Fixation of heavy metals in geopolymeric materials based on brown coal fly ash

The alkali-activated brown coal fly ash - geopolymer - can be used as a matrix for the fixation of heavy metals. The addition of heavy metals does not result in any substantial extension of the setting time as this happens in the case of Portland cement. The materials prepared by the alkali activation of fly ashes exhibit measurable properties in contrast to some cement mixes containing heavy metals. In contrast to the Portland cement mixes, the strength values of AA fly ashes containing metal admixtures (Zn2+, Cu2+, Cr3+, Cd2+ and Pb2+) do not drop in the time horizon of 180-520 days. The IR and NMR spectroscopy corroborate the penetration of aluminium ions into the original Si-O-Si structure (AlQ(4) (4Si), AlQ(2-3) (2-3Si) and Si(3Al) units). The salts of heavy metals are distributed uniformly within the geopolymer body. The leaching values characterizing the mixes containing salts of heavy metals are not the same for the individual investigated metals (Zn2+, Cu2+, Cr3+, Cd2(+) and Pb2+) and they vary within the ranges of the 1st-3rd class of durability (as per regulations valid in the Czech Republic). The best results could be achieved for the zinc fixation when the solidification in Portland cement was practically impossible. The resistance of AA fly ashes to leaching is influenced significantly by the presence of calcium ions in the geopolymer matrix.