“Alkaline Activation” as a procedure for the transformation of fly ashes into cementitious materials. Part II. Immobilisation of toxic and hazardous elements
نویسندگان
چکیده
In this study, the potential of alkali activated fly ash cements to immobilise metals that are generally not well immobilised in Portland cements was examined. A class F fly ash was activated with a solution of 8M NaOH to form a hardened cementitious paste. Prior to activation, fly ashes were doped with either: i) 5000 mg/kg (0.5%) Hg 2+ as HgCl2, ii) 10000 mg/kg (1.0%) Cs + as CsOH.H2O or iii) 10000 mg/kg (1.0%) As 3+ as NaAsO2. Mechanical strengths of the pastes were measured and the degree of metal immobilisation in pastes was determined by TCLP leaching tests. Finally, a study of the crystalline phases and paste microstructure was carried out to determine the fate of immobilised metals in the AAFA matrix. Microstructural analysis of Hg doped pastes showed a strong correlation between Hg and S, implying that formation of highly insoluble HgS or Hg2S precipitates is an important immobilisation mechanism. However, the coexistence of HgO could not be entirely ruled out. In Cs doped pastes, it is proposed that Cs + ions are associated with the N-A-S-H gel and zeolites formed in AAFA cements, suggesting that Cs is mainly chemically bound rather than physically encapsulated. With As doped pastes, generally poor performance was observed, both with and without the addition of Fe2O3 in an attempt to improve immobilisation.
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