Quantitative study of hydration of C<Subscript>3</Subscript>S and C<Subscript>2</Subscript>S by thermal analysis

This research is part of a European project (namely, CODICE project), main objective of which is modelling, at a multi-scale, the evolution of the mechanical performance of non-degraded and degraded cementitious matrices. For that, a series of experiments were planned with pure synthetic tri-calcium silicate (C₃S) and bi-calcium silicate (C₂S) (main components of the Portland cement clinker) to obtain different calcium–silicate–hydrate (C–S–H) gel structures during their hydration. The characterization of those C–S–H gels and matrices will provide experimental parameters for the validation of the multi-scale modelling scheme proposed. In this article, a quantitative method, based on thermal analyses, has been used for the determination of the chemical composition of the C–S–H gel together with the degree of hydration and quantitative evolution of all the components of the pastes. Besides, the microstructure and type of silicate tetrahedron and mean chain length (MCL) were studied by scanning electron microscopy (SEM) and ²⁹Si magic-angle-spinning (MAS) NMR, respectively. The main results showed that the chemical compositions for the C–S–H gels have a CaO/SiO₂ M ratio almost constant of 1.7 for both C₃S and C₂S compounds. Small differences were found in the gel water content: the H₂O/SiO₂ M ratio ranged from 2.9 ± 0.2 to 2.6 ± 0.2 for the C₃S (decrease) and from 2.4 ± 0.2 to 3.2 ± 0.2 for the C₂S (increase). The MCL values of the C–S–H gels, determined from ²⁹Si MAS NMR, were 3.5 and 4 silicate tetrahedron, for the hydrated C₃S and C₂S, respectively, remaining almost constant at all hydration periods.     

Non-isothermal kinetics of the thermal decomposition of sodium oxalate Na<Subscript>2</Subscript>C<Subscript>2</Subscript>O<Subscript>4</Subscript>

The thermal transformation of Na₂C₂O₄ was studied in N₂ atmosphere using thermo gravimetric (TG) analysis and differential thermal analysis (DTA). Na₂C₂O₄ and its decomposed product were characterized using a scanning electron microscope (SEM) and the X-ray diffraction technique (XRD). The non-isothermal kinetic of the decomposition was studied by the mean of Ozawa and Kissinger–Akahira–Sunose (KAS) methods. The activation energies (E _α) of Na₂C₂O₄ decomposition were found to be consistent. Decreasing E _α at increased decomposition temperature indicated the multi-step nature of the process. The possible conversion function estimated through the Liqing–Donghua method was ‘cylindrical symmetry (R₂ or F_1/2)’ of the phase boundary mechanism. Thermodynamic functions (ΔH*, ΔG* and ΔS*), calculated by the Activated complex theory and kinetic parameters, indicated that the decomposition step is a high energy pathway and revealed a very hard mechanism.     

Non-isothermal decomposition kinetics of NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles synthesized using egg white solution route

The thermal decomposition kinetics of nickel ferrite (NiFe₂O₄) precursor prepared using egg white solution route in dynamical air atmosphere was studied by means of TG with different heating rates. The activation energy (E _α) values of one reaction process were estimated using the methods of Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS), which were found to be consistent. The dependent activation energies on extent of conversions of the decomposition reaction indicate “multi-step” processes. XRD, SEM and FTIR showed that the synthesized NiFe₂O₄ precursor after calcination at 773 K has a pure spinel phase, having particle sizes of ~54 ± 29 nm.     

Some studies on the phase formation and kinetics in TiO<Subscript>2</Subscript> containing lithium aluminum silicate glasses nucleated by P<Subscript>2</Subscript>O<Subscript>5</Subscript>

Lithium aluminum silicate (LAS) glasses of compositions (wt%) 10.6Li₂O–71.7SiO₂–7.1Al₂O₃–4.9K₂O–3.2B₂O₃–1.25P₂O₅–1.25TiO₂ were prepared by the melt quench technique. Crystallization kinetics was investigated by the method of Kissinger and Augis–Bennett using differential thermal analysis (DTA). Based on the DTA data, glass ceramics were prepared by single-, two-, and three-step heat treatment schedules. The interdependence of different phases formed, microstructure, thermal expansion coefficient (TEC) and microhardness (MH) was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermo-mechanical analysis (TMA), and microhardness (MH) measurements. Crystallization kinetics revealed that Li₂SiO₃ is the kinetically favored phase with activation energy of 91.10 kJ/mol. An Avrami exponent of n = 3.33 indicated the dominance of bulk crystallization. Based upon the formation of phases, it was observed that the two-stage heat treatment results in highest TEC glass ceramics. The single-step heat treatment yielded glass ceramics with the highest MH.     

