The best replacement for mastic is mortar.

The best alternatives to mastic are rated as following:
★★★★★  mortar
★★★★★  thinset
★★  foil tape
★  grout
★  certainteed
★  tape
★  caulk
★  thinset mortar
★  copal
★  certainteed siding
★  adhesive
★  thin set
  aluminum tape
  thinset backsplash
  thinset shower
  duct tape
  simplemat
  thin set mortar
  thinset tile

Your alternative for mastic is not listed? Suggest!
Speaking about mastic, one probably shouldn't forget about natural stone.

Other search terms related to mastic and their ratings are:
★★★★★ natural stone
★★★★ glass tile
★★★ glass tile on backsplash
★ glass tiles
You have in mind a companion for mastic and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of mastic in all years. Click on graph for more.
mastic
mortar
thinset