Crumbles when soft

>> Instead of concrete or syncrete, why not basalt? Just melt down the
>> regolith, mold to shape, and voila, your "concrete" form. No mixing
>> in water, and I here that basalt is pretty tough stuff.

> How could we get a mold of about 100 m diameter which is able to take
> the heat into space?

I would imagine the mold would be built up through VVD or some other
such technique. The original "mold" could be a plastic inflatable
thing massing less than 10 tons.