I'm not 100% certain on this but this is what I understand to be the cause of SMS and other high CEC fired clays causing KH to drop.

SMS and other fired clays normally have high CEC's. CEC is Cation Exchange Capacity. Exchange is the important word here because cations (Mg++, Ca++, etc) are exchanged for hydrogen ions, H+. These H+ ions (acid) cause the KH and pH to drop.

If you are concerned with this then you can soak the SMS in a saturated solution of calcium and magnesium salts for a week along with some baking soda to prevent pH going too low. Rinse well and use. When I tried this KH hardly changed at all.

James