## 4 Materials Used in Construction
### Cement, Geopolymers, and Similar Compositions  
The results of studies conducted in different aquatic environments confirm that ceramic materials can be optimal for creating artificial reefs. The research found that ceramics and concrete were the most colonized by organisms compared with other materials such as metals or plastics, making them the most suitable habitats for a wide range of species, including fauna, flora, and microorganisms [6]. In addition, artificial structures made of ceramic materials are characterized by durability and the ability to form various types of reef structures. The complexity of the structure affects the associated fish communities, and the presence of holes and crevices provides shelter, breeding sites, and protection from predators [67]. The use of new technologies, in particular the use of 3D printing, is also of great importance.  
It is not always clear which cementitious material is the best for applications on artificial reefs. [PERSON] et al. [68] compared different cementitious materials, such as geopolymers and cement concrete, assessing their durability and biofouling (colonization of the structure by biological organisms) in seawater [68]. The results indicate that cement is a better material for building an artificial reef using 3D printing, due to its better mechanical properties and lower susceptibility to biofouling [68]. In turn, [PERSON] et al. [69] showed that geopolymers have better biological recetivity compared to cement [69]. This indicates the need for further research on the selection of materials, depending on the purpose of remediation [68; 69].  
Selecting the right materials is crucial, as some of them can have undesirable effects. The two main problems associated with the use of Portland cement (OPC) or geopolymers are the increase in water alkalinity and the precipitation of calcium carbonate. The high pH value around the surface of the material inhibits the settlement of marine life [70; 71]. OPC usually has pH of around 13, far exceeding the seawater pH of 8-9 [69; 72]. Such a high value negatively impacts the surrounding environment, including the organism's settlement [72; 73]. The process of neutralization in the natural environment typically takes 3-6 months to lower pH [73], where the colonization of the material is limited [74]. Therefore, during 3D printing, it is necessary to neutralize the print surface to ensure faster adjustment to the marine environment [75]. The second problem is mainly related to the use of OPC; in this case, we are dealing with carbonate formed as a result of the re-precipitation of soluble Ca\({}^{2+}\) from alkaline cement. It tends to cover the surface and prevents the growth of marine life, resulting in poor colonization with microalgae [70].  
A valuable trend is also the use of waste materials for artificial reef production, including industrial by-products, such as silica fume, which can result in lowering pH [72]. Alternative materials tested in artificial reefs manufacturing also include waste from machine-made sand production [76]. The investigation made by [PERSON] et al. [77] confirms the feasibility of applying this by-product in concrete for 3D-printed artificial reefs [76]. The replacement of 30% of sand with this waste does not influence the material's printability and only slightly decreases mechanical properties, while lowering carbon emissions [76].  
The influence on the fish behavior was not observed [76]. There are even more possibilities if industrial by-products are used in the geopolymers. [PERSON] et al. [77] investigated the material that was based on industrial waste-biomass of fly ash and red mud [77]. The 3D-printed samples confirm the printability of the created mixture and show that the mechanical properties are sufficient to create an artificial reef [77; 78].