I wouldn't use a dry mix on top of an existing hard surface. Assuming
that it's sound, then conventional mortar would be more appropriate.- Hide quoted text -

- Show quoted text -
I will start using conventional mortar now.

I am assuming that the patio is sound, It is monoblock and underneath
it has been leveled with sand. It has been in place for 10+ years.
What sould I be looking out for to make sure it is and what would be
the alternative?