Enhancing Early Strength in LC³ 3DCP with Advanced Chemical Admixtures

3D concrete printing (3DCP) holds promise for reducing carbon footprints through waste minimization, localized production, and structural optimization. However, current binder formulations face sustainability challenges, mainly due to the prevalent use of excessive amounts of Ordinary Portland Cement (OPC) to meet 3DCP's workability and pumpability requirements. Extrusion-based 3DCP can, thus, be deemed not environmentally friendly enough unless a substantial decrease in Portland cement content is achieved. Limestone Calcined Clay cement (LC3) presents a promising alternative to the traditional OPC from the environmental standpoint. Nevertheless, the use of LC3 binders imposes challenges in 3DCP applications due to their low early-stage strength. This is a crucial requirement for 3DCP to ensure fast and continuous printing as well as structural stability. To overcome that, we present a solution centered around the integration of innovative admixtures. Specifically, this involves incorporating novel additives, including a retarder admixture to regulate the rheological properties of fresh mortar/concrete and liquid accelerator introduced in the extrusion nozzle. This dual admixture (2k-system) approach aims to control cement hydration evolution selectively, promoting structural formation and enhancing early strength development. To validate the concept, we utilized a commercial LC3-binder. The printing outcome achieved a 3.0 m/h vertical build rate, validating the feasibility of the proposed solution in controlling workability, structural formation, and early strength development.