Hi,
Your sand looks fine enough and I don't think it will cause impaction problems to your axolotl (I've a similar sand for my axolotls).
From my own experience, shale makes problems because it traps poo and it's impossible to clean it.
In a mature aquarium, sand will "digest" poo and the live plants (I suppose they are real) will uptake resulting nitrogen compounds.
That being said, almost all rocks and sands can leak some minerals in the water.
With shale, the most common is sulfur, brought with iron sulfid, but I'm not sure it's a problem.