Performance, economic and environmental impact analysis of recycled materials and 3D printing process for sustainable construction

Abstract

The growing global population has resulted in significant growth in building construction and expansion of the construction sector. The demand for sustainable construction has been raised considering the construction sector's substantial impact on the environment and natural resource depletion. To overcome this impact, various green or recycled materials and smart technologies have been introduced in the construction sector. Geopolymer concretes have been considered to replace the traditional Portland cement-based concretes for a long time for sustainable materials. In the case of sustainable construction technologies, additive manufacturing/3D construction printing (3DCP) techniques have been focused on as a potential sustainable alternative to traditional construction processes. Geopolymer binders such as fly ash, blast furnace slag and silica fume are obtained as byproducts from various manufacturing processes. However, these materials are not adopted in the construction sector to a large extent; therefore, there is a need to conduct a comprehensive analysis based on the assessment of performance and the economic and environmental aspects of geopolymer concrete. In this study, geopolymer mixes were designed with varying proportions of alkaline activator; moreover, an alternative hybrid concrete was also studied for comparative analysis. The analysis shows that the alkaline activator used for activating recycled materials is the prime reason for the higher cost and environmental burden, which limits its applications on an industrial scale. Hybrid concrete with a limited proportion of alkaline activator and Portland cement offered better mechanical performance and competitive cost without compromising the environmental benefits of recycled materials to a greater extent. This thesis further extends the examination of geopolymer and hybrid cement mixes by investigating the durability features of the concretes through macro- and micro-porosity assessment. A comprehensive life cycle analysis was performed based on the manufacturing and transportation of materials in Hong Kong’s scenario. Analysis reveals that the hybrid cement mixes offer superior strength at later ages and the lowest microporosity due to the formation of more C-A-S-H and C-S-H gels. Moreover, hybrid cement mixes yield relatively lower environmental impact due to reliance on locally produced materials (such as fly ash and OPC), which reduces the environmental burden of transportation in the case of Hong Kong. In the construction processes, 3DCP has been widely adopted as an economical and environmentally friendly construction method. However, the transportation of the huge printing setup and its installation at the construction site has raised concerns related to the cost and environmental impact of the construction process. Therefore, this research compared the onsite 3DCP with the offsite 3DCP (a potential alternative) from the supply of materials and equipment to the final construction. The comprehensive analysis shows that the offsite 3DCP process even results in higher costs and environmental burdens, primarily due to the excessive transportation and additional assembly processes involved. The economic and environmental burden due to transportation can be reduced by selecting the adequate 3DCP process. For small-sized construction (containing <140 tonnes of raw material), offsite 3DCP can be the potential solution in minimizing the significant transportation impact on overall construction cost and environmental impact.