Causes of Efflorescence on Stamped Overlay

These two samples come from the same bucket of overlay mix. Yet only
the one on the left shows efflorescence.

Question:
A few days after placing a stamped overlay, I noticed that the overlay was covered with efflorescence. The picture shows two small samples of the overlay taken from material left in the mixing bucket. The sample on the left side shows material that was exposed to the air, and the sample on the right shows material that was up against the side of the bucket. Why is the sample exposed to the air loaded with efflorescence while the other sample has no effloresce and is the proper color?

Answer:
This is a great example of how efflorescence works. It also shows just how many variables decorative concrete installers deal with on any given project.

Efflorescence is a natural phenomenon that occurs in any material that contains cement (lime) and comes in contact with water. The process involves natural salts migrating out of the concrete (or mortar, block, or brick), in a. chain reaction triggered by water and carbon dioxide. In an attempt to escape from the concrete, efflorescence will take the path of least resistance. The concrete’s surface profile, color, water-cement ratio, and porosity all play a big part in the process.

The sample on the left side of the picture has a rough, open surface and is very porous, which allows for easy water migration. The sample on the right shows the backside of the piece that was up against the bucket, which was made of non-permeable plastic. This sample also has a smoother, denser surface that inhibits water migration. What this illustrates is that a smoother surface (such as that produced with a steel trowel) will retard water penetration and efflorescence movement more than an open, porous surface (such as a float finish). In addition, materials that are darker in color will show efflorescence more readily than lighter-colored materials. In fact, I have seen decorative concrete and concrete overlays that have severe efflorescence but are light in color, so the efflorescence is barely noticeable. When it comes to the water-cement ratio, less water is always better. And never add water to the surface. Apply an evaporation retarder instead.

While you can never eliminate efflorescence, controlling the factors outlined above will help minimize its effects.