Why are my pool deck pavers green?

Before naming anything that grows on it, the deck itself needs a material: calcareous stone such as travertine, or a concrete paver.

Travertine is a natural stone, and the Natural Stone Institute publishes care guidance for natural stone; concrete pavers are covered by ICPI's Tech Spec 5. A pool deck can be either. Techniseal's product table, for one, lists pool decks among the uses for its (iN) Natural Look Protector, alongside concrete pavers, natural stone, driveways, walkways and patios; that is a maker's use list for its own product, not a recommendation of which sealer fits which deck. Concrete pavers, travertine and clay brick are set out material by material on the service page.

Moisture is the thread in the documents that describe this growth. The EPA says none of the many types of mold will grow without water or moisture. In a nursery and greenhouse document, UF/IFAS describes Nostoc-like cyanobacteria forming mats on surfaces that include cement, drying to flakes in dry periods and swelling with any moisture into thick, dark green, gelatinous masses. No document cited here identifies the organism on a particular deck, so a pool deck with green on it is showing growth, with algae, moss or a cyanobacteria mat among the candidates.

A pool deck adds moisture of its own, from splash-out, wet feet, shade and nearby irrigation, and no document cited here measures how much faster growth appears beside a pool; whether pavers grow mold faster near the pool takes that question on its own. Green that sits in the joint sand rather than on the paver face is a separate case, at why the sand in paver joints turns green.

For outdoor pool, patio or hot tub areas, the Natural Stone Institute says stone is flushed with clear water and a mild bleach solution is used to remove algae or moss. It pairs that with a reminder never to mix bleach and ammonia.

The windows and doors of the house or a lanai beside the deck add a caution of their own: 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. A mild bleach solution on a stone deck therefore stops short of any glass door or window beside it.

Tech Spec 5, ICPI's cleaning document for concrete pavers, lists no algae or moss among its stains; its bleach entries are for leaf, wood rot, tobacco and smoke stains. Bleach on a concrete paver or its sealer is taken up at whether bleach hurts pavers.

On sealing, ICPI says the sealer manufacturer or supplier is consulted before any sealer is used, to confirm it will perform in the setting planned for it. ICPI's line on silane and siloxane sealers near pool chlorine, and what pool chemicals do to a paver or its sealer, are answered at whether pool chemicals damage pavers or sealer.