Natural Stone Porous Materials Protection Cleaning D.I.Y.

Alkali attack, and the sealer to survive it

We usually talk about the danger of acids to stone - but alkali attack can harm concrete.

Alkali attack: It sounds like name of a Star Trek episode doesn’t it?

But far from science fiction, alkali attack is very much science fact. And it can wreak havoc on your natural stone – in particular, any concrete features you have in your home, whether it’s a driveway or a garage floor.

The potential for alkali attack makes proper surface protection of your concrete of the utmost importance. But before we can get to that, allow us to explain what exactly alkali attack actually is.

What is an alkali?

You might remember the term ‘alkali’ from high school science class. Of course, depending on how long ago that was, your understanding of the term might range from ‘passing familiarity’ to ‘non-existent’.

Solutions – or a liquid mixture in which one substance has dissolved – can all be classed as either acids, alkalis, or neutral solutions. They are measured on what’s known as a pH scale. A neutral substance will measure exactly 7 on the pH scale. But if a substance is acidic – think citrus, vinegar, even plain old saliva – it will have a rating of 0-6.

Concentrated acids are highly corrosive, and if you’re a regular reader of Surfacepedia, you’ve probably heard about the damage acid etching can do to a once-beautiful stone feature. We’ve warned homeowners many times to keep acidic substances away from their natural stone countertops!

But alkalis have their own reputation for damage. Alkalis, occupy the 8-14 part of the pH scale, and have been used through history for a variety of important purposes.  Before the industrial revolution changed the face of modern life, alkalis were needed to create glass, convert fats and oils into soap, and numerous more uses. They were often derived from stale urine and burnt wood, though after industrialization they began to be manufactured.

Today, alkalis serve a variety of common household purposes. Items as diverse as baking powder, drain cleaner and bleach are all alkalis. But while these products are doubtless useful in their particular fields, they’re also perilous in the kind of damage they can do to concrete surfaces.

The science of alkali attack

The reason concrete is so particularly prone to alkali attack is because cement materials tend to have lots of calcium hydroxide – in other words, the thing that makes concrete what it is is highly alkaline.

The problem comes when some aggregates used in the creation of concrete react with these hydroxides. The vast majority of time, these aggregates will remain chemically unaffected – however, if you happen to be unlucky, the hydroxides will be affected by the silica in the aggregate.

A gel is the result, which increases in volume as it takes in water, resulting in an expansion of the concrete. As it expands, the concrete will also crack and spall, not only ruining the look of the concrete product, but also threatening its structural integrity.

What can be done about alkali attacks

According to the Durable Concrete Structures: Design Guide produced by the Comite Euro-International du beton, exposure of concrete to humidity can exacerbate this, as it allows more water to enter its surface. Therefore water-proofing your concrete with a sealer is essential.

Of course, sealing your concrete is important no matter what, to prevent stains and other kinds of damage. But it’s also true that some sealers don’t stand up as well against alkali attack. You’ve got to find a sealer that can repel water while also withstanding this hazard.

We’re pleased to report that Dry-Treat sealers like STAIN-PROOF Original™ are known for being able to withstand strong alkali attack. In one test, involving soaking a sample of concrete in a high pH solution for 21 days, Dry-Treat was the best performing sealer. Consider this next time you need to seal your stone.

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