The effect of temperature on the geopolymerization process of a metakaolin-based geopolymer

This paper describes the effect of different curing temperatures on the geopolymerization process, physical, mechanical and optical properties of a metakaolin-based geopolymer activated by alkali. The influence of different curing temperatures (within the range 30 to 90 °C) was studied systematically by means of differential scanning calorimetry (DSC), SEM, UV/Vis Spectrophotometry, Leaching analysis and Brunauer-Emmet-Teller method (BET). The results showed the existence of an optimum temperature at which the geopolymer presents the best physical and mechanical properties. The geopolymers cured at 30 and 90 °C presented high porosity, and were translucent to the Visible light, which makes possible to tailor this inorganic polymers for optical and photocatalytic applications.     

Synthesis of pentasil zeolites from sodium silicate solutions in the presence of quaternary imidazole compounds

The structure-selectivity of a class or organocations derived from imidazole is described for zeolite synthesis. In particular, high-silica zeolites are synthesized in sodium silicate solutions at reduced hydroxide concentrations. ZSM-12, 22, and 23 are produced by unique organocation structures. The same organocation can produce either ZSM-23 or 48 depending on other synthesis parameters. A discussion is given for the relationship of organocation size and the corresponding pore system produced. A kinetic analysis for the variable synthetic rates is presented. In addition, the crystallization chemistry for this crystallization field in the absence of organocation is also presented; fine-tuning produces magadiite, mordenite, or ZSM-5. Lastly, the inhibitory effect of aluminate in these syntheses is also described.     

Thermosensitive solutions and gels consisting of poly(vinyl alcohol) and sodium silicate

We have investigated the transparency for visible light of a poly(vinyl alcohol) (PVA) aqueous solution in the presence of sodium silicates (SS). The transparency of the aqueous solution dramatically changed by varying temperature despite that neither aqueous solutions of PVA nor SS is sensitive to temperature. The aqueous solution showed a lower critical solution temperature (LCST) around 30 °C. The LCST depended on both concentrations of PVA and sodium silicate. Below the LCST, the transmittance was thermally irreversible. By a further increase in the temperature, the solution was separated into two phases consisting of a phase rich in solvent and that in solute; that is a coacervation. In the region of coacervation, the transmittance was thermally reversible. Reversible change in the transmittance was realized corresponding to stepwise temperature changes between 10 and 40 °C. A PVA gel swollen by the solution demonstrated transparency change in response to temperature changes as well as the solution.     

Electromagneto-thermoelastic fundamental solutions in a two-dimensional problem for short time

Summary. The model of the two-dimensional equations of generalized magneto-thermoelasticity in a perfectly conducting medium is established. The analysis is carried out in the uncoupled framework under the classical Lord–Shulman [3] and Green–Lindsay [6] theories of thermoelasticity. Fundamental solutions of the corresponding differential equations are obtained using the inverse Laplace transform in an approximate manner for small values of time. The behavior of these solutions is discussed when we apply a continuous source of heat to an infinite elastic body. Numerical results in the presence of a magnetic field are presented graphically along with a comparison of the three theories. A discussion concerning which of the three theories is the most physically acceptable is given, with the Green–Lindsay formulation emerging as the clear choice.     

Mechanism of geopolymerization and factors influencing its development: a review

Geopolymerization is a developing field of research for utilizing solid waste and by-products. It provides a mature and cost-effective solution to many problems where hazardous residue has to be treated and stored under critical environmental conditions. Geopolymer involves the silicates and aluminates of by-products to undergo process of geopolymerization. It is environmentally friendly and need moderate energy to produce. This review presents the work carried out on the chemical reaction, the source materials, and the factor affecting geopolymerization. Literature demonstrates that certain mix compositions and reaction conditions such as Al₂O₃/SiO₂, alkali concentration, curing temperature with curing time, water/solid ratio and pH significantly influences the formation and properties of a geopolymer. It is utilized to manufacture precast structures and non-structural elements, concrete pavements, concrete products and immobilization of toxic metal bearing waste that are resistant to heat and aggressive environment. Geopolymers gain 70% of the final strength in first 3–4 h of curing.     

Phase separation behaviour of a metal-organic derived sodium silicate glass

The phase separation behaviour of an 18.56 mol % Na₂O-SiO₂ glass prepared via the standard glass preparation technique and the metallic-organic derived (MOD) method have been studied and contrasted. Our results suggest that at this composition the miscibility temperature is significantly elevated when a MOD preparation procedure is used. The origin of this effect appears to be related to the enhanced water content in the glass prepared by the MOD procedure.     

