The Hydrothermal Synthesis of Zeolites: History and Development from the Earliest Days to the Present Time

Geopolymers are a class of aluminosilicate materials synthesized by alkaline or alkali-silicate activation of solid alumina- and silica-containing precursor materials at ambient or higher temperature. These products have highly significant commercial and technological potential, but the exact physicochemical nature of the geopolymeric binder phase has never before been determined. Through analysis of existing experimental results from the literature and comparison with related systems, in particular the hydrothermal synthesis of zeolites, geopolymeric binders are identified as being comprised of agglomerates of nanocrystalline zeolites compacted by an amorphous gel phase. The degree of crystallinity is largely determined by product formulation and synthesis conditions. Results from powder X-ray diffraction, microscopy, electron diffraction, mechanical strength testing, and calorimetry are analyzed and shown to be consistent with the theory presented. The implications of nanocrystallinity are discussed, an...

Do Geopolymers Actually Contain Nanocrystalline Zeolites? A Reexamination of Existing Results

Structural models for the primary strength and durability-giving reaction product in modern cements, a calcium (alumino)silicate hydrate gel, have previously been based solely on non-cross-linked tobermorite structures. However, recent experimental studies of laboratory-synthesized and alkali-activated slag (AAS) binders have indicated that the calcium-sodium aluminosilicate hydrate [C-(N)-A-S-H] gel formed in these systems can be significantly cross-linked. Here, we propose a model that describes the C-(N)-A-S-H gel as a mixture of cross-linked and non-cross-linked tobermorite-based structures (the cross-linked substituted tobermorite model, CSTM), which can more appropriately describe the spectroscopic and density information available for this material. Analysis of the phase assemblage and Al coordination environments of AAS binders shows that it is not possible to fully account for the chemistry of AAS by use of the assumption that all of the tetrahedral Al is present in a tobermorite-type C-(N)-A-S-H gel, due to the structural constraints of the gel. Application of the CSTM can for the first time reconcile this information, indicating the presence of an additional activation product that contains highly connected four-coordinated silicate and aluminate species. The CSTM therefore provides a more advanced description of the chemistry and structure of calcium-sodium aluminosilicate gel structures than that previously established in the literature.

/pdf/generalized-structural-description-of-calcium-sodium-d1xiv5j5s0.pdf

Generalized Structural Description of Calcium–Sodium Aluminosilicate Hydrate Gels: The Cross-Linked Substituted Tobermorite Model

The development of the pore structure of geopolymers synthesized from class F fly ash was studied using electron microscopy and porosimetry. Fly-ash-based geopolymer can be classified as a mesoporous aluminosilicate material, with a Si/Al composition varying from 1.51 to 2.24. The Si/Al composition and pore structure of fly-ash-based geopolymer vary depending on the curing temperature and the silicate ratio of the activating solutions (SiO2/M2O, M = Na or K). A higher Si/Al ratio and finer pores are obtained in geopolymers synthesized at higher temperature and silicate ratios. Elevating the curing temperature increases the extent and rate of reaction, shown through an increase in mesopore volume, surface area, and an accelerated setting time. The kinetics appears to be temperature-controlled only before the material is hardened. Very high silicate ratios (SiO2/M2O ≥ 2.0) are also believed to slow the reactions. The pore structure of K-based geopolymer is more susceptible to change in temperature than that...

Effect of Curing Temperature and Silicate Concentration on Fly-Ash-Based Geopolymerization

This title covers quartz, zeolites, gemstones, perovskite type oxides, ferrite, carbon allotropes, complex coordinated compounds and many more products now being produced using hydrothermal technology. "Handbook of Hydrothermal Technology" brings together the latest techniques in this rapidly advancing field in one exceptionally useful, long-needed volume. The handbook provides a single source for understanding how aqueous solvents or mineralizers work under temperature and pressure to dissolve and recrystallize normally insoluble materials, and decompose or recycle any waste material. The result, as the authors show in the book, is technologically the most efficient method in crystal growth, materials processing, and waste treatment.The book gives scientists and technologists an overview of the entire subject including: Evolution of the technology from geology to widespread industrial use; Descriptions of equipment used in the process and how it works; Problems involved with the growth of crystals, processing of technological materials, environmental and safety issues; and Analysis of the direction of today's technology. In addition, readers get a close look at the hydrothermal synthesis of zeolites, fluorides, sulfides, tungstates, and molybdates, as well as native elements and simple oxides. Delving into the commercial production of various types, the authors clarify the effects of temperature, pressure, solvents, and various other chemical components on the hydrothermal processes.

http://www.coema.org.cn/bbs/UploadFile/2009-1/200911210302664295.pdf

Handbook of Hydrothermal Technology: A Technology for Crystal Growth and Materials Processing

The incorporation of aluminum into silicate ions in alkaline aqueous solutions, studied by aluminum-27 n.m.r.

Synthetic cubic faujasites (Si/Al = 4) are synthesized hydrothermally and compared with (1) materials of similar framework composition produced by secondary synthesis from zeolite Y using silicon hexafluoride and (2) zeolite ZSM-20. Solid-state NMR ({sup 29}Si and {sup 27}Al), XRD, FTIR, and surface analysis are used to characterize the materials, which are also evaluated as catalysts by using n-hexane conversion as a test reaction. {sup 29}Si NMR results suggest that the materials produced by primary synthesis have aluminum ordering differing from that observed in materials with similar framework composition (Si/Al) but produced by secondary synthesis. The difference evident in the {sup 29}Si NMR may account for the higher catalytic activity observed with the products of direct synthesis. Surface analysis using depth profiling shows that the siliceous frameworks produced by direct synthesis are more homogeneous than materials produced by secondary synthesis.

A comparison of siliceous faujasitic zeolites produced by direct synthesis or by secondary synthesis

Exchange reactions in aluminosilicate solutions

A New Silicate Clathrate Hydrate: An X-Ray Diffraction and Nuclear Magnetic Resonance Study of a System with Octameric Silicate Anions and Trivalent Cations

Comparison of Siliceous Faujasitic Zeolites Produced by Direct Synthesis or by Secondary Synthesis.

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