Scandium and Aluminum are two different metals. In the cycling industry what you find are aluminum alloys that contain Scandium (amongst other metals). Aluminum frames aren't pure aluminum, they are all some variant of a combination of metals. 6000 series, and 7000 series aluminum alloy contain different metals within the aluminum.
Scandium allows you to reduce the overall weight of the frame yet retain aluminums trademark rigidity and strength. The best part about Scandium is it's ability to absorb road vibration and shock. I had an older DeRosa Merak that was a scandium frame. I was very pleasantly surprised how nice the ride was, and this was prior to the Meraks having any carbon on the rear triangle.