Here is the full list of all 15 Portal achievements. It takes around 8-10 hours to unlock all of the achievements on Windows.
Earn bronze medals on all Portal challenges.
Earn silver medals on all Portal challenges.
Earn gold medals on all Portal challenges.
..?
Beat all six Portal advanced maps.
Beat four Portal advanced maps.
Beat two Portal advanced maps.
Complete Portal.
Make the correct party escort submission position decision.
Detach security cameras from the walls.
Do whatever it takes to survive.
Knock down a turret with another turret.
Jump 300 feet.
Fall 30,000 feet.
Acquire the fully powered Aperture Science Handheld Portal Device.