This study aimed to develop nine biocide mixtures that prevent the weathering and biofouling of stone monuments. Tetraethoxysilane (TEOS) was combined with one, two or three active components: chi-tosan (a biopolymer used for its antimicrobial potential), silver nitrate and hydrophobic silica. A laboratory test was set up, consisting of the inoculation of untreated (control) and treated limestone (" Dom stone ") slabs with an axenic suspension of the green alga Chlorella vulgaris. The biocide efficacy was evaluated by non-destructive methods such as colourimetry and chlorophyll a fluorescence analysis. The latter method supplements the information provided by the measurement of the colourimetric changes of stone surface and characterizes how the biocide acts on the alga PSII photosynthetic activity. Results revealed different patterns of algal development according to treatment efficacy. The combination of silver nitrate and hydrophobic silica, both at high dosages, provided the best biocide effect. When chitosan was added, a similar biocide effect was obtained using a lower concentration of chemicals. This synergy was not observed when hydrophobic silica was either absent or present at a higher dose.