How to seal waste pipe in bathroom?

noodles

Hello. 
I recently removed my bath and had a plumber seal the water pipes but he didn’t seal off the waste water pipe (I didn’t notice till after he left) I have purchased a cap for it and the concrete to fill the hole but I was wondering do I need to apply anything to the cap so it’s watertight? I have been given mixed advice. 

thanks for any helpful advice you can give 

MitchellMc

Welcome to the Bunnings Workshop community @noodles. It's sensational to have you join us, and many thanks for your question about waste pipes.

The cap would need to be glued to the pipe with solvent cement. Unfortunately, this likely comes under the realm of plumbing work requiring a licensed plumber. Perhaps that could pop back around on their way home and glue the cap on for you?

Please let me know if you have any questions.

Mitchell