One-step template-free solution route for Cu(OH)<Subscript>2</Subscript> nanowires

Cu(OH)₂ nanowires with a diameter of 8–10 nm and lengths of tens of micrometers were fabricated in the basic solution by dropping simply NaOH solution into CuCl₂ solution at ambient temperature. The formation mechanism of nanowires was discussed. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were used to characterize the samples. This article was submitted by the authors in English.     

Kinetics of the oxidation of diethyl sulfide in the B(OH)<Subscript>3</Subscript>-H<Subscript>2</Subscript>O<Subscript>2</Subscript>/H<Subscript>2</Subscript>O system

Data obtained for the kinetics of oxidation of diethyl sulfide (Et₂S) by hydrogen peroxide in aqueous solution catalyzed by boric acid indicate that monoperoxoborates B(O₂H)(OH) ₃ ⁻ and diperoxoborates B(O₂H)₂(OH) ₂ ⁻ are the active species. The rates of the reactions of Et₂S with B(O₂H)(OH) ₃ ⁻ and B(O₂H)₂(OH) ₂ ⁻ are 2.5 and 100 times greater than with H₂O₂. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 1, pp. 38–42, January–February, 2007.     

DSC investigation of nanocrystalline TiO<Subscript>2</Subscript> powder

The aim of the article is to investigate the influence of particle size on titanium dioxide phase transformations. Nanocrystalline titanium dioxide powder was obtained through a hydrothermal procedure in an aqueous media at high pressure (in the range 25–100 atm) and low temperature (≤200 °C). The as-prepared samples were characterized with respect to their composition by ICP (inductive coupled plasma), structure and morphology by XRD (X-ray diffraction), and TEM (transmission electron microscopy), thermal behavior by TG (thermogravimetry) coupled with DSC (differential scanning calorimetry). Thermal behavior of nanostructured TiO₂ was compared with three commercial TiO₂ samples. The sequence of brookite–anatase–rutile phase transformation in TiO₂ samples was investigated. The heat capacity of anatase and rutile in a large temperature range are reported.     

Heat capacity and heat content of BiNb<Subscript>5</Subscript>O<Subscript>14</Subscript>

The heat capacity and the heat content of bismuth niobate BiNb₅O₁₄ were measured by the relaxation time method, DSC and drop method, respectively. The temperature dependence of heat capacity in the form C _pm=455.84+0.06016T–7.7342·10⁶/T ² (J K^–1 mol^–1) was derived by the least squares method from the experimental data. Furthermore, the standard molar entropy at 298.15 K S _m=397.17 J K^–1 mol^–1 was derived from the low temperature heat capacity measurement.     

BaTi<Subscript>1–x</Subscript>Zr<Subscript>x</Subscript>O<Subscript>3</Subscript> nanopowders prepared by the modified Pechini method

The results reported here based on a study of BaTi_1–xZr_xO₃ (x=0, 0.2 and 1) nanometric powders prepared by the modified Pechini method. The powder samples annealed from 600 to 1000°C/2 h were characterized by thermogravimetric analysis (TG), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The decomposition reactions of resins were studied using thermal analysis measurements. The barium titanate zirconate system presented just one orthorhombic phase. Furthermore, this study produced BaTiO₃ powders with a tetragonal structure using shorter heat treatments and less expensive precursor materials than those required by the traditional methods.     

