I'm a little curious about that myself.
The art of underwater concrete casting was perfected by the Romans, but their concrete was quite different from the concretes used today. I've seen the process reproduced (on a video of course) and with the addition of water the mixture started to "boil". The Pantheon in Rome, which in its time (and for some time thereafter) was the largest dome in the world, and is still standing, was constructed (if memory serves) of concrete.
a translation online, by one Bill Thayer (I couldn't get it to load):
same work, different URL, now it's bedtime for 'Civ:
>>I've seen the process reproduced (on a video of course) and with the addition of water the mixture started to "boil".<<
Thanks for those links. Without looking further tonight---bad toothache---I'll guess the boiling was from using quick lime, calcium oxide:
1. n. [Drilling Fluids] ID: 1769
A chemical with formula CaO, commonly called quick lime or hot lime. When hydrated with one mole of water, it forms slaked lime, Ca(OH)2. Quick lime is used in preference to slaked lime at oil-mud mixing plants because it generates heat when it becomes slaked with water and therefore speeds up emulsification by the reaction to form calcium-fatty-acid soaps.
The adding of water would convert it to slaked lime, the prime ingredient of 'natural' (as opposed to modern 'Portland') cement.
[Quick lime was also used sparing to sprinkle down the hole of outhouses, to control oder, insects, etc. It is also the lime used to sprinkle on dead critters until [if ever] they can be picked up by the knacker or animal control. It will burn the skin, which is why pre-slaked lime is used for such things as white-wash & plaster; it is safer.]