Before we started building our wood-fired pizza oven, my only experience with concrete came from leaving the cake-batter bowl on the counter overnight. In general, though, I’m pretty careful about soaking used dishes. “Put that in the sink,” I’ll say to Kevin. “If you let it dry it’ll turn to concrete.”
And I suppose cake batter is a little like concrete. You take an amalgam of dry ingredients, mix it with water, and it hardens as it dries. That, though, is where the similarity ends. The thing about cake batter concrete is that, if you add back the water, it turns back into cake batter. Not so with actual concrete. And a good thing too, or our bridges, roads, and buildings would wash away in the first rainstorm.
What’s interesting and valuable about concrete is that, once it hardens, it’s hard for all time. You can pour it into a form when it’s wet, and then, when it’s dry, you’ve got what is essentially a rock in the shape of your form.
Astonishingly, both the Romans and the Egyptians figured out concrete thousands of years ago, and it figures in the pyramids and the Pantheon.
It’s astonishing because concrete is complicated.
The key ingredient in concrete is cement. The most common kind of cement is Portland cement, so called because the guy who invented a process to make it, a bricklayer named Joseph Aspdin, thought that the concrete made with it resembled limestone quarried from the Isle of Portland, a little landmass in the English Channel.
Cement starts by heating rock to something over 2500 degrees (F). But not just any old rock will do. It has to have the right balance of calcium and silicon, iron and aluminum. A mix of limestone and clay is a good starting point.
When you heat limestone and clay, you end up with a lot of complicated chemical reactions. I wish I could boil them down for you here, but my I’m afraid my chemical education isn’t up to the task even of understanding them, let alone explaining them. After those complicated chemical reactions have taken place, you end up with a substance called clinker, which looks like little stones. Grind those stones to a powder, and you’ve got cement.
But the chemistry doesn’t end there. When you add water to the cement, there are yet more complicated reactions I can’t explain, and some molecules bind to other molecules and you end up with something resembling stone.
There! Got that?
Not understanding the chemistry gave me pause. I thought our chances for a successful concrete pouring would be vastly increased if we actually understood what was going on. But ignorance hasn’t stopped us from launching into chickens, bees, mushrooms, or, most recently, turkeys, so why should it stop us from building a concrete pizza-making edifice?
We forged ahead.
To make concrete out of cement, you mix it with aggregate. Now, although aggregate sounded to me like something technical and specific, it turns out that ‘aggregate’ is just another word for ‘rubble.’ Even, colloquially, ‘crap.’ The point is to mix in a bunch of little hard things that the cement then bonds together, and just about any little hard thing will do.
The little hard things can be very little, like sand, or much less little, like rock and pieces of crushed brick. Usually, a combination of little things and less little things is what’s recommended.
Beyond the size of the aggregate, we had another concern. Since what we were pouring was the base for the deck of an oven, we needed some degree of heat-resistance. Ordinary concrete is good up to about 800 degrees (F), and then it starts doing things like cracking and, more alarmingly, spalling. Spalling is when a piece of the structure separates from the structure itself, sometimes explosively. It’s profoundly undesirable.
I read up on heat-resistance and found very little – ahem – concrete information. From what I could glean, it was generally made of a lightweight, insulating aggregate like perlite.
Okay, I reasoned. We have a supply of kiln bricks, which are lightweight and insulating. They crush relatively easily. Wouldn’t crushed kiln brick make a nice, heat-resistant aggregate?
I haven’t the foggiest idea whether this is true. I know you can’t reason out chemistry. But I had the kiln brick and I needed the heat-resistant concrete, so what the hell.
We set about crushing the kiln brick. When I say it crushes easily, I mean that if you drop a sledgehammer on it, it breaks into pieces. Drop it again, you get smaller pieces. You have to drop several times to get the crumbs we were looking for, and crushing enough kiln brick for our oven deck was a daunting task.
We looked for a way to mechanize the process, and found it in the form of a bedsheet and a truck. We wrapped the bricks in the sheet, and then ran over them, and over them and over them, with the truck.
When we unwound the bedsheet, we found that there were still a lot of pieces that were too big. Since this was going to be the base on which we put the kiln shelves that will be our oven floor, we needed a relatively smooth surface, and big chunks just won’t do. So we got out the compost sieve and ran the crushed brick through. What we ended up with was a mix that went from dust up to the size of peas. It was good enough.
This was the mix we used, a bastardization of a mix I read about on a wood-fired oven site:
3 parts crushed kiln brick
2 parts sand
2 parts Portland cement
½ part lime
There’s a good chance we used the wrong kind of aggregate, the wrong kind of cement, and the wrong kind of lime (we used whatever it was they sold at K-mart’s garden department). The only other mistake we could make was using either too little water, or too much.
I suspect we did both. Because we had to form a flat slab on an irregular surface, we made a wooden frame the size of the slab and set it on the stone base. Invariably, it wasn’t level and there were lots of gaps underneath.
To make sure the concrete didn’t leak out the gaps at the bottom, we made the first batch very stiff. We forced it into the places where the frame didn’t meet the stone (having stuffed newspaper underneath to prevent escape). Then, though, we needed to make a mix that was liquid enough to make a flat surface. We did that, and poured it on top. Will the two layers bond together properly? Who knows.
What we ended up with was a decent approximation of what we were trying to build, but I am not sanguine. I’m convinced that, at the first firing of the oven or the first frost, the whole thing is going to crack and collapse.
Kevin, on the other hand, is convinced that it’s way overengineered, and that when aliens land on the planet, long after humans are extinct, they will find our wood-fired oven base and believe it was some kind of altar for religious rituals.
All I can say is, this pizza better be good.