The KH absorption of STS is minimal and lasts for a very short period of time. It's not an aquatic substrate (we just use it for that) and each batch will actually vary from another. If you want a substrate that buffers, you'll have to buy a product specifically developed to do that like ADA Aquasoil.
Since it's calcined clay and dusty, I'm guessing you rinsed it quite heavily? That's going to cause it to break down a bit.
If you give it a couple days to settle into your tank, particulate matter in the water will eventually settle.
The KH dropped to zero in my tank, and the GH stayed the same.
The lowered KH allowed the pH to drop, too.
I also suspect this material removed the sodium hydroxide the water company adds to the tap water.