A Modular Concept of Crystal Structure Applied to the Thermal Transformation of α‐C2SH

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.     

Calcium silicate hydrates investigated by solid‐state high resolution H and Si nuclear magnetic resonance

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.     

The impact of Al2O3 amount on the synthesis of CASH samples and their influence on the early stage hydration of calcium aluminate cement

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.     

Formation characteristics of sodium calcium silicate compounds based on the solid-state reaction

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.     

From C–S–H to C–A–S–H: Experimental study and thermodynamic modelling

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.     

Evolution of C–S–H in lime mortars with nanoparticles: Nanostructural analysis of afwillite growth mechanisms by HRTEM

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.     

Hydration of dibarium silicate I. Stoicheiometry of hydration

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.     

Calcium hydroxide distribution and calcium silicate hydrate composition in tricalcium silicate and β-dicalcium silicate pastes

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 CSH were distinguished on a scale of 2–50 μm. The regions high in CSH 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 CSH 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 CSH 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.     

Oxidative Destruction of Hydrocarbons on Ca12Al14-x Si x O33+0.5x (0 ≤ x ≤ 4) with Radical Oxygen Occluded in Nanopores

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 (O₂^2-) 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 (O₂^2-).     

Molecular Simulation of Calcium Silicate Composites: Structure,Dynamics, and Mechanical Properties

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.     

Microstructure and properties of CaO–ZrO2–SiO2 glass–ceramics prepared by sintering

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 Ca_1.2Si_4.3Zr_0.2O₈. The CaO–ZrO₂–SiO₂ system showed good properties in wear and chemical resistance because the Ca₂ZrSi₄O₁₂ crystals positively affected physical and mechanical properties.     

Observations on silicate gardens

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 CSH 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 CSH phase coating hydrated Ca₃SiO₅.     

Crystallization of calcium silicate at elevated temperatures in highly alkaline system of Na2O–CaO–SiO2–H2O

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.     

Structure,Mechanisms of Reaction,and Strength of an Alkali‐Activated Blast‐Furnace Slag

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.     

Phase studies of pozzolanic stabilized calcium silicate hydrates at 180 °C

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.     

Reactivity and phase composition of Ca2SiO4 binders made by annealing of alpha-dicalcium silicate hydrate

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.     

煅烧磷石膏对蒸压硅酸盐制品水化过程的影响

磷石膏经高温煅烧改性后与粉煤灰、砂粉、石灰及水泥熟料等制备蒸压硅酸盐制品,研究了不同温度煅烧的磷石膏对蒸压硅酸盐制品水化过程的影响,用蒸压制品中未反应的Ca(OH)₂量及结合水量分析它们的反应速率,用XRD测定蒸压硅酸盐制品的水化产物,并结合SEM分析,结果表明,经煅烧的磷石膏对蒸压硅酸盐制品的水化有明显的促进作用,托勃莫来石与C-S-H(1)等水化产物的迅速生长而形成密实的水化产物结构是其增强蒸压硅酸盐制品的根本原因。     

Structural investigation relating to the cementitious activity of bauxite residue — Red mud

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.     

Structural and Crystal Chemical Investigation of Intermediate Phases in the System Ca2SiO4–Ca3(PO4)2–CaNaPO4

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 Ca_7?xNa_x(PO₄)_2+x(SiO₄)_2?x, (x ≤ 2), of nagelschmidtite was confirmed. Volume thermal expansion coefficients of Ca_6.8Na_0.2(PO₄)_2.2(SiO₄)_1.8 and Ca_5.4Na_1.5(PO₄)_3.7(SiO₄)_0.3 were determined using high‐temperature X‐ray powder diffraction, yielding mean α_V = 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 Ca_15?xNa_x(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.     

Energetics of Silica‐Poor Glasses in the Systems MgO–SiO2 and Mg0.5Ca0.5O–SiO2

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 Ca_0.5Mg_0.5O–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 Ca_0.5Mg_0.5O–SiO₂ system but not in the MgO–SiO₂ system. The thermodynamic observations support earlier structural studies in these systems.