Abstract: An animal feed binder, particularly for use in forming pellets. The binder is an anhydrous complex salt formed as a by-product in the desulfurization of fossil fuel combustion gases with magnesium oxide. The magnesium sludge from the desulfurization process is dewatered, dried and ground into a powder not larger than about minus 7 U.S. Sieve mesh. The anhydrous complex salt contains a major portion of magnesium sulfite (greater than 50% by weight to about 80% by weight) and a minor portion of magnesium sulfate (from about 15% by weight to about 45% by weight). An animal feed composition is formed by the admixture of a dry mixture of feed ingredients and the complex salt. When the animal feed composition is contacted with water or steam, it is conditioned by the heat of hydration of the complex salt and forms a hardened animal feed composition which can be extruded into pellets or formed into self-setting blocks.