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This sounds like a concrete nightmare:

If a material absorbs so much CO2 over it's lifespan, it significantly alters the chemical composition and therefore strength.

I doubt any builder will use this material unless it's been proven that the new material is sufficiently stable.

Example: as a geology student, I ran into an area in central spain with lots of Gypsum sediments (Ca|MG.SO4). Putting limestone and concrete buildings on this sediment wasn't done until the 20th century, but all the buildings built in that area are long gone, even though in nearby towns they still stand tall. Reason? The Gypsym in the soil chemically eats the mortar and limestone (CaCO3) out of the structure on top of it, making it crumble within a few decades. The Gypsum areas are largely a wasteland where only very few buildings remain.

Now, Mg.Ca-CO3 (dolomite limestone) is largely as stable or more stable than pure limestone, and certainly harder, but any new formula for the glue in concrete will have to pass the test of time before it will be widely adopted, especially in e.g. bridges and skyscrapers...

Perhaps we can start with the interstates, nobody would notice if they started to crumble early ;)

Fly ash, which is the ash waste from burning coal is also being used in concrete to lessen the amount of C02 concrete creates as well as improve strength. My question is since this fly ash has a high amount of toxins(heavy metals) in it, would the toxins be locked in the concrete or would they seep out if exposed to water or other stresses over time.
      I am curious to know this because apparently fly ash can make concrete easier to work with in insulated concrete form construction and because other types of materials that compete with concrete seem to be using it. Gigacrete.com ( supposedly 10,000 psi strength) though not for structural use is an example. I can't tell if they are using weasel words though because they claim there binder is nontoxic, I can't tell if they are purposely talking about the binder being non toxic and not the fly ash.
    I hope someday to build a house out of ICF's (insulated concrete forms), I guess I must have taken to heart that story of the three little pigs when I was young.

Novacem estimates that for every ton of Portland cement replaced by its product, around three-quarters of a ton of CO2 is saved, turning the cement industry from a big emitter to a big absorber of carbon

You mean turning the cement industry from a big emitter to a small emitter...

One of my dorm friends, Jakob Husum [www.dtu.dk], wrote his dissertation [nt.ntnu.no] on ways of optimizing cement productions.

One of the rather impressive/scary things about that, is that it is responsible for about 2% of the world's energy consumption. That's an insane amount of energy for something that isn't even an end product.

The first paragraph of the paper actually grabs you by the balls and twists firmly:

Production of cement is one of the most energy
intensive industrial processes, consuming up to 2 % of the worlds electricity due to several low eciency processes. The grinding of cement clinker from the kiln is the most inefficient process in the manufacturing, with an efficiency of 1 % (Benzer et al., 2001).

Can't quite remember how much of the energy if spent on the last bit, but I think it was something like 25%. That's 0.5% of the world's energy usage spent on a 1% efficient process. Now imagine you could up the efficiency to 10% or even 5%. That'd be a reduction of the world's energy usage of 0.45 or 0.4% respectively, simply by improving a single process.

Now, there are a lot of arguments for saving energy. Saving the environment, less pollution etc., but it's hard to overlook the economic incentive of cutting back energy costs of a production, where a large part of the process is 1% efficient.

The summary doesn't explain things very well. Just to set things straight, most of the CO2 emissions from portland cement production is not from the fuel burned in the kilns but from the gas released by the limestone itself during the calcination process. The only real incentive for the use of energy efficient kilns is to reduce fuel costs and not to reduce emissions. The upside is that cement will reabsorb much of the released CO2 as it cures over the course of time.

"...mostly because cement, the active ingredient in concrete, is made by baking limestone and clay powders under intense heat that is generally produced by the burning of fossil fuels."

This sentence got me to wondering. . . one of the big problems of thermal electric power plants (coal, natural gas, nuclear), is that we throw away 50-60% of the heat as waste heat into the environment (nearby body of water or the air). Could the waste heat from a coal or nuclear power plant be used to 'bake' the cement? In the case of coal, sure, you're still burning fossil fuels, but those were being burned *anyhow* to generate electricity, so why not put the waste heat to use? You are, *at least*, not burning any *additional* fossil fuels just for the cement, right? In the case of Nuclear, you are using a very low-carbon heat source, and again, doing something useful with the waste heat?

Why not just plant more trees around buildings made of concrete? That seems to me to be a more useful, long-term "incentive" program than some we've seen lately.