Why is concrete such a large source of emissions?

Because making cement, the binding ingredient in concrete, releases carbon dioxide through chemistry as well as through fuel — and there is no easy way around the chemistry.

Cement production accounts for roughly 7–8% of global CO₂ emissions, which would place it among the largest emitters if it were a country. Concrete is the most used manufactured material on Earth by mass.

Two separate sources of emissions:

Process emissions, which are the harder problem. Cement is made by heating limestone (calcium carbonate) to produce lime. The reaction is calcination: CaCO₃ → CaO + CO₂. Carbon dioxide is released from the rock itself, as an unavoidable product of the chemical reaction. This accounts for roughly 60% of cement's emissions, and switching to clean energy does nothing about it. You cannot make conventional cement without releasing this CO₂.

Fuel emissions. Kilns must reach around 1,450°C, traditionally using coal, petcoke or gas. This is roughly 40%, and is addressable through electrification, hydrogen or alternative fuels.

Why it is so hard to replace. Concrete is extraordinarily cheap, locally producible almost everywhere, strong in compression, durable, fire-resistant and requires no specialist skill to use. No alternative matches that combination at scale.

What is being tried:

Clinker substitution — replacing some cement with fly ash, blast furnace slag, or calcined clay. This is the most deployed measure, though supplies of industrial by-products are themselves shrinking as those industries decarbonise.

Carbon capture on cement plants, which addresses process emissions directly and is expensive.

Alternative chemistries — geopolymers, magnesium-based cements — technically promising and far from scale.

Using less. Better structural design, higher-strength mixes and reusing existing buildings avoid the emissions entirely, which is why embodied carbon in construction has become a central issue.

Concrete does reabsorb some CO₂ over its life through carbonation, but far less than was released.