Journal of Hazardous Materials 166 (2009) 561–566 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Coal fly ash-slag-based geopolymers: Microstructure and metal leaching Maria Izquierdo a,∗ , Xavier Querol a , Joseph Davidovits b , Diano Antenucci c , Henk Nugteren d , Constantino Fernández-Pereira e a Institute of Earth Sciences “Jaume Almera”–CSIC, Lluis Solé Sabaris s/n 08028 Barcelona, Spain b Cordi-Géopolymère, Espace Créatis, Z.A. Bois de la Chocque 02100 Saint-Quentin, France c Institut Scientifique de Service Public (ISSeP) 200, rue du Chéra, B-4000 Liège, Belgium d Delft University of Technology, Faculty of Applied Sciences, DelftChemTech, Particle Technology Group, Julianalaan 136, 2628 BL Delft, The Netherlands e University of Seville, School of Industrial Engineering, Department of Chemical and Environmental Engineering, Camino de los Descubrimientos s/n, 41092 Seville, Spain article info Article history: Received 31 May 2008 Received in revised form 21 October 2008 Accepted 20 November 2008 Available online 27 November 2008 Keywords: Geopolymers Leaching Trace pollutants Fly ash abstract This study deals with the use of fly ash as a starting material for geopolymeric matrices. The leachable concentrations of geopolymers were compared with those of the starting fly ash to evaluate the reten- tion of potentially harmful elements within the geopolymer matrix. Geopolymer matrices give rise to a leaching scenario characterised by a highly alkaline environment, which inhibits the leaching of heavy metals but may enhance the mobilization of certain oxyanionic species. Thus, fly ash-based geopolymers were found to immobilise a number of trace pollutants such as Be, Bi, Cd, Co, Cr, Cu, Nb, Ni, Pb, Sn, Th, U, Y, Zr and rare earth elements. However, the leachable levels of elements occurring in their oxyanionic form such as As, B, Mo, Se, V and W were increased after geopolymerization. This suggests that an optimal dosage, synthesis and curing conditions are essential in order to obtain a long-term stable final product that ensures an efficient physical encapsulation. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Over 40 million tonnes per year of fly ash are produced in Europe [1]. As far as the environment is concerned, the disposal of fly ash should not be regarded as a sustainable management strategy. Given the EU regulations, it is crucial to develop new technologies that allow the recycling of coal fly ash into added-value products. In this regard, the synthesis of geopolymer binders may be a success- ful alternative, resulting in low-cost and environmentally friendly materials with cementing properties resembling those of OPC [2]. Geopolymers were introduced by Davidovits in the early 1970s to describe inorganic materials obtained from the chemical reaction of alumino-silicate oxides with alkali silicates, yielding polymeric Si O Al bonds [3]. The network consists of SiO 4 and AlO 4 tetrahe- dra linked alternately by sharing all the oxygen. The Al 3+ in IV-fold coordination demands the presence of cations in the framework to balance the negative charge. According to Davidovits [2], the empirical formula of geopolymers, also known as poly(sialates) is as follows Eq. (1): M n {–(SiO 2 ) z –AlO 2 } n ·wH 2 O (1) ∗ Corresponding author. Tel.: +34 93 409 5410; fax: +34 93 411 0012. E-mail address: mariaizq@ija.csic.es (M. Izquierdo). where M is a cation such as K + , Na + or Ca 2+ ; n, the degree of polycondensation and z is 1, 2 or 3. Other cations such as Li + , Ba 2+ , NH 4 + and H 3 0 + may also be present. Research on the geopolymer green-chemistry conducted in recent years has resulted in the development of new cements based on natural materials (mainly kaolinite and metakaolin) or industrial wastes, with particular emphasis on pulverised coal combustion (PCC) fly ash [4–16]. Geopolymerization of fly ash may involve envi- ronmental benefits such as (1) the reduction in consumption of natural resources and (2) the decrease in the net production of CO 2 since it is estimated that the geopolymer cement synthesis emits 5–6 times less CO 2 when compared with Portland cement [11]. Given the physical performance of the final product, geopoly- merization has become a promising technology that offers attractive possibilities for commercial applications, i.e. fast hard- ening, high and early compressive strength, optimal acid resistance and long term durability [17]. The synthesis of geopolymer matrices has proved to be an optimal and feasible option to stabilize certain metallic and radioactive wastes [18–20] or industrial wastewater [21]. Although the microstructure, chemistry and mechanical prop- erties of fly ash-based geopolymers have been extensively studied, less attention has been paid to the leaching behaviour. The leach- ing properties have been assessed mainly on pulverised material under acidic conditions and essentially focused on Cd, Cr, Cu and Pb [9–10,12–14,22]. It could be expected that the acidic conditions and the size reduction could overestimate the releases of a number 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.11.063