Synthesis and supercapacitance of flower-like Co(OH)<Subscript>2</Subscript> hierarchical superstructures self-assembled by mesoporous nanobelts

A facile hydrothermal strategy was first proposed to synthesize flower-like Co(OH)₂ hierarchical microspheres. Further physical characterizations revealed that the flower-like Co(OH)₂ microspherical superstructures were self-assembled by one-dimension nanobelts with rich mesopores. Electrochemical performance of the flower-like Co(OH)₂ hierarchical superstructures were investigated by cyclic voltammgoram, galvanostatic charge–discharge and electrochemical impedance spectroscopy in 3 M KOH aqueous electrolyte. Electrochemical data indicated that the flower-like Co(OH)₂ superstructures delivered a specific capacitance of 434 F g⁻¹ at 10 mA cm⁻² (about 1.33 A g⁻¹), and even kept it as high as 365 F g⁻¹ at about 5.33 A g⁻¹. Furthermore, the SC degradation of about 8% after 1,500 continuous charge–discharge cycles at 5.33 A g⁻¹ demonstrates their good electrochemical stability at large current densities.     

Thermal study of TiO<Subscript>2</Subscript>–CeO<Subscript>2</Subscript> yellow ceramic pigment obtained by the Pechini method

TiO₂–CeO₂ oxides for application as ceramic pigments were synthesized by the Pechini method. In the present work the polymeric network of the pigment precursor was studied using thermal analysis. Results obtained using TG and DTA showed the occurrence of three main mass loss stages and profiles associated to the decomposition of the organic matter and crystallization. The kinetics of the degradation was evaluated by means of TG applying different heating rates. The activation energies (E _a) and reaction order (n) for each stage were determined using Horowitz–Metzger, Coats–Redfern, Kissinger and Broido methods. Values of E _a varying between 257–267 kJ mol^–1 and n=0–1 were found. According to the kinetic analysis the decomposition reactions were diffusion controlled.     

Study of the hydration of Portland cement blended with blast-furnace slag by calorimetry and thermogravimetry

The hydration of ordinary Portland cement (OPC) blended with blast-furnace slag (BFS) is a complex process since both materials have their own reactions which are, however, influenced by each other. Moreover, the effect of the slag on the hydration process is still not entirely known and little research concerning the separation of both reactions can be found in the literature. Therefore, this article presents an investigation of the hydration process of mixes in which 0–85% of the OPC is replaced by BFS. At early ages, isothermal, semi-adiabatic and adiabatic calorimetric measurements were performed to determine the heat of hydration. At later ages, thermogravimetric (TG) analyses are more suitable to follow up the hydration by assessment of the bound water content w _b. In addition, the microstructure development was visualized by backscattered electron (BSE) microscopy. Isothermal calorimetric test results show an enhancement of the cement hydration and an additional hydration peak in the presence of BFS, whilst (semi-)adiabatic calorimetric measurements clearly indicate a decreasing temperature rise with increasing BFS content. Based on the cumulative heat production curves, the OPC and BFS reactions were separated to determine the reaction degree Q(t)/Q _∞ (Q = cumulative heat production) of the cement, slag and total binder. Moreover, thermogravimetry also allowed to calculate the reaction degree by w _b(t)/w _b∞. The reaction degrees w _b(t)/w _b∞, Q(t)/Q _∞ and the hydration degrees determined by BSE-image analysis showed quite good correspondence.     

Synthesis,crystal structure,and thermal decomposition kinetics of complex [Nd(BA)<Subscript>3</Subscript>bipy]<Subscript>2</Subscript>

The complex of [Nd(BA)₃bipy]₂ (BA = benzoic acid; bipy = 2,2′-bipyridine) has been synthesized and characterized by elemental analysis, IR spectra, single crystal X-ray diffraction, and TG/DTG techniques. The crystal is monoclinic with space group P2(1)/n. The two–eight coordinated Nd³⁺ ions are linked together by four bridged BA ligands and each Nd³⁺ ion is further bonded to one chelated bidentate BA ligand and one 2,2′-bipyridine molecule. The thermal decomposition process of the title complex was discussed by TG/DTG and IR techniques. The non-isothermal kinetics was investigated by using double equal-double step method. The kinetic equation for the first stage can be expressed as dα/dt = A exp(−E/RT)(1 − α). The thermodynamic parameters (ΔH ^≠, ΔG ^≠, and ΔS ^≠) and kinetic parameters (activation energy E and pre-exponential factor A) were also calculated.     

