Do you need to change the ph of the soil, or are you worried you might change the ph?
There are a number of soil amendments that are good to use to improve your soil, expanded shale is not one I have used (it has good reviews, I've just never tried it). My soil used to be hard as concrete (here in North Texas) and over the years I've added decomposed granite, Texas green sand and lavasand to improve the consistency.
Look in the Natural Organic Library (the green box on the home page) for answers to things like
expanded shale.