Efflorescence

The Interlocking Concrete Pavement Institute (ICPI) explains efflorescence as cement chemistry, one step feeding the next.

First, cement hydrates as it hardens, and ICPI says that reaction forms “a significant amount of calcium hydroxide.” Second, water dissolves that calcium hydroxide and carries it by capillary action toward the surface; Tech Spec 6 describes water entering the microscopic capillaries in the pavers and bringing it up. Third, at the surface it reacts with carbon dioxide from the air and becomes calcium carbonate. Fourth, as the water evaporates, what is left behind reads as a white haze. The glossary names calcium carbonate as the most common type, and the same compound turns up in stone and in hard-water scale, which the calcium carbonate page keeps apart by body and by surface.

The water step is why weather matters. ICPI adds that when pavers go in during a dry part of the year, it can sometimes take the next wet spell before any efflorescence shows.

ICPI is direct about what efflorescence does and does not say about a paver.

Its glossary puts it in one sentence: “The presence of efflorescence does not compromise the structural integrity and is not indicative of a flawed product.” Tech Spec 6, the institute's maintenance guide, frames it the same way, as an aesthetic concern rather than a structural one.

When it shows up, how long it stays and whether it leaves by itself are ICPI timing lines with “may” in every clause, and they are read in full on whether efflorescence goes away on its own. A milky film inside a sealer is a different condition with its own name, blushing.

One is the haze a homeowner sees; the other is a line in the sealing procedure.

The first is the haze itself, on a concrete driveway, walkway or patio after wet weather has moved water through the pavers. Whether a particular white deposit is efflorescence is a question for the paver supplier, the party ICPI names, along with the chemical company, for the dilution and application of a remover. How the trade takes it off with a buffered proprietary cleaner is set out on the paver cleaning page, under what a paver cleaning removes.

The second is the sealing procedure. ICPI lists “a source of efflorescence under the pavers” among the conditions under which a sealer may whiten, which is why whether pavers with efflorescence can be sealed is its own question, and why freshly moved soil comes up on whether pavers should be sealed before or after new landscaping.

A white powder on concrete roof tile is a roof question, not a paver one, and it is taken up on what the white powder on roof tiles is.