Reddit reviews Roberts 10294 4" Floor / Wall Scraper (18" Handle)

Mapei's Kerapoxy cleaner and a blade scraper. Personally, I actually prefer the longer handled ones as I find the extra expense is well outweighed by how much more comfortable they are to use (and I've used them a lot..). You don't want a wider blade because it takes more force to take things off with a wider blade.

That'll remove the majority of it, use some of those abrasive pads with the kerapoxy cleaner to get rid of the rest. It'll still take a lot of elbow grease. And I do mean a lot. The longer you leave it the worse it'll get too.

Would recommend ventilating the area and/or using a respirator with vapour filters, and some decent rubber gloves. Kerapoxy cleaner isn't the nicest of stuff. But it's nicer than muriatic acid for sure, and a lot less can go wrong.

The next step up is muriatic acid, but it's really not something I'd ever recommend for a DIYer. Americans on reddit seem to have a fetish for the stuff, but almost no tile place openly sells it locally to me and you have to be known to them to be able to buy it, because so many people don't take the stuff half as seriously as they should. Tl;dr it's super aggressive stuff and I'd rather you try something that won't do as much damage if you fuck up working with it.