Tobermorite Structure Research Articles

Atomic and nano-scale characterization of a 50-year-old hydrated C3S paste

This paper investigates the atomic and nano-scale structures of a 50-year-old hydrated alite paste. Imaged by TEM, the outer product C–S–H fibers are composed of particles that are 1.5–2nm thick and several tens of nanometers long. 29Si NMR shows 47.9% Q1 and 52.1% Q2, with a mean SiO4 tetrahedron chain length (MCL) of 4.18, indicating a limited degree of polymerization after 50years' hydration. A Scanning Transmission X-ray Microscopy (STXM) study was conducted on this late-age paste and a 1.5year old hydrated C3S solution. Near Edge X-ray Absorption Fine Structure (NEXAFS) at Ca L3,2-edge indicates that Ca2+ in C–S–H is in an irregular symmetric coordination, which agrees more with the atomic structure of tobermorite than that of jennite. At Si K-edge, multi-scattering phenomenon is sensitive to the degree of polymerization, which has the potential to unveil the structure of the SiO44− tetrahedron chain.

Structural changes in C–S–H gel during dissolution: Small-angle neutron scattering and Si-NMR characterization

Flow-through experiments were conducted to study the calcium–silicate–hydrate (C–S–H) gel dissolution kinetics. During C–S–H gel dissolution the initial aqueous Ca/Si ratio decreases to reach the stoichiometric value of the Ca/Si ratio of a tobermorite-like phase (Ca/Si=0.83). As the Ca/Si ratio decreases, the solid C–S–H dissolution rate increases from (4.5×10−14 to 6.7×10−12) molm−2s−1. The changes in the microstructure of the dissolving C–S–H gel were characterized by small-angle neutron scattering (SANS) and 29Si magic-angle-spinning nuclear magnetic resonance (29Si-MAS NMR). The SANS data were fitted using a fractal model. The SANS specific surface area tends to increase with time and the obtained fit parameters reflect the changes in the nanostructure of the dissolving solid C–S–H within the gel. The 29Si MAS NMR analyses show that with dissolution the solid C–S–H structure tends to a more ordered tobermorite structure, in agreement with the Ca/Si ratio evolution.

Hydrothermal synthesis of tobermorite nanowires from porcelain stone

A facile and efficient hydrothermal process was explored to obtain tobermorite nanowires using porcelain stone and CaO powders as raw materials. The as-synthesised nanowires have a diameter of 50–100 nm and a length of tens of micrometres under conditions of a porcelain stone/CaO weight ratio of 3, a water/solid weight ratio of 22, a reaction temperature of 240°C and a reaction time of 24 h. The results indicate that the weight ratio of porcelain stone/CaO and hydrothermal temperature play important roles regarding the morphology and structure of tobermorite. Furthermore, a possible growth mechanism of the tobermorite nanowires is proposed. This approach might open up a simple and economic route to prepare tobermorite nanowires using inexpensive porcelain stone as the silicon source.

Synthesis of calcium silicate hydrate based on steel slag with various alkalinities

This study aimed to improve the hydraulic potential properties of the slag. Therefore, a method of dynamic hydrothermal synthesis was applied to synthesize calcium silicate hydrate. The phases and nanostructures were characterized by XRD, FTIR, TEM, and BET nitrogen adsorption. The influence of alkalinity of steel slag on its structures and properties was discussed. The experimental results show that, the main product is amorphous calcium silicate hydrate gel with flocculent or fibrous pattern with a BET specific surface area up to 77 m2/g and pore volume of 0.34 mL/g. Compared with low alkalinity steel slag, calcium silicate hydrate synthesized from higher alkalinity steel slag is prone to transform to tobermorite structure.

High-temperature phase transition and the activity of tobermorite

The high-temperature phase transition of tobermorite was investigated by TGA/DSC, X-ray diffraction and Infrared spectroscopy (IR), respectively. The experimental results showed that Si-OH bonds were cleaved at 724 °C and dehydroxylation occured at the same time, implying that the crystal structure of tobermorite was broken. As a result, the dehydroxylation tobermorite was metastable state, exhibiting obviously hydrolysis activity. The suspension was alkaline and Ca2+ ions content reached a maximum value 4.76% after heat treatment at 724 °C. The dehydroxylation tobermorite had potential reactive activity due to the strong hydrolysis activity. The disordered structure recombined to wollastonite, and the crystal structure became ordering and stable at 861 °C Finally, 2M-wollastonite structure can be found in the sample as the temperature reached up to 1 000 °C.

