Buildability Enhancement of 3D Printed Concrete Using Carbonated Water and Partial Replacement of OPC with Reactive MgO

3D concrete printing (3DcP) offers a transformative approach to modern construction, providing unique benefits such as faster construction, lower labour costs, and minimal material waste. Despite these advantages, high cement consumption and the challenge of achieving conflicting rheological requirements limit its widespread application. This study addresses these challenges by developing an Ordinary Portland Cement (OPC)-based 3D printable mortar incorporating magnesium oxide (MgO) as a partial cement replacement and using carbonated water (CW) as the mixing water. MgO is an abundant material capable of reacting with CO2 to form stable carbonates, while CW promotes early hydration and carbonation. Unlike previous approaches that require hydration agents to overcome the low dissolution of MgO, this work combines MgO and CW to meet the rheological demands of 3DcP without additional additives. Rheological properties such as static yield stress, buildability, and workability were evaluated alongside compressive strength and microstructural characteristics using XRD, SEM, TGA, and FTIR analyses. Results showed that MgO increased static yield stress due to higher water absorption, while CW further enhanced it through accelerated hydration. The MgO-CW combinations, particularly mixes with 25% (25MCW) and 50% MgO (50MCW), exhibited yield stress increases of 505% and 425%, respectively, after 30 minutes compared to a control mix with pure water (CPW). Among the tested mixes, 25MCW achieves a practical balance of extrudability, buildability and compressive strength, confirming its applicability for 3D printing applications under the tested conditions. Moreover, microstructural analysis confirms enhanced hydration and carbonation reactions in the MgO-CW modified mix, which contribute to the improved performance of the developed sustainable material. This study offers insights for advancing eco-friendly 3D printable materials and sustainable construction.