How is concrete made and why is it so hard?
Growing Stone
How is concrete made and why is it so hard?
You've walked on sidewalks, leaned against buildings, maybe even drawn on a driveway with chalk. All of that is concrete. It looks like plain gray rock, but it's not rock at all — it's something humans invented, something we mixed up and poured into place while it was still wet. How does a liquid turn into something harder than most stones?
Concrete starts as a recipe with four ingredients. First, cement — a fine gray powder made by baking limestone and clay in a giant oven until they crumble into dust. Second, sand. Third, gravel or crushed stone, for bulk. Fourth, water. When you mix them together, something magical happens: a chemical reaction kicks off, and the wet gray mud begins to turn itself into artificial stone.
