A method is provided for reducing the amount of ammonia evolved from a cementitious or pozzolanic mixture containing contaminated fly ash, including: providing fly ash contaminated with ammonia or ammonium-containing compounds; and adding a halogenated hydantoin to the contaminated fly ash, wherein upon the formation of a slurry, the halogenated hydantoin reacts with ammonia to reduce the evolution of ammonia gas from the slurry. Another method includes adding a halogenated succinimide to the contaminated fly ash, adding sodium dichloroisocyanurate to the contaminated fly ash, or adding a halogenated sulfamate to the contaminated fly ash.