Green Concrete

Making cement for concrete is energy-
intensive. Extremely energy-intensive. Here's
how it works: you heat pulverized limestone
clay — which is heavy in carbon — along with
sand to 1,450°C (2,600°F), usually with a
fossil fuel like coal or natural gas.
Unsurprisingly, that process generates a lot of
carbon dioxide: manufacturing one metric ton
of cement releases 650 to 920 kilograms of
CO2. The nearly 3 billion metric tons of
cement that were produced worldwide last
year accounted for about 5% of all CO2
emissions.
The good news is that there are enormous
carbon savings that could be realized by
making cement production more energy
efficient. For example, the company Hycrete
had reformulated the products used to
waterproof concrete in a way that allows for
recycling in the future, reducing the lifetime
energy footprint of a building. The London-
based startup Novacem is going further,
working on a new cement production method
that would actually absorb more CO2 than it
releases, by substituting cabon-rich limestone
with magnesium silicates that contain no
stored carbon. As the cement hardens, CO2 in
the air actually reacts to make solid
carbonates that strengthen the cement while
holding onto the gas. Novacem can't yet use
its process on a commercial scale, but if it
can, concrete could become carbon negative.