Unfortunately, our long love affair with concrete has also added to our climate problem. The variety of caementum thats most commonly used to bind today’s concrete, a 19th-century innovation known as Portland cement, is made in energy-intensive kilns that generate more than half a ton of carbon dioxide for every ton of product. Multiply that by gigaton global usage rates, and cement-making turns out to contribute about 8 percent of total CO2 emissions.

Source: The road to low-carbon concrete

This doesn’t get talked about enough.