Cambridge electric cement

 by Ian Page.

http://www.eng.cam.ac.uk/news/cambridge-engineers-invent-world-s-first-zero-emissions-cement

Cement without CO2 production seems impossible. After all the basic chemistry of cement production is 

CaCO3+ red Heat=> CaO + CO2

It's hard to capture the Co2 because of the process conditions.

The process from this Cambridge group is a neat fiddle.  They have noticed that as cement turns into concrete it basically undergoes this reaction 

CaO+H2O=> Ca(OH)2 

which is the glue that holds the aggregate and iron reinforcement together.  If you reprocess old concrete, remove the iron reinforcing bars, and the aggregate, you end up with just calcium hydroxide.  The advantage of this is that the reaction 

Ca(OH)2+ low temperature heat=> CaO + H2O

Is easy and produces no CO2.  They have (in the lab) gone through this process and demonstrated that the resulting cement is equivalent to standard Portland cement.

In addition, even electric steel production requires CaO as a flux on top of the molten steel to stop it oxidizing, which turns into clinker, and is identical to the CaO needed for Portland cement.

This leaves us with an overall process:

Limestone (CaCO3) converted to CaO with production of CO2 as usual, but after use in steel foundries made into cement. (Overall production of CO. Bad!). Then as buildings are demolished new cement is made from the old concrete instead of more limestone being cooked and more CO2 produced.  In a steady state this would merely eliminate CO2 from building construction and might work effectively in mature countries.  It won't help at all in rapidly industrializing countries unfortunately.

There are the usual questions.

1. What will it cost to process old concrete in money and energy, and can it all be done electronically? Practically concrete is heavy, and you need to extract the cement portion at site rather than move everything to a processing plant. Since the new building is probably going to be put where the old building is knocked down, you actually want to make the new cement on site and reuse the aggregate as well to save costs. How much would such an onsite machine/process save in new construction/ material transport costs  
2. How much impact would this have if it were cost effective on CO2 reduction in reality - how much Concrete escapes from the cycle.

From PRESS RELEASE: Cambridge engineers invent world’s first zero emissions cement