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Ind. Eng. Chem. Res. 2002, 41, 4242-4251
Characterization of Fly-Ash-Based Geopolymeric Binders Activated with Sodium Aluminate J. W. Phair and J. S. J. van Deventer* Department of Chemical Engineering, The University of Melbourne, 3010 Victoria, Australia
Properties and characteristics of sodium silicate activated fly-ash-based geopolymers have been extensively investigated, but the utilization of sodium aluminate activation has received little attention. The present work, therefore, examines the properties of the starting materials and sodium aluminate solutions and their effect on the final material properties and microstructure of fly-ash-based geopolymers. To achieve this, starting sodium aluminate solutions were characterized with ATR-FTIR and 27Al NMR spectroscopies to determine the coordination state of aluminum as a function of solution concentration and [OH]/[Al] ratio. Various fly-ash-based geopolymeric matrixes were synthesized utilizing fly ash, kaolinite, K-feldspar, and slag as the mineral aluminum sources. The matrixes were activated with alkali silicate or alkali aluminate solutions as a function of pH, concentration, and alkali ion (Na+ or K+). FTIR and XRD studies combined with compressive strength tests demonstrated that in certain cases aluminate activated geopolymers were mechanically superior to traditional silicate activated geopolymers. Of specific interest was whether the superior properties could be related to a polysialate or calciumaluminate-hydrate phase, which both contain 6-coordinate aluminum. A discussion of commercially viable industrial aluminate waste sources as potential large-scale feedstock for geopolymers was included. 1. Introduction The process design and synthesis of fly-ash-based geopolymers will continue to undergo intense research and development until it reaches the stage where it may be industrially applied. To achieve this, any systematic method of process and synthesis control that is adopted must ensure that the output products are competitive with other materials available on the market. Motivation for the present work is focused on reducing the energy costs associated with the mixing and proportioning of fly-ash-based geopolymers, by introducing an alternative and more cost-effective alkali activator that reduces the costs for input materials. Given the projected size and scale at which these materials may be manufactured and the requirement for large quantities of ingredients to activate the geopolymerization reaction, the price of alkali activators will always be at the forefront of costs to be minimized. The utilization of inexpensive or waste alkali activators is an effective approach to ensure the competitiveness of the manufacturing process and overall production of fly-ash-based geopolymers. Activation of Fly-Ash-Based Geopolymers. The process of geopolymerization and geopolymers, originally developed by Joseph Davidovits in the late 1970s and early 1980s,1,2 involves the addition of sodium silicate to promote, initiate, or activate the geopolymerization reaction. Recently, interest in geopolymers has increased because of the potential application of geopolymer technology to a wide variety of industrial aluminum or silicon-rich waste sources to produce mineralpolymeric products as construction materials with outstanding fire resistant properties, compressive strength, * To whom correspondence should be addressed. Phone: +61-3-83446620.Fax: +61-3-83444153.E-mail:jannie@unimelb. edu.au.
