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? (2,600?), 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.

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Source : http://www.time.com/time/specials/packages/article/0,28804,2030137_2030135_2021648,00.html