Wave propagation in a simplified modelled poroelastic continuum: fundamental solutions and a time domain boundary element formulation

In finite element formulations for poroelastic continua a representation of Biot's theory using the unknowns solid displacement and pore pressure is preferred. Such a formulation is possible either for quasi‐static problems or for dynamic problems if the inertia effects of the fluid are neglected. Contrary to these formulations a boundary element method (BEM) for the general case of Biot's theory in time domain has been published (Wave Propagation in Viscoelastic and Poroelastic Continua: A Boundary Element Approach. Lecture Notes in Applied Mechanics. Springer: Berlin, Heidelberg, New York, 2001.). If the advantages of both methods are required it is common practice to couple both methods. However, for such a coupled FE/BE procedure a BEM for the simplified dynamic Biot theory as used in FEM must be developed. Therefore, here, the fundamental solutions as well as a BE time stepping procedure is presented for the simplified dynamic theory where the inertia effects of the fluid are neglected. Further, a semi‐analytical one‐dimensional solution is presented to check the proposed BE formulation. Finally, wave propagation problems are studied using either the complete Biot theory as well as the simplified theory. These examples show that no significant differences occur for the selected material. Copyright © 2005 John Wiley & Sons, Ltd.     

On the fundamental solutions in micropolar elasticity with voids

Summary By means of an uncoupling representation, we derive the fundamental solution for the differential system of micropolar elasticity with voids in the steady vibration case. Reciprocity properties are also explored.     

Modified fundamental solutions for the scattering of elastic waves by a cavity: Numerical results

The boundary integral equation method is very often used to solve exterior problems of scattering of waves (elastic waves, acoustic waves, water waves and electromagnetic waves). It is known, however, that this method fails to provide a unique solution at the so-called irregular frequencies. This difficulty is inherent to the method used rather than the nature of the problem. In the context of elastodynamics. we proposed, in a recent work¹, two methods for eliminating these irregular frequencies. Both are based on modifying the fundamental solution. Here we present numerical results pertaining to the solutions of the modified and unmodified integral equations.     

Solution of potential flow problems by the modified method of fundamental solutions: Formulations with the single layer and the double layer fundamental solutions

This paper describes an application of the recently proposed modified method of fundamental solutions (MMFS) to potential flow problems. The solution in two-dimensional Cartesian coordinates is represented in terms of the single layer and the double layer fundamental solutions. Collocation is used for the determination of the expansion coefficients. This novel method does not require a fictitious boundary as the conventional method of fundamental solutions (MFS). The source and the collocation points thus coincide on the physical boundary of the system. The desingularised values, consistent with the fundamental solutions used, are deduced from the direct boundary element method (BEM) integral equations by assuming a linear shape of the boundary between the collocation points. The respective values of the derivatives of the fundamental solution in the coordinate directions, as required in potential flow calculations, are calculated indirectly from the considerations of the constant potential field. The normal on the boundary is calculated by parametrisation of its length and the use of the cubic radial basis functions with the second-order polynomial augmentation. The components of the normal are calculated in an analytical way. A numerical example of potential flow around a two-dimensional circular region is presented. The results with the new MMFS are compared with the results of the classical MFS and the analytical solution. It is shown that the MMFS gives better accuracy for the potential, velocity components (partial derivatives of the potential), and absolute value of the velocity as compared with the classical MFS. The results with the single layer fundamental solution are more accurate than the results with the double layer fundamental solution.     

The method of fundamental solutions for a biharmonic inverse boundary determination problem

In this paper, a nonlinear inverse boundary value problem associated to the biharmonic equation is investigated. This problem consists of determining an unknown boundary portion of a solution domain by using additional data on the remaining known part of the boundary. The method of fundamental solutions (MFS), in combination with the Tikhonov zeroth order regularization technique, are employed. It is shown that the MFS regularization numerical technique produces a stable and accurate numerical solution for an optimal choice of the regularization parameter. A. Zeb on study leave visiting the University of Leeds.     

Hydrolytic stability of sodium silicate gels in the presence of aluminum

Polycondensation in alkali silicate solutions comprises a fundamental process of the geopolymerization technology. Previous works had shown that the hydrolytic stability of sodium silicate gels depends on the SiO₂/Na₂O ratio. Sodium silicate gels totally insoluble in water can be produced at SiO₂/Na₂O molar ratios higher than 4.4. This article aims at elucidating the effect of tetra-coordinated aluminum addition on the hydrolytic stability of sodium silicate gels. According to the results, the aluminum addition stabilizes the sodium silicate gels in an aqueous environment. A sodium silicate gel with SiO₂/Na₂O molar ratio 3.48, which is totally soluble in deionized water at ambient temperature, can be transformed to insoluble sodium hydroaluminosilicates with the addition of tetrahedral aluminum at Al/Si molar ratios higher than 0.08. In addition, this article studies the structure of prepared sodium hydroaluminosilicates and draws very useful conclusions for the geopolymerization technology.     

