Tailoring geopolymer chemistry for value-added applications

• Main Author: Ye, Kai
• Other Authors: Aravind Dasari
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering; Materials
• Online Access: https://hdl.handle.net/10356/180586
• Institution: Nanyang Technological University

From the viewpoint of carbon footprint, geopolymer technology has shown excellent promise as an alternative to traditional ordinary Portland cement (OPC). Additionally, with its ease of manufacturability, inherent fire resistance, chemical stability, durability and compressive strength, the interest in these materials has grown. Despite this, their usage has been narrow. The focus of this thesis is to go beyond the traditional area of using the geopolymer technology to replace OPC, and exploit their potential in value-added applications. The geopolymer concept is demonstrated in applications ranging from fire protective composite panels, transparent interlayer system for fire-resistant glass to durability improvement of deteriorated concrete. This is achieved by systematically understanding the chemistry, tuning the compositions of these silicate systems and exploring how they behave in targeted scenarios. In composite panels, metakaolin geopolymers were reinforced with 0°/90° oriented glass fiber fabrics. Despite the thermal and structural stability of these composites even up to 700 °C and integrity even after fire exposure, underperformance in insulation was highlighted when exposing the composite panels to a heating profile matching ISO 834 fire curve. The panel thickness varying from 12 to 17 mm showed limited effect on the fire protection timing. This was attributed to the formation of micro- to meso-scale cracks in the geopolymer matrix during dehydration because of stress relief and mismatched thermal behavior between matrix and fibers. The finding emphasized the shortcoming of relying on endothermic effect from these composites. In other words, significant increase of panel thickness is necessary in achieving desirable fire insulation, which is consistent with the results reported in literature. Based on such understanding, a sandwich concept was explored, using these composites as thin (~2 mm thick) face sheets and geopolymer-infused polyurethane (PU) foam/traditional aluminum honeycomb as core. These sandwich systems showed significant improvement in fire insulation. The benefit was attributed to the transition of PU’s flammable nature to mild pyrolysis after infusion with geopolymers and the excellent integrity of inorganic cell walls, which accounted for a delayed heat transfer process. These sandwich composites also demonstrated advantages in low density, damage resistance and soundproofing. The best performed sandwich system (~21 mm thick) using infused PU foam showed comparable impact resistance and fire insulation to those of a conventional 31 mm-thick steel-rock wool panel, but better compression strength and sound insulation. In the application of fire-resistant glass, soluble alkali silicates have been used as interlayer materials. This is owing to their capability of swelling into a foamed siliceous structure when exposed to fire as a consequence of rapid dehydration. However, haze and cloudiness in these silicate interlayers has been a recognized durability issue. In this thesis, decreasing the molar ratio SiO2:K2O from 30 to 6 was found to enhance the stability of transparency at ambient conditions and trap more bonded water. The route of molar ratio to affect the transition was further confirmed as light scattering caused by the inter-particle distance. Decreasing the molar ratio induced the formation of more hydrous matrix after curing, which provided sufficient inter-particle. However, conditioning these low molar ratio systems at 50 °C influenced the clarity by inter-particle networking and aggregate formation. Yet, adding water-soluble additives with polyhydroxy groups showed improvement even after conditioning, as their presence partially replaced water and prevented silicate networking. A lower molar ratio was also responsible for the dehydration and intumescent behavior, providing fire resistance potential. Using these systems as interlayers between panels of soda lime glass, the laminated glass showed similar fire performance to commercial fire-resistant glass, good stability and no susceptibility to carbonation. Geopolymer-based materials have also been demonstrated to improve durability of mildly deteriorated concrete in repair application. By varying viscosity through dilution of geopolymer mixture, infusion into concrete surface cracks facilitated by capillary action was proved feasible. These diluted mixtures after curing could maintain the three-dimensional network and offer a stable improvement in permeability resistance during the 10-month monitoring period at a test bedding site. Characterization of the geopolymer-concrete interface suggested formation of incorporating silicate species into a new ordered and polymerized C-S-H phase and into a geopolymer network. Cation exchanges between the concrete phase and alkali ions in the geopolymer mixture also occurred. The thesis has provided a comprehensive analysis of exploiting and tailoring geopolymer technology for three value-added and distinctive applications. In composite panels, thermal insulation was deteriorated by the formation of cracks. A sandwich composite panel approach was adopted that exhibited stability in addition to positive thermal insulation performance. The work on interlayers in fire-resistant glass further revealed the effect of particle size and their dispersion state on optical clarity. Additives like xylitol and glycerol were further incorporated into the silicate interlayers to prevent networking and aggregate formation to achieve stable transparency. The geopolymer chemistry has also been tailored to improve the permeability resistance of mildly deteriorated concrete at an early stage by means of infusion. The complete knowledge of the exploration presented in this thesis can serve as an instruction guide for employing the concept for a far wider market.