The shale may affect water chemistry, so will check that first with this coarse cut…but also have to get a finer screen of it to know if it is good for the trees texture-wise. This cut could be used as coarse base layer for drainage, but not much more.
I use a shale product as the primary in my bonsai mix due to the highly humid climate I live in, coastal South Carolina. The product is called Permatill, it’s what is called expanded shale, heated to 2200 degrees. It has a good CEC, and has done well for me the last 2 seasons. Doesn’t break down at all, down side it is a bit heavy, and the size typically is on the larger size when sifted. Hard to get enough small material if you’re doing lots of smaller trees.