Light weight cockle shell aggregate

I'm building a roof garden and need to incorporate an aggregate with the soil and compost I have to help with drainage and reduce weight.  People often use expanded clay which is quite expensive and it takes a lot of heat energy to make and so isn't great environmentally. I have heard you can use cockle shells, has anyone had any experience with them? Do they work in a similar way by adding drainage?  Are there any downsides?