In a combustion facility, a portion of the bottom ash (BA) is mixed with substantially all of the fly ash (FA) as necessary to obtain a stabilized material that prevents toxic metals solubility and achieves a desired TCLP extraction pH value, preferably in the range of 8-11. The actual quantities of BA to be mixed with FA will vary depending upon various factors, so embodiments of the present invention include testing the ash to determine the desired distribution to achieve sufficient alkalinity to prevent the leaching of toxic metals in landfills. Because the resulting ash has a desired alkalinity by design, minimal additional alkali reagent amendment is necessary for safe disposal of the waste ash. The remaining BA may be recycled or deposited in a landfill without additional processing. Valuable materials in the waste ash may also be more easily recovered and reused.