An in-situ Raman spectroscopy study of the hydration of tricalcium silicate

Raman spectroscopy has been used to study the hydration kinetics of tricalcium silicate (C₃S). The progress of the reaction was followed by two different methods: the first one is based on measurement of the intensity of characteristic C₃S Raman bands compared with that of an internal Standard Raman peak (TiO₂); the second one consists in measuring the relative intensity of Raman peaks associated with C₃S and calcium hydroxide. These two concordant data sets show that a change in the hydration mechanism occurs at about 13 hours, and enable the kinetic parameters associated with the two hydration processes to be derived.     

Hyphenation of gas chromatography to microcoil 1H nuclear magnetic resonance spectroscopy

Whereas the hyphenation of gas chromatography (GC) with mass spectrometry is of great importance, little is known about the coupling to nuclear magnetic resonance spectroscopy (NMR). The investigation of this technique is an attractive proposition because of the valuable information given by NMR on molecular structure. The experiments shown here are to our knowledge the first hyphenating capillary GC to microcoil NMR. In contrast to liquids, gases have rarely been investigated by NMR, mainly due to the experimental difficulties in handling gases and the low signal-to-noise-ratio (SNR) of the NMR signal obtained at atmospheric pressure. With advances in NMR sensitivity (higher magnetic fields and solenoidal microprobes), this limitation can be largely overcome. In this paper, we describe the use of a custom-built solenoidal NMR microprobe with an active volume of 2 microL for the NMR detection of several compounds at 400 MHz, first in a mixture, and then with full coupling to capillary GC to identify them separately. The injected amounts of each analyte in the hyphenated experiments are in the range of 15-50 micromol, resulting in reasonable SNR for sample masses of 1-2 microg.     

Modelling of the hydration of tricalcium silicate

A previously developed mathematical procedure is used to illustrate the kinetics and mechanism of cement hydration. By using the experimentally obtained degree of hydration against time curve and the weight percentage particle size distribution (PSD) of a cement, the procedure can isolate an individual cement particle and characterize its different regimes or modes of hydration reaction. Specifically, one can identify an accelerating mode followed by a decelerating mode of reaction, and the changeover period, maxima, from one mode to the other.     

Hexavalent chromium in tricalcium silicate: Part II Effects of CrVI on the hydration of tricalcium silicate

The composition and structure of hydrated tricalcium silicate (C3S) pastes admixed with CrVI have been studied. The resultant mixture simulates CrVI waste forms stabilized in ordinary Portland cement. Scanning electron microscopy and transmission electron microscopy were used to identify the microstructural changes accompanying the addition of CrVI solutions to C3S. Energy-dispersive X-ray spectroscopy was used to probe the distribution of chromium in the phases within the hazardous waste forms. Elucidation of the molecular structure of the reaction products was accomplished with Fourier transform infrared and nuclear magnetic resonance spectroscopies.CrVI was found to be contained in the waste form as soluble Ca2CrO5·3H2O and partially chemically bonded within the calcium silicate hydrate (C–S–H) phase. CrVI was also found to increase the condensation of C–S–H and porosity of the waste form.     

Effect of CaCl2 on the hydration of tricalcium silicate

The hydration of tricalcium silicate has been studied at 30° C in the presence and absence of 2 wt % CaCl₂ with a water∶solid ratio of 0.8. Free lime determinations, X-ray diffraction analysis, thermal analysis, infra-red spectroscopy, scanning electron microscopy and zeta potential measurements were used for hydration studies. The results indicate that the accelerating action of CaCl₂ is due to higher diffusivity of chloride ions.     

Pre-induction and induction hydration of tricalcium silicate: an environmental scanning electron microscopy study

Environmental scanning electron microscopy (ESEM) has been used to study the very early pre-induction, and induction physical processes that occur in the hydration of tricalcium silicate. An in situ experimental technique is described which allows direct, real-time observation of the sub-micrometre morphological changes that take place during this reaction. The results of this investigation are correlated with kinetic data obtained by differential scanning calorimetry (DSC). In this way, microstructural evolution has been identified with the stages of very early hydration. Upon first contact with water, a gelatinous coating was seen to form at grain surfaces and a crystalline secondary product was observed at the end of an extensive dormant period. These findings are viewed in the light of previous wet and dry microscopy studies, and are discussed within the framework of ordinary Portland cement as a possible explanation of induction. Comment is made as to the suitability of environmental SEM for analysis of such materials.     

