Development of Low Alkalinity Activated Construction and Demolition Waste Based Geopolymer Binder Systems for 3D Printing Application

Abstract

The extent of environmental damage caused by construction activities executed to address the increasing population's housing and infrastructure needs and to construct a more livable world has been more clearly observed in recent years, especially as the magnitude of encountered natural phenomena is witnessed. The rises in greenhouse gas emissions, escalating soil and water pollution, the excavation use of clean and arable lands, significant depletion of natural resources, and various other factors can be detected as aspects missed by the construction sector in its pursuit of building a more livable world. Concrete stands out as the dominant material in the construction sector, with an annual global consumption of approximately 30 billion tons, making it the most commonly utilized material in the world after water. This widespread use contributes significantly to the consumption of Portland cement, the primary binder in traditional concrete production, which is energy-intensive and has high carbon emissions. To slow down this negative trend and lower the environmental burdens associated with the construction sector, various steps have been taken. At this point, some studies are being conducted on decreasing the clinker ratio in cement production, changing fuels and raw materials in cement production, substituting industrial by-products for cement, utilizing waste in cementitious systems, and developing alternative binding systems to cement.

As a result of these studies, it has been discovered that a new generation binding system, synthesized through the alkali activation of precursor materials called geopolymers, which reduces the environmental impact of cementitious systems, can be used as an alternative to cementitious systems in the construction sector. Its ability to utilize industrial waste as precursor materials in synthesis has led to rapid progress in its use as a construction material, providing both environmental and economic benefits. While the precursors used in traditional geopolymer synthesis are industrial by-products, their elevated demand by the cement/concrete industry, as they can serve as substitutes, has diminished the ready availability of these materials and climbed their costs. Increasing costs and supply-related issues have induced researchers to explore the incorporation of innovative aluminosilicate precursors in geopolymer synthesis. The objective is to use easily accessible and affordable materials while dealing with the challenge of currently unvalorized wastes. These dual objectives endeavor to design more economical and environmentally friendly materials while simultaneously addressing issues coupled with generated wastes. In this context, considering that construction and demolition wastes (CDW) are predominantly composed of aluminum, silicon, and calcium sources, coupled with the global prevalence of CDW generation where it is mostly either disposed of in appointed landfill areas or used for low-tech applications, the incorporation of CDW-based materials in geopolymer production emerges as a promising avenue for effective and innovative material development, along with the high-quality valorization of wastes.

In addition to material-sourced issues in the construction sector, despite holding a considerable market share among industrial sectors, the construction industry has fallen behind in keeping pace with emerging technology in the past few decades and has been indecisive in taking required steps in traditional production processes to improve productivity compared to the other sectors. The growing demand for housing because of ongoing population growth, disasters, and wars, has resulted in the recognition in recent years by academics and companies that manufacturing processes in the construction sector required to be more efficient, cheap, and fast. Given this context, three-dimensional additive manufacturing (3D-AM), whose usage is rapidly gaining popularity due to the numerous advantages it offers in other sectors, has started to be employed in the construction industry. 3D-AM systematically constructs tailored structures through the successive deposition of layers following the digital models, offering a high degree of design freedom. With the adaptation of 3D-AM in the construction industry, digitalization, automation and individualization in built environments can be acquired. The key benefits provided by the 3D-AM technique involve the reduction of production-related errors, shortened production time, decreased occupational safety risks, reduced need for skilled workforce, increased customization, and lowered costs. Furthermore, 3D-AM which provides moldless production and the reduced generation of waste materials during the operation process, contributes significantly to mitigating the high environmental burdens caused by the construction sector.

Considering all these circumstances, within the scope of this thesis, studies have been conducted to (i) evaluate the effects of incorporation of the industrial wastes into CDW-based geopolymer mixtures, (ii) to provide comprehensive insights and knowledge about tailoring the fresh and hardened properties of the CDW-based geopolymer mixtures considering environmental burdens, (iii) to develop more environmentally friendly, economical, sustainable, and value-added 3D-AM-compatible geopolymer binder systems using CDW, (iv) to minimize the use of alkaline activators and maximize waste materials, (v) to investigate the effects of various additives on the engineering properties of geopolymer mortars, (vi) to mitigate shrinkage-related crack formation in 3D-printed filaments and efflorescence issues in CDW-based geopolymer mixtures, (vii) to investigate reinforcing strategies for 3D-printed structures and possibility of modular system in 3D-printed construction. The study utilized a comprehensive testing approach, encompassing a range of assessments such as flow table, setting time, flow curve, three-interval thixotropy, ram extruder, LCA, compressive and flexural strength, direct tensile, water absorption, sorptivity, efflorescence, drying shrinkage, wet-dry and freeze-thaw cycling tests. This comprehensive methodology facilitated a thorough exploration of the properties and performance of the geopolymer mixtures, effectively fulfilling the research objectives.