OPTION-1 : Bitumen impregnated soft board which is available in various thickness and % of bitumen contents.
OPTION-2 : Polyethylene foam board
OPTION-3 : Foam backer rods can be used the diameter of the backer rod should be 1/8 inch larger than the width of your concrete expansion joint. These will save you huge quantity of filler material which is unnecessarily used to fill the expansion joint.
OPTION-4 : PU foam boards.