The hydration of tricalcium silicate in the presence of colloidal silica

Reactions of colloidal silica fumes with calcium hydroxide or hydrating tricalcium silicate (C₃S) have been studied using calorimetry, chemical analyses, and scanning electron microscopy. Silica fume reacts immediately with calcium hydroxide forming a colloidal calcium silicate hydrate (C-S-H) similar to that formed by the hydration of C₃S. When excess silica is present it reacts with C-S-H already formed to produce a new, highly polymerized C-S-H, having a very low C/S ratio (1.0). Silica fume accelerates the hydration of C₃S, reduces the amount of calcium hydroxide formed by reacting with it, and slightly lowers the C-S-H ratio of the C-S-H formed by hydration. When large amounts of silica fume are present the formation of calcium hydroxide may be entirely suppressed and a highly polymerized C-S-H is formed. Silica fume is considered a good model for reactive pozzolans used in concrete.     

Characterisation of products of tricalcium silicate hydration in the presence of heavy metals

The hydration of tricalcium silicate (C(3)S) in the presence of heavy metal is very important to cement-based solidification/stabilisation (s/s) of waste. In this work, tricalcium silicate pastes and aqueous suspensions doped with nitrate salts of Zn(2+), Pb(2+), Cu(2+) and Cr(3+) were examined at different ages by X-ray powder diffraction (XRD), thermal analysis (DTA/TG) and (29)Si solid-state magic angle spinning/nuclear magnetic resonance (MAS/NMR). It was found that heavy metal doping accelerated C(3)S hydration, even though Zn(2+) doping exhibited a severe retardation effect at an early period of time of C(3)S hydration. Heavy metals retarded the precipitation of portlandite due to the reduction of pH resulted from the hydrolysis of heavy metal ions during C(3)S hydration. The contents of portlandite in the control, Cr(3+)-doped, Cu(2+)-doped, Pb(2+)-doped and Zn(2+)-doped C(3)S pastes aged 28 days were 16.7, 5.5, 5.5, 5.5, and <0.7%, respectively. Heavy metals co-precipitated with calcium as double hydroxides such as (Ca(2)Cr(OH)(7).3H(2)O, Ca(2)(OH)(4)4Cu(OH)(2).2H(2)O and CaZn(2)(OH)(6).2H(2)O). These compounds were identified as crystalline phases in heavy metal doping C(3)S suspensions and amorphous phases in heavy metal doping C(3)S pastes. (29)Si NMR data confirmed that heavy metals promoted the polymerisation of C-S-H gel in 1-year-old of C(3)S pastes. The average numbers of Si in C-S-H gel for the Zn(2+)-doped, Cu(2+)-doped, Cr(3+)-doped, control, and Pb(2+)-doped C(3)S pastes were 5.86, 5.11, 3.66, 3.62, and 3.52. And the corresponding Ca/Si ratios were 1.36, 1.41, 1.56, 1.57 and 1.56, respectively. This study also revealed that the presence of heavy metal facilitated the formation of calcium carbonate during C(3)S hydration process in the presence of carbon dioxide.     

29Si MAS NMR study of the hydration of tricalcium silicate in the presence of finely divided silica

²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy has been used to investigate the effect of finely divided silica on the hydration of tricalcium silicate (C₃S). In order to reduce the time needed to obtain quantitative results, low levels of paramagnetic iron oxide were added to materials to increase the nuclear relaxation rate. The reactions of the C₃S and the finely divided silica could then be monitored separately and without chemical modification. The results were correlated with data from microcalorimetry, thermogravimetric analysis and electron microscopy. Under the conditions used here the presence of the silica accelerates greatly the hydration process, and results in a markedly increased degree of polymerization in the resulting gel, without significantly affecting the induction period of the reaction. The significance of these results for understanding the hydration process of C₃S in the presence of silica is outlined.     

