The production of cement is a major source of anthropogenic (i.e. human-made) carbon dioxide. In fact, its impact is comparable to that of using fossil fuels to transport goods and heat buildings.
It uses limestone, which contains carbon dioxide that has been locked away for a geologic time, and a portion of this is released in the process of cooking it into a material called clinker. Eventually, the cement will re-absorb some, but not all of the CO2 released this way. Cooking the limestone to 1500 C takes a lot of energy too, and in an intensity that makes it difficult to produce cleanly. There's also the costs and impact of the limestone quarrying and transportation to consider.
Another problem is disposal: Construction waste makes up a large part of what goes into landfills, and cement products like concrete and mortar potentially make up a large part of that. This is from buildings being demolished or renovated, from road construction, and from the occasional truckload of concrete that is mixed but can't be poured at the right time.
So I wonder, in my limited understanding, if it's possible to take cement products and reclaim some of the cement. Currently, concrete is recycled to reduce its landfill impact and the need to mine gravel and other fill. However, that seems to be all it's used for - simple rocks rather than the magic gluestone stuff that holds skyscrapers up.
Curing is a one-way chemical process (I think), and that there's a lot of fill that's added to cement to make concrete, so maybe it's just too hard to be profitable. Fresh cement powder is so fine that it would be a stretch to call it dust, so a tremendous amount of mechanical grinding would be necessary to get concrete down to a point where the fill could be removed from what was cement powder before curing.
Has anyone given serious thought to a chemical or biological means of doing this however? If lichen can break down solid rock, could concrete gravel be broken down or separated into something finer with a plant, enzyme, or type of bacterium? Can the curing process be undone by similar means that leaves behind clinker as a waste product?
Similarly, could wet concrete mix be saved for another time in the cases where it spoils from water contamination or when it can't be poured when intended? Could aging or damaged infrastructure be reinforced or renewed by drilling into it and injecting something to force it to re-cure?
Just some thoughts from someone ignorant on the limitations of infrastructure. Comments, corrections and discussion about this would be very welcome.
Here's a general statement to close -- climate change won't simply be fixed by driving hybrids and using recycle bins. It's a complex problem and solving it will be the great work of this and the next generation.