Alakali-activated cements as immobilisation matrices for arsenic wastes

A variety of natural and industrial waste product materials rich in SiO ₂ and Al ₂C ₃ may be activated with certain alkaline materials to produce cementitious systems which when cured under mild temperature conditions, set and harden, in the presence of appropriate amounts of water, to give a very compact paste. In some cases, e.g. where fly ash is the activated material, fast setting and rapid strength development have been reported. In addition to their application in construction, their role as host matrices for waste materials is beginning to attract interest and it is this application that the study reported here addresses. Arsenic immobilisation is not achieved satisfactorily in Portland cement matrices because calcium arsenate phases, which form readily, decompose when the matrix carbonates. This paper describes the results of some of the leaching and microstructural studies on alkali-activated fly ash matrices containing arsenic with additional iron and manganese. Arsenic leaching from fly ash matrices is high, and considerably increased by the presence of manganese. The role of iron is more complicated and depends on its speciation.