The composite building material — a mixture of aggregate, water, and a binding agent — that has been the primary construction medium of every major civilization since Rome, and whose modern form, reinforced with steel, is the material substrate of the contemporary built world: its dams, bridges, highways, apartment blocks, and infrastructure.
Concrete is the most widely used material on earth after water. Humanity now produces roughly 4 billion tonnes of cement annually — the binding agent in concrete — which works out to about half a tonne per person per year. The built environment of the 20th century is almost entirely made of it: the highway interchanges, the apartment towers, the dams, the bridges, the airport runways, the harbor walls. Concrete is so ubiquitous that it has become invisible. But it is also responsible for approximately 8% of global CO₂ emissions — more than aviation, more than shipping — and its production is accelerating in the developing world at precisely the moment when carbon budgets demand it decelerate. The material that built the modern world may be incompatible with the world's survival.