Photo: limestone
Global cement production accounts for 7% of annual greenhouse gas emissions in large part through burning of quarried limestone. Production can now be made carbon neutral by pulling CO2 out of the air with the help of microalgae. To make portland cement, limestone is burned at high temperatures, releasing large amounts of carbon dioxide. Replacing quarried limestone with biologically grown limestone, a natural process that some species of calcareous microalgae complete through photosynthesis, creates a net carbon neutral way to make the cement. The CO2 released equals what the microalgae already captured. Portland cement could actually become not only net neutral, but also carbon negative by pulling carbon dioxide out of the atmosphere and storing it permanently in concrete.