Feasibility of underwater 3D printing: Effects of anti-washout admixtures on printability and strength of mortar

The dispersion and reduction in strength of concrete underwater pose significant challenges to advancing underwater 3D concrete printing technology. While employing anti-washout admixtures holds promise, there is a dearth of relevant research. Herein, a study using various potential anti-washout admixtures, such as micro silica (MS), nano silica (NS), glass fibers (GF), and hydroxypropyl methyl cellulose (HPMC), in mortar mixes was conducted. These mortar mixes were then printed underwater, and their performance in both fresh and hardened states was evaluated to develop underwater 3D printable mortars. The findings revealed that both MS and NS enhance the washout resistance, extrudability, and buildability of fresh mortar, while also improving the flexural, compressive, and bond strengths of hardened mortar. HPMC enhances underwater printability and bond strength, although it may negatively impact flexural and compressive strengths. GF exhibits adverse effects on underwater printability and mechanical properties and is incompatible with HPMC. Noticeably, the combination of MS, NS, and HPMC greatly enhances underwater printability (no dispersion and lowest extrudability/buildability variation coefficients) and mechanical properties (increasing flexural strength and bond strength by 21 % and 151 %, respectively). These findings led to the proposal of several mortar mixes deemed suitable for underwater 3D printing.