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Calorimetric comparison of portland cements containing silica fume and metakaolin

Is silica fume, like metakaolin, characterized by pozzolanic activity that is more specific than generic?

Journal of Thermal Analysis and Calorimetry
Authors: R. Talero and V. Rahhal

Abstract  

This new study must be regarded to be a direct outcome of two previous studies published by these same authors, which were conducted to respond to interesting questions brought out about the effect of silica fume, SF and metakaolins, M and MQ, on the heat of hydration of portland cements, PC, with very different C3A and C3S contents. The answer to these so interesting questions has been the primary objective of the present research. For this purpose, the same PC, PC1 (14% C3A) and PC2 (≈0% C3A), metakaolins, silica fume and blended cements were once again used more 60/40 for sulphate attack, and the same analytical techniques (CC, pozzolanicity and XRD analysis) and parameters determined as well. In this new research, the sulphate attack was determined by two accelerated methods: Le Chatelier-Ansttet and ASTM C 452-68. The experimental results of sulphate attack mainly, have demonstrated definitively that the high, rapid and early pozzolanic activity exhibited by SF also is, as in the case of the two metakaolins, more specific than generic, for it indirectly stimulated greater C3A than C3S hydration, but only in the first 16 h monitored in this study. Thereafter it is the contrary, i.e., anti- or contra-specific for the same purpose. And the longer the hydration time, the more anti- or contra-specific it became, since, when exposed to sulphate attack, SF blended cements resisted or even prevented the aggressive attack against PC1 which, with a higher C3A content than PC2, was the more vulnerable of the two. By contrast, metakaolin MQ not only failed to hinder or prevent the attack, but heightened its effects, rendering it more intense, aggressive and rapid, leading to what could be called a rapid gypsum attack.

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Journal of Thermal Analysis and Calorimetry
Authors: R. Campostrini, A. Sicurelli, M. Ischia, and G. Carturan

Abstract  

A homogeneous silica gel sample bearing ≡Si-H groups was prepared, via sol-gel method, by hydrolysis of trimethoxysilane under acid condition in tetrahydrofuran. Preliminary NMR experiments in liquid phase indicated an immediate and complete hydrolysis of Si-OCH3, followed by a slower condensation of the Si-OH groups, with maintenance of Si-H bonds. The crude-gel, and samples heated to various temperatures, were characterized by different instrumental methods, including FTIR, density, porosity, and specific surface area. These data indicate that the crude-gel was a dense material which, on heating, increases porosity and surface area up to ca 500�C. The thermal behavior was studied in inert atmosphere by means of coupled thermogravimetric, gas chromatographic, mass spectrometric analyses. The pyrolysis process was described by the fundamental chemical reactions occurring among the siloxane chains of the gel network and by the qualitative and semiquantitative chemical analysis of the compounds released in gas-phase. The proposed pyrolysis mechanism was discussed and interpreted in agreement with the change of the morphological properties of the gel. The pyrolysis data and the mass balance between the compounds released in gas-phase and the solid residue at 1000�C allowed the determination of a nominal chemical formula to describe the crude-gel composition.

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The reduction process of silica supported cobalt catalyst was studied by thermal analysis technique. The reduction of the catalyst proceeds in two steps:

\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$Co_3 O_4 + H_2 \to 3CoO + H_2 O, 3CoO + 3H_2 \to 3Co + 3H_2 O$$ \end{document}
which was validated by the TPR and in-situ XRD experiments. The kinetic parameters of the reduction process were obtained with a comparative method. For the first step, the activation energy, E a, and the pre-exponential factor, A, were found to be 104.35 kJ mol−1 and 1.18�106∼2.45�109 s−1 respectively. The kinetic model was random nucleation and growth and the most probable mechanism function was found to be f(α)=3/2(1−α)[−ln(1−α)]1/3 or in the integral form: g(α)=[−ln(1−α)]2/3. For the second step, the activation energy, E a, and the pre-exponential factor, A, were found to be 118.20 kJ mol−1 and 1.75�107∼2.45 � 109s−1 respectively. The kinetic model was a second order reaction and the probable mechanism function was f(α)=(1−α)2 or in the integral form: g(α)=[1−α]−1−1.

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Journal of Thermal Analysis and Calorimetry
Authors: R. Campostrini, A. Sicurelli, M. Ischia, and G. Carturan

Abstract  

The thermal behaviour of a sol-gel prepared hybride silica gel (HSiO sample) in the 20–1000°C interval was studied by coupled thermogravimetric-mass spectrometric (TG-MS) analyses carried out at various heating rates. Thermogravimetric curve elaboration allowed the determination of the flex temperatures, corresponding to the maximum release rate of gas-evolved compounds, and to calculate the activation energy of the overall process. The mass spectrometric data, registered in the TG-MS measurements, were treated to discriminate the single reactions accounting for the release of each compound, among which water, dihydrofuran and various silane-and siloxane-derived species. These results were used to calculate the comprehensive activation energy and also those of each of the released species. Different methods of data processing were used to achieve better reliability of calculated activation energies. The discussion focuses on the high extension of kinetic information arising from MS data processing and on the advantage of identifying the contribution of single reactions, although they occur simultaneously during the heating process. In this respect, good agreement was found between the activation energies of the overall process calculated by separately processing TG and MS data. By processing MS data, the same agreement was observed in the comparison between the activation energy calculated for the overall thermal process and in the sum of the weighed activation energies of the reaction of each released compound.

