Friday Rock Blogging: Portland Cement
One of these days I should find a nice petrology textbook, and turn Friday Rock Blogs into an actual structured way to study for my qualifying exams. Until that day, I’ll be getting ideas from casual conversations and asides during class, when a professor lifts an eyebrow and says “you know, this would be a good thing to ask about during orals” – which happens all. The. Time. Agh!
Anyway. The Balclutha, featured ship in film versions of Mutiny on the Bounty and otherwise of historical significance, is currently docked at the San Francisco Maritime National Historical Park. At the turn of the last century, before the completion of the Panama Canal, the Balclutha brought Portland cement (among other things) from Europe to San Francisco. Cement is an essential part of concretes and mortars, but it’s also hella heavy. Why was it economically viable to haul such a load on a 140-day voyage?
The short answer is ballast. The long answer won’t appear in this entry, and probably not during orals either, but I’m going to ramble around the production and composition of Portland cement anyway.
Portland cement is a particular sort of cement, patented in 1824 and named for its similarity to building stone quarried from the Isle of Portland. It’s made from a mixture of calcium (mostly from limestone) and silica (from sand or clay), with bits of iron, aluminum, and other stuff thrown in for flavor. To make cement, you throw dump trucks full of sand and limestone and flavorings in a kiln and heat it up, without worrying too much about minor impurities like, oh, arsenic. Cement plants are thus completely gross and a huge health hazard.
The finished cement powder is a jumble of calcium, aluminum, sodium, potassium, and whatever-else silicates and oxides. When you add water to this, you get a bunch of calcium, sodium, and potassium hydroxides in solution, with a pH of about 13. Noxious!
As the cement sets, it turns back into a jumble of calcium and whatever-else silicates, oxides, and hydroxides – but this time it’s a hydrated jumble, which means that water has been incorporated into the molecular structure in such a way that it isn’t wet. Then you have solid cement and everybody’s happy (except the people living next to the cement plant).
Concrete is made of cement plus a bunch of crap thrown in for bulk – mostly sand and gravel. But it’s important to use the right kind of sand and gravel, because if you use the wrong kind, your bulk material will react with the cement. Instead of forming a fully hydrated solid, potassium and sodium in the aggregate will mix in with the cement and form a gel, which gradually expands into the concrete and creates cracks. Although the chemistry behind this isn’t completely understood yet, it’s similar to the process that causes some stained glass windows to deteriorate.