The Chemistry Of Brick Acid: How It Works & Its Applications

by Kate Onissiphorou

Brick acid is simply a solution of hydrogen chloride, otherwise known as hydrochloric acid. As the name suggests, it’s commonly used for removing dirt, limescale, cement, grease, and surface debris from bricks.

A highly corrosive and powerful cleaner, brick acid removes the sodium carbonate from walls through a neutralisation reaction between an acid and a base. Read on to learn more about how brick acid works, its applications, and how to use it correctly.

What is brick acid?

Brick acid is a highly-concentrated solution of hydrogen chloride in water. Most brick acids typically contain 36% hydrochloric acid as this tends to be the most effective concentration for cleaning brick walls or patios.

A brick wall

Less pure hydrogen chloride solution, otherwise known as muriatic acid, can also be used but it may not perform as well. Muriatic acid has a yellowish colour because of its iron impurities.

Brick acid is very effective at dissolving calcium carbonate on the surface of bricks and mortar. The chemical reaction of hydrochloric acid is sustained until the acid completely consumes a proportional amount of calcium carbonate. As shown below, the chemical reaction produces calcium chloride, carbon dioxide, and water:

CaCO3 + 2HCl → CaCl2 + CO2 + H2O

How to use brick acid

You should be very careful if you intend to use brick acid to treat mortar stains. If the process isn’t carried out correctly, it can actually produce stains that are more difficult to remove. For this reason, it’s usually best to avoid treating indoor or aesthetic brickwork with hydrochloric acid and muriatic acid. However, if you can’t remove the stain by washing it with detergent solutions or a pressurised water spray, you may want to try using brick acid. 

Before you start applying the brick acid, you’ll first need to treat any vanadium stains. These are water-soluble salts of vanadium from the bricks themselves that are left behind after the water has evaporated. You can easily remove vanadium stains with detergent solutions and a hard-bristled brush. Close up of person wearing gloves rubbing down a brick wall with a brush

Once these stains are gone, it’s time for the brick acid. Remember, brick acid is a hazardous substance so you’ll need to make sure you’re wearing protective clothing like an apron, a long-sleeved top, trousers, gloves, and goggles. You might also want to wear a mask or other form of face protection to avoid inhaling any dangerous fumes. 

How to clean brickwork step-by-step

You can follow the instructions below for cleaning brickwork:

  1. Start by removing all the mortar drags using a scraper
  2. Make sure non-brick areas like window frames and aluminium damp courses are protected against the brick acid
  3. Apply water to the brickwork until it’s saturated
  4. Measure the appropriate proportions as recommended by the manufacturer on the label. For example, for light-coloured bricks, use a 1:20 ratio between the cleaning agent and water
  5. You’ll need to prevent the bricks from being exposed to direct sunlight while applying the cleaning agent. You can use a tarpaulin sheet to provide shade in the area where you’re working
  6. When cleaning a brick wall or fence, always start from the top so the cleaning agent can drip down
  7. Focus on cleaning small areas at a time, such as one square metre. This means you’ll have plenty of time to wash off the cleaning solution without staining the other areas
  8. Let the cleaning solution react with the dirt for about three to six minutes before scrubbing
  9. Once you’ve finished scrubbing, wash the brickwork thoroughly to remove any traces of brick acid. When working on lightly-coloured bricks, use a bicarbonate solution to neutralise any residual acid.

How to apply brick acid

To apply brick acid effectively, it’s important to follow the recommended ratio of brick acid and water as stated on the container. This is crucial to prevent the bricks from becoming too brittle or stained. Typically, the recommended ratios are as follows:

  • Light-coloured bricks – 1:20 
  • Dark-coloured bricks – 1:10

These are general guidelines so do check the product label carefully before you begin.

How long do you leave brick acid on for?

You’ll usually need to leave brick acid on for three to six minutes before you start scrubbing the brickwork. Once you’ve finished scrubbing, wash the area thoroughly with water. You may also want to use a sodium bicarbonate solution to neutralise any remaining acid.

Does brick acid damage bricks?

As brick acid is highly corrosive, it can damage certain types of brickwork if it’s not used correctly. Brick acid effectively dissolves the top layer of brick, so it shouldn’t be used on natural stone surfaces, including stone like limestone and marble. 

Clay-baked bricks can also be damaged if the brick acid isn’t washed away properly. That’s because these types of bricks contain minerals that react with acid, thereby compromising the structural integrity of the bricks. Some of the minerals commonly found in clay-baked bricks include:

  • Quartz
  • Diopside
  • Wollastonite
  • Orthoclase
  • Minor anhydrite
  • Calcite
  • Illite
  • Smectite
  • Kaolinite

When these minerals react with hydrochloric acid, the bricks can become brittle. If multiple bricks become brittle, the brickwork itself may collapse. This is one of the reasons why it’s crucial to wash away all the acid and neutralise any residual cleaning agent.

What is the pH of brick acid?

The pH of brick acid varies depending on factors such as temperature and concentration. A highly-concentrated acidic solution of 36% can have a pH of below zero or a negative pH. 

Is brick acid dangerous to humans?

Brick acid is a strong acid, which means it’s highly corrosive even at low concentrations. It’s dangerous to handle because it can burn your skin and mucous membranes. The fumes are toxic and shouldn’t be inhaled. This is why it’s important to take the proper precautions and always wear the appropriate PPE. Brick acid must also be stored correctly as it can react with other materials.


Brick acid is a solution of hydrochloric acid and water that’s used to remove dirt and other surface debris from brickwork. The acid reacts with alkaline mortar and calcium carbonate, effectively stripping away the top layer of the surface on which it’s applied. Brick acid is a hazardous chemical that can cause severe injuries if it’s not used correctly. Its highly corrosive nature means it’s unsuitable for use on natural materials like limestone.


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