Geopolymers Beyond Construction: Enhanced Adsorption and Flame Retardancy in Nanocomposite Systems.

INTRODUCTION Geopolymers, inorganic materials formed by the polymerization of silicates and aluminates, resulting in a three-dimensional structure composed of bonds between silicon (Si), aluminum (Al), and oxygen (O), have garnered significant attention due to their excellent mechanical strength, with the potential to replace Portland cement, whose production accounts for 5-7% of global CO2 emission1 2. However, geopolymers can be more than just construction materials and can be exploited to create composite systems with diverse properties. These include providing flame-retardant properties to highly flammable polymers like cellulose3 and enhancing pollutant absorption due to their increased porosity compared to the starting material. In this study, two nanocomposite systems exhibiting these characteristics are presented and compared with a pure inorganic clay-based film that was geopolymerized to assess its morphological and CO2 adsorption properties. EXPERIMENTAL Materials Gel Beads. The alginic acid sodium salt (Mw=70–100 kDa), Halloysite nanotubes and Sodium Hydroxide (CAS 1310-73-2) were Sigma Aldrich products. Calcium Chloride (CaCl2·2H2O) was a Merck product. HPC/HNTs composite geopolymers. Ultra HalloPure Halloysite was a gift by I-Minerals Inc. mined in the geological deposit of Latah County. Hydroxypropyl cellulose (HPC) (Mw = 80kg·mol-1 CAS 9004-64-2) and Sodium Hydroxide (CAS 1310-73-2) were Sigma Aldrich products. Halloysite Inorganic Films. Patch Halloysite (Kalgoorie, Western Australia) is a kind donation provided by Dr. Keith Norrish from his collection and research on Patch (CSIRO Soils, Adelaide). Sodium Hydroxide (CAS 1310-73-2) was Sigma Aldrich product. Preparation Gel Beads. Gel beads were prepared by dissolving 2% sodium alginate in distilled water at 60°C. HNTs were added to achieve a 1:1 Alg ratio. The beads were formed by dropping the solution into a 0.1 M CaCl2 solution, then washing and vacuum drying. Geopolymerized gel beads were treated in 12 M NaOH at various times and then cured at 50°C. HPC/HNTs composite geopolymers. The composite film of HPC and HNTs was prepared by dissolving 2% by weight of HPC in water at 70°C. Halloysite was added to the polymer solution to achieve 30% HPC. The dispersion was stirred overnight, poured into a glass Petri dish, and kept at 80°C until the solvent evaporated. Geopolymerized samples were prepared by immersing the composite film in a 12 M NaOH solution for varying times (5 seconds or 2 hours). Halloysite Inorganic Film. Patch Halloysite was dispersed in water with different concentrations (0.5%, 1.5%, 3.33%, 5%). The dispersions were stirred for 20 minutes, poured into a plastic dish (LDPE), and placed in an oven at 50°C overnight. The film was recovered by overturning the dish. PT_Hal was treated by immersing it in a 12 M NaOH solution, followed by another night in the oven at 50°C. Structural and morphological characterizations For structural characterization, XRD was used. TGA was employed to investigate the geopolymerization process, and SEM was used for morphological characterization. Other measurements Contact angle measurements, transmittance measurements, CO2 and dodecane adsorption measurements, flame resistance tests, and DMA (dynamic mechanical analysis) were conducted. RESULTS AND DISCUSSION XRD, TGA, and SEM measurements were conducted to characterize the structure of all samples and confirm the occurrence of geopolymerization. Adsorption tests were performed on Gel Beads and inorganic films before and after geopolymerization. The latter, being non-composite, provides insights into the direct effect of geopolymerization. In all cases, the samples absorbed more CO2 at equilibrium compared to non-geopolymerized samples (Fig.1a), attributed to the increased porosity, qualitatively evaluated through SEM measurements. This trend is also confirmed by dodecane adsorption tests on the beads (Fig 1b), showing that geopolymerized samples absorb more hydrocarbons than non-geopolymerized ones. The flame-retardant effect was evaluated on geopolymerized HPC/HNTs composite samples (Fig. 1c), which demonstrated excellent properties in this regard compared to both the HPC film and the non-geopolymerized composite, extinguishing the flame approximately 0.5 seconds after the ignition source was removed.