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Abstract  

Mesoporous silica (MCM-41) with d (100) interplanar distance of 38 Å was prepared by a room temperature process through low surfactant templation technique. The surface of MCM-41 was functionalized with dithiocarbamate (dtc) ligand, named as MCM-41-dtc and this was characterized by X-ray diffraction, BET surface area, particle size analysis, 29Si MAS NMR spectra and sulphur analysis. The sorption of mercury from 0.1M HCl solution by MCM-41-dtc was studied as a function of pH, [Hg2+], time and temperature. The sorption data obtained at various initial concentrations of mercury were fitted into Langmuir adsorption model. Mercury speciation in solution and the sorption capacity measurements indicated possible formation of a 1 : 1 square planar complex in the solid phase. A very rapid sorption of mercury was observed in the initial stages of equilibration, which can be attributed to the large surface area, wide porosity and fine particle size of MCM-41-dtc, facilitating facile accessibility of mercury into the inner pores of the sorbent. The enthalpy change accompanied by the sorption of mercury was found to decrease from 83.7 to 6.2 kJ/mol, when the initial concentration of mercury was increased from 5.10-4M to 1.5.10-3M.

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Abstract  

Mortars taken from the walls of three historical buildings in Seville: Pond of Patio de las Doncellas in Real Alcazar of Seville, the Monastery of Santa Maria de las Cuevas and the Church of El Salvador were investigated. The techniques employed were thermogravimetry (TG), differential thermal analysis (DTA), XRD, FTIR, SEM with EDAX, Bernard calcimeter, granulometry, mercury intrusion porosimetry and mechanical strength tests. The majority of the studied mortars consist of calcite and silica. Gypsum was detected in samples of four mortars from the Santa Maria de las Cuevas Monastery and two from the El Salvador Church, whose samples were taken from the upper layers of the walls, but gypsum was not detected in the internal mortars layers. Only in two of the samples of the Monastery, the presence of cellulosic material as an organic additive was detected. All the studied mortars could be regarded hydraulic, so much by results from ratios between mass loss due to CO2 and H2O, hydraulic module and assays of compressive strength. The values obtained by these three techniques are related, providing good agreements between them. These results give useful information that aids in understanding the technology of historic mortars, and how to plan the restoration of these wall paintings.

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Abstract  

A convenient method for introducing desired trace elements into a silica matrix is described in order to have an ideal standard for instrumental neutron activation analysis.

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Abstract  

Zirconium in simulated high level radioactive liquid waste (HLLW) was selectively adsorbed and separated by self-made high adsorption activity silica gel. The selective adsorption mechanism was analyzed according to the structure character of self-made silica gel and performance of zirconium in acid simulated HLLW. The results show that the adsorption selectivity of self-made silica gel for zirconium is strong, because zirconium has higher positive charge and zirconium ion hydrolyzes easily. Distribution coefficient of self-made silica gels for zirconium is 53.5 ml/g. There are 6.5 (OH)/nm2 on the surface on self-made silica gels which provide more adsorption activity places, thus self-made silica gels have higher adsorption capacity for zirconium (31.4 mg/g). The elution rate of the adsorption of zirconium on self-made silica gel by 0.2 mol/l H2C2O4 is more than 99%. The solubility of the self-made silica gel in nitric acid is low, the chemical stability of self-made silica gel is very strong.

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Abstract  

Isothermal heat conduction microcalorimetry was adopted as a novel characterization method to investigate the polymerization processes of silica when the combination of silica sol and potassium sodium silicate was stirred at 25.0, 35.0, and 45.0 °C. Thermodynamic and kinetic parameters were simultaneously obtained. The enthalpy change was greater at each higher temperature. The reaction orders (m, n) instantaneously varied, up and down in an alternate manner. At 25.0, 35.0, and 45.0 °C, the rate constants were different; the maximum rate constant occurred at 25.0 °C. These phenomena reflect a two-stage oligomeric mechanism of silica monomers. The measurements of particle size showed the complex chemical composition of aqueous silicates, which can be qualitatively designated by the particle size distribution in two parts. The results further indicate that the colloidal particles in the mixed silica sol and silicates first dissolved. Then the “active” silica in the silicates redeposited to make a distinct particle size distribution influenced by K+ and Na+ ions as well as by temperature.

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Abstract  

Mesoporous silica material of MCM-41 type was synthesized by co-condensation of highly concentrated octyltriethoxysilane (OTEOS), octadecyltriethoxysilane (ODTEOS) and tetraethoxysilane (TEOS). The obtained hybrid materials were characterized using XRD, TG-DSC and low temperature adsorption/desorption of nitrogen. It was shown that the applied method of synthesis allows to obtain silica of MCM-41 type with a high degree of hydrocarbon saturation.

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