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Muriatic Acid and Cleaning Masonry Surfaces

This article has two parts.  The first part, Getting to know muriatic acid, deals with the properties of muriatic acid, how it works and suggestions for safer, alternative cleaning methods.  The second part, Special Masonry Cleaning Situations gives a few more specific examples, such as efflorescence removal and paint preparation.

Do-it-yourselfer's should avoid muriatic acid where possible.  Instead, try the chemical or mechanical cleaning alternatives suggested below.  However, I am a realist and understand that some of you will need muriatic acid's special strength.  If you consider using muriatic acid, please heed all safety recommendations both here and on the product's label.

Getting to know muriatic acid...

What is muriatic acid and how dangerous is it?

Muriatic acid as poison

Muriatic acid is a highly reactive liquid acid, and one of the MOST DANGEROUS CHEMICALS you can buy for home use. It is an industrial-strength solution of hydrogen chloride gas dissolved in water, also known as hydrochloric acid. Yep, muriatic acid is "super stomach acid"!

With the exception of some plastics, muriatic acid can damage most anything it touches, including clothing, metal, and skin! It emits a suffocating odor that can quickly burn the lining of the nose, throat and even the lungs.

Typical home uses include heavy-duty masonry cleaning, preparation of masonry for painting or sealing, removal of efflorescence or mineral deposits and pH reduction in swimming pools.  Its reactive power makes it the chemical of choice for some types of masonry cleaning.

Muriatic acid is sold in a standardized concentration of 31.45% acid and 68.55% inert ingredients, primarily water.  This is the concentration you are going to find in your local hardware store.  Our mixing suggestions are based on this concentration... if the muriatic acid you purchase is stronger, adjust the dilution proportions for the job accordingly.

A short anecdote... A hardware store in my area stopped storing muriatic acid. Over a period of years, gaseous seepage from the old containers had begun to dissolve the metal shelving it was stored upon, as well as the metal containers of other products nearby!

Fortunately, most muriatic acid sold now is in plastic bottles with safety seals to prevent leakage.  And the moral of this story?  It is wise to dispose of leftover acid properly and immediately. (See disposal tips at the end of this article.)

How does muriatic acid clean masonry and affect paint adhesion?

A masonry surface becomes rough, or "etched", when it reacts to strong acids.  When the powder residue of the reaction (calcium carbonate) is rinsed off, you are left with a very clean surface.

Masonry tends to be alkaline, which prevents proper adhesion of many coatings and paint products.  Acid washing neutralizes this alkalinity, leaving a coating-friendly surface. (Note:  Before coating masonry check the coating's label for acceptable surface alkalinity.  pH Test kits are available for precise measurements.)

Are there safer chemical alternatives to muriatic acid?

Muriatic acid is not the first choice for masonry cleaning but the last resort.  Do not use this dangerous chemical unless you are sure you have no other choice.

For example, the most popular concrete and grout cleaners on the market contain phosphoric acid.  This acid will, under most circumstances, do as good a job as muriatic acid... but with less danger.  Phosphoric acid cleaners also contain chemicals which emulsify oils to help the acid work more effectively and safely increase its cleaning properties.

Corrosive chemical warning symbol

TSP (trisodium phosphate) is another formidable heavy-duty cleaning product and can be used to clean masonry surfaces that are going to be left uncoated.  TSP does not etch or neutralize the surface alkalinity, so surface testing for pH should be done before coating TSP-cleaned masonry.  Due to environmental concerns of phosphate pollution, TSP cannot be legally used in some areas.

DO NOT MIX TSP WITH ANY ACID!!  A violent reaction can occur and the release of noxious gas.  You can use both products, but they must be used separately with a thorough rinsing with water between applications.

Can masonry be cleaned without chemicals?

Warning - corrosive material

Mechanical cleaning methods such as sandblasting, abrasive wheels and special power tools for cleaning mortar lines in brick and concrete block may be preferable to muriatic acid.  These can be rented at many rental centers or hardware stores, where you will also receive instruction in their use.

Can muriatic acid be used safely?

Only with proper preparation!  Muriatic acid must be used with EXTREME CAUTION!!  I cannot emphasize this enough. Contact with the eyes, for example, can cause irreversible damage and permanent blindness. Contact with the skin even for a few moments can cause severe burns. Got it?

1) Dress appropriately.

Don't wear your "Sunday-go-to-meeting" clothes. Gauntlet-style acid-resistant gloves are a must, as is eye and/or full-face protection.  A NIOSH-approved respirator equipped with the appropriate acid-grade filter should be worn.  Many hardware and paint stores sell vinyl-coated coveralls that offer some acid resistance.  Rubber boots are also recommended.

