It's listed on his site. About at the end of the page, you will find the following text.
Expanded shale
Expanded shale
So I was still stuck for a solution, when I was buying some construction materials for the renovation of my house. And that is where I came a across a product called expanded shale (In German: Blähton). I found them as a product made to level floors, in a good coarseness for the pots. The kernels are broken and sifted to size. This material drains perfectly. Has a large surface area due to the broken nature of it. And dries quickly. This is now the core material of my substrate. On its own it is too dry. So I mix in kitten litter and bark to keep it more evenly moist without having a constantly wet surface area.