If it's gonna cost a lot, perhaps perlite is an alternative? It's a siliceous rock and it's extremely light and much cheaper. I bought a 100 liter bag for 17,50 euro at my local garden centre to use it in my hypertuffa mix (roads, rocks, platforms etc). I think it weighted about 5 kilo!
It's also sometimes called 'Agra Perlite' (for agricultural usage).
Here's more information on it: http://www.perlite.net
However, it's always white or light gray so you have to color it. I think when you use some sort of mixture of diluted white glue and black paint you can glue and color it the same time (the same way ballasting smaller scale tracks).
But be aware; you can buy perlite on Ebay or on many shady "greenery" sites for very high prices in much smaller doses (like 20 euro for a 5 litre bag!!!). This because it's very popular to use it planting cannabis (perhaps because those folks smoke that trash so much they buy it for any price?).
So, you can better buy it from a garden centre or directly from a supplier (like for instance in The Netherlands: Pull Rhenen -
don't know any French supplier).
EDIT: I did find a French site where you can order it: http://www.perlite.fr/