Thread by @jasoncrawford

Someone asked me recently what's a boring topic that I'm excited/fascinated by.

That's easy: cement.

If it's wasn't thousands of years old, cement would seem futuristic. Think about it: it's *liquid rock*!

Here's how it works:

Cement comes from limestone, a soft, light-colored rock with a grainy texture. The key ingredient in limestone is calcium; it's a sedimentary rock typically formed from crushed seashells. Chalk is a form of limestone (“calcium” is basically “chalk-element”).

To make cement, we crush limestone and burn it, to turn the calcium carbonate (CaCO3) into calcium oxide (CaO). Calcium oxide is a caustic powder, traditionally known as “quicklime” (“quick” meaning “active, alive”).

When quicklime is combined with water, the water does cool it—instead, it reacts with the water, giving off heat! The result is Ca(OH)2, calcium hydroxide, aka “slaked” or “hydrated” lime.

If you then pour slaked lime into a mold, it will re-absorb CO2 from the air… turning back into calcium carbonate, CaCO3 (and giving off water vapor).

In other words, when cement sets, it's basically turning *back into limestone.*

Got that? We crush and burn rock, and then re-constitute it at a time and place and in a *form* of our choosing.

Like coffee or pancake mix, it's “instant stone—just add water!”

Isn't that basically *magic*?

Now, a pure lime mixture like this isn't actually good for much. To use it in construction, you need to mix it with something to give volume and strength.

Add sand → mortar (like between bricks)

Add sand + gravel → concrete

Sand/gravel in this context are called “aggregate”.

Those are the basics, but there are many advanced formulas for cement. The Romans found that mixing in volcanic ash, which they called *pozzalana*, created a cement that set even underwater (without oxygen). They were masters of cement.

One problem any form of cement has: it's strong under compression, but not under tension. You can build columns and arches with that, but not, say, cantilevered balconies. But during the Industrial Revolution, we figured out how to reinforce concrete with iron bars.

These reinforcing bars, or “rebar”, give concrete tensile strength, and let you make things like Frank Lloyd Wright's Fallingwater:

So that's why I think cement is underrated.

More details and image credits here: rootsofprogress.org/instant-stone-…

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