I don't know if I would really call this 3D printing, in as much as we usually do. Unless you call swimming pools 3D printing. They have been using "shotcrete" for years, for making swimming pools. Basically the same thing, the concrete is stiff enough to stand up with out a form.

Something that would go great with this idea is "papercrete"(Google papecrete), it is pretty much paper mache with cement. I have seen this technology, it really works well. It is VERY insulative, and fire resistant.