Sandtastik is made from feldspar, which a lot of people use in freshwater tanks. Occasionally there is some mention of it raising pH, which means it must be at least a little bit reactive in freshwater, not sure if it's reactive in saltwater. If it's non-reactive in saltwater it should be safe for DIY rocks.
There are some other threads on R2R about using in a sandbed, and the main drawback mentioned is that it would not have the buffering capability of an aragonite sandbed.