I'm contemplating purchasing beach adjacent cottages in Hawaii that have noticeable efflorescence (it is powdery and thick, in some places actually "standing" 1/2" or so high, like cotton fibre). The cottages have bare concrete floors. The floors show some sign of water staining along outside facing walls. The buildings are not new but were substantially remodeled approx. 2 years ago. How do I know if this is "typical" or more significant which may lead to more problems in the future? I'm a bit perplexed as all the online info. I read say it's not unusual and nothing seems to say that it can cause damage. The current owners say they simply vacuum up the powder and are not aware that it is causing any problems.
Normally, Efflorescence by itself does not mean that the concrete is damaged.
Normal efflorescence is made of dissolved salts from the concrete reacting at the surface and crystallizing there.
Efflorescence is considered a problem when:
1. It contains sulfates. This is an indication that the concrete is exposed to potential attack. Unless the concrete was designed to withstand sulfates, it could be a long term problem leading to deterioration and failure. It is simple enough to collect a sample of the powder and have it analyzed for sulfates by a testing laboratory.
2. It is excessive. When the quantities of 'leached' salts is very large the concrete may actually lose some mass and strength, while becoming more permeable. This is highly unusual, but your description does sound like a 'lot' of crystals. If you clean this buildup (sweep it) and it comes back within a week - I would consider that a potential problem.
3. The concrete transmits excessive amounts of moisture. This would be a concern in residential construction where interior flooring or belongings can be damaged. If you are dealing with beach cottages that are not fully enclosed in Hawaii, this is probably not an issue.
I would not be too concerned about 'staining' of bare concrete. If the surface starts spalling or dusting it is an indication that the concrete is undergoing significant internal changes which may get worse over time.
To put these things in perspective - even if you have one of these problems, the concrete slab will probably outlast the rest of the structure by many years. It may not look so pretty, but it will be structurally sound and perform its role as slab.
You need to understand that you will not be able to install any type of flooring without eliminating the efflorescence (or the moisture transmission) first.
If you want to minimize the buildup of efflorescence you will need to apply some type of sealant to the surface of the slab. Since it is a bare concrete floor the cost should be minimal. If you decide to do that make sure that the product you choose is recommended by the manufacturer for such application to wet concrete and follow all the instruction. It is very important to NOT leave untreated areas in the slab because the moisture will concentrate there.
Hope that helped.