Environmental, physical and structural characterisation of geopolymer matrixes synthesised from coal (co-)combustion fly ashes

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Abstract

The synthesis of geopolymer matrixes from coal (co-)combustion fly ashes as the sole source of silica and alumina has been studied in order to assess both their capacity to immobilise the potentially toxic elements contained in these coal (co-)combustion by-products and their suitability to be used as cement replacements. The geopolymerisation process has been performed using (5, 8 and 12 M) NaOH solutions as activation media and different curing time (6–48 h) and temperature (40–80 °C) conditions. Synthesised geopolymers have been characterised with regard to their leaching behaviour, following the DIN 38414-S4 [DIN 38414-S4, Determination of leachability by water (S4), group S: sludge and sediments. German standard methods for the examination of water, waste water and sludge. Institut für Normung, Berlin, 1984] and NEN 7375 [NEN 7375, Leaching characteristics of moulded or monolithic building and waste materials. Determination of leaching of inorganic components with the diffusion test. Netherlands Normalisation Institute, Delft, 2004] procedures, and to their structural stability by means of compressive strength measurements. In addition, geopolymer mineralogy, morphology and structure have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), respectively. It was found that synthesised geopolymer matrixes were only effective in the chemical immobilisation of a number of elements of environmental concern contained in fly ashes, reducing (especially for Ba), or maintaining their leachable contents after the geopolymerisation process, but not for those elements present as oxyanions. Physical entrapment does not seem either to contribute in an important way, in general, to the immobilisation of oxyanions. The structural stability of synthesised geopolymers was mainly dependent on the glass content of fly ashes, attaining at the optimal activation conditions (12 M NaOH, 48 h, 80 °C) compressive strength values about 60 MPa when the fly ash glass content was higher than 90%.

Introduction

Worldwide millions of tons of fly ash are generated each year by coal-fired power plants satisfying the large demand for industrial and domestic energy. The management of this by-product is always matter of concern. Only about 20–30% of the generated fly ash is used, mainly as additive in cement and concrete and as filling material, the rest is disposed of, landfilling is currently the processing technique used. Therefore, strategies to deal with this waste safely are required. Especial attention should be paid not only to prevent environmental pollution, but to treat fly ash as a valuable resource material. In this regard, the synthesis of geopolymers seems an interesting approach.

Geopolymers, also called polysialates, are amorphous to semi-crystalline three-dimensional aluminosilicate polymers considered as the analogues of certain zeolites. Thus, geopolymers consist of a polymeric silicon–oxygen–aluminium framework with alternating silicon and aluminium tetrahedra joined together in three directions by sharing all the oxygen atoms. The fact that aluminium is four coordinated with respect to oxygen creates a negative charge imbalance, and, therefore, the presence of cations is essential to maintain electric neutrality in the matrix [1]. The general formula to describe the chemical composition of these mineral polymers is Mn[–(SiO2)z–AlO2]n·wH2O, where z is 1, 2 or 3, M is an alkali cation (such as potassium or sodium), and n is the degree of polymerisation. Accordingly, in order to better describe the geopolymeric structures, a terminology has been proposed: poly(sialate) (–Si–O–Al–O–), poly(sialate-siloxo) (–Si–O–Al–O–Si–O–), and poly(sialate-disiloxo) (–Si–O–Al–O–Si–O–Si–O–) [2]. The main properties of geopolymers are: quick compressive strength development, low permeability, resistance to acid attack, good resistance to freeze-thaw cycles, and tendency to drastically decrease the mobility of most heavy metal ions contained within the geopolymeric structure [3]. Such properties make them interesting structural products, such as concrete replacements in various environments, and immobilisation systems for heavy metal containment.

Geopolymerisation occurs in alkaline solutions with aluminosilicate oxides and silicates (either solid or liquid) as reactants. Geopolymerisation takes place through a mechanism involving the dissolution of aluminium and silicon species from the surfaces of source materials as well as the surface hydration of undissolved particles. Afterwards, the polymerisation of active surface groups and soluble species takes place to form a gel, generating subsequently a hardened geopolymer structure. In most cases, only a small amount of the silica and alumina present in particles needs to dissolve and take part in the reaction for the whole mixture to solidify.

Many materials containing large amounts of silica and alumina that partially dissolve in alkaline solutions have been used as reagents for geopolymerisation reactions. These include natural minerals (kaolinite, feldspar, albite, stilbite) [4], [5], [6], treated minerals (metakaolinite) [7] and waste materials (building waste, blast furnace slag, fly ash) [8], [9], [10].

Although geopolymerisation is not a new concept, the application of this technology to waste materials is relatively recent. Thus, only in the last decade it has been applied to fly ashes, remaining still quite to know for geopolymerisation to became an actual environmental and commercial approach to treat them. A typical fly ash-based geopolymer mix consists of approximately 60% mass dry fly ash, and approximately 15% mass dry additional Al–Si source (usually kaolinite or metakoalinite), the rest of the mix is the alkali or alkali silicate mixing solution [8], [11], [12], [13], [14], [15], [16]. However, this percentage can vary greatly depending on the characteristics of fly ash (bulk and soluble Si and Al contents). Thus, percentages of fly ash as low as 10% have been employed in some geopolymer reactions [17], [18], in contrast to those geosyntheses where fly ash was the only solid source material [10]. It is evident that previous research on geopolymerisation concerning fly ash as the sole source of silica and alumina is quite limited, remaining still a lot unknown about the physical and chemical properties of derived products as well as about the factors governing the formation of these geopolymers.

The main objectives of this work are: (a) to study the suitability of different class F fly ashes to be used as the sole source material in geopolymer synthesis, both from a physical point of view (structural stability), aimed to explore their use as concrete replacements, and from a chemical standpoint (leaching behaviour), aimed to assess their stabilization and (b) to better understand the processes involved in this synthesis.

Section snippets

Fly ashes

Four fly ashes from Spanish power plants with the chemical composition given in Table 1 and with the particle size distribution indicated in Table 2 were used in the synthesis of geopolymer matrixes. Three of the fly ashes were generated from coal combustion processes (FA-1, FA-2 and FA-3 from Unión FENOSA-Narcea, ENDESA-Teruel and ENDESA-Los Barrios power plants, respectively), and the remaining one from a co-combustion process where petroleum coke is employed as co-combustible (FA-4 from

Compressive strength measurements

Fig. 1 represents the compressive strength development of synthesised geopolymers at the different concentrations of activation medium. The compressive strength shown by the geopolymers synthesised using the most concentrated alkaline solution (12 M NaOH) was the highest for all the fly ashes subject of study, except for the FA-3 sample for which a less concentrated medium (8 M NaOH) appeared as the most suitable for this aim. The compressive strength developed in the optimal NaOH concentrations

Conclusions

Compressive strength development of geopolymers synthesised using coal (co-)combustion fly ashes as the sole source of silica and alumina was greatly dependent on both the synthesis conditions and the characteristics of fly ashes subjected to the activation process. Higher compressive strength values were attained at curing time of 48 h, curing temperature of 80 °C and 8 and 12 M NaOH solutions as activation media. Such conditions were necessary for the required extent of fly ash dissolution for

Acknowledgements

The present study was carried out under the project Geoash supported by RFCS (Research Fund for Coal and Steel), EU (RFC-CR-04005). The authors thank to ENDESA and Unión FENOSA for supplying the fly ash samples employed in this study. The authors also acknowledge R. Bartroli, M. Cabañas, S. Martínez and J. Elvira (Institute of Earth Sciences “Jaume Almera”) and E. Beyret (UPC) for their technical assistance during the development of this work.

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