# Additive Manufacturing in Underwater Applications
## 3 Materials
### Ceramics, Concrete, Geopolymers, and Similar  
The most intensive research was carried out on the 3D printing of concrete materials for underwater application in the area of infrastructure, as well as coral reef re-revolution [18; 26]. Moreover, 3D printing technology was developed only for concrete in situ. AM technology supports the direct manufacturing of concrete structures underwater, offering a more convenient and efficient way to build [21; 26; 27]. However, while this technology seems to be very promising, the investigation of materials printed in situ underwater shows some challenges--see Table 1. \begin{table}
\begin{tabular}{c c c c c} \hline
**No.** & **Material** & **Investigated Properties** & **Main Findings** & **Reference** \\ \hline \multirow{4}{*}{1} & OPC (CEM 1 52.5 N) + & Assessment of the feasibility of & Decrease of 7\% in CS and 11\% in YM & \multirow{4}{*}{[28]} \\  & superplasticizer + & underwater 3D printing. & when comparing 3D-printable mortar & \\  & anti-washout & Compressive strength (CS). & specimens in water with those printed & \\  & admixture & Young’s modulus (YM). & in the air. & \\  & & & & An increased number of defects and \\  & & & & discontinuities occurred in samples of \\  & & & & 3D printing underwater. & \\  & & & & The buildability decreased—reduction & \\  & & In the fresh state: printability & in the layer height due to changes in & \\  & OPC + & and buildability. & weight and pressure underwater. & \\
2 & viscosity-modifying & Samples after curing: density, CS, & The density and CS of the elements & \\  & agent & interlayer bond strength, flexural & printed underwater were smaller than & \\  & & and splitting tensile strengths. & in those printed in the air, but the & \\  & & & & thicker board strength was better for \\  & & & & those printed underwater. \\  & & & & The flexural tensile strength was also \\  & & & & better for samples printed underwater. \\  & & & & The interlayer adhesion for samples \\
3 & superplasticizer + & Buildability and interlayer & printed underwater was 20.4\% less & \\  & anti-washout & & building. & \\  & admixture & & & \\  & & & Nozzle construction & \\
4 & Cement + sand + silica & optimization and material & \\  & & & properties during the printing & \\  & & & process. & \\ \hline \end{tabular}
\end{table} 
Table 1: Investigations of cementitious materials for application in underwater AM. Additionally, it is worth noting that the buildability in the case of 3D printing technology is a critical aspect. It requires special attention because the buoyancy of water reduces the material's deformation and density and influences the interlayer bonding [26; 32]. Moreover, the factor of printability seems to be crucial, due to interlayer adhesion [26; 29]. The research also shows that the behavior of the samples in time is different for 3D printing in the air and underwater. In the air, the interlayer adhesion of 3D-printed samples is correlated with the speed of printing. In the case of samples 3D-printed underwater, the interlayer adhesion first decreases and then increases with the speed limitation [26; 29]. The results confirm that obtaining elements of satisfactory quality requires more effort for 3D-printed elements [18; 19; 29]. During 3D printing underwater, more defects and discontinuities occur compared with specimens 3D-printed in the air. Additionally, when the same material is used, the samples 3D-printed underwater have worse mechanical properties than those 3D-printed in the air, especially compressive strength [29]. To reduce the above phenomenon, it is possible to modify the concrete mixture, especially through the reduction of the water/cement (W/C) ratio and the selection of proper admixtures, including superplasticizers and anti-washout components--for example, cellulose and Welan gum [26]. It is also worth mentioning that slightly different behavior is observed in the underwater investigation of concrete mixtures produced in freshwater and seawater. These investigations are focused on the washout resistance [33].