Competition behaviour of metal uptake in cementitious systems: An XRD and EXAFS investigation of Nd- and Zn-loaded 11 Å tobermorite

Cement-based materials play an important role in multi-barrier concepts developed worldwide for the safe disposal of hazardous and radioactive wastes. Cement is used to condition and stabilize the waste materials and to construct the engineered barrier systems (container, backfill and liner materials) of repositories for radioactive waste. In this study, bulk-X-ray absorption spectroscopy (XAS) was used to investigate the uptake mechanism of Nd on the crystalline C–S–H phase 11Å tobermorite in the presence of Zn (co-absorbing metal), and vice versa, as potential competitor under strongly alkaline conditions (pH=12.5–13.3). The Zn and Nd concentration in all samples was 50,000ppm, whereas the reaction times varied from 1 to 6months.Extended X-ray absorption fine structure (EXAFS) data of the Nd LIII-edge indicate that the local structural environment of Nd consists of ∼7–8 O atoms at 2.42Å, ∼7–8 Si at ∼3.67Å and ∼5–6 Ca at ∼3.8Å, and that this environment remains unchanged in the presence and absence of Zn. In contrary, Zn K-edge EXAFS data exhibit distinct differences in the presence and absence of Nd as co-absorbing element. Data analysis indicates that Zn is tetrahedrally coordinated (∼4 O at ∼1.96Å) and the obtained structural data in the simultaneous presence of Nd and Zn are consistent with the formation of mixed Zn surface complexes and Zn bound in the interlayer remaining in these positions also with prolonged reaction times (up to 6months). However, without the co-absorbing element Nd, strong structural changes in the uptake mechanisms of Zn are observable, e.g., after 3month reaction time Zn–Zn backscattering pairs can be observed. These findings suggest that Nd has an influence on the incorporation of Zn in the tobermorite structure. In addition, the results of this study indicate that competitive uptake of metal cations with similar sorption behaviour by C–S–H phases can take place, deserving further attention in future assessments of the safe disposal of radioactive wastes in cement-based repositories.

Generalized Structural Description of Calcium–Sodium Aluminosilicate Hydrate Gels: The Cross-Linked Substituted Tobermorite Model

Structural models for the primary strength and durability-giving reaction product in modern cements, a calcium (alumino)silicate hydrate gel, have previously been based solely on non-cross-linked tobermorite structures. However, recent experimental studies of laboratory-synthesized and alkali-activated slag (AAS) binders have indicated that the calcium-sodium aluminosilicate hydrate [C-(N)-A-S-H] gel formed in these systems can be significantly cross-linked. Here, we propose a model that describes the C-(N)-A-S-H gel as a mixture of cross-linked and non-cross-linked tobermorite-based structures (the cross-linked substituted tobermorite model, CSTM), which can more appropriately describe the spectroscopic and density information available for this material. Analysis of the phase assemblage and Al coordination environments of AAS binders shows that it is not possible to fully account for the chemistry of AAS by use of the assumption that all of the tetrahedral Al is present in a tobermorite-type C-(N)-A-S-H gel, due to the structural constraints of the gel. Application of the CSTM can for the first time reconcile this information, indicating the presence of an additional activation product that contains highly connected four-coordinated silicate and aluminate species. The CSTM therefore provides a more advanced description of the chemistry and structure of calcium-sodium aluminosilicate gel structures than that previously established in the literature.

Highly active calcium-silicate phases for application in endodontics

Introduction. Mineral trioxide aggregate (MTA) is one of the most commonly used materials in endodontics. Given its shortcomings, there is an intensive search for new materials. Calcium-silicate phase (CS phase) is a new material synthesized by the method based on a combination of sol-gel process and self-propagation synthesis which can significantly improve setting time through accelerated hydration. The aim of this study was to explain the mechanisms of hardening of CS phase in aqueous medium as similar mechanism is expected in contact with body fluids. Materials and Methods. CS phases Ca3SiO5 (C3S) and 2?-CaSiO4 (?-C2S) were synthesized from CaCl2?5H2O (Merck, Germany). To investigate the process of hydration, CS were mixed with water and kept at 37 ?C for 28 days in closed polyethylene containers. Analysis of the composition of samples before and after immersion in water for 1, 3, 7 and 28 days was performed using X - ray diffractometry and IR. Results. XRD patterns for hydrated samples during 1, 3, 7 and 28 days showed that the amount of hydrated tobermorite phases relative to the amount of CS phase changes with the time of hydration. After 1 day, in addition to the hydrated phases, a significant amount of untransformed ?-C2S and C3S were found while after 28 days hydrated CS phase was completely transformed to tobermorite with a small amount of portlandite. Conclusion. Using combined method of sol-gel and self-propagating waves at high temperature very active nanostructured silicate phases were obtained. Hydration process of CS phases was analyzed using XRD and FTIR, the mechanism of hydration was proposed and it was pointed to the difficulties in determining the exact reaction as well as the problem of determining the exact structure of tobermorite.

