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

Abstract

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.