Susceptibility of phosphogypsum to fungal growth and the effect of various biocides

Natural gypsum (NG) and phosphogypsum (PG) were tested for resistance to fungal growth based on standard test ASTM D 3273-86, with the recommended mixture of three fungal species, and using the same test modified by the use of a Cladosporium sp. A, isolated from a gypsum plaster ceiling. In the standard test little growth occurred on any of the test specimens. However, abundant fungal growth was produced by the Cladosporium sp. A on phosphogypsum, which was much more susceptible than natural gypsum. Phosphogypsum heated to 600°C to destroy organic residues was resistant to growth of Cladosporium sp. A, as well as other fungi isolated from phosphogypsum panels stored in the environment for 2 years: Cladosporium sp. B, Aspergillus niger and Trichoderma sp. Phosphogypsum moulded in Petri dishes was susceptible to growth of a wide range of fungi, although Fusarium sp. and Rhizopus sp. caused practically no discoloration of the substrate. Six biocides were separately incorporated into the phosphogypsum at concentrations recommended by the suppliers and test specimens incubated on Sabouraud agar inoculated with various fungal isolates. The biocide 2-N-octyl-4-isothiazolin-3-one was the most efficient compound. It prevented the growth of the fungi most likely to cause health problems in buildings, but not that of Helminthosporium sp., isolated from powdered phosphogypsum in the factory. This was the most resistant fungus showing growth on all biocide-containing specimens.