## 4 Discussion  
### Effect of Surface Roughness on the Attachment of Marine Organisms  
The biofouling process typically involves four stages, as follows: conditioning film formation; microbial biofilm development; protozoan attachment; and marine organism settlement [32]. Despite changes in the surface properties due to biofilm formation, macro-fouiler larvae often directly penetrate the conditioning and microbial biofilms to contact the substrate [33; 34]. Thus, the increase in marine organism attachment on DL can ultimately be attributed to changes in the surface properties.  
Sample DL was originally glazed only on its outer wall, making it flat and smooth, while the unglazed inner wall was rough. Previous research [13] indicated that DL experienced prolonged mechanical abrasion from sea wave-borne debris in an isolated, dynamic marine environment, leading to its degradation. Consequently, the surface of DL became increasingly rough. The glaze on DL, a calcium glaze belonging to alumina silicate glass with unfused quartz particles and precipitated anorthite crystals [13], underwent two primary corrosion processes: ion exchange and hydrolysis. When the pH was \(<\)9, Cl\({}^{-}\) or hydronium ions in the water exchanged with alkali metal ions in the glass, forming a de-alkalinized hydrated layer on the glass surface [35]. When the pH was \(>\)9, OH\({}^{-}\) ions disrupted Si-O-Si bonds in the silicate network, leading to gradual dissolution [36]. The seawater in Fujian, China is a weakly alkaline environment, with a pH of about 8.2. However, the low K and Na content and high Ca content in the DL glaze contributed to the stability of the Si-O-Si network, making ion exchange slow [10]. Nevertheless, the high pH in the localized microenvironment of iron rusts and marine organism remains accelerated the hydrolysis of the Si-O-Si network [13].  
Only trace amounts of S and Cl were detected on the inner wall surface of DL, originating from sulfate and chloride salts in the marine environment. These factors are significant for concrete corrosion. Sulfate reacts with Ca\({}^{2+}\) in concrete to produce ettrinigite, gypsum, and calcium silicate, leading to expansive corrosion and structural damage [37; 38]. Chloride ions can destroy passive layers on rebar surfaces, accelerating electrochemical corrosion [39]. The low Ca content in DL was insufficient to react with the sulfate, and the sintered body was not affected by chlorate.  
Marine fouling organisms typically adhere using wet adhesion [40]. They synthesize and secrete a viscous adhesive that wets and disperses on the substrate surface, forming a firm attachment through chemical bonding, electrostatic interactions, mechanical interlocking, and spreading [41]. The increased surface roughness of DL, resulting in numerous micron-level pores and cracks, raised its specific surface area and wettability. The adhesive filled defects or permeated into the sample surface, forming a strong bond. When marine organisms completed their life cycles or were displaced by seawater, significant detachment of the biofouling occurred. The adhesive layer, however, often took away some of the weak glaze, further roughening the surface [11]. This cyclic process led to continuous biological colonization, increasing the biomass. Coralline algae contributed to this increase by providing a preferred matrix for larval attachment. The polysaccharide sulfides secreted by coralline algae induced larval attachment and growth [31; 42; 43].  
The difference in marine organism attachment between the inner and outer walls is related to their surface properties. The unglazed inner wall, being rougher, more readily accommodated macrofouiler larvae compared to the glazed outer wall. Marine fouling organisms more effectively adhered to the rough surface of the inner wall, which directly faced the marine environment, accelerating biofouling formation. Consequently, the inner wall accumulated more marine organism remains than the outer wall.