On the Laplace transform of a matrix of fundamental solutions for thermoelastic plates

The Laplace transform of a matrix D(x,t) of fundamental solutions for the partial differential operator describing the time-dependent bending of thermoelastic plates with transverse shear deformation is constructed, and its asymptotic behavior near the origin is investigated. The differential system is reduced to an algebraic one through the application of the Laplace and then Fourier transformations, and all possible cases of roots of the determinant of the latter system are considered. It is shown that in every case, the asymptotic expansion of near the origin has the same dominant term. This is an important step in the construction of boundary-element methods for the above time-dependent model because it determines the nature of the singularity of the kernel of the boundary-integral-equations associated with various initial-boundary-value problems for the governing system.     

The method of fundamental solutions for a time-dependent two-dimensional Cauchy heat conduction problem

We investigate an application of the method of fundamental solutions (MFS) to the time-dependent two-dimensional Cauchy heat conduction problem, which is an inverse ill-posed problem. Data in the form of the solution and its normal derivative is given on a part of the boundary and no data is prescribed on the remaining part of the boundary of the solution domain. To generate a numerical approximation we generalize the work for the stationary case in Marin (2011) [23] to the time-dependent setting building on the MFS proposed in Johansson and Lesnic (2008) [15], for the one-dimensional heat conduction problem. We incorporate Tikhonov regularization to obtain stable results. The proposed approach is flexible and can be adjusted rather easily to various solution domains and data. An additional advantage is that the initial data does not need to be known a priori, but can be reconstructed as well.     

Wet chemical etching of silicate glasses in hydrofluoric acid based solutions

The etching of silicate glasses in aqueous hydrofluoric acid solutions is applied in many technological fields. In this review most of the aspects of the wet chemical etching process of silicate glasses are discussed. The mechanism of the dissolution reaction is governed by the adsorption of the two reactive species: HF and HF ₂ ^- and the catalytic action of H⁺ ions, resulting in the breakage of the siloxane bonds in the silicate network. The etch rate is determined by the composition of the etchant as well as by the glass, although the mechanism of dissolution is not influenced. In the second part of this review, diverse applications of etching glass objects in technology are described. Etching of SiO₂ and doped SiO₂ thin films, studied extensively for integrated circuit technology, is discussed separately.     

Solution of a two-dimensional bubble shape in potential flow by the method of fundamental solutions

This paper describes an application of the method of fundamental solutions to steady-state free boundary problems arising in potential flow around deformable bodies. The solution in two-dimensional Cartesian coordinates is represented in terms of the fundamental solution of the Laplace equation together with the first-order polynomial augmentation. The collocation is used for determination of the expansion coefficients. The shape of the free boundary is interpolated in the global sense by parameterisation of its length and use of the cubic radial basis functions with the second-order polynomial augmentation. The components of the normal and curvature are calculated in an analytical way. A special algorithm, based on Bernoulli equation, is used for the iterative reshaping of the free boundary towards its equilibrium position. The algorithm is divided into pressure equilibrium, incompressibility, node relocation, and smoothing steps. A numerical example of a two-dimensional deformed incompressible bubble in potential flow is shown.     

Frequency response analyses in vibroacoustics using the method of fundamental solutions

The method of fundamental solutions, one of the promising boundary-type meshless methods, is proposed as a direct procedure to formulate and analyze the vibroacoustic problem. The coupled system discussed in this study is composed of an acoustic-cavity and excited by an external force or an internal sound source harmonically. The wall of cavity consists of the beam or the plate components, respectively, in two- and three-dimensional problems. The two independent sub-systems interact at the interface simultaneously by satisfying the necessary equilibrium and compatibility conditions. The mathematical formulations described by the presented meshless method demonstrate straightforwardly the frequency responses of the vibroacoustic problems with no boundary integrals. General characteristics of the dynamic coupling effect are displayed, based on the systematic natural frequencies and mode shapes. Feasible results simulated by the presented numerical scheme are validated through meshless numerical experiments including the acoustic-wave propagation problems and the vibroacoustic problems.