Efflorescence appears as a white deposit of mineral salts on face of concrete or plaster walls and/or floors.

It is caused by moisture moving through the concrete or plaster.   As the moisture enters and moves through the wall or floor, it dissolves mineral salts present in the cement.  This mineral salt solution eventually finds its way to the surface of the concrete or plaster.  As the water evaporates, it leaves behind a white deposit of mineral salts on the surface.

How Serious Is The Efflorescence Problem?

If efflorescence is present, then this is evidence that moisture ingress is occurring from external sources, and therein lays a serious problem. The efflorescence will progressively worsen with time.  

In severe cases, the salt solution can travel down the face of the wall, leaving a trail of mineral salts.

Other problems are discoloration of the concrete or plaster, the proliferation of mould, moss, mildew and other organic growth, persistent musty smells and continual dampness.  These can adversely affect the health of occupants.

But the problem is not limited to occupational health and safety – it is also structural.

Moisture moving through the wall or floor will eventually wash out all the soluble salts and cause the breakdown of the cement matrix, leaving the concrete or plaster weak and sandy.

In addition the alkalinity of the concrete will drop, and with it, the ability of the concrete to protect the embedded reinforcing steel from corrosion.  Without protection, steel rapidly corrodes in the presence of moisture, ions and oxygen. The corrosion of reinforcing steel in concrete results in spalling (aka “concrete cancer”), and is difficult and expensive to rectify.

Can Efflorescence Be Painted Over?

The short answer is no.  Any efflorescence on the surface will provide a poor, friable base for coatings, preventing effective adhesion from occurring.  Even if all the efflorescence is thoroughly removed, moisture will still be present in the concrete.  Where there is moisture in concrete, there is hydrostatic pressure. Hydrostatic pressure will force the moisture out of the concrete and collect between surface of the concrete and the coating system, forming blisters under the coating and causing the coating to delaminate

How Can I Fix Efflorescence?

Efflorescence will stop forming when the movement of moisture through the concrete stops. Existing efflorescence can be removed by a number of options.  But before you start painting, here is a suggested procedure.

Remove all efflorescence, other surface contaminants and loose material.  Any one of a number of techniques may be used, including, but not limited to, wire brushing (hand or power tool), dry abrasive blasting or high-pressure water jet. (The use of water will reintroduce some moisture into the concrete and can cause some reappearance of efflorescence.  Allow concrete to dry, and wire brush any residual efflorescence.)  Check for any reoccurring efflorescence, as this may indicate a leakage problem.

Check moisture content of concrete. This should be <10%, as measured with a standard moisture meter. If so, the concrete is ready to coat. If not, then the concrete or plaster needs further drying or may need further waterproofing measures.

Once the concrete / plaster is dry and the moisture source that caused the efflorescence has been permanently fixed then the surfaces can be prepared and repainted.