To promote sustainable 3D concrete printing (3DCP), this study develops an extrusion-based 3D printable geopolymer concrete and evaluates the effectiveness of four chemical retarders, namely tartaric acid, sucrose, sodium tripolyphosphate, and barium chloride, in improving its printability. Based on a comparative assessment of time-dependent flowability and compressive strength of the mixture, the barium chloride demonstrated the most favorable overall performance among the four retarders and was therefore selected for further investigation. When the barium chloride dosage exceeded 2.5%, the mixtures satisfied the early-age strength requirements for printing and demonstrated stable extrudability and good buildability. For mixtures containing 2.5% and 3.5% barium chloride, the open time reached approximately 30 minutes and 60 minutes, respectively. A higher dosage (3.5%) shows better printing quality, resulting in printed structures with compressive and tensile strengths surpassing those of mold-cast specimens and exhibiting reduced mechanical anisotropy. Furthermore, the printed concrete showed the highest compressive strength along with the printing direction, whereas its tensile strength in this direction was lower due to the influence of interlayer interfaces. Overall, a dosage of 3.5% barium chloride provided superior flowability and extended open time, achieving an optimal balance between printability and mechanical performance. This formulation offers a promising retarder strategy for extrusion-based geopolymer concrete 3D printing.
Zhang et al. (Wed,) studied this question.