Proceedings of the EUROCOALASH 2012 Conference, Thessaloniki Greece, September 25-27 2012 http:// www.evipar.org/ “Alkaline Activation” as a procedure for the transformation of fly ashes into cementitious materials. Part II. Immobilisation of toxic and hazardous elements S. Donatello 1 , A. Fernández-Jiménez 2 , and A. Palomo 3 1 Eduardo Torroja Institute (CSIC), Serrano Galvache Nº 4, 28033 Madrid, Spain, email: shanedonatello@ietcc.csic.es 2 Eduardo Torroja Institute (CSIC), Serrano Galvache Nº 4, 28033 Madrid, Spain, email: anafj@ietcc.csic.es 3 Eduardo Torroja Institute (CSIC), Serrano Galvache Nº 4, 28033 Madrid, Spain, email: palomo@ietcc.csic.es Abstract 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 HgCl 2 , ii) 10000 mg/kg (1.0%) Cs + as CsOH.H 2 O or iii) 10000 mg/kg (1.0%) As 3+ as NaAsO 2 . 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 Hg 2 S 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 Fe 2 O 3 in an attempt to improve immobilisation. Keywords: fly ash, alkali activation, geopolymer, zeolite, Cesium, Mercury, Arsenic, TCLP, leaching. 1 Introduction 1.1. Alkali activated fly ash cements Alkali activated fly ash (AAFA) cements are Portland cement free binders that consist entirely of coal fly ash as the solid starting material. The ashes are typically activated by a strongly alkaline NaOH solution with or without the addition of sodium silicate. Curing at a moderately elevated temperature leads to the formation of hardened pastes with high early strengths that can be used in mortars and concretes (see Figure 1). During the setting and hardening process, the reactive aluminosilicates present in the fly ash release Al and Si to the liquid phase, which quickly precipitate into crosslinked, 3-D aluminosilicate tetrahedral networks (cementitious gel). In the tetrahedral co-ordination, aluminate groups carry a negative change, which is balanced by Na + cations provided by the NaOH activator solution. This has led to the gel being referred to as N-A-S-H gel (in contrast to C-S-H gel with Portland cement, which consists of Ca, Si and