My Pyrrhotite in Concrete Story (Part 1)

In 1989 my family and I moved from a pleasant Boston suburb to rural Ellington in north central Connecticut. We loved Ellington and quickly made good friends, primarily the parents from our daughter’s play group. Our house was set on 5 acres of mostly forested land, I installed a playground set with swings for the kids and we had many enjoyable times. Life was good, or so it seemed.

However, unbeknownst to us, something insidious was happening to our house that was beyond our wildest imaginings; the concrete support structure (i.e. the foundation) was quietly crumbling away beneath our feet. Who even knew that this was possible? I’m an environmental chemist with a lot of experience and the idea that concrete could literally corrode away over the course of a few years was news to me.

It’s Even Worse than You Thought

In 2005 during a routine trip to the basement I stopped to look at the concrete walls and it became apparent that something was very wrong. Big vertical, horizontal and diagonal cracks had opened up in the concrete walls and the formerly solid concrete had become shockingly friable – you could easily extract a piece of concrete with your hand and rub it to dust between your hands. There was also a snowy-white efflorescence covering most of the walls.

The next day at work I told OTO’s senior engineer Mike Talbot about what I had observed and asked him to take a look. When he did he was dumbfounded and recommended that I have a structural engineer friend of his, Rob Johnson, come by to give his opinion.

A few days later Rob was in the basement saying that he was uncertain what the precise cause of the problem was, but I needed to immediately hire a contractor to install temporary shoring to forestall the imminent collapse of the house. This was bad enough, but the next piece of news from Rob was even worse: I needed to start making plans for the total replacement of the basement walls and foundation footings because the concrete was clearly disintegrating rapidly.

Rob sketched up some plans for me to give to the contractor and soon we had these large wooden supports holding up the basement walls. Speculating, Rob suggested the underlying problem with the concrete could be ASR (alkali-silica reaction), but to find out for sure would require collecting concrete core samples and the petrographic analysis of the cores. Petrographic analysis involves making thin slices from the cores, staining them, and reviewing them under a special microscope.

Concrete coring, showing wall damage

Pyrrhotite Revealed

While we were not this first ones in the area to have a problem with crumbling concrete, to my knowledge, we were the very first to collect core samples for petrographic analysis.

When we got the lab results back it turned out, there was no ASR in the core samples, instead the problem was the presence of pyrrhotite in the concrete’s coarse aggregate.

To put this all in an understandable context, it’s helpful to know a little bit about concrete. Concrete is made from four basic ingredients: cement, sand, coarse aggregate (eg small stones), and water. Concrete is a very strong and durable building material (think Roman Colosseum), but there are two types of stress that concrete cannot tolerate: corrosive acids and tension forces. It turns out that pyrrhotite provides both of these stressors in abundance.

Pyrrhotite is basically a chemically unstable form of iron pyrite, made up of iron and sulfur. When pyrrhotite is mined out of the ground and is exposed to air and moisture, it begins a long slow degradation reaction. As the pyrrhotite degradation progresses, the sulfur turns into sulfuric acid and the iron becomes the mineral hematite. Any exposure to acid is bad news for concrete, but sulfuric acid is by far the worst. It immediately begins to dissolve the cement paste that binds the other concrete ingredients together.

The problem with hematite, which is effectively a type of ferric oxide or rust, is that it takes up more space in the concrete matrix than was occupied by the pyrrhotite it replaced. This results in internal pressure and expansionary forces. These expansionary forces are more than the acid-weakened concrete can withstand and massive cracking begins to appear. At first the cracks are narrow, but they soon expand to an inch or more across. What I saw on my trip to the basement that day was the characteristic cracking pattern referred to as “map cracking”, so named because the irregular cracking resemble roads on a map.

As the cracks widen, the basement walls effectively grow taller, which pushes the sides of the house upwards. Windows no long open and close and doors become crooked, no longer able to shut. The sides of the house became higher than the middle of the rooms. Welcome to Alice in Wonderland.

As has now been shown by so many homes and other buildings in north central Connecticut and south central Massachusetts, the pyrrhotite containing aggregate originating from Becker’s quarry in Willington, CT has laid to waste real estate values.

What’s next?

This story is getting long, but it wouldn’t be complete without the parts about the so-called insurance company (names changed to protect the unbelievably unhelpful), and the parts about the actual reconstruction of the foundation and basement walls. So stay tuned for parts 2 and 3 – and BTW, yes this all really happened to us and it is still happening to many folks in north central Connecticut and south central Massachusetts.