Eben - will the slab take any water? If you drop some water on it does it absorb or just sit on top?
If it doesn't absorb it has a sealer of some kind on it. You need to remove it - either chemically or mechanically. With only 25sf you can do this with your grinder - use a scarifier or steel brush attachment that can be bought at your local box store. Be sure to wear personal protective equipment - safety glasses, respirator. Also seal the room off & have a box fan in any exterior window to move the dust out or/& use a shop vac to keep things dusted down to a minimum. You can also use products such as Soy Gel to remove this sealer - this will create much less mess & isn't so hard on your eyes or lungs.
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John
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