Concrete and Acid (a damaging combination)

The nature of concrete-based materials dictates the risk of contact with acidic solutions. In this “white paper” on the subject, industry educators, Paolo Benedetti and Jon Temple offer a bold reasoning for turning away from long-practices that expose plaster to acidic water.

By Paolo Benedetti & Jon Temple

Understanding the basic chemical relationship between fresh concrete and pH is essential for those who plaster swimming pools and fill them with water. Specifically, the nature of acidic water and the highly soluble composition of concrete and plaster is highly problematic.

Despite the well-known damaging effects of acidic water and fresh concrete, acid-based start-ups and acid washing for the purpose of exposing aggregate remain common practice. The authors believe that the use of acid or “hot” startups, and acidic water used in exposing aggregate in pebble-finish applications, are highly detrimental for newly placed cementitious materials, be it plaster, pebble or polished aggregate surfaces.

It is our position that those practices should be abandoned in favor of more beneficial procedures.

The reasoning here all comes down to well-known and indisputable facts. Freshly placed concrete has a high pH of 12.5-13. That high pH originates from the alkaline cement binder that creates concrete. The dissolved cement carries its high pH throughout the matrix of the concrete. The same chemistry occurs within pool plaster, which is also cement based.

ESSENTIAL CHEMISTRY

Fresh concrete’s high pH makes it more susceptible to damage in response to chemical exposure because it consists of highly soluble compounds such as calcium hydroxide. It is so sensitive to acidic conditions, that it even reacts with the carbon dioxide in the air, a process called carbonization. Through this process, concrete/plaster pH decreases as the soluble compounds are transformed to more stable chemical species as a result of the carbonization process.

Writing for aboutcivil.org, civil engineer and researcher Haseeb Jamal explains: “If pH decreases to values lower than stability limits of cement hydrates, then the corresponding hydrate loses calcium and decomposes to amorphous hydrogel. The final reaction products of acid attack are the corresponding calcium salts of the acid as well as hydrogels of silicium, aluminum, and ferric oxides. When acid attack concrete it dissolves both hydrated and un-hydrated cement compounds as well as calcareous aggregates. In many of the cases the chemical reaction results in water soluble calcium compounds which are leached away.”

In specific terms, concrete vulnerability to acid attack increases as the pH of the acid in contact decreases from 6.5. Degree of aggression is “slight” for pH: 6.5 to 5.5, “severe” for pH: 5.5to 4.5 and “very severe” for pH less than 4.5.

It’s important to keep in mind that pH of concrete is itself a moving target. According to Yousuf Sumra et al. in an article for the “Journal of Wuhan University of Technology,” The pH of Cement-Bassed Materials: A Review: “The pH of concrete does not remain constant due to aging and other defect-causing factors, such a chloride ingress, alkali leaching, carbonation, corrosion, acid attach, moisture and biodegradation process. Reducing the concrete pH has negative impact on strength, durability and service life.”

The report further explains that as the pH of concrete drops below 9, the chemical nature of the cement is altered so much that it begins to lose its ability to bind. This is most evident on the surface of the concrete, as it begins to chalk, flake, spall or release aggregates. As these layers decay, they further expose the underlying layers to degradation.

This can allow water to migrate into concrete, which in turn leads to a host of potential problem that can range from purely aesthetic to structural.

EXPOSING AGGREGATE

Washing virgin plaster or cement surfaces with an acidic solution less than 4.5, and certainly anything approaching zero, obviously will be extremely detrimental to the lifespan of the concrete.

The more diluted the acid, meaning the higher the pH of the solution, the lesser the impact on concrete. But contact with any water that has a low pH or a negative saturation index value, will to some extent damage fresh concrete.

To expose aggregates, a high pH solution should be utilized to minimize the damage to the cement. In other words, exposing aggregate should be performed mechanically with a brush and rinse water.

Again, it’s basic chemistry. A rinse solution with a pH of 7.5 is far better than lower ranges, because the pH scale is logarithmic – that is each 1.0 change on the scale is a 10-fold change in the intensity.  A final rinse with a pH solution of 13, will restore the surface of the cement to its initial pH level of 13.

Exposing and rinsing the cement surfaces with solutions that are closer to the 13 pH of cement, will ensure that the plaster company does not prematurely damage or etch the cement surface.

PLASTER DAMAGE

With plaster, the impact is exacerbated by the thinner veneer, which becomes even more susceptible to long term damage by acid exposure. Plaster problems and their causes have been debated for decades. At the heart of those often-heated arguments is this issue of exposing high-pH compounds to acidic conditions.

As mentioned above, one of the most common malpractices involves using low-pH start-up procedures, i.e., “hot” start-ups. The idea is to use low pH water to burn up plaster dust that forms after plaster is placed and exposed to water, and to carbonate the surface.

The problem is the acidic water damages the components of the cement matrix that gives it strength, this is just as true for plaster as it is for structural concrete. With plaster, however, it’s such a crucial visual element, that cosmetic imperfections, which are vastly exacerbated by low pH conditions, often become serious issues with homeowners.

Finally, it’s important to note that low pH exposure also have a negative impact on pigments used in colored plaster. A topic unto itself, colored plaster can become extremely uneven in appearance as a direct result to exposure an acid start-up.

PURPOSEFUL ETCHING

There are times when the impact of acid washing is entirely intentional. Service technicians often turn to acid washing as a way to remove scale, stains and other surface blemishes. While exposure to low-pH solutions in this context does impact the long-term viability of the plaster surface, the procedure is done for the relatively short-term improvement in appearance.

It’s important, we believe, that homeowners realize that this type of cosmetic acid washing is in effect a temporary fix for tired-looking plaster. It is not our intention to impugn service technicians performing this basic maintenance procedure.

We do hold, that exposing brand-new concrete and plaster surfaces to acidic conditions is a bad idea and should be forever abandoned.

References:

Acid Attach on Concrete by Haseeb Jamal, Concrete Technology, Feb 2017.

Characterisation and Control of Cementitious Mixes with Colour Pigment Admixtures by Mehreen Z. Heerah a, Isaac Galobardes b, Graham Dawson, Science Direct, December 2021 

Acidic Attack of Cement Based Materials (a review) Part 1. Principle of Acidic Attack by Vladimı́r Zivica a, Adolf Bajza , Science Direct, December 2001.

What Happens if You Don’t Neutralize Muriatic Acid on Concrete? By Christina Trautman, materialhow.com May 2023

The Effects Of pH & Moisture on Concrete by Al Manaratain, almaratain.com, October 2019

Paolo Benedetti founded Aquatic Technology in 1993. He is an instructor for Watershape University and and has authored a myriad of articles on the finer points of pool construction and design. Jon Temple is president and founder of Tempool, a pool-surfacing firm located in Jacksonville, FL. Temple founded the company in 1996, which has plastered more than 30,000 pools.