Calcined clay and slag based alkali-activated materials

Abstract

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.