Concrete | Can’t Sleep? Learn About the Foundation of Modern Cities

Concrete is the second most used substance on Earth after water, the most widely used building material, and the most manufactured material in the world. This episode of I Can’t Sleep offers a calm, fact-filled tour of concrete’s history, chemistry, and engineering, from ancient Roman domes to modern reinforced skyscrapers. The central argument is that concrete is not a single, static invention but a continuously evolving composite whose properties—strength, durability, workability—are determined by the precise formulation of its ingredients and the ingenuity of its makers across millennia.

What Concrete Is and How It Works

Concrete is a composite material: a binder (typically cement paste) glues together a filler of aggregate (gravel, sand, crushed stone). When dry portland cement, water, and aggregate are mixed, they form a fluid slurry that can be poured into molds. The cement then reacts with water in a process called hydration—a chemical reaction that is exothermic, meaning it gives off heat. This reaction hardens the mixture over several hours into a solid, stone-like matrix. The ambient temperature significantly affects how long this curing takes. Additives are often included to delay or accelerate curing, or to modify the finished material’s properties. Because concrete is strong in compression but weak in tension, most structural concrete is reinforced with ste