Question: A driveway I built recently with a reddish integral color has white splotchy patches that I'm told is efflorescence. What went wrong that this happened and how do I get rid of it?
Answer: You're probably right that this is efflorescence. Efflorescence happens when calcium hydroxide, a chemical produced in the hydration reaction of portland cement, is dissolved into water moving through the concrete and transported to the slab surface where it precipitates and then combines with carbon dioxide from the air to produce calcium carbonate (a white residue). The calcium hydroxide is initially water-soluble and can be removed, but once it forms a carbonate, it is more difficult to remove.
Preventing efflorescence under some conditions is probably impossible, and there is no additive or admixture that will prevent the migration of salts to the surface of concrete. Prolonged bleeding causes more serious efflorescence problems. In cold weather, accelerators or winter admixtures will greatly reduce the time needed for initial set, and there will be less bleeding.
The only effective preventative action is to use concrete with a low water/cement ratio to help control primary efflorescence (from bleed water). The use of pozzolan in the mix, such as fly ash, will consume some of the calcium hydroxide and may be somewhat beneficial for controlling secondary efflorescence (caused by water in the subbase migrating through the slab). Less water in the mix means less bleed water that can migrate to the surface, therefore use water-reducing admixtures, midrange water reducers, and superplasticizers to make placement easier. Mix designs with well-graded aggregates and enough cement paste to fill the voids between sand particles minimizes capillary diameters in the plastic concrete and should also help prevent water from soaking into the concrete then coming back out bringing calcium hydroxide to the surface. The resulting slab is more impermeable to water.
Placing an acrylic sealer (with a high water vapor transmission rate) shortly after initial set helps to prevent efflorescence by preventing calcium hydroxide from reaching the surface. Though water vapor can pass through the coating, liquid water cannot, so water on top of a decorative surface can't penetrate the slab. Read the manufacturer's label carefully, however. Some products caution against use before the concrete is several days old.
The best time to deal with the problem is before calcium hydroxide and carbon dioxide start to react. At this early stage of the reaction, the salts are water-soluble and can be washed off—usually during the first couple of days. If carbonation has already taken place, a mild wash with muriatic acid (1 part acid to 10 parts water) can reduce the problem and perhaps remove all the whiteness. Work it into the surface with a soft nylon brush, thoroughly flush the surface with clean water, and check the pH to make sure the acid residue is removed.
A plastic cover to protect the surface from rain causes uneven condensation of water on the surface, which results in serious efflorescence problems. If you are forced to use a cover, remove it as soon as possible and scrub the surface carefully with water.
There are few other options. If an owner will allow light sandblasting, all efflorescence can be removed, but the surface will be textured. Colored sealers can be used to cover over the problem, but this is like painting over concrete—the color of the concrete isn't visible until the sealer wears off.
The passage of time also reduces efflorescence on exterior surfaces. Rain, by absorbing small amounts of CO2 as it passes through the atmosphere, becomes a mild solution of carbonic acid, which will gradually remove efflorescence.