For this specific use case, you need to heat to far above the boiling point of water to retain some thermal efficiency. Sand/rock is better suited for storing the thermal energy at ~500 celcius.
If it were pure silica sand, you could presumably get even hotter before anything changes chemically, but at the that point you start having materials issues with metal parts of the system: 500C is about the limit for ordinary steels to lose strength (and many are less than that - heat effects can often start at 300C).
None, really. Pure, fine sand being mostly silicon dioxide, it melts at ~2000 and boils at ~3000 C, still without decomposing or reacting. It is really extremely chemically stable.
That said in practice, at scale... before filling up your storage tank you'd probably need to pre-heat it once to remove all moisture and volatile gunk adsorbed onto the sand.