Transmission Electron Microscopy and X-Ray Diffraction of Doped Tricalcium Silicate

Member of oxides, including MgO, ZnO, La₂O₃, Al₂O₃, TiO₂, and F were reacted with C₃S, either individually or in combination, in order to stabilize a variety of polymorphic forms. The resultant phases were examined by both XRD and TEM. It was found that the results obtained by the conventional means of observing groups of reflections in powder X-ray diffraction patterns were not in agreement with the structure types identified by electron diffraction. In particular, the rhombohedral form of alite, synthesized by the addition of either ZnO or Al₂ O₃+ F to C₃S, was not observed by TEM although the corresponding X-ray patterns all displayed single peaks characteristic of the rhombohedral cell. Electron diffraction of C,S doped with 2.02 and 4.43% ZnO gave mono-clinic patterns, whereas C₃S doped with Al + F gave several forms, all of them different from the form produced by ZnO-doped C₃S. The addition of La₂0₃ produced a new hexagonal lattice with cell dimensions of a = 2.575 and c = 2.50 nm. When Al + F was incorporated in C₃S, the modifications found included the triclinic lattice characteristic of pure C₃S, the hexagonal lattice characteristic of La-doped C₃S, and a complex lattice distinguished by the presence of incommensurate doublet reflections and sometimes extra weak spots. A similar incommensurate superlattice was found in alites containing Mg, Al, and a large amount of Ti (0.13 mole). The Jeffery cell for the Mm modification of C₃S doped with 1.63% MgO was confirmed.     

Hydration of Beta Dicalcium Silicate Alone and in the Presence of CaCl2 or C2H5OH

Beta C₂S was hydrated at room temperature with and without added CaCl₂ or C₂H₅OH by methods previously studied for the hydration of C₃S, i.e. paste, bottle, and ball-mill hydration. The amount of reacted β-C₂S, the Ca(OH)₂ concentration in the liquid phase, the CaO/SiO₂ molar ratio, and the specific surface area of the hydrate were investigated. A topochemical reaction occurs between water and β-C₂S, resulting in the appearance of solid Ca(OH)₂ and a hydrated silicate with a CaO/SiO₂ molar ratio of ≃1. As the liquid phase becomes richer in Ca(OH)₂, the first hydrate transforms to one with a higher CaO/SiO₂ ratio. Addition of CaCl₂ increases the reaction rate and the surface area of the hydrate but to a much lesser extent than in the hydration of C₃S, whereas C₂H₆OH strongly depresses the hydration rate of β-C₂S, as observed for C₃S hydration.     

Characterization of Ca2SiO4:Eu2+ Phosphors Synthesized by Polymeric Precursor Process

The green emitting Ca₂SiO₄:Eu²⁺ (C₂S:Eu) phosphors were synthesized by the polymeric precursor process (Pechini-type), and the effects of calcination temperature and europium (Eu) doping concentration on the luminescent properties were investigated. The crystalline β-C₂S was obtained in the calcination temperature of 1100°–1400°C, and Eu was reduced into Eu²⁺ by annealing in 5% H₂/N₂ atmosphere. The obtained C₂S:Eu²⁺ phosphors exhibited a strong emission at 504 nm under the excitation of λ_exc=350 nm. The highest photoluminescence (PL) intensity was observed in the C₂S:Eu²⁺ phosphors either calcined at 1300°C or doped with 3 mol% Eu. The obtained PL properties were discussed in terms of crystal structure, particle size and shape, surface roughness, and effect of concentration quenching.     

Influence of SO3 on the Phase Relationship in the System CaO–SiO2–Al2O3–Fe2O3

The influence of the additive SO₃ on the phase relationships in the quaternary system CaO-SiO₂-Al₂O₃-Fe₂O₃ was investigated by observing the change of volume ratio of 3CaOSiO₂ (C₃S) to 2CaOSiO₂ (C₂S) + CaO (C) in the sintered material with the increase of SO₃ content. The primary phase volume of C₃S in the quaternary phase diagram shrank with the increase of SO₃ and disappeared when the SO₃ content exceeded 2.6 wt% in the sintered material. Changes in the peritectic reaction relationship between CaO (C), 2CaOSiO₂ (C₂S), 3CaOSiO₂ (C₃S), 3CaOAl₂O₃ (C₃A), 4CaOAl₂O₃Fe₂O₃ (C₄AF), and liquid were also observed and discussed.     

