Is there a possibility of a chemical reaction if I use Keraquick over concrete? should I PVA to ensure a barrier? all outside work.

No it will be fine and DONT use PVA !
Try a tile primer. PVA will break down when wet.

primer g by mapei. do not use pva

Ultrarapid-setting flexible (S1) cement based powder adhesive for ceramic tiles (thicknesses of adhesive up to 7 mm).
Applications:
Interior and exterior bonding of all types of ceramic floor and wall tiles (double fired tiles, single fired, porcelain tiles etc) on:
– old floors and walls (ceramic tiles, natural stone, terrazzo, concrete) provided they are solid and clean;
– cast concrete, properly cured prefabricated concrete panels (at least 4 months old), heating panels, reinforced cement panels, plaster boards, provided they are firmly fixed. Particularly suitable for works where rapid repairs are required spacing. N.B.: mixing with Latex Plus
(6 kg of Latex Plus for each 20 kg bag of Keraquick) provides increased deformability (S2), without changing the other positive features of Keraquick. Coverage
2.5-4 kg/m2. Packaging
20 kg bags grey, 20 kg bags white, 4x5 kg packs.


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