Improving buildability and reducing carbon emission of 3D printed concrete via CO₂ mixing and waste glass powder: a synergistic approach

The challenge of achieving simultaneous pumpability and buildability remains one of the fundamental obstacles in advancing 3D printed concrete (3DPC) technology. This study investigates a novel approach combining CO₂ mixing technique with waste glass powder (WGP) incorporation to address this challenge while simultaneously reducing the carbon footprint of construction materials. Through systematic examination of fresh and hardened properties of both cast concrete and 3D printed specimens prepared with varying WGP contents (0-30% cement replacement), it is demonstrated that while WGP initially enhances workability but reduces early-age strength development, the application of CO₂ mixing creates a remarkable synergistic effect that reverses these trends. The results reveal that CO₂ mixing substantially enhances the yield stress and early-age penetration resistance of concrete which reflects a better buildability of 3DPC, with these improvements being dramatically amplified in the presence of WGP. Specifically, concrete containing 30% WGP showed a 1188% increase in penetration resistance after CO₂ mixing, compared to only 304% for WGP-free concrete. Microstructural investigations indicate that this synergistic enhancement occurs through a dual mechanism. WGP releases alkali ions in the carbonation-induced acidic environment, promoting additional calcium carbonate formation, while simultaneously providing silica-rich nucleation sites that accelerate cement hydration. Furthermore, the combination of WGP and CO₂ mixing effectively addresses the interlayer bonding challenges typically associated with 3DPC, showing improved interfacial strength compared to either treatment alone. These findings establish a practical strategy for developing sustainable 3DPC with enhanced buildability characteristics.