Cement without the CO2 release?

If you didn't know, and if you want to read this article in its entirety, making cement gives off a lot of greenhouse gas CO2.  Nice quote from the article I'm about to link:

"If the cement industry were a country, it would be the third-largest emitter of carbon, behind China and the United States."

So, here's the article:

Reinventing the world’s favorite building material

The idea is to use an industrial byproduct -- slag -- as a substitute for the limestone that is used to make cement in the traditional process. Because limestone is CaCO3, the process releases CO2.  A lot, as the quote above demonstrates.

So is this alternative feasible?

Perhaps.  As the article alludes to, at commercial scale, a lot of slag would be necessary. And I'm not sure how the amount of available slag compares to the amount of available limestone.

But if it could, it would probably be a significant improvement on the current situation., 

"The inventors at Material Evolution are using slag to make their cement because it has been already been super-heated and contains silica and alumina, materials that were transformed and are now “chemically activated,” ready to add strength to the concrete when water is added."

If you're curious, here's how cement is made: