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1.
Krassimir Garbev Biliana Gasharova Peter Stemmermann 《Journal of the American Ceramic Society》2014,97(7):2286-2297
A single crystal of α‐Ca2[HSiO4](OH) (α‐C2SH) was repeatedly imaged at room temperature with synchrotron mid‐infrared microscopy after heating to 310°C, 340°C, 370°C, and 400°C respectively. The mechanisms of the observed phase transformations are discussed on the basis of a modular concept of the crystal structures. All images show domains of dellaite, Ca6[Si2O7][SiO4](OH)2, which are predominantly formed in the core of the crystal. In the crystal rim area α‐C2SH persists in higher abundance. The mechanism of the phase transformation of α‐C2SH into dellaite includes the following: (1) Partial formation of killalaite (Ca3[HSi2O7](OH)) as nuclei according to the isochemical reaction 2Ca2[HSiO4](OH) → Ca3[HSi2O7](OH) + Ca(OH)2 probably induced by anisotropic thermal expansion, local chemical fluctuations, structural (proton) disorder, and different bond strengths of the OH groups in the α‐C2SH structure. (2) Further dehydration of killalaite and α‐C2SH domains results in the formation of dellaite according to Ca3[HSi2O7](OH) + Ca(OH)2 + Ca2[HSiO4](OH) – 2H2O → Ca6[Si2O7][SiO4](OH)2. The results suggest that the polymerization of two isolated [HSiO4] tetrahedra takes place without dehydration according to reaction (1) rather than through condensation with simultaneous H2O release: 2[HSiO4] → [Si2O7] + H2O. We suggest that reaction (1) cannot be completed at ambient pressure. Thus in the regions close to the rim of the crystals we expect the formation of x‐C2S, which starts along the crystal edges according to Ca2[HSiO4](OH) → Ca2SiO4 + H2O. Based on a modular concept, a structural relationship between α‐C2SH, killalaite, dellaite, and x‐C2S has been established. Similarities and differences in the thermal behavior of α‐C2SH and afwillite have been highlighted. 相似文献
2.
Fabienne Méducin Bruno Bresson Marie‐Noëlle de Noirfontaine 《Cement and Concrete Research》2007,37(5):631-638
This work focuses on phases formed during cement hydration under high pressure and temperature: portlandite Ca(OH)2 (CH); hillebrandite Ca2(SiO3)(OH)2 (β‐dicalcium silicate hydrate); calcium silicate hydrate (C-S-H); jaffeite Ca6(Si2O7)(OH)6 (tricalcium silicate hydrate); α‐C2SH Ca2(SiO3)(OH)2 (α‐dicalcium silicate hydrate); xonotlite Ca6(Si6O17)(OH)2 and kilchoanite Ca6(SiO4)(Si3O10). Portlandite and hillebrandite were synthesized and characterised by high resolution solid‐state 1H and 29Si Nuclear Magnetic Resonance. In addition, information from the literature concerning the last five phases was gathered. In certain cases, a schematic 3D‐structure could be determined. These data allow identification of the other phases present in a mixture. Their morphology was also observed by Scanning Electron Microscopy. 相似文献
3.
Kestutis Baltakys Anatolijus Eisinas Jolanta Doneliene Tadas Dambrauskas Gabriele Sarapajevaite 《Ceramics International》2019,45(2):2881-2886
In this work the impact of Al2O3 amount on the synthesis (200?°C; 4–8?h) of calcium aluminium silicate hydrates (CSAH) samples and their influence on the early stage hydration of calcium aluminate cement (CAC) was examined. It was found that the amount of Al2O3 plays an important role in the formation of calcium aluminate hydrates (CAH) because in the mixtures with 2.7% Al2O3 only calcium silicate hydrates (CSH) intercalated with Al3+ ions were formed. While in the mixtures with a higher amount of Al2O3 (5.3–15.4%), calcium aluminate hydrate – C3AH6, is formed under all experimental conditions. It is worth noting that the largest quantity of mentioned compound was obtained after 4?h of hydrothermal treatment, in the mixtures with 15.4% of Al2O3. It was proved that synthesized C3AH6 remain stable up to 300?°C and at higher temperature (945?°C) recrystallized to mayenite (Ca12Al14O33), which reacted with the rest part of CaO and amorphous structure compound, resulting in the formation of gehlenite (Ca2Al2SiO7). Moreover, the synthesized C3AH6 addition induced the early stage of CAC hydration. Besides, in the samples with an addition, the induction period was effectively shortened: in a case of pure CAC (G70) paste, hydration takes about 6–6.5?h, while with addition – only 2–2.5?h. The synthesized and calcinated compounds was characterized by using XRD and STA analysis. 相似文献
4.
