Recycling Waste Glass to Develop Low-CO2 Foamed Composites: Advancing Sustainability

Abstract

Over 750,000 vehicles in Australia reach the end of their lifespan yearly, leading to the disposal of 22,500 tonnes of waste glass (WG) from their windshields and windows. These materials are usually sent to landfills due to their complex structure and costly recycling process. However, this thesis proposes an alternative by utilising WG as the primary raw material for producing sustainable Low-CO2 foamed composites (FC). The FCs, particularly glass foam and alkali-activated foamed composites (AFCs) are increasingly used in insulation, water treatment, and energy sectors, replacing conventional materials that are flammable, energy-intensive, and expensive. Since glass foam manufacturing is energy-intensive and non-eco-friendly due to its involvement with different chemicals and high-temperature melting-annealing (1400℃), researchers tried to develop alternative methods such as powder sintering and gel casting. These methods enable the sintering of glass foam mixtures at lower temperatures (700-1000℃), aiming to reduce energy consumption, reduce emissions from materials and create a sustainable manufacturing process. Uniformly distributed, finely sized, and homogeneous pores play a crucial role in the properties and application of FC. However, the powder sintering method, which relies on stabilising chemicals to enhance pore characteristics, emits CO2; and faces limitations in industrial applicability due to the need for pelletisation of dry glass powder for achieving uniform particle contact. On the other hand, gel-casting is recognised as an eco-friendly method but requires a lengthy gelation process at elevated temperatures, which is energy-inefficient. Additionally, the controlling parameters that influence foaming, reactivity, and fresh properties of the mix, and their correlation with pore formation and distribution in the final FCs, are not well understood. Hence, this thesis aims to develop sustainable and eco-friendly methods to enhance the pore characteristics of glass foam and AFCs, addressing these challenges and knowledge gaps. The main objectives include comprehensive investigations into the parameters controlling activation, foaming, and fresh and final properties of glass foam and AFCs, fostering a thorough understanding and bridging existing knowledge gaps. In this thesis, a curing-sintering method is proposed to eliminate the use of a chemical stabilising agent and the associated emissions from the materials during sintering. The method involves a process wherein glass powder, fly ash additive, and calcium carbonate foaming agent were mixed with water and cured in sealed plastic wraps. After the curing process, the samples were sintered at 800℃. During curing, physical interlocking, filler effects of the particles and the alkalinity of the glass and calcium carbonate aid in forming weak bonds along the particle surfaces. These weak bonds ensure uniform contact and stability among the particles, eliminating the need for pelletisation. Moreover, this stabilisation process helps maintain pore stability during sintering and achieve homogenous pore size distribution. Additionally, it contributed to reducing the leaching of metals from the glass foam. It is noteworthy that during sintering, regardless of the energy source perspective, the decomposition of calcium carbonate resulted in CO2 emissions detected in the gas analysis test. This thesis presents a novel combined mechanical and chemical foaming technique to completely eliminate emissions from materials during foaming, reduce energy consumption during gelation, and enhance pore characteristics in glass foams. The process involves rapid alkali-activation of precursors, followed by controlled foaming and subsequent hardening. The resulting glass foams were then sintered at temperatures ranging from 700°C to 800°C. The low-speed mechanical foaming applies minimal shearing stress to the activated paste, while surfactants reduce surface tension, preventing pore coalescence. Additionally, chemical foaming using low-concentration hydrogen peroxide minimises anisotropic pore formation. As a result, the desired pore distribution was achieved without the need for lengthy gelation. The correlation between controlling parameters, reactivity in the mix, foaming, and their impact on the final properties of FC was investigated through chemical, rheological, microstructural, and mechanical characterisations at different stages of the process. The activated mix underwent percolation and partial dissolution of precursor particles. During the hardening process, inter-particle gel interactions, cross-linking of the gels, and rigidification of the network occur sequentially and concurrently with foaming caused by hydrogen peroxide decomposition. The formation and cross-linking of the gels contribute to the structural build-up of the system, ensuring pore stability in fresh FC. By promoting the early-age reactivity of the precursor mix, the pore structure in these foams can be controlled. Key parameters for controlling reactivity include water-to-binder ratio, rapid-setting binder (slag), activator, and curing conditions. Through optimised mix design and controlled parameters in the combined foaming method, sustainable AFCs can be developed at ambient conditions using a high volume of WG as the primary raw material, without requiring sintering. It is anticipated that the findings of this research will contribute to a comprehensive understanding of the control of process parameters and pore structure in glass foam and alkali-activated foamed materials using sustainable and environmentally friendly approaches. Ultimately, this study presents a commercially viable and more eco-friendly method for recycling waste glasses from vehicle windows and windshields, transforming them into low-CO2 foamed composites for use in various industries