Showing papers on "Sodium silicate published in 2005"

Geopolymer formation processes at room temperature studied by 29Si and 27Al MAS-NMR

Abstract: A systematic study of 29 Si and 27 Al MAS-NMR has been carried out in an attempt to understand the reaction mechanism of the geopolymer formation at room temperature. For selected compositions, reactions were followed at selected times after the initial mixing of the reactants until the final products formed. The reactants were silicate anions, metakaolin and aqueous alkali. Silicate anions from various sources, such as sodium orthosilicate (Na 4 SiO 4 ) and two types of sodium silicate solutions (2SiO 2 ·Na 2 O and 4SiO 2 ·Na 2 O) were investigated. Further, silicate anions formed from the reaction of colloidal silica and aqueous alkali was also studied as a source for silicate anions. Interaction of silicate anions with Al sites of metakaolin soon after the mixing was obvious as 29 Si MAS-NMR resonance lines of silicate anions were shifted to less shielded values. As the reaction progressed, the coordination of Al (IV, V, VI) in metakaolin changed almost completely to IV. It was indicated by NMR observation that polymeric silicate anions of high cross-linked densities were better than monomeric silicate anion in the geopolymerisation process.

Structural studies of geopolymers by 29Si and 27Al MAS-NMR

Abstract: A systematic study of geopolymers by 29Si and 27Al MAS NMR has been carried out in an attempt to understand polymer structural details. 27Al MAS NMR data shows that transient aluminium species are formed during the reaction of metakaolin with NaOH. Interaction of silicate anions with the aluminium sites of metakaolin was evident during the synthesis of geopolymers as observed from low field shift of 29Si MAS NMR resonance lines of silicate centres. As the reaction progresses, the coordination of aluminium (IV, V and VI) in metakaolin changes almost completely to IV. 29Si MAS NMR of selected compositions of the ternary system of sodium silicate, metakaolin and aqueous alkali reveals that geopolymerisation occurs in a distinct compositional region. At high alkalinity [> 30% (mol/mol) overall Na2O content], connectivity of silicate anions is reduced, consistent with poor polymerisation. At low alkalinity [<10% (mol/mol) overall Na2O content], a clear 29Si NMR resonance line due to unconverted metakaolin is observed. NMR spectra were recorded from a series of samples with a fixed Na2O content (20 mol%) and varied SiO2/Al2O3 ratio to observe aluminium substitution in the cross-linked silicon tetrahedra of polymer network. Aluminium insertion into the silicate network is confirmed from the observed 29Si NMR shift as a function of Si/Al ratio. The identification of the presence or absence of metakaolin in the cured geopolymer product is not possible even by 29Si NMR as the signal from metakaolin is indistinguishable from a broad 29Si NMR peak consisting of many resonance lines from the network of cross-linked silicon/aluminium tetrahedra. In an attempt to identify metakaolin signal, we prepared geopolymers with higher SiO2/Al2O3 molar ratios. Since aluminium substitutions in the silicate tetrahedral network are decreased, this results in better-resolved 29Si NMR lines. The 29Si NMR signal due to metakaolin is then distinguishable in the spectra of cured products in a series of samples with 3 to 11 mol% metakaolin. These results indicate that a geopolymer structure is a network of silicon/aluminium tetrahedra with some presence of unreacted metakaolin. The silicon/aluminium tetrahedra might have connectivity ranging from 1 to 4.

Preparation of silica aerogel from rice hull ash by supercritical carbon dioxide drying

Abstract: Silica aerogel, which is a mesoporous light solid material, was prepared from the rice hull ash by sol–gel followed supercritical carbon dioxide drying. The rice hull ash, which is rich in silica, was extracted using sodium hydroxide solution to produce a sodium silicate solution. The solution was neutralized with sulfuric acid solution to form a silica gel. After washing with water and the solvent exchange with ethanol, the aged gel was dried to produce aerogel using supercritical carbon dioxide drying. The prepared silica aerogel was characterized using SEM, TEM and BET measurements. The specific surface area of the rice hull ash aerogel was as high as 597.7 m2/g with a bulk density of 38.0 kg/m3. The diameters of the pores inside the aerogel are between 10 and 60 nm.

Molecular manipulation of two- and three-dimensional silica nanostructures by alkoxysilylation of a layered silicate octosilicate and subsequent hydrolysis of alkoxy groups.

