Re: What?
Cinder blocks are the concrete building blocks sometimes called "concrete masonry units". They are so named because they used to use cinders from defunct cinder cone volcanoes as the aggregate in the concrete mix. You can still see cinder operations in the volcano fields in the southern California desert.
Note that they are only strong in one axis. Somewhat counter-intuitively[0], only the two "open" ends can take any pressure. The four "flat" sides are quite weak. Thus my questioning their use as flooring.
[0] As any number of shade-tree mechanics have discovered after attempting to support a vehicle on them.