You can't build sky-scrapers without rebar (i.e. with un-reinforced concrete), but you can build some pretty large structures if you use curves, arches, widenin

You can't build sky-scrapers without rebar (i.e. with un-reinforced concrete), but you can build some pretty large structures if you use curves, arches, widening bases, buttresses, etc. The Pantheon is pretty big, built from un-reinforced concrete, and nearly 2000 years old. You have to adapt your building style to the material you're working with and tall, thin structures depend on the tensile strength of steel; concrete doesn't have much tensile strength, but does have tremendous compressive strength, so your structure will have to be wider at the bottom, although not necessarily wider than it's tall. It's all about directing the vectors of forces in a way that they stay inside the material of the structure, so no flying slabs, upper floors have to have arches or domes supporting them from below (or lots of pillars that widen into a small arch at the ends). Here is an idea for a technique may be useful for building with un-reinforced concrete: instead of pouring whole walls into a mold, pour "lego"-style interlocking (large) blocks, layer by layer. Between layers you paint the surface with a thin layer of weak but flexible mortar or glue before pouring the next layer. This way you keep enough room for the structure to shift and settle without cracking and you can use the angle of contact between blocks to deflect the vectors of force back into the material. The article mentions that the Roman-style concrete hardens much faster, so that'll work well with this idea (you don't have to wait too long between pours).