I am building a thin set stone veneer wall for my back patio. The place where I bought the stones said I could use the same mortar mix for the cap stones as well the veneer. The mortar mix is 4 parts sand, 1 part type S mortar and 1 part portland cement. I am now finding the capstones are loose. The capstones are made of Pennsylvania blue stone. Should I use just a straight type S mortar to set the stones?
I should mention the wall underneath is cinder block. The sand/mortar mix sticks very well to the cinder block, I just spent the past hour chiseling off the mortar from the top of the wall where some of the cap stones come off. Should I pretreat the PA bluestone with anything like muriatic acid to clean it?
What about non-cement masonry adhesives? Could I just "glue" the stone back on without removing the mortar underneath?
The mortar mix you are describing will only work well with a dense and un-absorptive stone, and even then it is not particularly suited for the use. If it is sandstone or limestone, use either a lime mortar or a 1-3 type N masonry cement/sand mix.