Go ahead and try anything. Make sure it is not contaminated. The expanded clay will probably be a great hydroponic media once you get the seed started. Using a piece of sponge might work, and it sounds like a good idea. You can also start seedlings in a plug of soil or sand and then remove to the expanded clay after they are about two inches tall.
Sand is a recognized hydroponic media.
It can waterlog ans sea sand is usually too salty to work, but river sand works well. Wood coal and regular coal is another new idea for me, but go ahead and try. I think I have heard of activated charcoal working. All of things would make great experiements for the children. I have a good chemical skill.