Lightweight concrete
Concrete made with expanded shale, clay, or slate aggregate, weighing substantially less than normal concrete while retaining structural strength.
What does lightweight concrete mean?
Reducing dead load lets designers use smaller beams, columns, and foundations, which often pays for the more expensive aggregate. Lightweight aggregate is porous and must be pre-wetted, since dry aggregate will pull water out of the mix and cause rapid slump loss, especially in a pump line. Its internal moisture also provides a curing benefit from within the concrete.
In practice
Elevated composite decks in multi-story buildings are frequently lightweight, and pumping them requires a pre-soaked aggregate and a supplier who does it routinely.