A rapid setting, controlled low strength composition of Class C fly ash is provided having a quantity of hydrated lime and an iron chelating compound in an amount sufficient to accelerate the hydration and set time of the fly ash. In some examples, a filler material is added. A method for acceleration of the hydration and set time of a cementitious mixture is provided wherein hydrated lime is added to the cementitious mixture in an amount in the range of 0.1% to 15% by weight and an iron chelating compound in an amount in the range of 0.01% to 5.0% by weight of the cementitious material. Further, a calcium source and an iron chelating compound may be added to a Class C fly ash to accelerate the hydration and set time of the ash.