Hi Enrique, plastic to concrete, it is best to go to a plumbing supplier, 
after all their are many types of Silicone Sealants, and it is best to get 
advice from a specialist.
But you will need a special type of sealant for Plastic/Concrete, not just 
any type will do, that is if you wan't to do it properly.
Take care.
Garry Lawson
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