We have a concrete sectional garage. This was sealed by the supplier with
what looked like sand and cement which quickly came detached from the
concrete base and leaked.

We have resealed about 18 months ago by first painting the base with pva
then adding a sand cement mortar mixed with pva at a 45 degree angle on the
inside to the panels. No more leaks but the mortar is now black with mould
and is beginning to crumble.

Any ideas of a mix or cure?