Finnish City Inaugurates 1 MW/100 MWh Sand Battery

It doesn't list the advantages over water, which seems the most common in https://en.wikipedia.org/wiki/Thermal_energy_storage systems.

You'd think water would be easier to exchange heat with since it can slosh around the heat exchanger elements in the tank more easily. Which should translate to lower costs since you don't need as many exchanger structures in the medium.

Any guesses for the motivation in using sand? Maybe it's that you can heat it over 100C? But then big heat differences to the environment mean high conductive/radiation losses or heavier insulation requirements.

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.

I wonder if there are any chemical effects from heating the sand to 500 degrees Celsius. Finely roasted sand.

This is crushed soapstone, so it's mostly talc. Talc is apparently more or less stable up to about 800C, where it starts to break down into enstatite and silica: https://nvlpubs.nist.gov/nistpubs/jres/15/jresv15n5p551_A1b....

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).