The Mechanism of Surface Blackening and Deterioration of a Traditional Construction Material, CATC, for Coastal Stone Masonry Jointing

Crust Ash Triad Clay (CATC) is a traditional construction material commonly used for jointing coastal stone masonry in Southeast China. Its surface is prone to blackening in coastal environments. This study focused on traditional stone masonry residences within the protection area of Quanzhou Shihu Ancient Wharf. A systematic detection and analysis were conducted using combined technologies: XRD, Raman, SEM-EDS, and 16S rRNA sequencing. The results revealed that the CATC substrate is mainly composed of quartz and feldspar minerals, with calcite and other substances as binding components. The black coating on the surface is a loose material attached to the substrate, retaining some of the original minerals. The core mechanism of blackening lies in the coastal environment’s abundance of salt spray and humidity. The sulfate substances carried by rainwater react synergistically with metal ions such as Cu, Fe, and Mn in the substrate under the metabolic action of anaerobic bacteria, producing metal sulfide minerals. Photoautotrophic bacteria generate oxygen through photosynthesis, promoting the oxidation and acidification of metal sulfide. This process directly triggers the chain deterioration of the CATC substrate. Based on the principle of “minimal intervention”, physical waterproofing or laser stain removal can be implemented. This study provides scientific support for optimizing the durability and achieving precise protection of traditional building materials in coastal stone structure heritage.