Why does efflorescence happen, and how can I minimize it?

What is efflorescence?

Efflorescence in masonry units is usually observed as a white deposit on the surface that can present as a slight haze all the way through to a crusty layer. Efflorescence is not a structural problem for the concrete, it does not affect strength or durability. But it is unsightly, and lowers the perceived value of the concrete. 

Efflorescence is usually composed of salts that are deposited on a concrete surface. It is the result of chemical reactions where calcium hydroxide (free lime salts) particles dissolve into water contained within voids  in the concrete matrix. Over time the water and dissolved lime salts migrate back to the concrete surface where the solution reacts with carbon dioxide in the air to form calcium carbonate. The water evaporates leaving the calcium carbonate residue on the surface as efflorescence.

It’s worth noting that any concrete product that contains cement will produce calcium hydroxide as a  by product of the cement hydration reaction. 

The four factors  

1. Source of soluble salts - Portland Cement
2. Sufficient water to dissolve the salts
3. Path for the salt solution to migrate to the surface
4. Driving force to move the salt solution through the pathway

 1. Source of soluble salts

Most efflorescence is due to calcium hydroxide or lime that is always present in hydrated cement based products. When water is added to Portland cement it hydrates and forms a calcium silicate hydrate (CSH) or “good” gel that is the glue that holds concrete together. Unfortunately the reaction is not very efficient and a by product of calcium hydroxide “bad” gel is also formed. Calcium hydroxide is soluble in water and may dissolve into water that is contained in voids in the concrete matrix.

Efflorescence can also result from other soluble salts, called alkalis, found in the aggregates.

2. Sufficient water to dissolve the salts

It is very important for concrete strength and durability that there is sufficient water available during mixing to hydrate the cement particles properly. Therefore, maximizing water during production is highly desirable. Adding as much water as possible short of pulling, picking or slumping during production will lead to products with higher strength and lower absorption. 

The concern over excessive water that will dissolve the salts comes into play once the concrete products reach the curing stage - in the kiln or out in the yard.

Units that are exposed to repeated cycles of wetting and drying in the yard, such as during a wet spring or fall, or after they have been installed, for example through landscaping irrigation, may also develop efflorescence.

3. Pathways for salt migration

All concrete products have a network of interconnected voids.  Connected voids serve as pathways for moisture migration into and out of the concrete unit. Voids can be minimized but not eliminated, even if you have the best mix design, materials, equipment and compaction possible. Because there are always voids, there will always be pathways for salt solutions to migrate to the surface. 

These pathways can be reduced by making the concrete more dense, and thus less able to absorb water.  ASTM C936 Standards for quality manufactured concrete products specify 5% maximum water absorption.  Producers can reduce absorption by optimizing concrete mix designs, including aggregate grading and shape so aggregates will pack together better, and using as much water during production as possible to help lubricate the mix and hydrate the cement. On the machine side, it is important to feed and fill molds properly to achieve a dense concrete matrix. 

4. Driving forces to move the salt solution

When two things occur in different conditions (for example one hot and one cold) the environment will try to find balance between the two situations and will react to reach the middle. This reaction is the driving force that causes the salt solution to move to the surface of the concrete. During curing and storage of concrete units, many common practices can cause a driving force to occur. These can include exposing units to differences in humidity and temperature between the units and their environment, such as might occur in kilns with very high humidity, or when units are moved from a heated kiln environment to a cold yard without allow the units time for their internal temperature to lower and match that of the outside environment, 

What does this mean to me?