Concrete sequesters carbon. Not as a design feature — as chemistry. Atmospheric CO₂ penetrates the material, reacts with calcium hydroxide and calcium silicate hydrates, and forms stable calcium carbonate. The process is called carbonation, and it happens spontaneously in every concrete structure on Earth.
U.S. cement infrastructure passively sequesters 6.5 million metric tonnes of CO₂ per year — roughly 13% of process emissions from cement manufacturing. In Mexico, the figure reaches approximately 25% despite using half the cement. The difference: Mexico uses more mortar, which is more porous than structural concrete.
Porous mortar absorbs CO₂ an order of magnitude faster than dense concrete. The mechanism is surface area. CO₂ must diffuse through the material to reach reactive calcium compounds. Porosity increases the internal surface area available for reaction and provides channels for gas diffusion. Dense concrete, optimized for compressive strength, limits both.
The weakest cement product is the best carbon sink. Structural engineers specify dense concrete to maximize strength. Atmospheric chemists would prefer porous mortar to maximize carbonation. The optimization targets are orthogonal.
This doesn't mean buildings should be made weaker. It means the non-structural cement products — plasters, renders, mortars, repair fills — deserve attention as carbon sinks. They are already deployed at enormous scale, already porous by design, and already sequestering carbon without anyone optimizing for it. The opportunity is in the material nobody optimizes, doing the job nobody assigned it.