Q.: Is there a concrete admixture that will prevent ammonium nitrate fertilizer from attacking the portland cement in concrete?
A.: You're unlikely to find an admixture that will prevent ammonium nitrate attack of concrete. Water-reducing admixtures and pozzolans such as fly ash, silica fume or metakaolin may increase resistance to a limited degree by reducing concrete permeability. However, the surface still will deteriorate, and the deterioration will be progressive.
For the best protection, apply a coating that will prevent contact between the ammonium nitrate and the concrete.