《Ceramics International》2022,48(18):25958-25967
The formation thermodynamics, phase transition and stability of sodium calcium silicate compounds under different calcination parameters in the Na2O–CaO–SiO2 system were studied using XRD, FTIR and SEM-EDS methods. As the Na2O/SiO2 ratio increases from 0.3 to 0.7 when the CaO/SiO2 ratio is 1.0, the formation sequence of sodium calcium silicate compounds is Na2Ca3Si2O8→Na6Ca3Si6O18→Na2Ca2Si2O7→Na2CaSiO4; as the CaO/SiO2 ratio increases from 0.3 to 1.2 when the Na2O/SiO2 ratio is 0.5, the formation sequence is Na6Ca3Si6O18→Na2Ca2Si2O7→Na2Ca3Si2O8. As the most stable sodium calcium silicate compound, Na6Ca3Si6O18 forms by the solid-state reaction of preformed Na2SiO3 with CaO and SiO2, while increasing the calcination temperature and holding time can promote its crystal stability. The decomposition of Na6Ca3Si6O18 in sodium aluminate solution follows the mixed control of the film diffusion and chemical reaction, and the corresponding activation energy is between 40 and 41 kJ/mol. 相似文献
5.
It has long been known that the stoichiometry of C–S–H varies with the calcium hydroxide concentration in solution. However, this issue is still far from understood. We revisit it in both experimental and modelling aspects. A careful analysis of the solubility confirms the existence of three different C–S–H phases, defined as Ca4H4Si5O16, Ca2H2Si2O7 and Ca6(HSi2O7)2(OH)2, respectively. The variation of the Ca/Si ratio of the three phases has been described by surface reactions: the increase of the Si content is accounted for by silicate bridging, the increase of calcium content and the surface charge are accounted for by reactions involving silanol groups via deprotonation and complexation with calcium. In the presence of Al in solution, the uptake of Al by C–S–H is experimentally observed. The Al content increases with Al concentration. C–A–S–H formation is modelled by the competition between silicate and aluminate tetrahedra for the bridging of the dimeric silicates in C–S–H. 相似文献
6.
Luz S. Gomez-Villalba Aranzazu Sierra-Fernandez M. del Mar Barbero Barrera Duygu Ergenç Rafael Fort 《Journal of the American Ceramic Society》2022,105(8):5472-5489
This work studies the formation of calcium–silicate–hydrates (C–S–H) in lime mortars prepared with additions of nano-Ca(OH)2 and nano-SiO2 at 23.3°C. Mineral identification was carried out by X-ray diffraction after 10, 30, 90, and 120 days of curing. The nanoscale study starts from the generation of amorphous phases until the development of crystalline phases. Observations of binder mortar made by transmission electron microscopy (low magnification and high resolution TEM) after 30 and 110 days of curing showed the formation of two types of C–S–H with different degrees of crystallinity depending on the curing time. The development of short-range order C–S–H globular phases was visible after 30 days. C–S–H evolved into lamellar crystalline phases visible after 110 days of curing. The crystalline phase corresponds to the C–S–H known as afwillite (Ca3(SiO3OH)2·2H2O), first reported to affect cement and concrete's mechanical and hydration properties. It appears as isolated fibers, growing epitaxially along the edges of the calcite (product of the carbonation of Ca(OH)2), and advancing inward aided by atomic defects (grain boundaries, stacking faults). In addition, high-resolution transmission electron microscopy tools and electron diffraction simulation confirmed a monoclinic symmetry for afwillite crystals. These results contribute to analyzing the presence of crystalline/amorphous C–S–H in lime mortars, providing information on the structure of afwillite and its possible effects on the binder materials. 相似文献
7.
