We have a concrete slab, covered patio and would like to install composite decking. Can we just glue them to the concrete?

Not unless you never have freezing, where do you live?

Just install a platform deck over the patio. You won't regret it.

Can I glue hardwood decking to concrete

No you can't gavin. You should also know that you can start your own fresh post by clicking on ASK ABOUT EXTERIOR HOME IMPROVEMENT rather than tagging onto an old post. Wood in contact with concrete produces one thing, rot. Give more details about what you want/need to do