Wanting to protect your floor from welder and/or cutting torch slag??? I don't use anything on my floor. Small particles are no problem. If I'm using a Plasma Cutter or Cutting Torch I am conscious of larger molten pieces that land on the floor. A gentle nudge to move them a couple times til they cool off prevents popping of the concrete. Been in my shop 7 years with 99% of my metal work happening in one area. You hafta look very closely to find any damage.
When my shop first got built. I went through a stage of trying to protect the floor. Then I just gave up. But if I'm doing a lot of cutting in one place I lay down a good size sheet of 16-GA to protect the floor.
I weld and cut all the time in my shop. The only time I have damaged the floor was when I did about 50' of cutting on 2" plate. The massive amounts of slag hitting the floor made it pop even though what I was cutting was 4' off the ground. Normal light duty stuff has never hurt my floor.
Hardiboard or equivalent in other brands in comes in 1/4", 4'x8' sheets. I use it on my table to keep from burning the plywood below and it's better than a metal top because your slag won't stick. It works!
I believe the 1/2" thick is only in 3'x5' sheets. It will show burn marks if a torch if left in place for too long but will not burn through unless you are careless.
A couple of sheets will cover a 8x8 area.........set your project on top and you are good to go.