This is one of those things I was told as a kid and only recently tried to look up. The story goes that the blood and fat of some animal were part of the recipe for Roman Concrete, and because this would turn into soap during the mixing process, it would help aerate the concrete and make it stronger.
I haven't found any credible source that backs that up, and the credible stuff I found said that volcanic ash was the important ingredient. Is there any truth to the aerated-with-blood story that creeped me out as a kid?
This seems very unlikely. As the wiki article you've cited notes, if the Romans wanted bubbles in their concrete all they had to do was toss in pumice. That being said, the explanation you were given doesn't make much sense: solid concrete is stronger than concrete with significant voids, and saponification doesn't spontaneously generate soap bubbles.