Theoretical study of the gas-phase Fe<Superscript>+</Superscript>-mediated oxidation of ethane by N<Subscript>2</Subscript>O

We report herein a comprehensive study of the gas-phase Fe⁺-mediated oxidation of ethane by N₂O on both the sextet and quartet potential energy surfaces (PESs) using density functional theory. The geometries and energies of all the relevant stationary points are located. Initial oxygen-atom transfer from N₂O to iron yields FeO⁺. Then, ethane oxidation by the nascent oxide involves C–H activation forming the key intermediate of (C₂H₅)Fe⁺(OH), which can either undergo C–O coupling to Fe⁺ + ethanol or experience β-H shift giving the energetically favorable product of FeC₂H₄ ⁺ + H₂O. Reaction of FeC₂H₄ ⁺ with another N₂O constitutes the third step of the oxidation. N₂O coordinates to FeC₂H₄ ⁺ and gets activated by the metal ion to yield (C₂H₄)Fe⁺O(N₂). After releasing N₂ through the direct H abstraction and/or cyclization pathways, the system would be oxidized to ethenol, acetaldehyde, and oxirane, regenerating Fe⁺. Oxidation to acetaldehyde along the cyclization –C–to–C hydrogen shift pathway is the most energetically favored channel.     

Effects of nano-limestone on hydration properties of tricalcium silicate

In this research, the influences of nano-limestone (NC) on tricalcium silicate (C₃S) hydration, including early hydration rate, setting time, Ca(OH)₂ content, quantification of C–S–H, micrographs of hydration products and compressive strength, were investigated by different methods. Calorimetry results show that the first and second exothermic peak of C₃S with 1 mass% NC is ~29 and ~5% higher than that of control sample and the second peak appears 32 min in advance. Besides, the total hydration heat of C₃S with 1 mass% NC increases by 10.08% compared to control sample at 72 h. The initial and final setting time of C₃S with 1 mass% NC shortens by 23 and 17 min, respectively. In addition, thermal analysis results show that adding NC leads to higher Ca(OH)₂ and C–S–H content at all ages (1, 3 and 28 days). All of the above results imply that the hydration progress of C₃S is accelerated by NC. Correspondingly, the compressive strength of C₃S with 1 mass% NC increases by 9.88% at 28 days. Besides, EDS results reveal that the Ca/Si ratio of C–S–H gel decreases after NC modification. However, excessive content NC (4 mass%) reduces the acceleration effect of NC on C₃S hydration and finally leads to relative low content of Ca(OH)₂ and C–S–H compared with C₃S sample containing 1 mass% NC.     

Synthesis of highly dispersed super-refractory tantalum-zirconium carbide Ta<Subscript>4</Subscript>ZrC<Subscript>5</Subscript> and tantalum-hafnium carbide Ta<Subscript>4</Subscript>HfC<Subscript>5</Subscript> via sol-gel technology

Nanocrystalline powders of super-refractory complex carbides Ta₄HfC₅ and Ta₄ZrC₅ were synthesized using a hybrid method comprising sol-gel technology for preparing highly dispersed metal oxidescarbon starting mixtures and a relatively low-temperature (1300–1500°C) carbothermal synthesis under a dynamic vacuum (P = 1 × 10⁻³ to 1 × 10⁻⁵ MPa). The elemental and phase compositions of the products and average crystallite sizes were determined. TEM was used to study particle morphology and dispersion. Microstructures were observed by SEM. BET specific surface areas were determined for powders prepared at 1400°C.     

