Concrete has been a widely used building material for centuries; however, its production is associated with a significant carbon footprint, contributing at least 8% of global CO₂ emissions. Additive manufacturing (AM) presents opportunities to enhance the sustainability of concrete by reducing labor, on-site construction time, and material waste, while allowing for the creation of complex geometries. This approach not only reduces overall costs but also enables more sustainable construction processes. Sustainability can be further improved by replacing Ordinary Portland Cement (OPC) with alkali-activated materials (AAMs), which are low-carbon alternatives incorporating industrial by-products such as fly ash and blast furnace slag. These materials prevent resource depletion, reduce landfill deposition, and lower CO₂ emissions. Nevertheless, the future availability of fly ash and slag is expected to decline after 2030 due to decreasing coal-fired power generation and increasing demand for blending cements.

Metakaolin, a high-purity calcined clay, has been extensively studied as an aluminosilicate source for concrete. However, competing demands make it less practical, and lower-grade calcined clay is considered a cost-effective alternative due to its larger supply and lower purification requirements. This research focuses on developing an alkali-activated calcined clay and slag-based material specifically designed for 3D-printing applications. The study investigates the relationships between calcined clay content, fresh and mechanical properties, hydration, and shrinkage, with particular attention to 3D concrete printing (3DCP) applications.

Results indicate that partial replacement of blast furnace slag with calcined clay delays setting, due to its lower CaO content and reduced reactivity compared to slag. Incorporating calcined clay increases the static and dynamic yield stress as well as the initial storage modulus, attributed to the larger specific surface area and porosity, which reduces the effective water-to-binder ratio and limits particle separation. While extrusion is positively affected due to the extended printability window and reduced extrusion pressure, calcined clay does not enhance early-age green strength evolution. Moreover, both drying and autogenous shrinkage increase with calcined clay addition, and mixtures with 20% calcined clay exhibit drying shrinkage four times higher than the target value.

Mechanical properties, including compressive strength, improve with increasing calcined clay content, likely due to pore refinement and a denser microstructure. Isothermal calorimetry and thermogravimetric analysis suggest that calcined clay acts primarily as a filler at early ages rather than actively contributing to gel formation. Despite improved rheology, workability, and mechanical properties, the high shrinkage and delayed setting limit the applicability of calcined clay and slag-based AAMs for 3DCP.

Future research should focus on introducing admixtures and optimizing alkali activator concentrations to promote gel formation, reduce shrinkage, and improve early-age performance. For optimal results, calcined clay content should remain below 20% by mass of the binder unless activator formulations are adjusted. Overall, this study provides a foundation for the continued development of sustainable alkali-activated materials suitable for additive manufacturing, particularly as traditional by-product sources like fly ash and slag become less available. ... Read more

After water, concrete is the most used substance on the planet. Approximately three tonnes of concrete is produced per person annually (Gagg, 2014). Traditionally, large amounts of Ordinary Portland Cement (OPC) are needed for the production of concrete. The production of OPC is very energy intensive and therefore a major generator of carbon dioxide, which is considered to be a potent greenhouse gas. High CO2-emissions are caused by calcination of limestone and combustion of fossil fuel during an energy intensive production process.
To reduce the CO2-emissions caused by the concrete manufacturing, there is a growing interest in alternative and low CO2-binders. A great example of alternative binders are alkali-activated materials (AAMs), often referred to as geopolymers. Alkali-activated materials are inorganic polymers acting as the binder agent in concrete, often containing by-products from the industry such as fly ash and blast furnace slag (Davidovits, 1989). Unlike OPC, AAMs are synthesized by activation of an aluminosilicate source (fly ash, slag, metakaolin) with alkaline activators. AAMs have attracted a lot of attention in the industry because of its superior mechanical properties, excellent resistance to sulphate attack, low creep and low drying shrinkage compared to OPC.
Besides sustainability, the total amount of costs is also a relevant factor for the construction industry. 3D-printing of concrete removes the need for formwork and enables the industry to create complex shapes as well as optimization of material use. It also provides an opportunity for an automated building process with a minimal amount of labour and material wastage. Combining the use of alkali-activated materials (AAMs) in an innovative and automated 3D-printing process may therefore offer many advantages for the construction industry...
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