I think it must be sawdust, which was used in many types of places to absorb waste and spills, and prevent the floor becoming slippery.

Butchers shops in the UK used it until at least the 60s. It would be spread in the morning and swept up at the end of the day, having absorbed splashed blood, water from customers shoes etc.

Lower-class pubs would also use sawdust. Someone of my age (50s) might still refer to a particularly rough pub as 'spit 'n' sawdust'.