While it's interesting to explore ancient concrete, actually scalable improvements of concrete production will be made elsewhere. For instance HeidelbergCement has successfully piloted and is currently implementing technologies that bring the CO2 emissions from cement production close to zero. The OxyCal hybrid process that they are deploying in Bulgaria and Belgium will capture >97% of all emissions from the cement…
Does it turn into rusted out rubble in 100 years like the reinforced concrete we use today? I remember reading an article about the longevity of Roman concrete. The best they could come up with was the use of volcanic ash and much slower curing in salt water.
But many methods are available both to extend life, and to monitor the health of reinforced concrete. The failures that do occur are typically due to negligent maintenance.
The Romans didn't use steel reinforcement, of course, they just used massive concrete structures. We can do the same today, but it would mean no skyscrapers, no long span bridges, etc.
To be able to build the Pantheon, at 43 meters height, they used walls with a thickness going from 6.4 meters at the base down to just 1.2 meters at the top. In addition they reduced the density of the concrete by almost 50% towards the top, by using very light aggregates.
In comparison the Burj Khalifa at 828 meters heigt has less than a tenth of the wall thickness at the base, just 0.6 meters of reinforced concrete wall thickness in the hexagonal frame.