Hydrothermal preparation of tobermorite incorporating phosphate species

Slurries of quartz, slaked lime, and sodium dihydrogen phosphate containing hydrochloric acid as the solvent were hydrothermally treated to prepare tobermorite incorporating phosphate species. X-ray diffraction and X-ray fluorescence analyses showed that tobermorite with a trace of phosphate species formed after the treatment. The sizes of plate-like tobermorite depended on the sodium phosphate additive and solvent. There were almost no effects observed of the additive and solvent on the morphology of tobermorite. Sodium and/or phosphate species promoted the transformation from calcium silicate hydrate gel as the precursor to tobermorite. Phosphate species were introduced into the tobermorite structure, resulting in the distortion of its structure.

Insight into elastic behavior of calcium silicate hydrated oxide (C–S–H) under pressure and composition effect

The present work relates to the study of structural and elastic properties of Tobermorite 11Å as a function of external pressure and composition in terms of calcium to silicon ratio. Basing on the lattice dynamics method, the main aim of this work is precisely to shed light, for the first time, on the high pressure structural phase transition in Tobermorite 11Å and the possible correlation with some elastic quantities. In order to check the transferability of the potentials used we have, additionally, performed a single calculation based on the density functional theory (DFT) for a pressure of 15GPa in the case of Ca/Si=1. The variation of the unit cell parameters with pressure indicates that Tobermorite 11Å undergoes a structural instability around 15GPa along b-axis and around 20GPa along a-axis which is confirmed from our calculations of X-Rays diffraction patterns at various pressure values. We have also observed the anisotropic character of the Tobermorite structure for both cases (Ca/Si=1 and Ca/Si=0.83). Our results show that around 20GPa an important change appears in the elastic behaviour of Tobermorite. As pressure increases the calculated elastic quantities for Ca/Si=1 became closer to those evaluated for Ca/Si=0.83, which may stimulate further experimental and theoretical research on the matter.

Preparation of autoclaved aerated concrete using copper tailings and blast furnace slag

Based on the background that large amount of copper tailings are stockpiled in China, the utilization of skarn-type copper tailings to prepare autoclaved aerated concrete (AAC) was studied. The AAC samples were prepared on a laboratory scale with a dry density of 610.2 kg m −3 and compressive strength of 4.0 MPa. Compared with the traditional AAC, lime was totally substituted by skarn-type copper tailings and blast furnace slag in order to develop a potential technique of reducing CO 2 emission during the AAC production process. The samples of different curing stage were examined by XRD, FESEM as well as 29Si and 27Al NMR analyses. Results show that the main minerals in the AAC product are tobermorite-11 Å, anhydrite, augite, quartz, calcite and dolomite, with small amount of other minerals brought in by the copper tailings. It was also suggested that most minerals in the copper tailings participated in the hydration reaction during the procuring process, and the chemical elements in them got into the structure of platy tobermorite in the subsequent autoclaving process.

Hydrothermal synthesis of aluminum substituted tobermorite by using various crystal phases of alumina

Slurries of quartz and slaked lime containing various crystal phases of alumina were hydrothermally reacted to synthesize aluminum-substituted tobermorite. X-ray diffraction results confirmed the formation of aluminum-substituted tobermorite during the reaction. The aluminum species were substituted at the Q2 or Q3 sites in tobermorite. The amount of aluminum substituted in the tobermorite depended on the reactivity of alumina. The tobermorite formed showed fiber- and plate-like morphologies. The reactivity of the alumina used as an additive influenced the substitution site and the amount of aluminum substituted into the tobermorite structure, resulting in controlling its morphology.

