Thanks to Dale Grover for pointing me at this article about Grancrete, a new type of spray-on concrete replacement which the developers claim could transform house-building in the developing world.
To build a house, you construct it out of polystyrene panels, cut holes where you want doors and windows, and then spray the panels, both inside and out, with Grancrete.
The mixture is about 50% sand, 25% ash, and 25% their special binding compound. A few hours later, the house is usable, and the foam core of the walls makes for good insulation.
More info on the Grancrete site, especially the video page.