Re: Fly Ash
Use of fly ash seems to be becoming increasingly popular as a cement replacement material
AFAIK both fly ash and 'slag' have long been used in making breezeblocks.. Especially I guess if you can collocate block production with a plant that produces a lot of fly & bottom ash. But now I'm curious how much paperwork might be involved in turning 'industrial waste' into useful products. I was talking to some folks about glass recycling a while back and how little of that ends up as 'recycled' glass bottles and jars. That's used in making block products, and I've also seen it used as a sand alternative for laying paving. Even though it's ending up as landfill, it avoids the landfill taxes.
Rest seems to be the usual Greenwash. Instead of taking a waste product and doing something useful with it, we're shutting down the 'waste' producers, replacing them with more expensive/less efficient alternatives and adding transport carbon costs into it in the process. Of course if we ignored the 'renewables' scumbags, we could reduced CO2 emissions by 30%+ by building modern supercritical coal power stations and keep a supply of building materials.
Also I read an interesting report from one of the US biosphere projects. It failed after CO2 levels dropped unexpectedly and plants started dieing. That was due to the concrete absorbing CO2, which lead me down a rabbit hole of learning about concrete science.