Calcium Silicate Phases Research Articles

High temperature resistance of a very high volume fly ash cement paste

The physical and chemical changes taking place in a very high volume fly ash cement paste (FAN-4) following exposure to temperatures up to 1000°C are presented. Tests were repeated with commercially available CEM II/A-M Portland composite-cement (MS). FAN-4 pastes showed impressive residual strengths after heat exposure which increased dramatically when heated previously at 800 or 1000°C. This was in stark contrast to the MS paste, which showed a continual decrease in residual strength following heating and subsequent cooling. The increase in residual strength with FAN-4 paste coincided with a major shrinkage event, which was associated with sintering and the formation of the new mineral phases anorthite, gehlenite, wollastonite, diopside and albite. The MS cement formed generally non-hydraulic calcium silicate phases upon heating at ⩾800°C. The differences in phase formation were linked to different starting elemental compositions. The FAN-4 paste was considerably richer in Si, Al, Fe and alkalis but poorer in Ca. The elemental composition of the binder phase will be an important factor to take into account when determining the high-temperature performance of future cements and concretes, which are likely to contain significantly higher contents of supplementary cementitious materials than is presently the case.

Effects of various mixing techniques on physical properties of White Mineral Trioxide Aggregate

The aim of this study was to evaluate the effects of three different mixing techniques on surface microhardness, initial setting time, and phase formation of white mineral trioxide aggregate. Twenty-one cylindrical glass tubes were selected and divided into three groups of seven in each (n = 7). White mineral trioxide aggregate (WMTA) in groups A, B, and C were mixed by conventional, trituration, and ultrasonic techniques, respectively. Cements were mixed and packed into the glass tubes and incubated at 37°C for 3 days. After incubation, samples were subjected to microhardness evaluation, and four specimens from each group were prepared and observed under a scanning electron microscopy and X-ray diffraction. For setting time assessment, WMTA was mixed in three parts again, and Gilmore needle test was performed until the initial setting time of cement. Data were analyzed by one-way anova and post hoc Tukey's test. Samples mixed by trituration technique significantly showed the highest microhardness (P < 0.001) and the lowest setting time, while the ultrasonically mixed specimens significantly revealed the highest setting time in comparison with other groups (P < 0.001). Samples mixed by trituration technique showed the highest uniformity in the bulk and highest amount of hydrated calcium silicate (CS) phases, while the lowest value was observed in ultrasonically mixed cements, which also showed the highest amount of anhydrated CS phases. Trituration and conventional techniques were more suitable mixing methods for mineral trioxide aggregate in comparison with ultrasonic technique. Trituration resulted in better hydration and crystallization, which prevents clustering of powder and reduces voids and setting time of mixed cement.

Structural and surface studies on calcium phospho-silicate glass-ceramics containing zinc and iron oxide

Effects of ZnO addition on structural and microstructural properties of glass/glass-ceramic with nominal composition 25SiO2–50CaO–15P2O5–(10−x) Fe2O3−x ZnO (where x=0–5mol%) have been studied. Glasses were prepared by melt-quench technique and converted into glass-ceramics by controlled heat treatment. The glass-ceramics samples were immersed in bovine serum albumin (BSA) and their surfaces were investigated using Fourier transform infrared spectroscopy (FTIR) and time of flight-secondary ion mass spectrometry (TOF-SIMS). Microstructure of glass-ceramic exhibited a granular microstructure. Calcium phosphate, calcium silicate (Ca3Si2O7), hematite and magnetite phases were observed. Surface morphology of glass-ceramics after immersion in BSA showed the formation of SiOH functional groups. Absorption of BSA took place by interacting with the silanol groups present at the surface. The fraction of non-bridging oxygen decreased with the increase in the amount of ZnO, which led to a decrease in the surface dissolution; thereby decreasing the adhesion of BSA.

