What is Lime Bloom and How Can You Remove it?

If you are completing an external wall insulation project, you will inevitably use some sort of topcoat. Our most popular solution is silicone render which produces a weatherproof envelope for your home. Whatever the chosen topcoat, it must be a flawless finish. However, lime bloom can blight your efforts and leave unsightly streaks on your façade. A façade without streaks is not just to satisfy a customer’s vision, but a point of personal pride in your work. As such, you must be able to recognise, understand, and be able to rectify it when it occurs. Today’s blog will hopefully provide the information to allow you to do so. It is important to note that lime bloom is not limited to rendered surfaces. It crops up on exposed bricks and also other substrates.

What is lime bloom?

“Lime bloom” or “efflorescence” is a commonly encountered problem with concrete and brick structures. It is the result of a chemical reaction involving various salts present within the material.

Concrete and brick are made using calcium hydroxide (also known as lime), among other materials. When these calcium hydroxide-containing materials are exposed to water – either from rain, groundwater, condensation, or even from the original construction process – the water can dissolve some of the calcium hydroxides, creating a saturated solution that can migrate towards the surface.

When this solution comes to the surface and evaporates, it leaves behind a deposit of calcium carbonate, also known as lime bloom.

Ca(OH)2 + CO2 → CaCO3 + H2O

Calcium hydroxide in concrete or brick reacts with carbon dioxide in the air to produce calcium carbonate and water. This reaction is an example of carbonation.

It is worth noting that although lime bloom can be unsightly, especially on coloured or textured surfaces, it doesn’t usually cause any significant structural damage. The white, chalky substance is mostly a cosmetic issue. There are various methods to remove or reduce it, like using acid-based cleaners or pressure washing, though these can sometimes damage the surface as well.

Factors that affect lime bloom

Much like the application of basecoats and topcoats, lime bloom is influenced by several factors.

Environmental conditions

Humidity: High levels of humidity can enhance the solubility of calcium hydroxide in the construction material and its migration to the surface. The moisture in the air can also facilitate the reaction with carbon dioxide, leading to a more extensive lime bloom.

Temperature: Higher temperatures increase the rate of chemical reactions, including the reaction between calcium hydroxide and carbon dioxide. On the other hand, warmer conditions also lead to faster evaporation, which may limit the amount of water available to dissolve calcium hydroxide.

Carbon dioxide levels: The availability of carbon dioxide is essential for the carbonation reaction that leads to lime bloom. Higher levels of carbon dioxide in the air will enhance the rate and extent of lime bloom.

Composition of the material

Calcium hydroxide content: Different types of cement and mortar can have varying amounts of calcium hydroxide. Materials with higher levels will be more susceptible to lime bloom.

Soluble salts: The presence of other soluble salts in the construction materials can contribute to efflorescence, a similar phenomenon to lime bloom. These salts can dissolve in the water, migrate to the surface, and then crystallise as the water evaporates.

Construction practices

Curing process: The curing process of cement is crucial to limit lime bloom. Adequate curing allows for complete hydration of the cement, which reduces the amount of free calcium hydroxide that can react to form lime bloom.

Water content: Using too much water in the cement mixture or exposing the material to excessive water during construction can lead to increased dissolution and migration of calcium hydroxide.

Quality of the sealants and coatings: Quality sealants and coatings can limit the penetration of water into the construction material. Thereby reducing the dissolution of calcium hydroxide. They can also act as a barrier to carbon dioxide, limiting the carbonation reaction.

Exposure to weather and time

Over time, with consistent exposure to weather and environmental conditions, more lime can leach out from the concrete or brick, leading to a more significant lime bloom. This is why lime bloom is often more prevalent in older structures.

How to recognise it

Recognising lime bloom involves a visual inspection of the surface of the material, typically concrete or brickwork. Lime bloom is a white, powdery, or crystalline deposit on the surface. It can sometimes have a fluffy or chalky texture. Lime bloom is most commonly seen on the surface of concrete, brickwork, or mortar. It’s often more prevalent in areas that are exposed to weather and moisture, such as exterior walls, driveways, patios, and sidewalks.

Lime bloom often appears in a patchy pattern and is unevenly distributed. It may also follow the lines of the mortar in brickwork. This is often where water can penetrate the structure most easily. Lime bloom becomes more pronounced after rain or in periods of high humidity, as these conditions promote the migration of calcium hydroxide to the surface and its subsequent reaction with carbon dioxide.

If you touch the wall, it typically feels gritty or powdery and may come off onto your hand. It can often be brushed off the surface, although this won’t necessarily remove it entirely, as the underlying issue may still be present.

How to remove it

Removing lime bloom requires careful treatment so as not to damage the underlying material.

1. Dry brushing: The simplest method is to use a stiff brush to scrub off the bloom. This may work for minor cases, but can also be the first step before other treatments for more severe cases. Be aware that some powdery residue may remain after brushing, and the lime bloom may return if the underlying cause is not addressed.
2. Water washing: After brushing, you can wash the area with water. Use a low-pressure water spray, as high-pressure washing can drive the water deeper into the material and potentially make the problem worse in the future.
3. Mild acid washing: If the lime bloom is persistent, a solution of mild acid and water can be used. The acid can react with the calcium carbonate to dissolve it. Vinegar (acetic acid) or a solution of citric acid can be used. However, always test a small, hidden area first to ensure the acid does not discolour or damage the material.
4. Commercial efflorescence removers: There are also commercial products available that are designed to remove lime bloom or efflorescence. These are usually acid-based and should be used according to the manufacturer’s instructions. Be sure to rinse thoroughly after use to remove all of the acid.
5. Professional cleaning: Professionals have access to specialised products and equipment and are trained to deal with these kinds of issues without causing damage.

It’s important to note that these methods remove the visible signs of lime bloom, but they do not address the underlying cause. If the structure continues to be exposed to moisture, the lime bloom is likely to recur. Therefore, prevention methods are crucial. These include improved drainage, the use of water-resistant sealants, or changes in landscaping to divert water away from the structure.