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Portland cement‐based materials are frequently used for fixing heavy metals but are not very effective for arsenic. Alternatively, alkali activated metakaolin and fly ash systems were tested for arsenic retention by the toxic characteristic leaching procedure and American Nuclear Society 16.1 methods. The influence of Fe and Mn on As leaching rates was assessed. These materials perform well in aggressive environments (pH<3) but the activated metakaolin matrix is less able to retain As than the activated fly ash matrix. The leaching in fly ash matrices occurs in two steps: immediate initial leaching followed by a slow diffusion‐controlled process.