Grout is what you are describing----it is used to fill voids ---
Are you making a tiled niche ? If so--low expanding foam will do---blue can (window and door)
Then waterproof it well using a paint on waterproofing like Red Guard or Hydro Ban
Are you making a tiled niche ? If so--low expanding foam will do---blue can (window and door)
Then waterproof it well using a paint on waterproofing like Red Guard or Hydro Ban