Name: Concrete
Description: Concrete would be a new fluid that would be somewhat similar to lava. Once placed it would harden after a few seconds, once hardened it would be rather strong (somewhere between obsidian and reinforced stone?). Because it is liquid it would need to have a cast so it is in the right shape (similar to using lava and water to make obsidian structures w/o a diamond pick.
Recipe: Cement; I have no idea how to implement this though... anyone know how its actually made?
Sand, Gravel, and/or Cobblestone
Water
This would be a relatively cheap material since it would spread from the source tile so you get multiple blocks from 1 bucket, however making the cast would be very time consuming making reinforced stone still a viable option.
EDIT: I just realised that the tar suggestion is extremely similar... well I did check for people suggesting concrete.