K. M. Hanna R. L. Cook D. L. Kantro 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1970,20(11):334-340
Dibarium silicate, Ba2SiO4, was hydrated in two ways: in paste form at 25° using a water/solid weight ratio of 0.7:1, and in a polyethylene bottle rotated on a wheel at 5°, 25° and 50°, using a water/solid weight ratio of 9.0:1. When Ba2SiO4 is hydrated in paste form, the stoicheiometry of the reaction at 25° is the same as in bottle-hydration at 50°: 2BaO.SiO2+2.2H2O = 1.2BaO.SiO2.1.4H2O+0.8Ba(OH)2. The stoicheiometry of bottle-hydration at 5° and 25° is represented by the equation: 2BaO.SiO2+2.2H2O = BaO.SiO.1.2H2O+Ba(OH)2. Barium silicate hydrate, 1.2BaO.SiO2.1.4H2O, is well crystallised and has a specific surface area of ? 3m2/g. The crystals are plate-like and have a tendency to form clusters. The low-baria hydrate, BaO.SiO2.1.2H2O, is poorly crystallised and consists of thin platelets. It has a specific surface area of ? 35m2/g. The thermal dehydration of fully hydrated barium silicate and of the barium silicate hydrates was investigated by thermogravimetric and differential thermal analysis techniques. The similarities and differences between the barium silicate hydrates obtained in the hydration of barium silicate and the calcium silicate hydrates obtained in the hydration of β-dicalcium silicate and Ca3SiO5 are discussed. A mechanism of hydration of barium silicate is proposed which involves solution, precipitation and crystallisation steps. 相似文献
8.
Pastes of tricalcium silicate (C3S) and β-dicalcium silicate (C2S) 23 years old were studied by electron probe microanalysis. In both cases, regions consisting entirely or largely of calcium hydroxide and of CSH were distinguished on a scale of 2–50 μm. The regions high in CSH accounted for 75–80 percent of the whole in the C3S paste and about 96 percent in the C2S paste; these values are much higher than those initially occupied by anhydrous starting materials. Within the high CSH areas, no compositional variation was detected that could have corresponded to the so-called inner and outer hydrates. The ratio of mean Ca to mean Si in the high CSH areas was found to be 1.72 for the C3S paste and 1.78 for the C2S paste with an exciting beam energy of 10 keV. 相似文献
9.
Satoru Fujita Hideyuki Nakano Kenzi Suzuki Toshiaki Mori Hideki Masuda 《Catalysis Letters》2006,106(3-4):139-143
Three types of calcium aluminosilicate, Ca12Al14O33 (C12A7), Ca12Al14Si2O34 (C12A6), and Ca12Al10Si4O35 (C12A5), were prepared by calcining the respective hydrothermally synthesized hydrogarnet, Ca3Al2(OH)12, Ca3Al2(SiO>4)1/3(OH)32/3, and Ca3Al2(SiO4)0.8(OH)8.8. Different amounts of superoxide (O2-) and peroxide (O22-) were occluded in the lattice of these calcium aluminosilicates. No activity improvement was observed for the oxidation of
propylene and benzene by increasing the amounts of (O2-) and (O22-). 相似文献
10.
Molecular Simulation of Calcium Silicate Composites: Structure,Dynamics, and Mechanical Properties
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Dongshuai Hou Tiejun Zhao Zuquan Jin Hongyan Ma Zongjin Li 《Journal of the American Ceramic Society》2015,98(3):758-769
Calcium silicate composite (CaO)x(SiO2)1?x has significant applications in the bioactive materials in medical treatment and cementitious materials in construction engineering. In this study, to unravel the role of calcium atoms on the silicate composite, the molecular dynamics (MD) technique was used to simulate the structures, dynamics, and mechanical properties of (CaO)x(SiO2)1?x systems, with x varying from 0 to 0.6. The Feuston–Garofalini model was employed to describe the interatomic interactions in the systems. Q species, the connectivity factor, shows that the increase in calcium content in the silicate composite can lead to the depolymerization of the silicate network. Due to the high diffusion rate, the presence of Ca atoms also weakens the stability of the chemical bonds in the system. With the increasing calcium content, the molecular structure of the silicate skeleton is transformed from an integrity network to separated short chains, which significantly decreases the stiffness and cohesive force of the calcium silicate composites. On the other hand, the uniaxial tension response of the calcium silicate composites suggests that at the postfailure stage, Ca atoms associate with the nonbridging oxygen atoms and the reconstructed Ca–O connection slows down the irreversible damage of the composite, hereby enhancing the plasticity. 相似文献
11.
《Journal of the European Ceramic Society》2003,23(13):2193-2202
For the development of a new wear resistant and chemically stable glass-ceramic glaze, the CaO–ZrO2–SiO2 system was studied. Compositions consisting of CaO, ZrO2, and SiO2 were used for frit, which formed a glass-ceramic under a single stage heat treatment in electric furnace. In the sintered glass-ceramic, wollastonite (CaSiO3) and calcium zirconium silicate (Ca2ZrSi4O12) were crystalline phases composed of surface and internal crystals in the microstructure. The internal crystal formed with nuclei having a composition of Ca1.2Si4.3Zr0.2O8. The CaO–ZrO2–SiO2 system showed good properties in wear and chemical resistance because the Ca2ZrSi4O12 crystals positively affected physical and mechanical properties. 相似文献
12.
