All methods in your article are duly noted. I'm a recently retired highway resident engineer (who designed/built many bridges and roads). My opinions:
1) We allowed up to 25% fly ash on our projects, which were readily accepted by the contractors (its cheaper). We weren't thrilled about 50%, as too much fly-ash can create the risk of cracking. At this point note that many things related to concrete "in the lab" don't work well "in the field" as remote locations or temptations of suppliers to overwater mix (to cover the fact that mix is old or to make more workable for finishers) defeats the engineer's intent (we catch them and reject product, then claims ensue which complicates projects). When this happens, society's intent to "use less CO2" comes into conflict with "building viable, long lasting structures".
2) I've read about "greener" concrete, and the simple fact is that it always takes heat to create the cement (even fly ash is created from heat). As such, I'd say paying the extra 30% to scrub the CO2 from the flue is the best and most immediate solution. In fact I don't understand why they don't do that in natural gas powered electricity generation...that 30% extra could almost immediately solve about 80% of our CO2 problem, gradually giving way to renewables or something like your algae process...isn't that worth it?
3) Concrete is good stuff. Obviously, I've paved with asphalt concrete also, and there are parallel arguments about the CO2 release from that (fortunately, there are "warm mix" and many lower net CO2 recycling ideas in the asphalt world). However, most of us feel that concrete is the much superior paving material (especially for major roadways, now that expansion/contraction are fully taken into account and the product is placed with machinery).
4) In the future, look for plastics/polymers to increase in the construction world (polymerized concrete in this case, but also plastic coated rebar). Plastic industry really needs to clean up its environmental act also, but at least concrete/steel/plastic combos may make for thinner and longer lasting elements. Also, there is hope for recycled plastic roadways (probably in base layers, as top layers can release microplastics)
5) I was recently in Tanzania where they were lime treating many roads, a really cheap way to make a non deteriorating surface (good for remote roads that would otherwise erode). Unfortunately that lime is also made with heat.
6) This article about Roman concrete claims they were able to create a superior product using high amounts of pozzolan (volcanic ash from Vesuvius, similar to fly ash), lime, and sea water. It states that the Romans were able to make lime with 2/3rds less heat, and use in smaller amounts: https://newscenter.lbl.gov/2013/06/04/roman-concrete/#:~:text=The%20Romans%20made%20concrete%20by,were%20packed%20into%20wooden%20forms.
7) I'm hoping someday, cars will "float" (perhaps via maglev or some other tech) which may make much of this concrete and asphalt unnecessary!
