So here’s an idea: life-size Linkin Logs you can build a house out of.
As it rains, the logs absorb water, which bonds with the material to slowly but steadily turn it into a heavy solid material.
But it’s super light foam during construction. You construct it out of super light foam members, then just those it down or wait for it to rain. As it absorbs water, it hardens and gets heavier.
In order to ensure saturation of the whole cross section, the crystal is formed in such a way that it transmits bonded water downward with gravity, but the water can travel outside of the matrix. It stays bonded but can travel within the crystal, and therefore settles down with gravity.
This is just to ensure even and complete hydration of the linkin log.