I came across this post recently regarding a new super concrete out of Iran. During a contest hosted here in the states, the Iranians were able to build several samples of UNREINFORCED CONCRETE that had a 28 day cure strength of 50,000-60,000 psi. That's 2x that of most granite!
The article can be seen here:
http://blog.wired.com/defense/2007/04/irans_superconc.html
This reminds me of a hypothesis about the massive stones in Peru that weigh 20,000 tons each and were stacked 2 high, with such precision you could barely stick a credit card between them. My thought was that these were actually cast in place, perhaps using a similar arrangement (sans the metal fibers used in the Iranian sample).
Does anyone here have any idea how the ancients made such permanent concrete (even if the Peruvian hypothesis is incorrect). Supposedly, according to wikipedia, the mystery on how to form concrete was lost for over a 1,000 years!?!?!?