I'm sure that's the case in some manner - funny in that the shop and garage were both poured at the same time, same thickness, heck they even came off the same truck! That's why I'll wager it is
road salt - and MN uses more than anyplace in the nation I'm pretty sure. In winter the roads look like the snow around them - pure white from salt. Not good on autos either. Or snowblowers for that ...