Abstract: A novel methodology for constructing molecularly ordered silica nanostructures with two-dimensional (2-D) and three-dimensional (3-D) networks has been developed by using a stepwise process involving silylation of a layered silicate octosilicate with alkoxytrichlorosilanes [ROSiCl3, R = alkyl] and subsequent reaction within the interlayer spaces. Alkoxytrichlorosilanes react almost completely with octosilicate, bridging two closest Si−OH (or −O-) sites on the silicate layers, to form new five-membered rings. The unreacted functional groups, Si−Cl and Si−OR, are readily hydrolyzed by the posttreatment with a water/dimethyl sulfoxide (DMSO) or water/acetone mixture, leading to the formation of two types of silicate structures. The treatment with a water/DMSO mixture produced a unique crystalline 2-D silicate framework with geminal silanol groups, whereas a water/acetone mixture induced hydrolysis and subsequent condensation between adjacent layers to form a new 3-D silicate framework. The 2-D structure is r...

The structure of erbium doped sodium silicate glasses

Abstract: The structure of erbium doped sodium silicate glasses 1Er2O3–xNa2O–(99 − x)SiO2 (x = 0, 10, 20, 30) were studied by molecular dynamics simulations. The simulated glasses were compared with neutron diffraction and other experimental structural data. Silicon oxygen network ring size distributions and Qn distributions, the local structures around erbium ions, including the pair distribution functions, bond angle distributions, coordination number, dipole and quadrupole moments, were calculated. Their changes with sodium concentration were studied. It was found that both the Er–O bond length and the erbium coordination number increases from silica to sodium silicate glasses. Erbium ions adopt more favorable and uniform coordination environments in sodium silicate glasses. Erbium and sodium distribution and erbium clustering in the glasses were characterized. Erbium clustering is less obvious in sodium silicate glasses than in silica. Bond valences calculations of the simulated glasses provide a useful way to study the simulated glass structure. The comparison of bond valence of simulated glasses with related crystalline materials showed that the biggest difference be on the modifier sites.

Cation diffusion and ionic conductivity in soda-lime silicate glasses

Abstract: We have studied the mobilities of calcium and sodium ions in silicate glasses of compositions xNa2O·(3 − x)CaO·4SiO2 with x = 0.0, 0.1, 0.3, 1.0 and 3.0 by means of radiotracer diffusion, electrical conductivity measurements, and dynamic mechanical thermal analyses. In glasses containing sodium oxide, the Na+ ions are much more mobile than the Ca2+ ions, and are, therefore, governing the electrical conductivity. In the pure calcium silicate glass, the activation energy of Ca2+ diffusion is higher than the activation energy of the electrical conductivity. This provides strong evidence that the electrical conductivity of this glass is not determined by the migration of Ca2+ ions, but by impurity charge carriers, which are most likely Na+ ions. We sketch the composition-dependent mobilities of Na+ and Ca2+ ions in soda-lime silicate glasses with variable Na2O and CaO content. Our results indicate that the coordination environment of Ca2+ ions remains unchanged when CaO is replaced by Na2O which is consistent with recent results of molecular dynamic simulations. Moreover, our results confirm the formation of dissimilar Na–Ca pairs which lead to a non-random mixing of the cations in the glass. The formation of such pairs was recently deduced from nuclear magnetic resonance spectra of soda-lime silicate glasses.

Acid-resistant cements for geothermal wells: sodium silicate activated slag/fly ash blends

Abstract: Sodium silicate activated slag/class F fly ash (SSASF) blend cements were prepared by varying two parameters, the SiO2/Na2O mol. ratio in the sodium silicate activator, and the slag/fly ash weight ratio, and then autoclaving them at 100, 200 and 300°C. The usefulness of the autoclaved SSASF cements as acid-resistant geothermal well cements was evaluated by exposing them for 15 days to 90°C CO2-laden H2SO4 (pH 1·1). Among the combination of these two parameters, the most effective one in inhibiting acid erosion consisted of 50/50 slag/fly ash ratio cement and 2·50 SiO2/Na2O ratio activator. The weight loss by acid erosion of this cement was less than 7%. The major contributor to such minimum acid erosion was the zeolite phase formed by interactions between the mullite in fly ash and the Na ion liberated from the activator. Further characteristics of this cement included a long setting time for its slurry of approximately 1770 min at room temperature, and a water permeability ranging from 2·4 × 10−4 to 1·9 ...