Thermochemistry of Bu<Subscript>4</Subscript>NBr solutions in binary solvents containing formamide

The heats of solution of tetrabutylammonium bromide have been measured in mixtures of formamide (FA) with methanol (MeOH) and ethylene glycol (EG) at 313.15 K by calorimetric method. The standard enthalpies of solution in binary mixtures have been extrapolated to infinite dilution by Redlich–Rosenfeld–Meyer type equation using the literary data at 298.15 K and the present paper data at 313.15 K. The Debye–Hückel limiting law slope A _H required for calculation of the ∆_sol H ⁰ value has been obtained with application the new additive scheme of determination of the physic-chemical characteristics of binaries. The scheme is tested on the example of Bu₄NBr solutions in FA–MeOH mixture at 298.15 K. Its application yields the ∆_sol H ⁰ value very closed on the ones determined with the real (non-additive) characteristics of binaries. The standard enthalpies of solution extrapolated by Redlich–Rosenfeld–Meyer type equation are in a good agreement with the ones computed in terms of the Debye–Hückel theory in the second approximation. The heat capacities characteristics of Bu₄NBr have been calculated in H₂O–FA, MeOH–FA and EG–FA mixtures using the literary and present data. The sequence of solvents H₂O > FA > EG > MeOH located on their ability to solvophobic solvation found by us earlier for enthalpic characteristics is confirmed by the ∆C _p ⁰ values. The comparison of thermochemical characteristics of Bu₄NBr solutions in aqueous and non-aqueous mixtures containing FA has been carried out. The own structure of water remains in the region of small additions of formamide to co-solvents. It considerably differs the H₂O–FA mixture from the investigated non-aqueous systems.     

Degree of hydration in cement paste and C<Subscript>3</Subscript>A-sodium carbonate-water systems

This research provides a fundamental understanding of the early stage hydration of Portland cement paste, tricalcium aluminate (C₃A) paste at water to cement ratio of 0.5 and C₃A suspension at water to cement ratio of 5.0 modified by 2 or 4 mass% of sodium carbonate. A high conversion of unreacted clinker minerals to gel-like hydration products in the cement-Na₂CO₃ pastes takes place rapidly between 1^st to 24^th h. Contrary the Ca(OH)₂ formation within the same time interval is retarded in the excess of CO₃²⁻ ions due to intensive rise and growth of CaCO₃ crystals in hydrated cement. Later, the conversion of clinker minerals to the hydrate phase is reduced and higher contents of calcite and vaterite relative to that of Ca(OH)₂ in comparison with those found in the Portland cement paste are observed. As a consequence a decrease in strength and an increase in porosity between hardened Portland cement paste without sodium carbonate and those modified by Na₂CO₃ are observed. C₃A hydrates very quickly with sodium carbonate between 1^st and 24^th h forming hydration products rich in bound water and characterized also by complex salts of (x)C₃A·(y)CO₂·(zH₂O type, whereas C₃A-H₂O system offers C₃AH₆ as the main hydration product. Higher content of the formed calcium aluminate hydrates in C₃A-Na₂CO₃-H₂O system also contributes to early strength increase of Portland cement paste.     

Reduction of N<Subscript>2</Subscript>O and NO over H-ZSM-5- and ZrO<Subscript>2</Subscript>-supported iron- and cobalt-containing catalysts

The combined conversion conditions were examined for the reactions of decomposition and reduction of N₂O and NO with C₁,C₃–C₄ hydrocarbons, in particular, in gas mixtures containing oxygen and sulfur dioxide, over Fe- and Co-containing catalysts supported on zeolites and zirconia, as well as on structured honeycomb monoliths.     

Plasma Composition by Mass Spectrometry in a Ar-SiH<Subscript>4</Subscript>-H<Subscript>2</Subscript> LEPECVD Process During nc-Si Deposition

Mass spectrometry has been used to assess plasma composition during a low-energy plasma-enhanced chemical vapor deposition (LEPECVD) process using argon-silane-hydrogen (Ar-SiH₄-H₂) gas mixtures with input flows of 50 sccm Ar, 2–20 sccm SiH₄ and 0–50 sccm H₂ at total pressures of 1–4 Pa. Energy-integrated ion densities, residual gas analysis and threshold ionization mass spectrometry have been used to characterize the transition from amorphous (a-Si) to nano-crystalline silicon (nc-Si) deposition at constant LEPECVD operating parameters. While relative ion densities have a marked decrease with H₂ input, the densities of SiH_x (x < 4) radicals show evolution trends depending on the SiH₄ and H₂ input. For conditions leading to nc-Si growth a turning point is reached above which SiH is the main radical. Observed SiH_x density trends with H₂ input are explained based on kinetic reaction rates calculated from previously obtained Langmuir probe data.