and durability.3 However, the extent to which alternative alkali activators may be utilized to reduce costs and improve performance of geopolymers has received considerably less attention. To investigate the effect of novel alkali activators on geopolymerization, it is impossible to rely on traditional concepts, methodologies, and models central to the understanding of calcareous-based cementitious systems. The purpose of the present research is to therefore not only further the commercial and economic viability of geopolymers as an added-value construction material but to try to develop the means for characterizing flyash-based geopolymers as a function of alkali activator. An emphasis will be placed on systematically evaluating experimental data describing both the microstructure and material properties of fly-ash-based geopolymers. Efforts will be made to quantify the effects of aluminate activation on the fundamental polysialate model, established by Davidovits to describe the basic chemistry of geopolymer matrixes. Microstructure of Fly-Ash-Based Geopolymers Activated with Sodium Aluminate. The physical microstructure of fly-ash-based geopolymers activated with sodium aluminate is not expected to be dramatically different to the microstructure found in conventional sodium silicate activated geopolymers. Chemical differences in the microstructure, though, are expected to be significant when comparing aluminate-activated binders to silicate-activated binders. The basis for quantifying this chemical difference is to investigate the manner by which a particular alkali activator may or may not be incorporated into the polysialate network of the geopolymer. It is becoming more and more common for researchers to utilize the polysialate model of Davidovits,4 to rationalize and characterize the role of the chemical microstructure in determining the various applications and properties of geopolymeric matrixes. A “sialate” is
10.1021/ie010937o CCC: $22.00 © 2002 American Chemical Society Published on Web 07/17/2002
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defined as a silicon-oxo-aluminate repeating unit found in any polysialate macromolecule and is commonly denoted by the formula: (-Si-O-Al-O-)n. Polysialates have an empirical formula of Mn{-(SiO2)zAlO2}n,wH2O where M is a cation and n is the degree of polycondensation with the most common types being the following: poly(sialate): Mn-(-Si-O-Al-O-)-n [(M)-PS]; poly(sialate-siloxo): Mn-(-Si-O-Al-O-SiO-)-n [(M)-PSS]; and poly(sialate-disiloxo): Mn-(-SiO-Al-O-Si-O-Al-O-)-n [(M)-PSDS]. The defining properties of polysialates are the IV-fold aluminum coordination and that they can exist as ring or chain polymers. Compared to alternative approaches of describing geopolymeric matrixes such as phase diagrams/compositions,5 ingredient ratios, or dissolution properties of the starting materials,6 the polysialate model aims to provide a fundamental molecular interpretation of the various microscopic, macroscopic, and physicochemical properties of geopolymeric binders. Polysialate chemistry, therefore, serves as a monostructural tool in a similar way to which calcium silicate hydrates/calcium hydroxide chemistry describes Portland cement-type systems or calcium oxide/aluminum oxide primarily define calcium aluminate cements. The advantages of such an approach are multiple; namely, in defining polysialate chemistry, attention is drawn to the molecular and physicochemical features upon which material, mechanical, and physical advantages are based. Alternative approaches to date have failed to provide such information. For instance, phase diagrams assuming either equilibrium or pseudoequilibrium conditions have yet to include dissolution into compositional predictions. Moreover, consideration of dissolution properties of the aluminosilicate starting materials alone has ignored the final relationship between the geopolymeric phase and the remaining particulates and the consequences this may have for the physical properties.7 Polysialate chemistry itself, however, still remains in a developmental stage and as a consequence has central assumptions requiring clarification before it can be applied to predict mechanical properties. One such unclarified assumption of polysialate chemistry is that chemically bound aluminum is restricted to a tetrahedral (4-fold) coordination environment within the polysialate macromolecules.3,8 Conversely, the appearance of 6-coordinate Al in fly-ash-based geopolymers has been well documented.7,10 Substantial research is therefore required to decipher this anomaly and establish the role and requirement of 6-coordinate Al in polysialate chemistry in both purer and more applied geopolymer systems. It remains to be seen whether the polysialate phase can include 6-coordinate Al or whether additional phases must exist to permit octahedral Al. The present work will directly address this issue by utilizing a variety of techniques to characterize a series of solid and liquid aluminum sources with variable Al coordination for their effect on the process of polysialate formation and the final chemical and physical properties of fly-ash-based geopolymeric matrixes. 2. Experimental Section Materials. Sodium silicate (Vitrosol N(N40); weight ratio SiO2:Na2O ) 3.22, weight % SiO2 ) 28.7) and potassium silicate (KASIL 2236(K32); weight ratio SiO2: K2O ) 2.23, weight % SiO2 ) 24.8), were obtained from P. Q. Australia. All solutions were diluted freshly from stock solution using distilled H2O. Standard buffers for
Table 1. Composition and Properties of Aluminosilicate Source Minerals chemical composition (weight %) SiO2 Al2O3 CaO Fe2O3 MgO K2O Na2O SO3 MnO P2O5 TiO2 loss on ignition surface area (m2/g) mean particle size (d50, µm) density (g /m2)
kaolinite K-feldspar slag 52.4 28.6 0.1 1.2 0.2 0.2 0.1 0 0.02 0.1 2.8 14.28 21.05 0.9 2.65
67.1 17.6 0.2 0.2 0 10.6
