www.growingbonsai.net/characteristics-of-good-bonsai-substrate/
Cat litter
Based on these characteristics I started to look for solutions that were in use. One of the first I found was actually through the website of Harrie Harrington in the UK: bonsai4me. He grows his trees in pure baked diatomous earth. This product is sold in many European countries as a non-clumping cat litter. If you read the packaging it is actually recommended as a soil improvement addition. Note: This is a very specific type of cat litter. Most brands of cat litter are NOT suitable.
After growing everything in pure diatomous earth for some 2 years, I realized it stayed quite wet. In winter the water stored in the granules when frozen produces a number of icepeaks on the granules. And in summer the substrate did not dry out quick enough to stop algea to form. Watering only every other day was not an option: The substrate would be dry b the time I would rewater. Not ideal, unless you can use a sprinkler system on a 36 hours cycle.
Ground pine bark
I started to look for additives that could work. I started with adding ground up pine bark, which helped a bit, but did not make a big difference. I do keep it in there as I am convinced it helps with bacterial colonization. This in turn helps with the breakdown of organic fertilizer pellets. But.. I cannot find any real confirmation that this indeed helps.
Not lava, Expanded shale
Then I looked at lava. But although I could find it at the right size, it was very heavy. Because of the solidity of the material, it warms up very slowly, which in Spring for me is a downside. So I left that track quickly.
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.