We have been experimenting for a couple of months on the use of cat litter for ballast. We started by filling our cats tray with litter, then sand, and then soil. he used it every time which suggested it wasn't the material but the location!
Cat litter is made from many different things (including some which are banned in America!).
We used the litter from NETTO which is 85 centimes for 7 litres and is made from TURONIENS which are crushed fossils from the cretacious age (gives its name to Tours, and its other name is Angoumien, which gave its name to Angouleme)
It took about 20 bags to ballast all the line and mark out some extra roads: so pretty good value. it absorbs five times its own weight in moisture and doesnt break down;
heres a pic of it in use around the 'picket' siding: