successful incorporation of hydrophobic groups. SEM micrographs revealed improved matrix densification and reduced pore connectivity, particularly in TEVS-modified samples, while EDX spectra indicated carbon enrichment from 2.7% in the control to 28% and 25% in TEVS and HTS composites, respectively. Water absorption testing further highlighted the durability enhancement, with TEVS- and HTS-modified specimens restricting uptake to 0.34% and 0.40% after 28 days, compared to 1.5% in the control. The comparative analysis demonstrates that TEVS yields slightly superior hydrophobic performance due to stronger interfacial crosslinking, whereas HTS provides long-chain barrier effects. These findings establish silane-modified geopolymers as multifunctional composites with enhanced moisture durability, suitable for applications in marine infrastructure, wastewater systems, and chemically aggressive service environments.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e","manuscriptTitle":"Tailoring Water-Resistant Hybrid Geopolymers with triethoxyvinylsilane and Hexadecyl-trimethoxy-silane: A Comparative Study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-03 16:29:53","doi":"10.21203/rs.3.rs-9203964/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"4000c3f2-cb35-4826-b1f8-7060386488d9","owner":[],"postedDate":"April 3rd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-04-20T12:40:23+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-03 16:29:53","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9203964","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9203964","identity":"rs-9203964","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}