What is the white residue I keep seeing on my concrete? Efflorescence!

We often get calls from clients who are worried about a white powdery substance often found on garage floors, concrete slabs and brickwork etc. wondering why they can’t seem to remove it and is it salt damp?

What is this mystery white powder?

When we are out on building inspections, we often see a white powdery residue on concrete slabs, retaining walls or stone walls and it is not rising damp; it is something called efflorescence. The great news is that efflorescence is a cosmetic concern rather than a structural concern, so its nothing to panic about.

Efflorescence is a white powdery residue of crystallised salts found on the surfaces of concrete or masonry. Without wanting to get too technical, it’s often made up of a variety of compounds such as calcium carbonates, hydroxides and chlorides. These compounds such as lime (calcium hydroxide) are often found in a soluble form within the concrete mixture.

Efflorescence may present as a thick layer of salts across a large area of the concrete or it may be found as randomly scattered white patches.

What causes efflorescence?

Keeping the chemistry behind the reaction simple, the primary cause of efflorescence is lime (chemical name – calcium hydroxide) or other salts which are held within the concrete, dissolving into water and then reacting with carbon dioxide in the atmosphere.

Efflorescence is most likely to occur in the first few months after the concrete has been poured; however, it can occur at any time when the concrete is exposed to alternative cycles of wetting and drying.

When the concrete is drying (curing) excess water in the mix gets saturated with lime and migrates to the surface. As the water evaporates, it leaves the lime behind which then reacts with the carbon dioxide in the air to form calcium carbonate, a new compound which is no longer water soluble and hence cannot be washed off! This type of primary efflorescence is usually visible within the first 3-6 months of the concrete curing.

Long after the concrete has cured, if moisture gets into the concrete it can cause efflorescence. Concrete and masonry products are porous and if left unsealed they can absorb water. Efflorescence is most prevalent when concrete or masonry is exposed to alternative cool, wet weather followed by warm, dry days which is often the case in South Australia.

As the concrete repeatedly gets wet and draws moisture in, the lime and salts which are part of the concrete can leach out into the water. When the weather warms up, the continual heat draws the water and salt solution to the surface of the concrete or masonry where the water evaporates leaving behind a white crystalline residue of salts.

A concrete slab in a garage or carport is often a key area for efflorescence to occur. Vehicles can bring in moisture on the tyres or rainwater may pool after dripping off the car. If the surface is unsealed, the water may sit for a while and slowly permeate into the pores within the concrete and start to dissolve the salts within it. When the weather changes, the sun dries the concrete out causing the water to evaporate, leaving the crystalline salts behind as efflorescence.

Alternatively, if the waterproofing membrane between the concrete and subsoil is breached, water can track into and through the concrete, leaching out the salts as it goes. Some retaining walls have noticeable efflorescent patches where the waterproof barrier between the retaining wall and the soils it retains has been punctured or pierced, and water is able to seep into the concrete and escape through to the front face of the retaining wall.

How can I remove Efflorescence?

If you get to the efflorescence as soon as it appears you have a better likelihood of removing it as the calcium hydroxide (lime) may not have fully reacted with the carbon dioxide in the atmosphere which then makes it insoluble.

In the first instance, brush the crystalline deposits off with a stiff brush or wire brush and if required use a small amount of clean water. Do not allow water to pool on the concrete surface after cleaning as you risk further efflorescence.

If the efflorescence doesn’t seem to be coming off, then we would recommend using an acid-based solution such as 1-part white vinegar to 3-parts water and brush and scrub the area.

Need a second opinion?

If you have any concerns about the house you are buying, get a second opinion. Let one of the experienced professionals at Homemasters inspect the home so you can be confident and knowledgeable about what you are buying. Your house is a significant investment – don’t leave anything to chance, be informed before you buy.

To find out more or book a home inspection call in the experts at Homemasters (08) 8326 8885

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