Effect of Phosphorus Impurity on Tricalcium Silicate T1: From Synthesis to Structural Characterization

Alite is the major compound of anhydrous Portland cement: it is composed of tricalcium silicate Ca₃SiO₅ (C₃S) modified in composition and crystal structure by ionic substitutions. Alite is also the main hydraulic phase of cement and the most important for subsequent strength development. Using raw meals (rich in Ca₃P₂O₈) as alternative fuels in cement plants raises the question about the effect of phosphorus on C₃S and its consequences on reactivity with water. This paper deals with a systematic study of C₃S triclinic T₁ polymorph doped with P₂O₅ in the range 0–0.9 wt%. All the samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and electron-microprobe analysis. The appearance of a phase rich in phosphorus is shown. It displays a structure derivative of the α'_H–Ca₂SiO₄ polymorph, noted α'_H–C₂S(P). As phosphorus content increases, C₃S is more and more decomposed into free lime and α'_H–C₂S(P). The α'_H phase was detected from 0.1 wt% P₂O₅ and located at the interfaces of C₃S grains. Two identification keys are proposed in order to highlight the α'_H–C₂S(P) phase: the XRD angular window at 2θ_Cu=32.8°–33.2° and a smooth aspect on SEM micrographs.     

Thermal Study of the Effect of Additives on the Hydration of Tricalcium Silicate

The rate of paste hydration of 3C_aO·SiO₂ (C₃S) and the effects of additions of CaCl₂, CdI₂, and CrCl₃, were studied by differential thermal analysis and thermogravimetry. X-ray analyses were used to identify the synthesized C₃S. The salts CaCl₂, CdI₂, and CrCl₃, accelerated the hydration of C₃S. The degree of hydration was estimated by the amount of Ca(OH)₂, formed, as determined by TG.     

Substituted Hydrated Calcium Silicates Obtained in Autoclave Hydration

Hydrated calcium silicates containing Al^3+] or Fe^3+] were prepared by autoclaving C₃S and β-C₂S in the presence of C₃A or C₂F at 190°C. Al^3+] and Fe^3+] diffuse into the crystal lattice of α-C₂SH and C₃SH_1.5. Solid solutions containing Al^3+] and Fe^3+] were placed in contact at 25°C with sources of sulfates, either in aqueous stirred suspensions or as pastes. Al^3+] and Fe^3+] remain stable in the solid solution, inhibiting the formation of ettringite. This absence of ettringite can explain the resistance of autoclaved cement pastes and concretes to sulfate attack.     

Early Hydration of Tricalcium Silicate

The hydration of tricalcium silicate (C₃S) in the preacceleration stages was studied. The C₃S particles carry a positive charge during the early stages of hydration. Following a rapid hydrolysis of C₃S, calcium ions adsorbed on the Si-rich surface of C₃S particles, greatly reducing their further dissolution, thus initiating the induction period. The [Ca²⁺] and [OH^-] continue to increase at lower rates and, because Ca(OH)₂ crystal growth is inhibited by silicate ions, become supersaturated with respect to Ca(OH)₂. When the supersaturation reaches a value of ∼1.5 to 2.0 times the saturation concentration, nuclei are formed, and rapid growth of Ca(OH)₂ and C-S-H is initiated. These products act as sinks for the ions in solution, thus enhancing the further dissolution of C₃S.     

Kinetics of the α-to-α'H Polymorphic Phase Transition of Ca2SiO4 Solid Solutions

The α-to-α'_H transition of Ca₂SiO₄ solid solutions (C₂S(ss)) is a nucleation and growth process. This process was shown on time–temperature–transformation (TTT) diagrams for C₂S(ss) with different concentrations of foreign oxides (Na₂O, Al₂O₃, and Fe₂O₃). The kinetic cutoff temperature and the activation energy for growth of the α'_H phase increase steadily with increasing concentration of impurities. The activation energy for nucleation also increases above 950°C. The α'_H phase, which exists in equilibrium with the α phase at 1280°C, is formed at a maximum rate at around 1100°C regardless of the chemical composition. The TTT diagrams enable us to predict, as a function of cooling rate, the phase constitution of C₂S(ss) at ambient temperature.     

Reaction of Beta-Dicalcium Silicate and Tricalcium Silicate with Carbon Dioxide and Water Vapor

The carbonation-reaction kinetics of beta-dicalcium silicate (2CaO·SiO₂ or β-C₂S) and tricalcium silicate (3CaO. SiO₂ or C₃S) powders were determined as a function of material parameters and reaction conditions and an equation was developed which predicted the degree of reaction. The effect of relative humidity, partial pressure of CO₂, surface area, reaction temperature, and reaction time on the degree of reaction was determined. Carbonation followed a decreasing-volume, diffusion-controlled kinetic model. The activation energies for carbonation of β-C₂S and C₃S were 16.9 and 9.8 kcal/mol, respectively. Aragonite was the principal carbonate formed during the reaction and the rate of carbonate formation was coincident with depletion of the calcium silicates; C-S-H gel formation was minimal.     

