I've seen people doing silicone with coco-fiber, bark, spaghnum and the like, but has anyone tried working with silicone and sand?

I'm looking to do a natural water feature in my vivarium, including a small waterfall. I do not want to attempt to coat the foam on the water feature using fiber, as I know it will wick moisture. I don't really want to use a lot of stone, because the "concept" of the vivarium is almost a "biotope" - and our local rock is very porous sandstone. This will not be a "rainforest" viv - considering that it'll be a habitat for a couple of Leopard Frogs.

It's my hope that the sand would wick and carry far less moisture, meaning that I could butt the waterfall directly against the coco-fiber'd back wall without soaking the whole thing. It's also my hope that I can coat the surface with local red sand and gravel, giving it the natural, local, look I want.

So, has anyone? Would you? Can you think of problems that might arise?