Antifungal activity of Ca[Zn(OH)3]2·2H2O coatings for the preservation of limestone monuments: An in vitro study

• The antifungal protective coatings were evaluated on different limestone materials.
• The limestones were inoculated with Penicillium oxalicum and Aspergillus niger.
• The antifungal protective coatings of Ca[Zn(OH)3]2·2H2O were evaluated.
• After 21 days of inoculation the Ca[Zn(OH)3]2·2H2O kept its antifungal properties.
• The samples were evaluated under photoperiods of dark and illumination conditions.

The antifungal properties of protective coatings based on Ca[Zn(OH)3]2·2H2O have been evaluated both on glass slides and on three different limestone materials, which have regional significance as the most commonly used building and restoration materials in the region, including for historic Mayan monuments and artifacts. The protective coatings were found to have excellent antifungal properties, inhibiting the growth of Penicillium oxalicum and Aspergillus niger, both in the dark and under illumination.Experiments were carried out at a laboratory test site under simulated photoperiod conditions, consisting of 24-h cycles of 12 h in the dark and 12 h under illumination: 21 days after inoculation and exposure to photoperiod conditions, no fungal growth was observed for any type of limestone, for both fungi. X-ray diffraction illustrated that the formation of calcium oxalate in the weddellite structure is an indicator of the fungal activity. Close inspection of samples with scanning electron microscopy (SEM) revealed that for all limestone substrates without protective coating, fungal growth was observed as well as fungal penetration into the limestone material. SEM images also show that the Ca[Zn(OH)3]2·2H2O coating is well-adhered to all substrates, without the presence of significant cracks in the coating. For the three limestone substrates with the protective coating, no fungal growth was observed, illustrating the excellent antifungal properties of the calcium zinc hydroxide dihydrate material.