I think it's the much greater PSI concrete... https://www.wired.com/2007/04/irans-superconc/ https://www.wired.com/2007/04/irans-superconc/ Regular concrete ca

I think it's the much greater PSI concrete... https://www.wired.com/2007/04/irans-superconc/ https://www.wired.com/2007/04/irans-superconc/ Regular concrete can last for 100 years and who knows how design /needs can change during that so it makes sense not to build with much more expensive 500 year concrete. Bridges are another story.