Is Acid Rain Ruining my Stone

 

Is Acid Rain Ruining my Stone

By Frederick M. Hueston

 

Many of us have heard the stories on the news about how acid rain can affect our waterways, potentially killing fish and plants and destroying the environment. But what about your exterior stone surfaces? Are Pool decks, sidewalks and buildings made of stone affected by acid rain. The simple answer is yes but let’s explore a little deeper.

 

What is acid rain?

Any type of precipitation that contains acidic elements, such as sulfuric or nitric acid, that falls to the ground from the atmosphere in wet or dry forms is referred to as acid rain, also known as acid deposition. Rain, snow, fog, hail, and even acidic dust might fall under this category

When sulfur dioxide (SO2) and nitrogen oxides (NOX) are released into the atmosphere and carried by wind and air currents, acid rain is the result. Nitric and sulfuric acids are created when the SO2 and NOX react with water, oxygen, and other substances. Then, before hitting the ground, they combine with water and other substances.

So, what causes acid rain?

fossil fuels are burned to produce electricity. Electric power generators are responsible for two thirds of the SO2 and one fourth of the NOX in the environment.

vehicles and large machinery.

businesses like manufacturing and oil refineries.

Acid rain is an issue for everyone, not just those who live close to these sources because winds can carry SO2 and NOX over large distances and across borders.

 

How does acid rain affect stone surfaces

First you need to know that marble and limestone and the most susceptible to acid rain. Granite and sandstone are resistant. Marble and limestone are made up primarily of calcium carbonate. This mineral easily dissolves in acid solutions. So, you can imagine if the rainwater that is exposed to these surfaces would dissolve the calcium carbonate. The result produces a rough surface on the stone and over time can even cause the stone to break apart and even turn to powder.

You could anticipate that acid precipitation wouldn't harm protected sections of stone home and buildings. However, protected portions of limestone and marble exhibit blackened crusts that, in some locations, have spalled (peeled off), exposing deteriorating stone beneath. Gypsum, a mineral created by the interaction of calcite, water, and sulfuric acid, makes up the majority of this black crust. Gypsum can form anywhere on carbonate stone surfaces that are exposed to sulfur dioxide gas (SO2) and is soluble in water; however, it is typically washed away. It only persists on covered areas that are not directly wet by rain. Gypsum is white, but the networks that form between the crystals capture dirt and impurities, giving the crust a black appearance. What this means is that even exterior stone not directly exposed to the weather can also be affected.

How do I protect Marble and Limestone from Acid Rain?

 

The answer is not that simple. There is protective coating that can be applied that will resist the acidity however coating any stone in an exterior can cause other problems.  See my article on how sealing exterior stone can be detrimental. If you suspect acid rain maybe the cause of your stone deterioration, then a stone consultant should be called in to evaluate the installation and possibly suggest a remedy. Don’t be surprised if that remedy is to replace the stone.