1. Introduction
3,4,5,6,7, The construction industry stands as a major contributor to global waste generation, with the accumulation of construction and demolition debris (C&D) posing a significant environmental challenge [ 1 ]. This ever-increasing waste volume necessitates a fundamental shift towards sustainable practices, driving research and development into innovative, eco-friendly construction materials [ 2 ]. Among these promising alternatives, geopolymers have emerged as a compelling substitute for traditional cement-based materials, offering a pathway to significantly reduce the industry’s carbon footprint and mitigate the environmental impact associated with C&D waste [ 2 ]. Geopolymers, formed through the alkali activation of aluminosilicate precursors, present a versatile platform for utilizing a wide range of source materials. These precursors can be derived from naturally occurring minerals, industrial by-products such as fly ash and slag, and, importantly, components of C&D waste like crushed concrete, brick, ceramic tile, and glass [ 2 8 ]. The geopolymerization process involves the reaction of these aluminosilicate materials with an alkaline activating solution, forming a dense and durable material that is often highly resistant [ 9 ].
11,12,13,14, Numerous studies have explored the feasibility of incorporating various waste materials into geopolymer production, investigating their influence on the resulting material’s mechanical properties (compressive and flexural strength), durability (resistance to chemical attack, freeze-thaw cycles), and overall performance [ 10 15 ]. Researchers have experimented with different combinations of waste streams, optimizing mix designs, varying activator concentrations, and adjusting curing conditions (temperature, humidity) to achieve desired material characteristics tailored to specific applications. For instance, studies have investigated the use of fly ash-based geopolymers for structural elements, slag-based geopolymers for pavements, and waste glass-based geopolymers for decorative applications. Furthermore, the development of activators derived from waste materials, such as employing waste glass to produce sodium silicate solutions, has gained increasing attention, further enhancing the sustainability and circularity of geopolymer production [ 16 ]. Life Cycle Assessments (LCA) are increasingly crucial in evaluating the holistic environmental impact of geopolymer materials, being compared to traditional cement to provide a comprehensive framework for guiding the development of truly sustainable construction solutions [ 17 ]. These assessments consider the entire material life cycle, from raw material extraction and processing to manufacturing, use, and end-of-life disposal or recycling.
While existing research has extensively investigated the use of waste materials in geopolymer synthesis, a critical gap remains in the comprehensive utilization of exclusively waste-derived components, spanning from the aluminosilicate source to the activating solution. The reliance on commercially produced chemicals for activation limits the full potential for waste valorization and the realization of a truly circular economy. Achieving a 100% waste-derived geopolymer would represent a significant step towards greater sustainability. Despite incorporating waste materials as the primary aluminosilicate source, many studies still rely on commercially produced chemicals in the activation process. For instance, while waste glass might be used to produce the silicate solution, the precursor material to be activated is often metakaolin [ 6 ], a commercially processed clay mineral. In other cases, waste may be used as a precursor, but they are activated with commercial sodium silicate or require high curing temperatures (T > 150 °C). This reliance on virgin materials, even in limited quantities, reduces the full potential for waste valorization and the realization of a truly circular economy within the construction sector.
This study aims to address this critical gap by focusing on developing geopolymers derived entirely from waste materials, effectively closing the loop on material flows. We explore the production of geopolymers using a carefully selected combination of waste cement and crushed brick as the primary aluminosilicate sources. These materials are readily available in C&D waste streams and offer diverse chemical compositions crucial for successful geopolymerization. We report on developing and thoroughly characterizing a novel, entirely waste-derived activator. This novel activator utilizes crushed glass waste to produce sodium silicate, a key component in the geopolymerization process. This approach aims to close the loop on material flows by using waste glass, another abundant component of C&D waste, to create the necessary alkaline environment for geopolymerization. Developing a reliable method to produce sodium silicate from waste glass on-site or regionally could significantly reduce the cost and environmental impact associated with transporting and using commercially produced activators. The resulting geopolymers are then tested to assess their chemical resistance under harsh environmental conditions. This assessment includes exposure to highly acidic and alkaline solutions simulating challenging scenarios to evaluate their potential for practical applications in various construction contexts, thus showing the feasibility of producing high-performance geopolymers entirely from waste materials.
This research aims to advance sustainable construction practices significantly, contribute to a circular economy, and pave the way for a more environmentally responsible built environment by demonstrating the feasibility of producing high-performance geopolymers entirely from waste materials.