The Pore Structure of Hydrated Cementitious Compounds of Different Chemical Composition

The pore structure ofβ-C₂S, C₃S, and portland cement pastes was investigated using mercury porosimetry and H₂O and N₂ adsorption. The β-C₂S had more total macro- and mesoporosities than C₃S and portland cement pastes of a similar degree of hydration. C₃S and portland cement pastes had similar total porosities but differed in the porosity size distribution. In the mesopore range, the various test methods gave different results. These differences are discussed on the basis of the various models proposed for cement paste. It is shown that shrinkage could be correlated with the volume of pores <0.03 μm, but not with total porosity.     

Structure Change of Ca2SiO4 Solid Solutions with Ba Concentration

A series of Ba-bearing Ca₂SiO₄ solid solutions (C₂S( ss )), (Ba _x Ca_{1− x} )₂SiO₄ with 0.075 x 0.30, were prepared and examined by X-ray and electron beam diffraction. They are all made up of orthorhombic domains 120° different in orientation around the common c axis of the former α phase. The C₂S solid solution with x = 0.075 shows a superstructure incommensurate along the a axis with λ (modulation wavelength) = 3.5 and commensurate along the c axis with Δ= 3. With x = 0.15, modulation is observed only along the a axis and Δ= 3.4. No evidence of superstructure is found with x = 0.24; the space group and cell dimensions are comparable with those of pure α '_H-C₂S. The C₂S( ss ) with x = 0.30 gives a superlattice with the cell-edge length of 3 b . All the C₂S( ss ), when reheated at 1000°C for 24 h, produced lamellae of the trigonal phase T nearly in parallel with (001) of the host α '_L phase. The crystallographic orientation between the two phases is

This indicates that the above Ba-bearing C₂S( ss ) phases occur as precursors to the thermodynamically more stable two-phase mixtures.     

Changes in Zeta Potential During Hydration of C3S With and Without CaCl2 Additions

C₃S and C₃S+2% CaCl₂ were hydrated for varied times; the degree of hydration and zeta potential were determined. In the absence of CaCl₂, the duration of the induction period was 5 h, whereas when CaCl₂ was added, an induction period of 1 h was observed. The zeta potential was positive, maximum, and constant during induction .     

Application of Selective 29Si Isotopic Enrichment to Studies of the Structure of Calcium Silicate Hydrate (C-S-H) Gels

Selective isotopic enrichment of SiO₂ with ²⁹Si in a mixture with tricalcium silicate (C₃S) has allowed the Si from this phase to be effectively labeled during the course of the hydration reaction, thus isolating its contribution to the reaction. A double Q₂ signal has been observed in ²⁹SI solid-state MAS NMR spectroscopy of C-S-H gels of relatively low Ca/Si ratio, prepared by hydration or by carbonation of a C₃S paste. The origin of the weaker, downfield peak is discussed and tentatively attributed to bridging tetrahedra of a dreierkette silicate chain structure.     

Melt Differentiation Induced by Zonal Structure Formation of Calcium Aluminoferrite in a CaO–SiO2–Al2O3–Fe2O3 Pseudoquaternary System

The phase stability in part of the P₂O₅-bearing pseudoquaternary system CaO–SiO₂–Al₂O₃–Fe₂O₃ has been studied by electron probe microanalysis, optical microscopy, and powder X-ray diffractometry. At 1973–1653 K, the α-Ca₂SiO₄ solid solution [α-C₂S(ss)] and melt coexisted in equilibrium, both chemical variations of which were determined as a function of temperature. The three phases of melt, calcium aluminoferrite solid solution (ferrite), and C₂S(ss) coexisted at 1673–1598 K. On the basis of the chemical compositions of these phases, a melt-differentiation mechanism has been, for the first time, suggested to account for the crystallization behavior of Ca₃Al₂O₆ solid solution [C₃A(ss)]. When the α-C₂S(ss) and melt were cooled from high temperatures, the melt would be induced to differentiate by the crystallization of ferrite. Because the local equilibrium would be continually attained between the rims of the precipitating ferrite and coexisting melt during further cooling, the melt would progressively become enriched in Al₂O₃ with respect to Fe₂O₃. The resulting ferrite crystals would show the zonal structure, with the Al/(Al+Fe) value steadily increasing up to 0.7 from the cores toward the rims. The C₃A(ss) would eventually crystallize out of the differentiated melt between the zoned ferrite crystals in contact with their rims.     

