How much would a piece of concrete from a renovation of a capitol be worth?

About $2 unless you can prove the block actually came from the capitol building. There should be some sort of certificate stating the authenticity of the block. At the very least, go back to the newspapers from Montgomery in 1991 and 1992 (Sunday papers would probably be best). You want to find the notice or ads for the sale of the blocks like you have and make a copy of it. On your copy, please note the source and date of publication. The sale price of the pieces will give you some idea of the value of your chunk.