How does Drain opener decompose?
The sodium hydroxide will not decompose appreciably, and leaves an area of concrete dangerously caustic. A person or pet walking across it could suffer serious burns.

You can neutralize the alkali with a mild acid such as vinegar or citric acid. Vinegar is only 5% acetic acid, so you'd need about 20 times as much as the amount of drain cleaner. Too much acid will attack concrete, so use pH indicator strips (e.g. for fish tanks or swimming pools) to adjust pH to neutral (about 7).

During the neutralization, a bit of chlorine gas will be driven off from the hypochlorite, but that smell should no last long. Any other smell would be from waste in the drain... that might last quite a while.