Understanding of bonding and mechanical characteristics of cementitious mineral tobermorite from first principles

This paper reports density functional theory study of the structural and mechanical properties of tobermorite mineral (9 Å phase) as one of the main component of cementitious materials in a concrete chemistry. Calculated bulk modulus and elastic constants reflect a relatively high resistance of the tobermorite structure with respect to external isostatic compression. Moreover, the elastic constants proved the anisotropic character of the tobermorite structure. The directions parallel to the axb plane are more resistant to the compression than the perpendicular direction. The largest contribution to this resistance comes from the "dreierketten" silicate chains. The bonding analysis linked macroscopic mechanical properties and the atomic structure of the tobermorite. It was found that polar covalent Si-O bonds are stiffer than iono-covalent Ca-O bonds. The SiO(4) tetrahedra are resistant with respect to the compression and the effect of external pressure is reflected by the large mutual tilting of these tetrahedra as it is shown by changes of the Si-O-Si bridging angles. Polyhedra with the seven-fold coordinated Ca(2+) cations undergo large structural changes. Especially, axial Ca-O bonds perpendicular to the axb plane are significantly shortened. Besides, it was shown that structural parameters, more or less in parallel orientation to the axb plane, are mainly responsible for the high resistance of the tobermorite structure to external pressure. The main mechanism of a dissipation of energy entered to the structure through the compression is proceeded by the tilting of the tetrahedra of the silicate chains and by large shortening of the axial Ca-O distances.

FTIR study and cation exchange capacity of Fe 3+- and Mg 2+-substituted calcium silicate hydrates

Single phases calcium silicate hydrates (1.1 nm tobermorite; Ca 5Si 6O 16(OH) 2·4H 2O), with Fe 3+ and Mg 2+ substitutions, were synthesized under hydrothermal conditions at 175 °C. The structure of different tobermorite samples was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. The results indicated that Fe and Mg ions played a key role in crystallization, morphology and structure of tobermorite. Mg 2+ increases crystallinity of tobermorite and changes its morphology from platy-shape at 4 h curing time to lamellar-shape at longer curing time. Fe 3+ increases imperfection of tobermerite at short curing time, however it increases crystallinity at longer curing time and the morphology of tobermorite changes magnificently from reticulated-shape to fiber-shape. FTIR proved that Fe + and Mg + increase silicate chains polymerization and increase the chain cross-linkage, which is consistent with tobermorite lamellar and fibers morphology that grow parallel to the b-axis (along the silicate chains). The cations exchange capacity (CEC) of Fe- and Mg-substituted tobermorites is lower than that of the unsubstituted tobermorite. Cross-linkage in the silicate chains was found to cause a reduction in cation exchange capacity.

Molecular dynamics modeling of the interface between surface functionalized graphitic structures and calcium–silicate–hydrate: Interaction energies, structure, and dynamics

Molecular dynamics simulations were performed to study the molecular-scale energetic, structural, and dynamic properties of the interface between surface functionalized graphitic structures and calcium–silicate–hydrate (C–S–H). The 9 Å tobermorite structure was used as a model for C–S–H, the main building block (“the glue”) that hold a cementitious matrix together. Six types of carbon surface structures were investigated: a pristine graphite plane and five graphite planes functionalized with hydroxyl ( OH), carboxyl ( COOH), carboxylate ( COO −, deprotonated carboxyl), carbonyl (C O), and amine ( NH 2) groups. Results demonstrated the dominant role of electrostatic forces in the interfacial interactions and indicated that the polarity of the functional group can be used as an indicator of affinity to C–S–H. MD simulations revealed that an optimal number of polar oxygen containing groups may exist for efficient graphitic structure/cement interaction and emphasized the mediating role of Ca 2+ counterions in the interfacial interactions.

The modular structure of dovyrenite, Ca6Zr[Si2O7]2(OH)4: Alternate stacking of tobermorite and rosenbuschite-like units

The average structure, space group Pnnm [subcell: A = 5.666(16), B = 18.844(5), C = 3.728(11) Å, V = 398.0(2) Å3, Z = 1], of the new mineral dovyrenite Ca6Zr[Si2O7]2(OH)4 has been refined from single-crystal X-ray data to R = 7.97%. The modular structure of dovyrenite is build by alternate stacking of Ca-polyhedral layers characteristic of the tobermorite structure and octahedral layers with attached disilicate groups known from the rosenbuschite group of minerals. No indications of ordered polytypes were detected for the potential OD-structure. Either the small crystal size producing only weak diffraction intensities did not allow detecting diffuse diffraction features (or “super-structure” reflections) or the structure is build by disordered stacks of OD layers. Nevertheless, the resolved average structure allowed unraveling the possible order patterns within the rosenbuschite-like octahedral layers. The key for understanding the polytypic character of this structure is the short periodicity of the tobermorite-like Ca polyhedral layer of only 3.73 Å along c, whereas the periodicity of the attached rosenbuschite-like octahedral layer is doubled. In dovyrenite Ca occurs in sixfold-, sevenfold-, and eightfold-coordination. The octahedral Ca site is only half occupied and may reveal additional vacancies, which must be charge balanced by disordered OH-groups replacing O. A corresponding modular structure with the same subunits but different composition and without octahedral vacancies exists for rinkite (Ti,Nb,Al,Zr)(Na,Ca)3(Ca,Ce)4[Si2O7]2(O,F)4, which has hitherto been considered as heterophyllosilicate.

