Is this Addy ok for 20mm marble slabs

[lucius]

Me neither, so...?

Just seen several other suggestions trolling through here and started doubting myself, wobt look no more.

 

[Huddersfield Tiling]

That's exactly what I've been using for years never had had a problem.

As long as the floor is stable, flat, and de-coupled and use good gear, you shouldn't have a worry in world.