When people talk about the Colosseum surviving for nearly two thousand years, they usually point to the obvious things: the travertine arches, the sheer mass of the walls, the fact that it’s just big. But the Colosseum doesn’t stand because of stone alone. Structurally, it’s a hybrid building. The showy outer shell, the travertine, carries some of the load-bearing work, but much of it happens behind the scenes. Roman concrete, often working with tuff and brick, was used to form vaults, ramps, internal walls, and load-bearing cores throughout the structure to do most of the work.

What’s remarkable isn’t just that Roman concrete is still holding together. It’s that, in many cases, it has gotten stronger over time.

The Process of Making Cement

Concrete is not a single material. It is a recipe.

Both Roman and modern concrete rely on the same three categories of ingredients: a binder that becomes the glue, aggregates—such as sand, gravel, or stone—that provide bulk, and water, which triggers the chemical reaction. They also follow the same three fundamental steps: cook, mix, and cure.

Where modern and Roman concrete differ are (1) the cooking temperature; (2) how the binder is prepared; and (3) how the binder is mixed with water.

The Cooking Process

The Modern “Aggressive” Cook (Portland Cement)

Today, limestone is cooked together with clay in massive rotary kilns that reach about 1,450°C (2,640°F). At these temperatures, the limestone and clay materials partially fuse. The result is clinker, a hard material with relatively large, dense, non-porous crystals.

The Roman “Gentle” Cook

The Romans only cooked limestone. They used wood-fired or charcoal-fired kilns, and generally operated around 900°C to 1,000°C. At those lower (relative to modern Portland-cement kilns) temperatures, the limestone did not melt or fuse. Instead, it became porous—like a sponge, full of tiny holes and tunnels. The result was simply cooked limestone: quicklime that remained chemically reactive and easy for water to penetrate later.

The Mixing Process and Self-Healing Properties

The Romans added ash, or pozzolana, from volcanic regions in central and southern Italy. This ash is rich in reactive silica and alumina.

When combined with quicklime and water, the volcanic ash triggers a chemical reaction that forms the cementing matrix. Within that matrix, plate-like crystals called strätlingite grow across some of its most vulnerable interfaces, knitting the material together and making it harder for cracks to spread. This gives Roman concrete a small but critical amount of toughness and flexibility. During earthquakes, the Colosseum’s concrete vaults absorb and distribute energy instead of snapping.

The Secret of “Hot Mixing”

Evidence from recent excavations suggests the Romans often used what researchers now call hot mixing, which involved adding quicklime and ash directly into the mix to release heat.

Adding water to quicklime makes the mix incredibly hot. These hot, relatively dry conditions keep some of the lime from fully dissolving. Instead, the lime creates white chunks called “lime clasts.”

For decades, archaeologists were puzzled by these lime clasts. They looked like mistakes or sloppy workmanship. That assumption turns out to be wrong.

Recent research suggests those lumps weren’t flaws at all. They were a way of allowing the concrete to not only resist damage, but in many cases, actively repair itself.

Here’s what happens over time. All large structures develop micro-cracks, where water can seep in. When that water reaches one of these lime inclusions, the lime partially dissolves and releases calcium-rich material. That material then re-crystallizes as calcium carbonate (the mineral material that makes up limestone), effectively sealing the crack.

In effect, the concrete re-glues itself.

The Cure: Why Roman Concrete Gets Stronger

Modern concrete reaches its intended design strength at about 28 days. Modern reinforced-concrete structures are typically designed around a service life of roughly fifty to sixty years, after which time the concrete is in a slow, brittle decline.

Roman concrete took months to reach its full strength. Because it set slowly, it didn’t shrink or warp as much as modern concrete. The heat released during hydration was spread out over a long period, greatly reducing internal stress.

Crucially, Roman concrete could also continue curing in wet conditions. The volcanic ash in the mix reacted chemically with lime and water to form a hydraulic binder. Instead of weakening the concrete, external moisture allowed the reaction to continue, strengthening the material over time.

Scientists have found that Roman concrete can continue increasing in strength over decades and even centuries because of this behavior.

Why This Mattered for the Colosseum Specifically

The Colosseum hasn’t survived because nothing bad ever happened to it. Quite the opposite.

It’s been shaken by earthquakes, stripped for stone and metal, turned into housing, fortified, quarried, partially collapsed, and excavated again. Not many ancient buildings survived that sort of treatment.

This one did because its concrete core didn’t fail catastrophically. The volcanic binder limited crack propagation. Lime inclusions could even help seal small cracks after they formed, giving the material a second chance to stabilize damage. Even when iron clamps were removed—or when entire sections of wall collapsed—the structure held.