The Effect of Silica Polymerization in Fly Ash on the Strength of Geopolymers
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The Effect of Silica Polymerization in Fly Ash on the Strength of Geopolymers N. Koshukhova1, I. Zhernovsky1 and K. Sobolev2 The Belgorod State Technological University named after V.G. Shoukhov, Russia. 2 Department of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee, USA. 1
ABSTRACT The formation of the structure of geopolymer binders based on low-calcium fly ash is a multifactorial process that depends on the degree of solubility of aluminosilicate components in the solution of alkali activator. It is observed that the geopolymer binders based on fly ash with an identical chemical and mineral composition, the same grain size, and also activated by the same alkalis can result in a different strength. This study is based on the assumption that there is dependence between the solubility of aluminosilicate components and the degree of polymerization of the silicates in glass phase. The degree of SiO2-polymerization is an integral parameter that is equal to the Si molar ratio (fSi) of the silicate component in the glass phase of fly ash. The degree of SiO2-polymerization can be estimated from the molar composition of glass phase, which is determined from the chemical composition and quantitative X-ray diffraction analysis including identification of the amorphous phase composition. The SiO2 polymerization rates of investigated fly ash specimens are confirmed by the IR results, specifically, by comparison of absorption bands of silicate fragments with varying levels of connectivity (Q0-4) in the range of wave numbers of 650–1350 cm-1. The comparative analysis of the correlation of 28-day strength of geopolymer binders based on fly ash from different sources and level of SiO2-polymerization demonstrated an inverse relationship with fSi molar ratio and compressive strength. INTRODUCTION Low-calcium fly ash (ASTM C618 class F) is a very fine aluminosilicate material formed by the combustion of a bituminous coal. Class F fly ash is an attractive source for geopolymer production [1–3]. Strength properties of geopolymers depend on many parameters: pH of alkali medium, alkali–aluminum ratio, particle size distribution, chemical and mineral composition of fly ash, content of vitreous phase of fly ash, the parameters of thermal curing, etc. The differences in chemical composition of fly ash are related to mineralogical and geochemical features of enclosing secondary rocks, syngenetic to coal formations. Different concentrations of crystal- and vitreous- components in fly ash are due to the difference in thermal evolution of the materials, such as different fuel combustion temperatures and also cooling rates of dispersed aluminosilicate pyrogenic melt. One of the basic characteristics of fly ash, reactivity (defined by solubility in alkali medium) is directly related to geopolymer structure [4–7]. The variation of fly ash based geopolymer binder properties is due to the variety of properties of aluminosilicate fly ash (especially, vitreous phase). In this work, an attempt to demonstrate the correlation between
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