Formation, composition and properties of silicate gardens originated from solid CaCl2.6H2O and sodium silicate solutions with Na2O/SiO2 = 0.5–1.5 and 0.5–2.5 moles SiO2/l are described. In this reaction cryst. Ca(OH)2 and CSH phases of different anion compositions are formed. The permeability of the products, covering the CaCl2 crystals, with respect to water is strongly influenced by the Na2O/SiO2 mole ratio and the SiO2 concentration in the sodium silicate solution. A parallel is drawn between the membranes of the gardens and the CSH phase coating hydrated Ca3SiO5. 相似文献
13.
In Na2O–CaO–SiO2–H2O system, systematic investigations of phase and morphology of calcium silicate in hydrothermal conditions were concisely conducted for high-value utilization of silicon resource in high-alumina fly ash (HAFA). The results show that crystal composition and phase may be affected by relatively low concentration of NaOH, and sodium ions are rearranged into the structure to form NaCaHSiO4 and Na2Ca3H8Si2O12 with different C/S ratio at high concentration of NaOH. In addition, phases in wollastonite group possess the morphology of nanofiber. Formation of nanofiber is attributed to the difference of surface energies between axial and radial direction, and higher temperatures lead to easier growth along radial direction. The preparation of C–S–H with different phases and morphologies can guide for the application of silicate solution with high alkalinity with different purposes. 相似文献
14.
Oswaldo Burciaga‐Díaz José Iván Escalante‐García 《Journal of the American Ceramic Society》2013,96(12):3939-3948
This research examines the chemical activation of blast‐furnace slag pastes with alkaline solutions by means of various characterization techniques. Pastes were activated using sodium silicate solutions with modulus (Ms) of 0, 1, 1.5, 2, and Na2O at 5%, 10%, and 15%. Compressive strengths of up to 108 MPa were achieved for Ms = 1–1.5 after 720 d of curing at 20°C. The addition of Na2O > 10% resulted in the formation of hydrotalcite and carbonated pastes with low compressive strength. X‐ray diffraction, microanalysis of outer products (OP), and nuclear magnetic resonance (NMR) results showed that the main reaction products in the activated cements with Ms = 1 and 5%Na2O had an average ratio Ca/Si = 0.71–0.9 and consisted of a mixture of two kinds of C–S–H; one similar to a 9 Å tobermorite‐type calcium silicate hydrate (Ca5Si6O16(OH)2 and other amorphous related to a cross‐linked structure of C–N–(A)–S–H gel. Both were intermixed with hydrotalcite and cross‐linked structures of silica gel. 相似文献
15.
K. Luke 《Cement and Concrete Research》2004,34(9):1725-1732
Phase studies of calcium silicate hydrates formed at elevated temperature and pressure have been well documented. At 180 °C, the initially formed amorphous calcium silicate gel [C-S-H] transforms into well-defined crystalline phases, the stability of which is primarily dependent on the C/S ratio in the CaO-SiO2-H2O system and the hydrothermal conditions. Hillebrandite [C2SH], α-dicalcium silicate hydrate [α-C2SH] and β-tricalcium silicate [β-C6S2H3] are predominantly the stable phases in the lime-rich part of the CaO-SiO2-H2O system and are typically associated with high permeability and compressive strength retrogression. Gyrolite [C2S3H~2], tobermorite [C5S6H5], truscottite [C7S12H~3] and xonotlite [C6S6H] have all been reported to coexist stably in aqueous solution with silica in the silica-rich part of the CaO-SiO2-H2O system.The addition of excess silica to the CaO-SiO2-H2O system is usually in the form of silicon dioxide [SiO2], either as microsilica or quartz flour, which, in theory, should not affect the equilibrium chemistry. This has not been found to be the case, and metastable phases formed in the early stages of reaction modify the long-term stability and phase equilibrium. Pozzolanic materials that are predominantly alumino-silicates have also been investigated as a source of excess silica. Partial replacement of aluminum for silicon occurred, but had no apparent influence on the stability of the calcium silicate hydrates formed. 相似文献
16.
