Dealing with Efflorescence on Masonry Surfaces
Efflorescence is a powdery looking substance that will sometimes appear on stone, concrete or tile surfaces. This shows up as irregular, whitish blotches and is especially frequent around joints and grout lines. All masonry is susceptible to efflorescence and it is an ugly and unwelcome flaw.
Unseen moisture is the problem.
Efflorescence happens when moisture becomes saturated with minerals and salt as it travels through a masonry material. When the moisture reaches the surface, it leaves behind the minerals as it evaporates. This is the unattractive whitish looking deposit on the finish.
I have seen this problem occur on both flat surfaces or vertical walls. And it can even happen when no obvious moisture is apparent. In fact, you may never see the moisture actually causing the problem because it is likely buried deep within the material.
“Travelling” is not really the right term to describe what the moisture is doing. Capillary action actually draws moisture to the surface with a very strong force. This movement happens because water tends to be attracted to, and migrate towards drier places. It also will “wick” vertically to great heights through small cracks or tubes. Capillary action is powerful enough that trees use it to pull moisture from the roots all the way up to the leaves. On some trees that’s a journey of perhaps 200-300’.
The best way to avoid this problem (as usual) is to prevent it from happening. You do this by using moisture barriers directly beneath any concrete or mortar base material. It can also help if you use materials with low salt or mineral content. Use low amounts of water in the concrete or mortar mixes and allow them to dry properly.
Also incorporate good drainage to eliminate water getting to the underside, backside or interior of concrete slabs or masonry walls. For exterior walls, drainage space and weep holes must be provided so that any water getting into the interior or backside of the masonry can escape out the bottom. Waterproof flashing or cap material should be installed along the top of any exposed walls to shed water off.
Repair is difficult
Efflorescence is very difficult to correct. It is often too late once it is noticed. In some rare cases, the moisture causing the problem will evaporate completely. If it does, some scrubbing with a stiff brush and muriatic acid will remove the spots. Unfortunately, the condition almost always returns. Dealing with the moisture is key to ensuring efflorescence will not recur.
To stop the problem the moisture source must be eliminated, and the masonry allowed to dry. Only then can the surface deposits be cleaned off permanently.