They aren't comparable. The compression strength is too low to prevent oxidation of iron rebar (though epoxy-basalt rebar could replace), limiting you only to forms of ancient Rome: primarily vaults, domes, and vertical walls one or two stories tall (upon which wooden structures went higher).

You also need much more limestone because the material is weaker, incurring higher environmental impact that way.

Compressive strength doesn't passivate steel. Alkalinity does. But pozzolanic cements don't absorb CO2 IIRC. Only cements with no acidic additives (e.g. silica, pozzolan, GGBFS) will absorb CO2, but in this case strength is always lost.

I don't know enough of the details but there were reports about using it for the Swansea lagoon barrier - a £1bn tidal energy project that was to be about 6 miles long. The government blocked the project just last week so we'll never know.

"The government blocked the project just last week so we'll never know."

To be more accurate - the government decided not to subsidise the project. It was emphatically the right decision - the economics of it were awful compared to wind, solar.

To be even more accurate the government decided to ignore the clear advice of the independent cross-party committee of MPs they appointed, and after the announcement chairman of said committee said the government were not even comparing like with like in their economic comparison.

Makes it look solely a political not economic or environmental decision.

It would perform admirably in that usage, though even modern concrete should be fine because you shouldn't need any rebar.

Can't find the source now, the problem is that Roman concrete hardens slower than modern one.

That's also what killed Rosendale cement. ~1 month cures as opposed to putting a structure on the slab as little as ~1 day after pour.

Yp, 42.5 cement was used for concrete and 3 hours later the forms were removed. It was around 30celcius outside and that helped but still, you can walk on that thing hour later

If it has a 1 month curing time, I would assume weather would affect outside structures more often as well.

Rosendale cement is a natural hydraulic cement - it was used for the foundations of the Brooklyn Bridge, the US Capitol, and the pedestal of the Statue of Liberty. It's actually more durable and weather-resistant than Portland cement.