Is it safe to make aquarium background out of silica sand aquarium silicone and polystyrene?

i have silica sand in mine at mo and my fish have being spawning regular, so nothing wrong on sand bit,the aquariam sili will be ok to and i cant see the poly causing any harm either. i guess you want the sili to stick the poly down as well as sticking the sand.,i have bog wood in mine, i used strips of lead to way it down. why dont you use plaster of paris instead of poly as a thought.