I also have an Ott light that stopping working. The ballast you are thinking are what submarines use to submerge. The ballast in your Ott is what electronically provides power to the bulb. The cement you are referring to is a weight in the base of your lamp that keeps it from tipping over. It seems that over time it either fell apart or was dropped or damaged in some way.
You can use stepping stone cement to make a new weight. Just adhere it to the casing in the bottom of your lamp. Very smart to use bags as weight, however if you are like me, I need all the floor space I can spare for my treasures. Good Luck.