I need some help determining if a material exists that is similar to concrete in that it can be formed into shapes, however it is designed to be formed as a light plaster like substance, then submerge in water to transform into a concrete like substance.  I need to make pre-made light weight blocks that I can easily transport and place underwater, then once submerged in water, it absorbs the necessary water to start the solidification process and becomes much denser concrete.  The main requirements I have are:

1.  Must be able to form into shapes prior to submerging.

2.  Must be much lighter prior to submerging, i.e. gains substantial density with addition of water.

3.  Is permanent under water.

Any help would be greatly appreciated.