Two-Part Print-Head Mixing of Hybrid Alkali-Activated Cement for Buildability Enhancement in 3D Concrete Printing (2024-07)¶

3D Concrete Printing (3DCP) technology demands contradicting rheological requirements associated with the high flowability of printable mixes during the pumping and extrusion stages followed by rapid early age strength development during the placement. These rheological requirements enable ease of pumping and extrusion while attaining high build heights without the plastic collapse or elastic buckling. This paper focuses on developing a two-part mixing system using Hybrid Alkali Activated Cement (HAAC), containing geopolymer slurry and Ordinary Portland Cement (OPC) slurry as two components, mixed at the print head for meeting the abovementioned rheological requirements. More precisely, the two-part mixes remain highly flowable slurries during the pumping stage, followed by rapid yield stress development after print head mixing to meet the buildability requirements. The proposed methodology is expected to achieve the contradicting rheological requirements for 3DCP while benefiting from the superior strength and durability properties of HAAC. Two HAAC mixes with fly ash replacement levels of 12.5% and 25% with OPC were prepared to compare the fresh and hardened properties with a control mix made of geopolymer 3D printable concrete. The early age yield stress development of HAAC mixes, measured by the slow penetration test, demonstrated that the HAAC mixes showed a 10-fold and 17-fold yield stress enhancement, compared to the control mix. Moreover, the measured yield stress of HAAC mixes suggests a print height of 8.7 m and 14.2 m without failing through plastic collapse. Meanwhile, the directional compressive strength of HAAC mixes was improved in all directions, compared to their control mix counterpart. This is demonstrated by the compressive strength enhancement of 4.2% and 23.1% in the longitudinal direction of 3D printed HAAC mixes with 12.5% and 25% fly ash replacement levels.