The synergistic effect of greenhouse gas CO2 and silica fume on the properties of 3D printed mortar

To fulfil the requirements of pumpability and buildability, the rheological properties of mortar/concrete for 3D printing should follow much stricter requirements than traditional-cast mortar/concrete. This study proposed to use a secondary CO ₂ mixing technique coupled with the addition of silica fume (SF) to achieve in-situ rheology control of 3D printed mortar. The influences of CO ₂ mixing on the properties of the cast mortar and the 3D printed mortar prepared with SF were systematically studied. The pH, conductivity and ion concentration were tested to revel the essential chemical reactions and phase evolutions during CO ₂ mixing. It was found that the CO ₂ mixing effectively enhanced the early-age penetration resistance and yield stress of mortar incorporated with SF, and thus significantly improved the buildability of 3D printed mortar, while the maximum printing layer increased higher than 33 layers. Meanwhile, the mechanical properties of the SF-incorporated mortar were improved by this process. It was revealed that during the CO ₂ mixing process, the injected CO ₂ rapidly reacted with calcium ions to form CaCO ₃, which further reacted with C ₃A to form monocarboaluminate (Mc). The fast precipitation of calcium ions during CO ₂ mixing accelerated the hydration of C ₃A and C ₃S, contributing to the rapid development of strength at early ages and the improved performance of hardened mortar. The addition of SF in mortar greatly amplified the beneficial effect of CO ₂ mixing, which due to SF as nucleation sites promoted the formation of CaCO ₃ and Mc during CO ₂ mixing.