Ind. Eng. Chem. Res. 2000, 39, 2925-2934
2925
Mechanism of Polysialation in the Incorporation of Zirconia into Fly Ash-Based Geopolymers J. W. Phair,† J. S. J. Van Deventer,*,† and J. D. Smith‡ Department of Chemical Engineering and School of Chemistry, The University of Melbourne, 3010 Victoria, Australia
Little attempt has been made to understand the chemical and physical consequences of incorporating non-aluminosilicate materials into geopolymers. In the present work zirconia was chosen as an inert reference to examine the effects of a non-aluminosilicate source on the chemical and physical properties of a geopolymer matrix. Fourier transform infrared (FTIR), X-ray diffraction, and compressive strength analyses were used to characterize the matrixes. Inclusion of only a small quantity of zirconia was shown to impart a substantial increase in compressive strength for fly ash-based geopolymers. The basis of this increase in strength has been hypothesized to be due to the formation of specific zirconia-associated, three-dimensional polysialate species, which reduce the mobility of sodium while maintaining the charge balance and structural stability of the matrix. Through a simplified study based on zirconium polysialate solubility and FTIR measurements, an attempt was made to rationalize and correlate the observed chemical and physical properties of the matrixes with established mechanisms of geopolymerization. 1. Introduction Geopolymeric systems are gaining increasing attention as a versatile solidification and immobilization technology as well as a durable construction material based on waste aluminum and silica-rich materials, such as fly ash.1,2 Several mechanisms have been postulated for geopolymerization,3 and considerable work has confirmed that dissolution of the starting materials is the major geopolymer-initializing step. Dissolution has two major roles in geopolymerization. First, it is the necessary process whereby polysialateforming species are liberated from the starting materials similarly to zeolite precursors.4,5 Second, it prepares or “activates” the surface for surface binding reactions which are hypothesized to significantly contribute to the final strength of the structure. Depending upon the quantities and nature of starting aluminum and silicate sources, the extent of dissolution will vary and therefore determine the chemical composition of the geopolymeric phase (M2O:Al2O3:SiO2), which controls the resultant bulk physical and chemical properties of the final product.6 To what extent the dissolution step in geopolymerization is required remains unknown, and to what extent other factors complement dissolution or predominate in the absence thereof remains largely unexamined. The present work therefore introduces insoluble non-aluminosilicate fillers as a template for investigating geopolymerization reactions in which the dissolution step has been removed as an integral process in geopolymer formation. Addition of various quantities of zirconia into a standard fly ash-based geopolymeric matrix provides the basis to examine the effect of the filler on the various chemical and physical properties of the matrix. * To whom all correspondence should be addressed. Tel.: +61-3-93446620. Fax: +61-3-93444153. E-mail: jsj.van_deventer @chemeng.unimelb.edu. † Department of Chemical Engineering. ‡ School of Chemistry.
Reasons for the Incorporation of Zirconia into Geopolymers. Incorporating additional or “filler” materials into geopolymers is necessary for a variety of structural and mechanical improvements for a range of applications.7 Filler materials and aggregates have widely been used in the construction industry to increase the compressive strength in concrete while reducing cement requirements. Few investigations have examined the interaction of the geopolymeric phase with filler materials despite the fact that there has been extensive work into the nature of geopolymers and the geopolymeric interphase region obtained from aluminosilicate starting materials.8 Zirconia is an immediate choice to investigate as a filler because of its high technological and practical importance that has been established for a variety of refractory and ceramic-type materials. Of substantial consequence are the well-defined physical characteristics (homogeneity, stability at high temperature, >300 °C, and under ionizing radiation doses) and chemical properties (i.e., relative insolubility in water