2)  Have a neutralizing agent and a reliable, steady source of water available

Baking soda or garden lime can quickly neutralize the acid if spilled. Water should be freely available in case you accidentally get acid on your skin.   (More information on neutralizing muriatic acid at the end of this article.)

Since muriatic acid can damage or kill foliage, cover or wet all nearby foliage with water before application of the acid.  If the foliage has been accidentally sprayed with acid, a neutralizing mix of lime and water can be applied to the plant and/or soil.  Be careful and check with a garden pro if necessary before doing any acid treatments or applying lime around sensitive plants!

3) You must have adequate ventilation...  Use a fan to bring fresh air to the work area if necessary. Muriatic acid is nonflammable, but the vapors are highly corrosive and irritating.  Also, there is always the chance of a chemical reaction that will produce hydrogen gas, which is extremely flammable.  Remember the tragic explosion of the hydrogen-filled zeppelin Hindenburg?

4) Using muriatic acid indoors is not recommended, since the corrosive vapors can begin chemical reactions in metals that are difficult to stop, leading to long-term, permanent damage in both appliances and electronics!

5) Accident cleanups... what if you spill muriatic acid?  Scroll to the end of this article on cleanup and disposal recommendations.

How is muriatic acid mixed and applied?

Muriatic acid is always diluted before use. The standard dilution for most applications is one part muriatic acid to ten parts water. Be careful when mixing to avoid splashing the acid. Do not mix in a paper, ceramic or metal bucket... use a plastic bucket.  Glass containers are also acceptable for measuring and mixing.

IMPORTANT: Always add acid to water... never add water to acid!!

To quote a knowledgeable reader, Tommy from Corsicana, Texas:

"When water is added to muriatic acid, an exothermic (heat is given off) reaction occurs.  This is often accompanied by a violent "belch" which propels the acid mixture out of the container and onto the person making the dilution!  This occurs because the heat generated by the reaction is under the cooler water and causing it to expand rapidly.  When the acid is poured slowly into the water, the cooler water layer is on bottom, so the heat generated is dissipated upwards at a slow speed."

Also, never pour acid into an empty container.  It can cause dangerous splashing.

Do not mix muriatic acid with any other chemicals. Unexpected violent reactions can occur which can spew acid everywhere... possibly on you!

Muriatic acid can be applied with a brush or sprayed, depending on the circumstances.  If used outside, do not spray acid if there is any significant wind. Any foliage near where you are working should be thoroughly dampened before applying the acid.  Since using a brush causes the acid to spatter, full-body protection is essential!

Do not use your best steel garden sprayer with any acid or you may destroy it. Purchase a cheap plastic sprayer for the job to throw away when you are finished. You may have less than an hour before the acid renders the sprayer useless, so plan to work quickly. Spray a little plain water through the sprayer to adjust the spray pattern before adding the acid..


Removing efflorescence deposits on masonry walls...

Efflorescence on concrete foundation below grade

 Efflorescence is a fancy name for white, crystalline deposits that can appear on masonry or concrete. These deposits occur when moisture within the masonry rises to the surface, carrying minerals with it. The moisture evaporates, leaving the minerals behind. This often occurs with new masonry work and usually stops when the masonry has thoroughly dried.

However, if there is a source of moisture pushing through the masonry such as a damp below-grade foundation, the efflorescence will continue building up. I've seen some basement walls that looked like the Carlsbad Caverns!!  The efflorescence on this wall (graphic left) is so thick (the white areas) that it falls off the wall like dandruff... and is piled on the foundation ledge like snow!

Severe, persistent efflorescence is not harmful, just a visible symptom of excessive moisture in the cement. Excessive moisture is the cause of a myriad of problems in the home, from stinky moldy basements to insect problems.

Today, there are various cement and non-cement-based products that can be used to seal foundation walls from the inside to reduce or eliminate efflorescence. However, efflorescence must be removed first for these products to stick properly.

Mother Nature does a great job on outdoor masonry via acidic rain, as efflorescence is rarely seen outdoors... but it takes years for a cement surface to be neutralized "naturally".  For indoor work, efflorescence can be removed with plain water and/or with TSP and a very stiff bristle brush.  A phosphoric acid masonry cleaner is also acceptable for indoor use. As mentioned earlier, don't mix TSP and acid!

If you plan on coating or painting the surface, consider using a phosphoric acid masonry cleaner before resorting to using muriatic acid!  Follow the dilution recommendations on the product label or at the manufacturer's website.

