Strategy for improving buildability of 3D printing concrete using CO₂ mixing and chemical admixtures

Polycarboxylate superplasticizer (PCE) and hydroxypropyl methylcellulose (HPMC) are commonly used chemical admixtures to control the rheological properties of concrete, such as 3D printing concrete (3DPC). CO₂ mixing is a promising technology that improves the rheological and mechanical properties of 3DPC and has the potential to sequestrate CO₂. But at present, the coupling effects of CO₂ mixing with PCE and HPMC on the properties of cementitious materials are unknown. Therefore, this study investigated the influence of CO₂ mixing on the fresh and hardened properties of cement mortar with varying amounts of PCE and HPMC. The results showed that after using CO₂ mixing, the effects of HPMC and PCE on the properties of cement mortar changed obviously. The PCE in cement mortar almost lost its effect after using CO₂ mixing. The addition of 0.2 %PCE increased the workability of mortar by 50.3 % and increased setting time by 84.6 %, but after using CO₂ mixing, the workability and setting time of mortar with 0.2 %PCE exhibited only limited increase compared to mortar without PCE. The effect of HPMC was also weakened after using CO₂ mixing. Furthermore, the heat evolution and microstructural properties of fresh and hardened mortar were analyzed to reveal the mechanism of the coupling effects of CO₂ mixing with PCE and HPMC. In addition, based on the coupling effect of CO₂ mixing and PCE, a strategy for improving the buildability of 3DPC using CO₂ mixing and PCE was proposed.