Use acid to remove excess mortar

Dear Tim: How can I remove brick mortar from the sidewalk I just built with paver bricks? My husband and I got the mortar smeared on the brick, and it looks terrible. What methods can I use so the brick is not damaged? Is there a nontoxic way to do this repair? – Sarah H., Kingsport, Tenn.

Dear Sarah: Smeared mortar on brick is a very common problem. Depending upon the type of brick, the job can be simple or a nightmare. I hope you don’t have a deeply grooved wire-cut brick, as these have many small crevices that can make cleaning a career choice.

Because you are concerned about the toxicity of different options, you may find it very hard to do this job. There are some pretty aggressive acids you can use, and the ones that work faster tend to be more toxic. But don’t panic: You may have no difficulty using an aggressive acid treatment if you take care to control the amount of liquid you use as you work.

Bricklayers typically use muriatic acid to dissolve mortar from most brick. This is simply a form of hydrochloric acid. When you buy this product you may notice a skull and crossbones on the label, warning you that it is a powerful chemical. But these products can be neutralized. That is something all of us should have discovered in our high-school chemistry class. In fact, the mortar neutralizes the acid as it works.

There are other acids that will react and dissolve the alkaline chemicals in the mortar. Even the white vinegar found in your kitchen will work to some degree. Because you eat vinegar in many foods, you know it is not a toxic acid. The challenge for you is to find an acid somewhere between vinegar and hydrochloric acid that will efficiently remove the mortar buildup. Consider calling a chemistry instructor at a local college or high school. You can also visit a building supply company and see if they stock a nontoxic acid that will dissolve the mortar.

The first step in the process is to remove as much of the mortar as possible with a scraper or chisel without scratching the brick face. Clear water flowing over the brick acts as an effective lubricant that will minimize damage to the brick. If you have to tap the scraper with a hammer, do so at a low angle with the face of the scraper nearly parallel with the face of the brick. You are trying to get the mortar film to a thickness less than that of a plastic credit card.

Once the majority of the mortar is off the brick, you then should consider bathing the brick with a diluted acid solution. As mentioned above, muriatic acid is highly effective, and it works on most brick. Usually you mix 1 part of the acid with 10 parts of water. Always read the instructions on the acid label, and pay particular attention to all safety instructions.

It also is worth the effort to contact the brick’s manufacturer. Some brick can be damaged and discolored by acids.

I have found in most cases you should dampen the brick with clear water before you apply the acid solution. Always try this in an out-of-the-way area, if you are unsure if the acid will harm the brick. If you use muriatic acid, you should see tiny bubbles forming where the solution contacts the smeared mortar. This tells you that the acid is reacting with the mortar paste and is starting to dissolve it into the liquid solution.

Use a scrub brush to help remove the mortar. Wait five or 10 minutes after the acid solution has been applied to the brick before you scrub. Allow the acid to do much of the work for you. It can take multiple applications of acid solution to remove all of the mortar. Rinse the acid solution from the brick with massive amounts of water. The more water you use, the better.