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Polished plaster? Seen it on retail jobs - https://armourcoat.com/en/products/polished-plaster
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Probably microcement which is what we've got in our toilet/bathroom. We have it floor to ceiling and it's waterproof so good for wet rooms etc. It's the grey stuff in the first four pics here:
https://www.dezeen.com/2016/11/14/sunken-bath-studio-304-glazed-garden-bathtub/I don't really know about the Venetian plaster @diable mentioned - I think it's different but it looks very similar.
The bathroom in a hotel that I stayed in recently had the appearance of floor to ceiling polished concrete but on closer inspection was some kind of resin finish (I think). It appeared to be seamless so I assume it had been poured/spread rather than laid.
Not sure exactly what it was but I thought it looked great.