Sustainable 3D Concrete Printing for Eco-conscious Construction: Recycled Aggregates and Low-carbon Binders
The construction industry faces increasing pressure from rapid urbanisation, rising infrastructure demand, and global climate imperatives, yet it remains a major contributor to environmental degradation. Concrete, and in particular Portland cement, contributes significantly to this challenge, accounting for nearly 10% of global CO₂ emissions, while inefficient waste management practices direct millions of tonnes of construction and demolition waste (CDW) to landfills annually. Additive manufacturing in the form of 3D concrete printing (3DCP) has emerged as a promising technology that can potentially address inefficiencies currently associated with the construction industry such as improving productivity and reducing labour, material use, construction time, and waste generation. However, the environmental advantages of 3DCP are constrained by the high binder content of printable mixtures, which continue to rely heavily on ordinary Portland cement and virgin aggregates. This dissertation investigates strategies to reduce cement dependency, incorporate recycled aggregates, and improve the sustainability of 3DCP without compromising printability or constructability. A total of thirteen novel, potentially sustainable, mixtures were designed of which eleven were fully characterised through rheological testing, 3D printing and mechanical testing. The research demonstrates that significant reductions in Portland cement can be achieved through the use of supplementary cementitious materials (SCMs) such as fly ash, silica fume, slag, and metakaolin, while simultaneously increasing the aggregate-to-binder ratio to 1.75. Printable mixtures were developed with 60% of the total binder volume consisting of SCMs. The first three mixtures developed using this technique achieved adequate compressive strengths, improved stiffness ranging from 29 – 37 GPa, and excellent surface quality in printed elements. A critical review of low-carbon printable mixtures confirms the practical potential of such formulations, identifying viable pathways for reducing embodied carbon and material costs while still satisfying the structural requirements demonstrated in completed 3DCP projects. The use of locally sourced materials can reduce the cost and emissions associated with the transportation of ready-made mixtures. The three mixtures are further modified to reduce the use of virgin materials by incorporating CDW as a replacement for fine natural aggregates. Replacement levels up to 100% are achieved, with results showing enhanced buildability and thixotropic behaviour, maintaining adequate pumpability and extrudability for 3DCP. The highly porous CDW leads to reduced stiffness as the CDW content increased. The suitability of the material developments made is proven through the fabrication of a full-scale sinusoidal cavity wall segment. The wall is produced with a low-carbon 3D printable concrete mixture with cement content of 348 kg/m3 (achieved by using a large volume of fly ash and silica fume as alternative binders combined with an aggregate-to-binder mass ratio of 1.75) and 25% of the aggregate being replaced with locally sourced CDW. The printed wall demonstrates equivalent stability to a conventional masonry wall while reducing material consumption by 55%. Load testing verifies strong structural performance under both vertical and lateral loading, withstanding wind pressures exceeding four times the ultimate limit state design load. By reducing cement content in printable concrete mixtures, reintegrating CDW as recycled aggregate, and employing structurally efficient geometries, 3DCP can evolve into a viable low-carbon construction method.
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