Efflorescence is a powdery deposit, usually white in color, that appears on the slab surface.
Efflorescence occurs when water in the concrete transports minerals up through the porous slab, leaving a mineral deposit on the slab surface. Efflorescence can cause paint and stucco to blister. The white spots created by efflorescence can create an unattractive appearance, resulting in dissatisfied homeowners. Homeowners may mistake the white spots for mold and call the service department complaining of mold growth on the slab.
Concrete naturally contains compounds of sodium, potassium, calcium, magnesium, and aluminum. Water in the slab dissolves these compounds, creating a salt solution inside the slab. Through capillary action, this solution rises to the surface of the slab, where it evaporates. As the moisture evaporates, the minerals precipitate out onto the surface and leave stains on the slab. The majority of efflorescence found on concrete is calcium carbonate.
Efflorescence during the curing process
Efflorescence can occur while the slab is curing. The slab cures through a chemical reaction between the water and cement. During this chemical reaction, a compound called calcium hydroxide is released. As the slab cures, the calcium hydroxide moves up, along with water, to the surface. Then, as the water evaporates on the surface, the calcium hydroxide is released and instantly reacts with carbon dioxide in the air to form calcium carbonate, which stains the slab surface.
Efflorescence after the curing process
Efflorescence more often takes place once the slab has cured. If the slab wicks up groundwater, the water can move upward through the porous slab, bringing calcium hydroxide with it. When the calcium hydroxide reaches the slab surface, it reacts with carbon dioxide in the air to form calcium carbonate, which stains the slab surface.
Cause 1:
Lack of gravel layer and vapor barrier
If the foundation crew doesn't install a layer of gravel and a vapor barrier in the foundation area, moisture will rise through the slab. The moisture will bring calcium hydroxide with it. Once the calcium hydroxide reaches the surface, it will react with the air to form calcium carbonate deposits on the slab.
Solution
Install a layer of gravel over the region the concrete will cover. The gravel layer will prevent bulk water from reaching the underside of the slab. Also, install a 10 mil vapor barrier over the gravel layer. The vapor barrier will prevent water vapor from reaching the underside of the slab.
Cause 2:
Excess water in concrete mix
If the foundation crew adds excess water to the mix to make it easier to work with, the excess water can move upward through the porous slab via capillary action and lead to efflorescence.
Solution
To solve this issue, add only enough water to the concrete mix to ensure complete curing.
Cause 3:
Reduced drying time
During the curing process, hydration between the cement and water produces calcium hydrate. As long as the calcium hydrate remains within the slab, it doesn't cause efflorescence. As the water in the concrete rises through the slab to evaporate on the surface, it brings calcium hydrate with it. The calcium hydrate reacts with carbon dioxide in the air, causing efflorescence. The quicker the water in the slab evaporates, the more calcium hydroxide will reach the slab surface.
Solution
Slow the evaporation of water from the slab by employing proper curing methods. Use mists, curing blankets, polyethylene sheeting, or curing compounds to slow down curing. More of the calcium hydroxide will be released inside the slab, rather than on the slab surface.
