Development of CO₂-integrated 3D printing concrete

3D printing concrete (3DPC) technology is a promising technique for construction due to its advantages such as no formwork is needed, fast production, automation, and high architectural freedom. However, the layer-by-layer extrusion method has stricter requirements on the rheological properties of concrete. One of challenges of this technology is how to improve the rheological properties of concrete to satisfy the conflicting requirements during pumping and after extrusion. This study proposed to use CO₂ as accelerator and rheology modifier by injecting CO₂ during secondary mixing to improve the rheological and mechanical properties of 3DPC. The influences of the secondary CO₂ mixing on the properties of poured concrete and 3DPC were investigated. After using the secondary CO₂ mixing, the setting time and workability of concrete were reduced, which contributed to the significantly improved buildability of 3DPC. This was because CO₂ accelerated the hydration of tricalcium aluminate (C₃A) and tricalcium silicate (C₃S) during the secondary mixing. After that, they were continuously accelerated by the calcium carbonate formed during CO₂ mixing. Also, the compressive strength of poured concrete was enhanced by the secondary CO₂ mixing because it reduced the volume of larger pores (>200 nm) and promoted the formation of calcium silicate hydrates (C-S-H), which simultaneously slightly increased the drying shrinkage. In addition, after using the secondary CO₂ mixing, the compressive strength and interlayer bond strength of 3DPC was enhanced.