The impact of biofouling on the water transport properties of porous building stones

Natural stones are essential for our built heritage. They are considered durable and permanent. However, they deteriorate through physical, chemical and biological action with water as the principal facilitator. Within a stone, water transport and retention depend on the stone's characteristics. It is expected that biofilms and extracellular polymeric substances change these properties and thus the water-stone relationship. To study the effect of the biofilms, three porous natural building stones: Ernzen, Euville and Savonnières, were biofouled with cyanobacteria Phormidium autumnale. The amount of biofilm was estimated by spectrophotometry, characterized by optical microscopy, and (environmental) scanning electron microscopy. Water transport and retention for biofouled and untreated samples were determined following European standards. Microscopy showed that the biofilms primarily covered the stone's outer surface. They experienced swelling and shrinkage dependent on the subjected relative humidity conditions, which could facilitate physical weathering. The biofilms slightly affected the water transport properties and induced a (near) hydrophobic surface. They reduced the drying rate and capillary coefficient. Moreover, biofouled samples absorbed more water vapor, had a higher water content, while no direct effect was measured on the water vapor diffusion and gas permeability. These changes could affect future weathering processes, including freeze-thaw and salt weathering.