If you must use muriatic acid to remove efflorescence, follow these steps (be sure to follow the safety recommendations earlier in this article):

  1. Dampen the wall.
  2. Mix the acid with water. 1 part acid to 10 parts water (by volume) is typical, but dilutions as light as 1 part acid to 16 parts water work well, too.  (1-16 is easier to measure... that's 1 cup acid to 1 gallon of water.)  Read the label on the product you buy and follow the recommendations, if any.  Remember... add the acid to the water, not water to acid!
  3. Brush or spray acid onto affected area. Do not use a metal sprayer.  A plastic sprayer will work for a while, but will eventually be destroyed by the acid.  Have a few extras nearby and throw away used sprayers when finished.
  4. Let the acid sit for no more than a few minutes, less if you can see the efflorescence lifting.
  5. Scrub off any remaining residue with a stiff brush while rinsing thoroughly with water. There are long handled masonry brushes ideal for this job.  To neutralize any remaining acid, you can spray a neutralizing rinse of one (1) cup household ammonia to one (1) gallon of water.

This is a nasty, sloppy and potentially dangerous job, so understand what you are getting into before you start this one!  And I do not recommend using muriatic acid indoors except in extreme circumstances and, hopefully, by experienced professionals who know how to deal with the dangerous fumes!

Etching masonry for paint and patching preparation

"Etching" is a process where a chemical is applied to a surface to degrade it slightly. Etching is used to make circuit boards, where a thin copper sheet is selectively dissolved away to form a path for electricity. In masonry work, etching is a way to chemically clean and prepare masonry for repair or painting.

Actually, the process of efflorescence removal discussed above also etches the masonry. Etching roughens the surface of the masonry, allowing better adhesion and absorption of both paints and patching materials.

Use the same procedure and cautions for etching as described above for efflorescence.

Removing mortar stains from masonry and/or ceramic tile

Muriatic acid can quickly remove mortar stains from masonry. Again, phosphoric acid masonry cleaner may be adequate if the staining is not too severe.  As any tile professional will tell you, phosphoric acid is routinely used for removal of grout residue from ceramic tile and stone.

Leave the acid in contact with the masonry for a minimal amount of time, in some cases just ten or fifteen seconds! New stains will release almost instantly. Quickly hose off all acid and reapply only as necessary until staining disappears.  When satisfied, rinse thoroughly and then use a neutralizing solution of one cup household ammonia to one gallon of water and then do a final thorough rinse with water.

How to safely dispose of muriatic acid or clean up muriatic acid spills...

My appreciation Bert Tisher for suggesting I add muriatic acid clean-up and disposal information in this article.  It was a huge oversight!  Again, an observant reader comes to my rescue!!

Muriatic acid should NEVER be poured down a storm drain, a sink or flushed down a toilet.  It can cause extreme damage to pipes, dissolve solder and damage the biological balance of your septic system.  Throwing away even a closed container of muriatic acid with the trash can be dangerous for trash handlers, their trucks and possibly cause unexpected chemical reactions in landfills.

Small quantities of spilled muriatic acid will not cause widespread environmental disaster, but it can cause severe damage to plants and animals that may come into contact with it.  It's easy to neutralize a muriatic acid spill common household and/or garden chemicals.

Here are some suggestions...

1) Recycle it!

Many counties or cities have drop sites for recycling hazardous chemicals such as oil-based paints and other household chemicals.  Most will also accept muriatic acid.  Call your local recycling center for more info.

2) Neutralize it!

Earlier, I mentioned using lime (the type used on lawns and gardens) to neutralize acid spills.  Spreading a generous quantity of lime (the powdered or crushed type used for lawn or gardens) or baking soda and adding water will cause a distinctive "fizz" as the lime reacts with the acid to produce a harmless salt, water and release carbon dioxide.  I prefer garden lime over baking soda since it is less expensive, is sold in larger bags and most gardeners have some laying around!

You can also use lime to neutralize leftover muriatic acid.  Get a large bucket.  I prefer 5-gallon size dangerous since the chance of dangerous spattering is minimized in a large bucket.  Put three of four cups of lime in the bottom and a gallon of water.  Give it a stir with a long disposable wooden stirrer (an old 1x2 is fine).  Slowly add the acid to the bucket keeping your face away while pouring (and wearing your respirator).  Stir, adding more acid and more lime until all chemical "fizzing" has stopped.  The fully neutralized acid can now be safely disposed down a sink or storm drain without fear of damage to your septic system or the environment. 

NOTE:  Remember... always add acid to water, not water to acid!!!!

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