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Synthesis and Microstructural Characterization of Fully-Reacted Potassium-Poly(sialate-Siloxo) Geopolymeric Cement Matrix

ACI Materials Journal, Mar/Apr 2008 by Yunsheng, Zhang, Wei, Sun, Zongjin, Li

In this paper, a total of nine potassium-poly(sialate-siloxo) (K-PSS) geopolymeric cement matrixes, with different molar ratios of SiO^sub 2^/ Al^sub 2^O^sub 3^, K^sub 2^O/Al^sub 2^O^sub 3^, and H2O/K^sub 2^O, is designated to investigate the influence of the three ratios on mechanical properties and microstructure in accordance with the orthogonal design principle. The experimental results show that SiO^sub 2^/Al^sub 2^O^sub 3^ has the most significant effect on compressive strength among the three ratios. The highest compressive strength (5.04 ksi [34.8MPa]) can be achieved when SiO^sub 2^/Al^sub 2^O^sub 3^ = 4.5, K^sub 2^O/Al^sub 2^O^sub 3^ = 0.8 and H2O/K^sub 2^O = 5.0. Comparing the infrared (IR) spectra of nine K-PSS geopolymeric cement matrixes also indicates that the geopolymeric cement matrix with the highest strength is the most fully-reacted one and possesses the largest amount of geopolymeric products. Subsequently, X-ray powder diffraction (XRD), environment-scanning electron microscope equipped with energy dispersion X-ray analysis (ESEM-EDXA), transmission electron microscopy-electron diffraction spectroscopy (TEM-EDS), and magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) techniques are employed to further characterize the microstructure of the fully-reacted geopolymeric cement matrix. The microscopic analysis reveals that the fully-reacted K-PSS geopolymeric cement matrix possesses structural characteristics similar to glassy or gel substances in having a wide range of Si endowments, but predominantly the framework molecular chains of Si partially replaced by four-coordinated Al tetrahedral. A three-dimensional (3D) molecular structural model is also proposed based on the decomposition of MAS-NMR spectrum of the fully-reacted K-PSS geopolymeric cement matrix synthesized from the optimum mixture proportion.

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Keywords: microstructure; preparation; strength.

(ProQuest: ... denotes formula omitted.)

INTRODUCTION

In recent years, there has been significant development worldwide in a new type of inorganic binder: geopolymeric cement. Geopolymeric cement is one type of three-dimensional (3D) CaO-free aluminosilicate gel binder, which was first introduced into the inorganic binder world by Glukhovsky in the former Soviet Union in the 1950s.1 In France, Davidovits2 also conducted extensive research in the late 1970s. Geopolymeric cement can be synthesized by mixing aluminosilicate reactive materials with no CaO component (such as metakaolin, dehydrated clay, and Class F fly ash) and strongly alkaline solutions (such as NaOH or KOH), and then curing it at room temperature. Under a strongly alkaline solution, aluminosilicate reactive materials are rapidly dissolved into solution to form free SiO^sub 4^ and AlO^sub 4^ tetrahedral units. With the development of reaction, mixture water is gradually split out and these SiO^sub 4^ and AlO^sub 4^ tetrahedral units are linked alternatively to yield polymeric precursors (-SiO^sub 4^-AlO^sub 4^-, -SiO^sub 4^-AlO^sub 4^-SiO^sub 4^-, or -SiO^sub 4^-AlO^sub 4^-SiO^sub 4^- SiO^sub 4^-) by sharing all oxygen atoms between two tetrahedral units, thereby forming monolithic-like geopolymeric products.3-7 According to the molecular structure, geopolymeric cement can be expressed in the following empirical formula3

R^sub n^{-(SiO^sub 2^)^sub z^-AlO^sub 2^-}^sub n^ × wH2O

where R is a cation such as potassium (K) or sodium (Na); n is degree of polycondensation; z is 1, 2, and 3; and w is binding water amount.

Geopolymeric cement made with reasonable mixturedesign and formulation can exhibit superior properties to portland cement; the production of geopolymeric cement requires much lower calcining temperature (1112 to 1472 °F [600 to 800 °C]) and emits 80 to 90% less CO2 than portland cement.3,8 Reasonable strength can be gained in a short period at room temperature. In most cases, 70% of the final compressive strength is developed in the first 12 hours.3,8-10 Low permeability, comparable with natural granite, is another property of geopolymeric cement.3,11,12 It is also reported that resistance to fire and acid attacks for geopolymeric cement are substantially superior to those for portland cement.13,14 Apart from the high early strength, low permeability, and good fire and acid resistance, geopolymeric cement also can attain higher unconfined compressive strength and shrink much less than portland cement.3,12 Other documented properties include good resistance to freezing-and-thawing cycles as well as excellent solidification of heavy metal ions.3,8-18 These properties make geopolymeric cement a strong candidate for substituting portland cement applied in the fields of civil, bridge, pavement, hydraulic, underground, and military engineering.19-20

In this study, a fully-reacted potassium-poly(sialatesiloxo) (K-PSS) geopolymeric cement matrix is attempted to synthesize at room temperature by optimizing three key molar ratios: SiO^sub 2^/Al^sub 2^O^sub 3^, K^sub 2^O/Al^sub 2^O^sub 3^, and H2O/K^sub 2^O. The compressive strength and microstructure of the hardened geopolymeric cement matrixes are evaluated as a function of the three ratios. The influencing extent of each ratio on the compressive strength will be quantitatively determined on basis of the statistical variance analysis. The microstructural changes as a function of ratios will also be investigated by using X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) techniques. Based on the macroscopic and microscopic experiments, an almost fully-reacted K-PSS geopolymeric cement matrix with the highest strength and optimum microstructure can be obtained by properly adjusting the three molar ratios. Subsequently, the coordination status of two main construction elements (Al and Si), micrographics and chemical compositions of the fully-reacted K-PSS geopolymeric cement matrix are further characterized and examined by using an environment scanning electron microscope equipped with energy dispersion X-ray analysis (ESEM-EDXA), highly sensitive magic angle spinning-nuclear magnetic resonance spectroscopy (MAS-NMR), and transmission electron microscopyelectron diffraction spectroscopy (TEM-EDS) techniques. Finally, a 3D molecular model is proposed for the fullyreacted K-PSS geopolymeric cement matrix based on the aforementioned microanalysis.

Synthesis and Microstructural Characterization of Fully-Reacted Potassium-Poly(sialate-Siloxo) Geopolymeric Cement Matrix

ACI Materials Journal, Mar/Apr 2008 by Yunsheng, Zhang, Wei, Sun, Zongjin, Li

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