text: Portland cement is approximately two-thirds calcium oxide by mass and its manufacture is responsible for approximately 8% of the world's CO  emissions. 60% of this CO  comes from the calcination of calcium carbonate as shown earlier in this article, and the other 40% from fossil fuel combustion. By replacing some or all of the calcium carbonate entering the plant with waste calcium oxide, the CO  caused from calcination can be avoided, as well as some of the CO  from fossil fuel combustion.
  This calcium oxide could be sourced from other point sources of CO  such as power stations, but most effort has been focussed on integrating calcium looping with Portland cement manufacture. By replacing the calciner in the cement plant with a calcium looping plant, it should be possible to capture 90% or more of the CO  relatively inexpensively. There are alternative set-ups such as placing the calcium looping plant in the preheater section so as to make the plant as efficient as possible.
  Some work has been undertaken into whether calcium looping affects the quality of the Portland cement produced, but results so far seem to suggest that the production of strength-giving phases such as alite are similar for calcium looped and non-calcium looped cement.