Role of Acidified Sodium Silicate in Low Temperature Bitumen Extraction from Poor-Processing Oil Sand Ores

Abstract: Process aids are generally required to improve bitumen recovery from poor-processing oil sand ores containing a relatively high amount of divalent metal ions and fine solids/clays. In this paper, the role of acidified sodium silicates as a dispersant/depressant of clay fines in bitumen extraction was evaluated using a laboratory hydrotransport extraction system (LHES) at low temperature (35 °C). Bitumen recovery experiments showed that adding acidified silicates during the bitumen extraction process resulted in a higher degree of bitumen liberation from sand grains, a faster bitumen flotation rate, and a better bitumen froth quality than adding caustic. Solution chemistry analysis demonstrated that acidified sodium silicate is a better process aid than caustic because it has three functions: to precipitate calcium and magnesium in the process water, which minimizes the synergistic effect of divalent cations in inducing a clay coating on the bitumen surface and clay gelation; to maintain an adequate pulp slurry pH for better bitumen-air bubble attachment; and to disperse/depress clay fines from flotation by its specific species.

Emulsion-Templated Hierarchically Porous Silica Beads Using Silica Nanoparticles as Building Blocks

Abstract: Uniform silica beads were produced using emulsion-templated polymer beads as templates and silica nanoparticles as building blocks. Hybrid silica/alumina and silica/titania beads were also prepared by immersing silica−polymer composite beads or silica beads in the corresponding precursor solutions. Inorganic beads were produced by calcining the polymer−inorganic composites at 520 °C in air. The resulting materials exhibited a hierarchical mesoporous−macroporous structure, with high pore volumes and high surface areas. The mechanical stability of the beads could be enhanced by sintering at 1450 °C under argon or by using sodium silicate as a precursor, but with a concomitant reduction in the total surface area of the materials.

Using magnesium hydroxide (Mg(OH) 2 ) as the alkali source in peroxide bleaching at Irving paper

Abstract: Mg(OH)2 was used to replace sodium hydroxide and sodium silicate in a TMP perox- ide bleaching process. Brightness gain with Mg(OH)2 as the sole alkali source is comparable to the caustic/silicate based process. The Mg(OH)2 process produced less anionic trash in the pulp and lower effluent COD content. The sequential addition of chemicals with hydrogen peroxide last was superior to the conventional peroxide bleaching process. Improved economics and performance have been proven in the mill application.

Raman scattering coefficients of symmetrical stretching modes of microstructural units in sodium silicate melts

Abstract: Quantitative analysis of Raman spectra of vitreous or molten silicate is always one of the key subjects in some fields, e.g. materials science, geological physi cs, etc. But there are many difficulties with this analysis, one of which is the determination of Raman scattering coefficient. Not long ago, we developed a new method for the calculation of Raman spectrum of amorphous silicate. Essentiall y, this method combines the molecular dynamics simulation with wilson's GF matri x method, electro_optical parameter method and bond polarizability model. With t his theoretical method, we have studied the Raman scattering coefficients of sym metrical stretching modes of Qi in sodium silicate melts, and concluded th at these coefficients are independent of composition. Meanwhile, the five values of the coefficients are also given as follows: S0=１, S1=0514, S2=0 242, S3=0090 and S4=0015 In order to apply these quantities to experiments, the Raman spectrum of sodium disilicate melt has been measured and its high_frequency envelope has been deconvolved into three Gaussian bands. Fina lly, the molar fractions of Qi were achieved by dividing the area fraction s of the three Gaussian bands by corresponding scattering coefficients. Addition ally, combining the results of theoretical calculation and quantitative analysis of experimental spectra, we found that the intensities of anti_symmetrical stre tching modes of Q0 and Q2 in sodium silicate melts are so strong tha t they cannot be neglected in the deconvolution process.

High-Temperature Plug Formation With Silicates

Abstract: A grouting system is being developed that is applicable to water shutoff, steam flooding, and hightemperature grouting/plugging for lost circulation. This grouting system uses silicate hydrates as the plugging material. We describe a means to introduce sodium silicate and an activator through a single tubular by encapsulating the activator in a material that is temperature sensitive, thus delaying the initiation of the reaction. The formation of silicate hydrate plugs has been studied over the range of 80C to 300C as well as the chemical stability of silicate hydrate for 10-12 weeks.