核磁共振技术在茶叶生化成分鉴定中的应用

核磁共振(NMR)在化合物结构鉴定中有着无可比拟的优势.文中就NMR技术在茶叶品质成分、有害成分及品质鉴定中的运用状况进行了阐述与分析,指出了NMR在茶叶鉴定中的技术特点及急待攻克的局限性,提出NMR与其他仪器的联用可为茶叶生化成分研究提供更加全面准确的信息.该文对从事茶叶生化组分鉴定及其生理活性机理研究的工作者有一定借鉴作用.     

Solid state spatially resolved 1H and 19F nuclear magnetic resonance spectroscopy of dental materials by stray-field imaging

As part of a program to evaluate the use of stray-field magnetic resonance microimaging (STRAFI) in dental materials research spatially resolved nuclear magnetic resonance (NMR) for solid dental cements has been investigated. By applying a quadrature echo pulse sequence to a specimen positioned in the stray-field of a NMR spectrometer superconducting magnet the magnetic resonance within a thin slice was obtained. The specimen was stepped through the field in 500 m increments to record ¹ and ¹⁹F profiles and T₂ values at each point. The specimens were fully cured cylinders made from four types of restorative material (glass ionomer, resin modified glass ionomer, compomer, composite). The values for F¹⁹T₂ varied with material type and reflected the nature of the matrix structure. For all materials containing ¹⁹F in the glass two values were calculated for ¹⁹F T₂, one short and one long. These were relatively invariant. Solid state magic angle spinning (MAS)-NMR showed that they came from the glass. This suggests that a proportion of the element is relatively mobile (in a glass phase) and the remainder is more tightly bound (in a compound dispersed in the glass). This demonstration, that NMR microimaging of both ¹H and ¹⁹F in solid dental cements is possible, opens up exciting new possibilities for investigating the distribution of these elements (in particular fluorine) in solid dental materials. ©©1999©Kluwer Academic Publishers     

The effect of a retarder on the early stages of the hydration of tricalcium silicate

Cement is used in the oil industry to line oil wells. The major component of oilwell cement is tricalcium silicate (C₃S), which is responsible for the initial thickening of the cement slurry. It is important to control the time that it takes for this slurry to thicken, and this is achieved in practice by the addition of chemical retarders, which delay the onset of thickening. In this paper, the action of a retarder whose main effect is to form a complex with calcium ions is investigated by use of a model for the hydration of C₃S previously investigated by Preece, Billingham and King (2001). It is found that such a retarder can significantly increase the thickening time of pure tricalcium silicate.     

Possible states of chloride in the hydration of tricalcium silicate in the presence of calcium chloride

Calcium chloride may be present in the free, adsorbed or interlayer state in hydrating tricalcium silicate. Attempts have been made to study these states to correlate some of the physical, chemical and mechanical properties.     

Low-temperature fabrication of macroporous scaffolds through foaming and hydration of tricalcium silicate paste and their bioactivity

A low-temperature fabrication method for highly porous bioactive scaffolds was developed. The two-step method involved the foaming of tricalcium silicate cement paste and hydration to form calcium silicate hydrate and calcium hydroxide. Scaffolds with a combination of interconnected macro- and micro-sized pores were fabricated by making use of the decomposition of a hydrogen peroxide (H₂O₂) solution that acted as a foaming agent and through the hydration of tricalcium silicate cement. It was found possible to control the porosity and pore sizes by adjusting the concentration of the H₂O₂ solution. The in vitro bioactivity of the highly porous scaffolds was investigated by immersion in simulated body fluid (SBF) for 7 days. Hydroxyapatite (HAp) was formed on the surface of the scaffolds. Their bioactivity could be expected to be as good as that of tricalcium silicate cement, making the material competent for the bone tissue engineering application.     