Synthesis and characterization of calcium–alumino-silicate hydrates from oil shale ash – Towards industrial applications

The influence of the alkaline medium on the hydrothermal activation of the oil shale fly ash with NaOH and KOH was studied using SEM/EDX, XRD, 29Si and 27Al high-resolution MAS-NMR spectra. In the presence of NaOH the silicon in the original fly ash was completely converted into calcium–aluminum–silicate–hydrates, mainly into 1.1nm tobermorite structure during 24-h treatment at 160°C. At similar reaction conditions, the activation with KOH resulted only to the formation of amorphous calcium–silica-hydrate gel on the surface of ash particles at temperature. The results obtained in this study indicate that the oil shale fly ash can be used for production of Al-substituted tobermorites when strongly alkaline media (NaOH) is applied. The synthesized product was used in a catalytic d-lactose isomerization reaction.

Hydrothermal synthesis and characterization of aluminium and sulfate substituted 1.1nm tobermorites

Substitutions of sulfate and aluminum ions in calcium silicate hydrate (tobermorite) were investigated at 175 °C under saturated steam pressure. Structure and morphology of different tobermorite samples were investigated using XRD, FTIR and SEM. A sample of pure xonotlite was prepared to assist in FTIR band assignment of different tobermorites. Al 3+ decreases the degree of crystallinity at short curing time and increases crystallinity at longer curing times, with a sort of stabilization of small crystals. Al 3+ changes the morphology of tobermorite from platy-shape to lath-shape. Sulfate ion increases imperfection of tobermorite crystal structure and the morphology of tobermorite change magnificently to leafy shape. The FTIR spectra of all 1.1 nm tobermorite samples are nearly similar. All tobermorite samples have a significant concentration of Q 3 Si sites, which indicates cross-linking of the chains. Sulfate substitution for silicate in tobermorite structure is associated with the appearance of the bands at 1610 and 631 cm −1 which indicates the incorporation of CaO–H group in tobermorite structure. This indicated the charge compensation mechanism; SiO 4 4− is substituted by SO 4 2− and 2OH − .

Structural model of the rare mineral tacharanite and a possible process of its formation

A large number of samples of the poorly studied tacharanite mineral have been comprehensively investigated. A model of the tacharanite structure is derived from the tobermorite structure. A new zeolite-like structure is derived from the tacharanite structure by removing calcium atoms and subsequent shifts and rotations of the silicon-oxygen fragments forming the tacharanite structure. The IR spectral features have been considered, which made it possible not only to show the inhomogeneity of the system studied but also to propose a hypothesis about the conditions of its formation, taking into account the overall close presence of zeolite as the initial material.

Structural Features of C–S–H(I) and Its Carbonation in Air—A Raman Spectroscopic Study. Part I: Fresh Phases

The Raman spectra of a series of mechanochemically prepared calcium silicate hydrate samples of type C–S–H(I) with C/S ratios ranging from 0.2 to 1.5 reveal changes in structure with changes in the C/S ratio. These support the model of Stade and Wieker based entirely on the tobermorite structure. The main characteristic feature of the spectra is the Si–O–Si bending vibration at about 670 cm−1. Comparisons with bending frequencies of some known crystalline phases composed of single silicate chains led to an estimation of the mean Si–O–Si angles in the C–S–H(I) phases to be ∼140°. Finite silicate chains (Q2) dominate the structures of the samples at C/S ratios 0.2–1.0, the spectra showing characteristic bands from 1010 to 1020 cm−1. When the samples are measured in air, the spectra exhibit carbonate bands arising from surface carbonation. The ν1[CO3] bands obscure the characteristic Raman scattering of silicate units near 1080 cm−1, which is clearly evident in the fresh samples analyzed in closed capillaries. At C/S>1.00, dimers (Q1) are the main building unit of the silicate anionic structure, with a characteristic band at 889 cm−1. At C/S ratios 1.33 and 1.50, portlandite (Ca(OH)2) is also observed.