Influence of Minor Ions on the Stability and Hydration Rates of β-Dicalcium Silicate

The effects of Al³⁺, B³⁺, P⁵⁺, Fe³⁺, S⁶⁺, and K⁺ ions on the stability of the β-phase and its hydration rate were studied in reactive dicalcium silicate (C₂S, Ca₂SiO₄) synthesized using the Pechini process. In particular, the dependences of the phase stability and degree of hydration on the calcination temperature (i.e., particle size) and the concentration of the stabilizing ions were investigated. The phase evolution in doped C₂S was determined using XRD, and the degree of hydration was estimated by the peak intensity ratio of the hydrates to the nonhydrates in ²⁹Si MAS NMR spectra. The stabilizing ability of the ions varied significantly, and the B³⁺ ions were quite effective in stabilizing the β-phase over a wide range of doping concentrations. The hydration results indicated that differently stabilized β-C₂S hydrated at different rates, and Al³⁺- and B³⁺-doped C₂S exhibited increased degree of hydration for all doping concentration ranges investigated. The effect of the doping concentration on degree of hydration was strongly dependent on the stabilizing ions.     

Effects of Seed Layer Characteristics on the Synthesis of ZnO Nanowires

Single crystalline zinc oxide (ZnO) nanowires were synthesized on sputter-deposited ZnO seed layers via hydrothermal reactions in an equimolar (20 mM) aqueous solutions of Zn(NO₃)₂·6H₂O and C₆H₁₂N₄ at 90°C. The sputter-deposited ZnO seed layers were prepared to exhibit different crystalline structures in order to examine their effects on the growth of ZnO nanowires. It was found that the nanowire diameter depends on the size of the (002) grains of the seed layer. This is attributed to the epitaxially growth of the nanowires from the columnar grains of the seed layer which is shown by the TEM analysis.     

Preparation of Portland Cement Components by Poly(vinyl alcohol) Solution Polymerization

The four components portland cement-dicalcium silicate, C₂S (Ca₂SiO₄); tricalcium silicate, C₃S (Ca₃SiO₅); tricalcium aluminate, C₃A (Ca₃Al₂O₆); and tetracalcium aluminate iron oxide, C₄AF (Ca₄Al₂Fe₃O₁₀)-were formed using a solution-polymerization route based on poly(vinyl alcohol) (PVA) as the polymer carrier. The powders were characterized using X-ray diffraction techniques, BET specific surface area measurements, and scanning electron microscopy. This method produced relatively pure, synthetic cement components of submicrometer or nanometer crystallite dimensions, high specific surface areas, as well as extremely high reactivity at relatively low calcining temperatures. The PVA content and its degree of polymerization had a significant influence on the homogeneity of the final powders. Two types of degree of polymerization (DP) PVA were used. Lower crystallization temperatures and smaller particle size powders were obtained from the low-DP-type PVA at optimum content.     

Characterization of Hot-Pressed Cement Pastes

Very-high-strength cement pastes (compressive strengths ∼100,000 psi) were prepared using hot-pressing techniques at temperatures up to 250°C. The results of characterization studies summarized include X-ray diffraction, DTA, ir spectra, and microstructural investigations. Much less Ca(OH)₂ is formed than in normally hydrated pastes. Some well- crystallized hydroxyl-containing hydrates, C₅S₂H, C₃SH_1.5, C₃AH₆, and C₄A₃H₃, as well as poorly crystalline CSH, are formed. The structure of the CSH, as revealed by ir absorption studies, does not appear to be very different from that in low- temperature hydrated cement pastes. Microstructures are extremely dense and, in some cases, the sulfate-containing phase concentrates along original grain boundaries.     

Borate Distribution in Stabilized Stainless-Steel Slag

Disintegration of stainless-steel slag during cooling is caused by the β→γ transformation of dicalcium silicate (2CaO·SiO₂ or C₂S). It is well established that disintegration can be avoided by borate additions, which are commonly believed to stabilize the high-temperature polymorphs by forming a solid solution with C₂S. In this study, the borate distributions in slowly cooled synthetic and industrial slag samples are determined. The slag microstructures are characterized using electron probe microanalysis (EPMA), quantitative X-ray diffraction (QXRD), and wavelength dispersive spectroscopy (WDS). The results show that only a limited fraction of the added borates forms a solid solution with the C₂S phase, while the main fraction is found in a calcium borosilicate phase. In combination with literature data, this suggests that slag stabilization involves more than the chemical stabilization of the high-temperature C₂S polymorphs.