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Ind. Eng. Chem. Res. 2003, 42, 1698-1706
Effect of Source Materials on Geopolymerization Hua Xu and Jannie S. J. van Deventer* Department of Chemical Engineering, The University of Melbourne, Parkville, Victoria, Australia 3010
Few attempts have been made to understand the effect of the structural and surface properties of source materials on geopolymerization. In the present work, kaolinite, albite, and fly ash have been chosen for a case study to answer this question. X-ray diffraction, X-ray fluoresence, X-ray photoelectron spectroscopy, 27Al and 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and scanning electron microscopy (SEM) have been used to characterize the source materials and geopolymers. The fly ash that has an amorphous structure and possesses the lowest binding energies in its structure shows the highest reactivity during geopolymerization. Apart from the reactivity, the desired Si/Al molar ratio in the gel phases and the leaching of the source materials are observed to significantly affect the geopolymerization. The geopolymerization of mixtures of two or three source materials demonstrates the benefits derived from the interaction between the source materials. Such interactions could optimize the gel phase composition and result in high strength geopolymers. It is expected that the geopolymers containing Ca and K and having Si/Al molar ratios of about 2 in the gel phase could have a high compressive strength. 1. Introduction Geopolymerization could convert a wide range of waste aluminosilicate materials into building materials with excellent physical and chemical properties as well as long-term durability.1-5 Similar to the synthesis of zeolite, geopolymers are formed by reacting silica- and alumina-rich solids with a solution of alkali salts through dissolution, nucleation, gelation, and polymerization.1 Reaction times, however, are substantially faster than that of a comparable zeolite, with the structure of the resultant geopolymers consisting of crystalline aggregates and semicrystallized and/or an amorphous poly(aluminosilicate) gel phase.1 Over the past 20 years, geopolymer technology has emerged as a promising replacement of construction materials as well as a possible solution to the treatment of toxic and radioactive wastes.1-8 Many wastes, such as fly ash, slag, construction residues, and mine tailings as well as a wide range of rock types, have been used successfully in geopolymerization.1-16 The source materials used in geopolymerization, in terms of pretreatment, can be divided into two main groups: (1) calcined materials, such as fly ash, metakaolinite, slag, construction residues, pozzolanic wastes,1-5,12-14,16 etc., and (2) noncalcined materials, for instance, kaolinite, feldspars, rock-type aluminosilicate minerals, mine tailings, etc.9-11 Previous research shows that geopolymerization of calcined source materials has a faster dissolution and gelation, with the resultant geopolymers demonstrating a higher early compressive strength.1,3-5,12,13 In contrast, geopolymers synthesized from noncalcined source materials display a higher increase in the compressive strength in the later stage of the reaction.9,10 Geopolymers synthesized from one calcined material (for instance, fly ash or metakaolinite) and one noncalcined material (such as kaolinite) are found to have both high early and high final compressive * To whom correspondence should be addressed. Tel.: 61-3-83446620. Fax: 61-3-83444153. E-mail: jannie@ unimelb.edu.au.
strengths as well as a reduced probability of cracking.16,17 This finding suggests that the calcination of source materials as well as the appropriate combination of calcined and noncalcined materials significantly influences the process of geopolymerization and the mechanical properties of the resultant geopolymers. Although the geopolymerizations of only calcined, only noncalcined, and both calcined and noncalcined materials have been investigated previously with an emphasis on obtaining geopolymers with a high compressive strength and/or a more stable structure,1,2,6,9-11,15-17 the effect of the nature of the source materials on geopolymerization has not yet been documented. This study, therefore, investigates three source materials that have been widely used in geopolymerization, i.e., kaolinite, albite (noncalcined materials), and fly ash (calcined material)1,2,6,9-11,15-17 using X-ray diffraction (XRD), X-ray fluoresence (XRF), X-ray photoelectron spectroscopy (XPS), 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR), and scanning electron microscopy (SEM) analyses in an attempt to determine the effect of the structural and surface properties of the source materials on geopolymerization. 2. Experimental Methods Materials. Kaolinite, grade HR1/F, was purchased from Commercial Minerals, Australia, and has a particle size of 50% less than 0.5 µm and 1% larger than 38 µm. Albite was obtained from Minerals Corp., Ltd., Broken Hill, NSW, Australia, with a particle size of 100% less than 53 µm. Fly ash (Gladstone) was bought from Pozzolanic Enterprises Pty. Ltd., Queensland, Australia, and possesses a particle size of 50% less than 8.47 µm and 1% greater than 110 µm. The elemental composition of three minerals, as detected from XRF analysis, and the average density and hardness are given in Table 1. Sodium silicate (Vitrosol No. 40) and potassium silicate (Kasil 2236) solutions were supplied by PQ Australia Pty. Ltd., Victoria, Australia. The average compositions,
10.1021/ie0206958 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/25/2003
Ind. Eng. Chem. Res., Vol. 42, No. 8, 2003 1699 Table 1. Mass Composition of Kaolinite, Albite, and Fly Ash Detected by XRF Analysis kaolinite (wt %)
albite (wt %)
fly ash (wt %)
54.4 29.4 0.20 0.20 0.20 1.40 0.20
70.90 17.00 9.75 0.43 0.31 0.07 0.04
