A low density annular grout composition for filling voids. The composition may consist of cementitious fly ash, water, set retarder and cellular foam. The composition may have a compressive strength of between 100 and 600 psi at seven days and less than 1500 psi at 28 days. The composition may have a density between 20 and 75 pcf. Also disclosed is a method of filling a void with a low density annular grout composition. The method can include determining the time necessary to fill the void, adding water and set retarder to a cementitious fly ash to make a wet mixture, adding air to the wet mixture, and adding the composition to the void.