17O quantitative nuclear magnetic resonance spectroscopy of gasoline and oxygenated additives

17O Nuclear magnetic resonance (NMR) spectroscopy allows exclusive detection and direct quantification of oxygenates in gasoline unaffected by its hydrocarbon content, using the internal standard quantitative NMR (QNMR) method. Chemical shifts of 24 oxygen-containing compounds as potential additives and contaminants have been measured in gasoline and corrected values of deltaO 18.1 and 3.9 determined for neat methyl tert-butyl ether (MTBE) and neat di-n-butyl ether, respectively. Quantification of ethanol in gasoline can be readily achieved by 17O QNMR with dimethyl sulfone as an internal standard reference material, at the levels currently used in retail gasolines (1-20%). In addition, the simultaneous detection and quantification of the oxygenates methanol, ethanol, 2-propanol, tert-butyl alcohol, and MTBE in gasoline has been established to further demonstrate the specificity of the method. 17O NMR has distinct advantages over 1H and 13C QNMR methods, and although it cannot reliably differentiate 1-propanol, 1-butanol, 1-pentanol, and isopentyl alcohol, 17O NMR does allow the rapid and unambiguous identification of unexpected oxygenates such as acetates and ketones found as contaminants in some retail gasoline.     

Microinhomogeneity of high-alkali aluminosilicate glasses studied by nuclear magnetic resonance spectroscopy

The structure of quenched aluminosilicate glasses with Na₂O/Al₂O₃ > 1 has been studied by high-resolution nuclear magnetic resonance spectroscopy. The aluminum in the structure of the glasses is shown to be in fourfold coordination. Increasing the sodium oxide content of the glasses reduces the degree of polymerization in their structure and leads to a nonuniform nonbridging oxygen distribution over the aluminosilicate glass network. The glasses have a locally microinhomogeneous structure due to the presence of both highly polymerized aluminosilicate anion groups and relatively depolymerized silicate anions.     

High-field silicon-29 nuclear magnetic resonance spectroscopic studies of dilute methanolic tetramethylammonium silicate solutions

By taking advantage of the increased sensitivity afforded by using samples prepared with silica-enriched in silicon-29, together with a high-field n.m.r. spectrometer, it is possible to monitor the behavior of the cubic octameric silicate anion in dilute tetramethylammonium silicate solutions. It is shown that the addition of methanol to 2:1 (N:Si) tetramethylammonium silicate solutions displaces the silicate anion equilibrium in favor of the cubic octameric cage, to the eventual exclusion of all other species and to the extent that this anion persists as the only silicate species present down to silica concentrations of 0.1 m. Moreover, the stability of this anion in such dilute methanolic tetramethylammonium solutions is such that it reforms after having been destroyed by boiling. Silicon-29 n.m.r. spectra of methanolic 2:1 (N:Si) tetramethylammonium silicate solutions that are 0.02 m in silica show that the cubic octamer gradually disappears from solution. No other n.m.r. signals are observed, indicating that the silica may be forced out of solution under these conditions. Alternatively, it may be incorporated into structures that are essentially n.m.r. invisible at such low concentrations, such as large or colloidal species.     

Spectral editing: A quantitative application of spin-echo nuclear magnetic resonance spectroscopy to the study of 27Al in zeolite catalysts

²⁷Al spin-echo nuclear magnetic resonance (n.m.r.) is used to measure the spin—spin relaxation times, T_2H, for a substantial number of model compounds, and a theory (with no adjustable parameters) based on AIAI dipolar interactions combined with crystallographically determined AIAI distances is used to estimate T_2H. The homonuclear magnetic dipole interaction explains the experimental data reasonably well for compounds with high Al levels, but much less well for compounds with low levels of AI, where structure-specific interactions are important. Such structure-specific interactions are exploited to edit zeolitic AI from the background binder in alumina-bound ZSM-5 and in dealuminated zeolite-Y catalysts containing nonframework (NFW) AI. Editing allows quantitative analysis of the zeolitic components. For dealuminated zeolite Y, it is concluded that peaks assigned by others to “pentacoordinate” AI may actually arise from NFW aluminum, based on the fact that their T_2H is short relative to framework (FW) AI. Theory and experimental results for the technically more demanding measurement of T_2H under conditions of “magic-angle” sample spinning (MAS) with synchronous sampling are also reported. Spin-echo editing of synchronously sampled ²⁷AI MAS n.m.r. spectra are shown to be useful for determining the FW zeolitic AI content of realistically formulated (kaolinite bound) and steamed/calcined fluidized bed cracking (FCC) catalysts. The loss of framework AI in two series of steamed FCC catalysts is less precipitous than the loss in catalytic activity, as measured by the hexane cracking α parameter.