Can mold be removed from pavers?

Before any cleaner is named, the EPA sets out the condition mold cannot grow without.

The EPA describes mold as part of the natural environment: outdoors it breaks down dead organic matter such as fallen leaves, and it reproduces by tiny spores that float through outdoor and indoor air. There are many types of mold, the EPA says, and none of them will grow without water or moisture; the key to mold control is moisture control.

For hard surfaces, the EPA's instruction is to scrub mold off with detergent and water and dry completely. Its caution for porous materials is that mold can grow on or fill in their empty spaces and crevices, where it may be difficult or impossible to remove completely, and a paver is a porous material.

UF/IFAS Extension, writing about mildew in the home, says mold and mildew grow from spores that exist almost everywhere, that mildew requires moist, warm air, and that it can grow in dark areas and places without much air circulation. UF/IFAS does not write about pavers, but on a paver surface those conditions describe the shaded corner more than the open drive in full sun.

ICPI's Tech Spec 5 stain list for concrete pavers has no entry for mold, mildew, algae or moss. Its nearest organic entry, for leaf, wood rot or tobacco stains, is household bleach and a stiff bristled brush. Travertine and marble belong to the calcareous stones, and the Natural Stone Institute publishes growth lines of its own for natural stone: a diluted solution for biological stains such as mildew, moss and algae, and a mild bleach solution for stone around an outdoor pool, the line taken up at why pool deck pavers turn green. For brick, the Brick Industry Association says a constantly damp masonry surface encourages moss, algae, lichen or other microbial growth, that masonry cleaning makers offer a biocide for it, and that household bleach, ammonium sulfate or weed killer applied per their directions can also remove organic material in some cases. Which paver material is underfoot decides which of these lines applies.

Cautions travel with any bleach use. UF/IFAS says to wear rubber gloves, avoid contact with skin and eyes and prolonged breathing of the vapors, and protect plants from the mixtures, and both UF/IFAS and the Natural Stone Institute say never to mix ammonia and bleach, a combination NSI says creates a toxic and lethal gas. Near windows there is one more: the National Glass Association lists bleach, and other solutions containing sodium hypochlorite, among materials that may harm laminated glass, and impact windows are built with laminated glass, so bleach solution on a walkway beside impact windows or doors has to stay clear of their glass. Whether bleach harms the paver or its sealer is answered at whether bleach hurts pavers.

UF/IFAS says mold and mildew spores exist almost everywhere, and the EPA says mold spores will not grow if moisture is not present. Read together, a cleaned paver that stays damp can grow the same thing again, and a cleaning does nothing about the spores in the air. Why pavers turn black in summer and whether pavers grow mold faster near the pool are separate questions with their own pages.

None of the documents cited here pairs a growth cleaner with a paver sealer, and none of the makers or associations cited here says what household bleach does to a cured sealer. ICPI's standing instruction is to read and follow the label of every cleaner and sealer, so on a sealed surface the sealer maker's label is where the product question is settled. Whether growth returns once a surface is sealed is answered at whether algae comes back after sealing.