This study investigates the production of highly reactive dicalcium silicate Ca2SiO4 (C2S).To that end, binders were synthesised by annealing of alpha-dicalcium silicate hydrate (α-C2SH) between 400 and 800 °C. Two different heating sequences were tested. The phase compositions were determined by means of XRD. Depending on the annealing temperature and the heating conditions the cementitious materials consist of an X-ray-amorphous content as well as x-Ca2SiO4 (x-C2S) and γ-Ca2SiO4 (γ-C2S). The hydration kinetics of some selected binders were investigated by means of isothermal calorimetry. The specific reactivity of the phases produced by means of annealing was determined during the first 40 h of hydration by use of XRD and TGA.The resulting binders show the highest reactivity when low annealing temperatures (< 500 °C) were used. After 72 h, degrees of hydration of about 89% are achieved. The most reactive component is the X-ray-amorphous content, followed by x-C2S. 相似文献
17.
18.
Xiaoming Liu Na Zhang Henghu Sun Jixiu Zhang Longtu Li 《Cement and Concrete Research》2011,41(8):847-853
The cementitious behavior of red mud derived from Bauxite-Calcination method was investigated in this research. Red mud were calcined in the interval 400–900 °C to enhance their pozzolanic activity and then characterized in depth through XRD, FTIR and 29Si MAS-NMR techniques with the aim to correlate phase transitions and structural features with the cementitious activity. The cementitious activity of calcined red mud was evaluated through testing the compressive strength of blended cement mortars. The results indicate that red mud calcined at 600 °C has good cementitious activity due to the formation of poorly-crystallized Ca2SiO4. The poorly-crystallized Ca2SiO4 is a metastable phase which will transform into highly-crystallized Ca2SiO4 with the increase of calcination temperature from 700 °C moving to 900 °C. It is the metastable phase that mainly contributes to the good cementitious activity of red mud. This paper points out another promising direction for the proper utilization of red mud. 相似文献
19.
Structural and Crystal Chemical Investigation of Intermediate Phases in the System Ca2SiO4–Ca3(PO4)2–CaNaPO4
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Remo Widmer Frank Gfeller Thomas Armbruster 《Journal of the American Ceramic Society》2015,98(12):3956-3965
Two intermediate compounds of the system Ca2SiO4–Ca3(PO4)2–CaNaPO4 were synthesized by reaction sintering at 1600°C and analyzed structurally, chemically, and optically. The structure of Ca7(PO4)2(SiO4)2 nagelschmidtite (space group P61, a = 10.7754(1) Å, c = 21.4166(3) Å) was determined by single crystal X‐ray analysis. Its unit cell can be interpreted as a supercell (≈ 2 × a, 3 × c) of the high‐temperature polymorph α‐Ca2SiO4. Evidence for pseudo‐hexagonal symmetry is shown. Using electron microprobe, the solid solution Ca7?xNax(PO4)2+x(SiO4)2?x, (x ≤ 2), of nagelschmidtite was confirmed. Volume thermal expansion coefficients of Ca6.8Na0.2(PO4)2.2(SiO4)1.8 and Ca5.4Na1.5(PO4)3.7(SiO4)0.3 were determined using high‐temperature X‐ray powder diffraction, yielding mean αV = 3.95 and 5.21 [×10?5/°C], respectively. Ca15(PO4)2(SiO4)6 is a distinct phase in the binary section Ca2SiO4–Ca3(PO4)2 and was found to extend into the ternary space according to Ca15?xNax(PO4)2+x(SiO4)6?x, (x ≤ 0.1). Quenching experiments of the latter allowed for structural analysis of a strongly disordered, defective high‐temperature polymorph of the α‐Ca2SiO4–α‐Ca3(PO4)2 solid solution. Structural relations between nagelschmidtite, Ca15(PO4)2(SiO4)6 and the end‐member compounds of the system are discussed. 相似文献
20.
Nektarios K. Nasikas Alexandra Navrotsky 《Journal of the American Ceramic Society》2014,97(2):451-456
A series of alkaline‐earth silicate glasses, with compositions ranging from the metasilicate to the ortho‐ and suborthosilicate, have been synthesized by aerodynamic levitation and CO2 laser melting. They have been studied by high‐temperature oxide melt solution calorimetry with 2PbO·B2O3 as solvent. The enthalpies of formation from the oxides at room temperature () have been calculated from the solution enthalpies. Glasses in the Ca0.5Mg0.5O–SiO2 system show greater energetic stability than those in the MgO–SiO2 system, with a more pronounced negative enthalpy of mixing near the orthosilicate composition. This stabilization may explain why it is possible to prepare glasses poorer in silica (suborthosilicate) in the Ca0.5Mg0.5O–SiO2 system but not in the MgO–SiO2 system. The thermodynamic observations support earlier structural studies in these systems. 相似文献