Thermal Behavior of Silver in Ion-Exchanged Soda-Lime Glasses

Abstract: The refractive index and the nonlinear optical properties of sodium silicate glass can be easily tailored by replacing the sodium with silver. However, the thermal responses of silver and the corresponding modifications of the silica (SiO2) network in ion-exchanged glass under heating are still not clear. X-ray photoelectron spectroscopy has been used to study the in-situ behavior of silver and SiO2networks on the surfaces of silver-ion-exchanged-content soda-lime glasses during heating and cooling processes under ultrahigh vacuum. Temperature-dependent concentration changes and oxidation states have been monitored. The results show that silver diffuses toward the surface, precipitates, and crystallizes during heating, and the total silver surface concentration is slowly increased during cooling. The concentration changes and binding-energy shifts of oxidized and neutral silver atoms, a new nonbridging oxygen species (NBO*), and a new silicon species (Si[a]) have been applied to deduce a disappropriation reaction mechanism of the Ag+ ion on the surface during annealing. The SiO2 network is modified at temperatures of <350°C to accommodate more silver on the surface and to balance the extra charge that is carried by the Ag+ ion. The fact that the SiO2 network polymerizes during annealing has been deduced from the results of the higher binding energies of Si 2p and O 1s after annealing. This observation is of importance in optimizing the distribution of the ion-exchanged silver and in the formation of silver-metal colloids in glass networks.

Structural rules of phase separation in alkali silicate melts analyzed by high-temperature Raman spectroscopy

Abstract: The structure of alkali silicate melts with the compositions around the immiscibility dome, i.e. xR2O-(100 − x)SiO2 (R = Li, Na; x = 15, 25, 33) were investigated by high-temperature Raman spectroscopy. The distributions of structural units of Qn were estimated as a function of temperature and composition through the curve fitting of spectra based on the Qn equilibration 2Q3 ↔ Q2 + Q4. The Qn distributions of lithium silicate melts were insensitive to temperature, while those of sodium silicate melts showed a temperature dependence especially in the high sodium concentration region. By using the temperature and composition dependences of the Qn distribution, a new expression of the non-ideal entropy of mixing (ΔSmix) for the silicate system is proposed. In order to clarify the relationship between the Qn equilibration and the entropy change of alkali silicate melts, the Qn distributions were summarized as isothermal Qn curves on the Q2–Q3–Q4 ternary diagram (Qn diagram) on a contour map of ΔSmix. The Qn diagrams reveal unique characteristics of the melt under the immiscibility condition. It is suggested that phase separation takes place in a specific composition range, where the system cannot decrease entropy by structural change via the equilibration of Qn species.

Structural model of gelation processes of a sodium silicate sol destabilized by calcium ions: combination of SAXS and rheological measurements

Abstract: This study deals with the physico-chemical processes involved in the formation of basic fractal silica gels derived from a sodium silicate sol destabilized by calcium ions. Using small-angle X-ray scattering and dynamic rheological measurements, structural and viscoelastic properties have been investigated in situ during aggregation and gelation processes. The experimental results lead to a consistent model that describes the structural features and aggregation mechanisms involved in the formation of these gels.

Collagen-silica hybrid materials: sodium silicate and sodium chloride effects on type I collagen fibrillogenesis.

Abstract: Collagen-silica hybrid materials have been considered for potential biomedical applications. Understanding of the collagen-silica interactions is the key to control hybrids structure and properties. For this purpose, the effect of sodium silicate and sodium chloride addition at two concentrations, 0.83 and 10 mM, on the kinetic of the type I collagen fibrillogenesis at 20 degrees C, and pH 7.4 were studied. Absorbance profiles of fibrillogenesis experiments were collected together with measures of silicic acid concentration and transmission electron microscopy analysis. The specific effect of silica addition on the collagen fibrils self-assembly mechanisms was demonstrated by comparison with the sodium chloride. Sodium silicate at 10 mM inhibited the collagen fibrillogenesis. At the same concentration, the sodium chloride decreased the rate of the collagen fibril assembly. Collagen fibrillogenesis kinetic was not significantly disturbed by the presence of 0.83 mM of sodium chloride. However, the same concentration of sodium silicate modified the collagen fibrillogenesis kinetic. Transmission electron microscopy indicated for experiment with 0.83 mM of sodium silicate, the formation of longer and wider fibrils than for the equivalent collagen fibrillogenesis experiment with sodium chloride. The effect of sodium chloride is explained in terms of osmotic exclusion and influence on electrostatic interactions between collagen fibrils. The specific involvement of silicic acid in collagen helices hydrogen-bond interactions is suggested. Finally, the results of this study are discussed regarding the preparation of composites by co-gelation of type I collagen and sodium silicate, for potential application as bone repair device.