Influence of Initial Silica Modulus of Na2SiO3 on the Compressive Strength of Alkali Activated Ultrafine Palm Oil Fuel Ash Based Mortar

The influence of silica modulus (Ms) on the compressive strength of alkali activated ultrafine palm oil fuel ash based mortar has been investigated. Two alkali activated mortar mixtures, S1 and S2 were prepared using sodium silicate (Na2SiO3) with initial silica moduli, Ms of 3.3 and 2, respectively. The Na2SiO3 was used in combination with sodium hydroxide (NaOH) with NaOH concentration of 10 M. The results indicate that the S2 mortar mixture has higher strength than the S1 mortar mixture at all testing ages, with 28 day strength of 27.18 MPa and 32.8 MPa recorded by S1 and S2 mortar mixture, respectively. Hence, lower Ms leads to higher compressive strength. The higher compressive strength of S2 is corroborated by the formation of more C-S-H as implied by the Fourier Transform Infra-red (FTIR) spectra analysis.

Smectite Dissolution Kinetics in High-Alkaline Conditions

ABSTRACTTo determine the initial rates and effects of silica in solution on the dissolution kinetics of smectite, short- and long-term batch experiments (0.5 hour to 30 days) were completed at three temperatures (T = 25, 50, and 75°C) using stock solutions pH adjusted by NaOH (pH = 12, 13, and 13.5) with varying initial silica concentrations (0, 30, 60, and 100 ppm). The following important characteristics were observed at pH = 12: (1) The concentrations of Al, Si, Mg, Fe, and Ti in solutions increase rapidly during the first ∼2 hours and reach steady state (equilibrium) within ∼5 days. (2) The concentration ratios of Al, Si, Fe, Mg, and Ti in solutions during the early (<2 hours) reaction phase differ significantly from those of smectite, indicating initial dissolution proceeds non-stoichiometrically; Al dissolves much faster than Si, Mg, Fe, and Ti. (3) Further dissolution of smectite proceeds nearly stoichiometrically, including Fe and Ti. (4) The high solubility of Ti in highly alkaline solutions may be due to the formation of aqueous complexes, such as TiO(OH)3− and TiO2(OH)22−, similar to aqueous silica species. (5) The initial rate of smectite dissolution increases with increasing pH, T, and initial silica content of solution. (6) The silica in solution acts as a promoter and a catalyst, rather than an inhibitor, of smectite dissolution in high-alkaline solutions. This role is easily recognizable when the solubility of smectite and amorphous silica are very high, i.e., at pH >∼9.

Unsaturated Zone Waters From the Nopal I Natural Analog, Chihuahua, Mexico - Implications for Radionuclide Mobility at Yucca Mountain

Chemical and U-Th isotopic data on unsaturated zone waters from the Nopal I natural analog reveal effects of water-rock interaction and help constrain models of radionuclide release and transport at the site and, by analogy, at the proposed nuclear waste repository at Yucca Mountain. Geochemical reaction-path modeling indicates that, under oxidizing conditions, dissolution of uraninite (spent fuel analog) by these waters will lead to eventual schoepite precipitation regardless of initial silica concentration provided that groundwater is not continuously replenished. Thus, less soluble uranyl silicates may not dominate the initial alteration assemblage and keep dissolved U concentrations low. Uranium-series activity ratios are consistent with models of U transport at the site and display varying degrees of leaching versus recoil mobilization. Thorium concentrations may reflect the importance of colloidal transport of low-solubility radionuclides in the unsaturated zone.

Gas Chromatographic Characterization of Silica Bonded with a Phenylpolysiloxane Film

A method of synthesizing modified silica with a dense grafted phenylpolysiloxane layer is described. Gas chromatography at zero surface coverage has been used for investigating the adsorption properties of both modified and initial silicas. It was shown that the thermodynamic parameters for retention (retention volume, Va; heat, q; and entropy, ΔS0, of adsorption) and the Kovats retention indices I for different classes of organic compounds are essentially changed with surface modification. The increase in Va and q for n-alkanes on phenyl-bonded silica was interpreted as due to penetration of the solutes into the grafted phenylpolysiloxane layer. The significant reduction in retention parameters for all compounds capable of specific interaction, including the formation of hydrogen bonds, on the phenyl-bonded silica relative to the initial silica has allowed conclusions to be drawn regarding the weak role of residual silanol groups in the retention process. These conclusions have been confirmed by a comparison of the chromatographic properties of phenyl-bonded silica and the polymethylphenylsiloxanes OV-17 and OV-25.

Surface Properties of Carbonaceous Solids as Examined by 1H NMR Spectroscopy Using the Bulk Freezing Procedure

It has been established that the type of adsorption sites on the surface of different carbonaceous materials [graphite oxide, thermo-expanded graphite, graphitized and non-graphitized carbons, carbon black, carbonated silicas (Carbosils)] can be determined using a chemical shift in the 1H NMR spectra of adsorbed test compounds possessing different donor–acceptor properties and polarizability. This is due to the existence of local magnetic anisotropy regions near the carbon surface. The 1H NMR measurements on adsorbent/adsorbed water/inert solvent interfaces provide an effective procedure for the differentiation of NMR signals from molecules bound with various types of surface sites. Variations in the free energy of water in the adsorption layer of Carbosils having different carbon contents on the silica surface and their free surface energy have been determined from measurements of the temperature dependence of the NMR signal for non-freezing water in the cooling process. A low degree of carbonization for the initial silica surface leads to a drastic increase in the aqueous interlayer thickness structurized by the surface.

Synthesis of Periodic Mesoporous Silica Thin Films

We have synthesized periodic mesoporous silica thin films (PMSTF) from homogeneous solutions. To synthesize the films a thin layer of a pH = 7 micellar coating solution that contains TMOS is dip- or spin-coated onto silicon wafers, borosilicate glass, or quartz substrates. Ammonia gas is diffused into the solution and causes rapid hydrolysis and condensation of the TMOS and the formation of periodic mesoporous thin films within ∼10 seconds. The combination of homogeneous solutions and rapid product formation maximizes the concentration of desired product and provides a controlled, predictable microstructure. The films have been made continuous and crack-free by optimizing initial silica concentration and film thickness.

Filler-Elastomer Interactions. Part V. Investigation of the Surface Energies of Silane-Modified Silicas

As shown in previous papers of this series, the main feature of silicas with regard to surface energy, which distinguishes them from carbon blacks, is a low dispersive component, γsd, and a high specific component, γssp, of surface energy. The low γsd, would result in a lack of interaction between filler and hydrocarbon rubbers, while the high γssp would suggest a high degree of agglomeration of the filler particles in the polymer matrix. In this study, the surface free energies and the energy distribution on the surfaces of precipitated silicas which had been modified with octadecyltrimethoxy silane, 3-thiocyanatopropyltriethoxy silane and bis(3-trimethoxysilylpropyl)-tetrasulfane, respectively, were investigated by chromatography at infinite dilution and at finite concentration. A comparison with the initial silica suggests a drastic decrease in surface energy, especially of the specific component, as a result of the modification and a strong dependence of surface energy on the chemical nature of the grafts and the ratio of these grafts. The energy distribution function shows that, while the surface of the ungrafted silica is heterogeneous, the heterogeneity of the fully modified surface is drastically reduced, particularly when the product was modified with octadecyltrimethoxy silane.