Perhaps masonry adhesive that comes in a caulking tube would work assuming the block sites on the cinder block evenly without huge gaps.
Have to make sure the surfaces of the block and the cinder block are clean, dry, and paint free.
use type s mortar and pin the caps to the block wall,
type s for the leveling bed and head joints between the caps
drill 2 holes on the bottom of each stone near the end and use either 3 inch tapcons or simmilar type concrete anchor. dab a bit of adhesive on the end of each screw and place the cap temporarly in place, the adhesive on the tips will mark your holes and then drill them slightly bigger fill the holes with adhesive or epoxy them place mortar then set cap. caps should always be pinned outside or inside where anybody can sit on them or lean up against them. if you look at most planters that arnt done like this the joints are cracked for sure.
A little off the subject for the post, but it might help some -
For segmental mortarless retaining walls the caps are frequently adhered with a 4 generous dabs of silicone adhesove on each cao. This allows some movement and flexibility and the caps can be removed with a long, thin knife. I even used my fish fillet knife and it worked well if the dab was generous.
I have seen this done on numerous private and municipal walls and is noted on the standard plans for "flexible" segmental retaining walls. A rigid wall with with a lot of abuse may be a different situation.
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