Roman concrete was more durable than most modern concrete, but was much, much weaker. It also relied on volcanic ash, which isn’t as readily available as the ingredients for portland cement. Being able to have larger freestanding spans and lower construction costs due to reinforcement is usually worth a much shorter design lifetime.

The reinforced aerated autoclaved concrete was clearly a mistake, trying to make a concrete foam to reduce weight meant that more water could get to the rebar and cause corrosion much more quickly than in normal reinforced concrete.