Molecular Dynamics Study of Water and Ions Transported during the Nanopore Calcium Silicate Phase: Case Study of Jennite

AbstractDurability is an important property that determines the long-term behavior of cement-based materials. Water and ions are transported in nanopores of calcium-silicate-hydrate (C-S-H) gels, the main element in cement-based material, which significantly influences the durability of cement. Because of its structural similarity, jennite, an important mineral analog of C-S-H gel, is first taken to investigate the transport behavior at a molecular level. In this paper, structural and dynamical properties of the water/ions and the jennite interface are studied by the molecular dynamics (MD) simulation method. On the (001) surface of jennite, water molecules diffusing in the channel between silicate chains demonstrate the following structural water features: large density, good orientation preference, ordered interfacial organization, and low diffusion rate. The channel water molecules have more H-bonds connected with the neighboring water molecules and solid surface. As the distance from the channel incre...

Precipitation Behavior of V-Enrichment Mineral in Steelmaking Slag Bearing Vanadium Modified by SiO&lt;sub&gt;2&lt;/sub&gt;

In order to concentrate vanadium in converter slag bearing vanadium, the effect of SiO2 on behavior of vanadium in the process of enriching was studied. Experimental results show that, during cooling, dicalcium silicate containing vanadium and phosphorus precipitate first from original slag. For original with 7% SiO2, most of vanadium dissolves in calcium silicate phase. However, the proportion of dicalcium silicate is big. For slag modified by 12% SiO2, during cooling, two vanadium-rich minerals precipitate firstly, i.e. Ca3(V,P)2O8(30-33%V2O5) and Ca3(V,P)2O8•nCa2SiO4 (14-31%V2O5), then 3CaO•2SiO2 (2-4%V2O5) generates. With addition amount of SiO2 increasing, the proportion of 3CaO•2SiO2 decreases gradually, and the content of vanadium in vanadium-rich phase increases gradually. So appropriate SiO2 addition can suppress the precipitation of Ca2SiO4 and Ca3•Si2O7, and promote the generation of vanadium-rich phase.

New periclase-spinel refractories from densely sintered high purity magnesite and new synthetic compositions based on spinel. Part 1. Study of mineral composition, microstructure, thermal expansion, and ultimate strength in compression

Six specimens of magnesia-spinel material are made under laboratory conditions. Specimens are prepared at 1600°C from sintered magnesite and three different compositions based on spinel, previously synthesized at 1760 °C. Sintered magnesite and synthesized spinel properties are provided. Raw material properties in relation to different spinel content within them, apparent density, additions introduced, and also their effect on final product, are shown. Three forms of final product contain 8 – 11 wt.% Al2O3, and the rest contain from 19 – 21 wt.% Al2O3. Photographs taken in a scanning electron microscope, and results of microstructural analysis of raw material and final product, point to formation of direct periclase–periclase and periclase–spinel bonds, and also secondary calcium aluminate and calcium silicate phases. Secondary spinel is detected in the form of a solution within periclase, and also at grain boundaries within magnesia-spinel refractories. On the basis of petrographic criteria (mineral composition, microstructure), and also linear thermal expansion coefficient (LTEC) and ultimate strength in compression for final materials, the authors of this article propose a most suitable material with respect to quality for testing in high-temperature branches of industry.

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.

Creating of highly active calcium-silicate phases for application in endodontics

The synthesis of active silicate phases by combined sol gel and high-temperature selfpropagating wave method, is described in this paper. They show a significant decrease of setting time and good mechanical properties, which are very important for its potential application in endodontic practice. Particularly, process of hydration of calcium silicate phases is carefully analyzed, from the aspect of phase changes during their soaking in water for 1, 3, 7 and 28 days. XRD and FTIR methods were used for phase analysis of all samples, while morphological characteristics and chemical composition of the given phases were investigated by SEM and EDS.

Reaction of clinker surfaces investigated with atomic force microscopy

The application of microscopy to investigate cement hydration has widely spread in the last decades. For instance, scanning electron microscopy (SEM) is of primary importance to detect the formation of microstructures and detect their chemical composition. In this work, atomic force microscopy (AFM), which is a relatively less diffused technique, is applied to illustrate and estimate changes of the surface roughness of a clinker substrate, treated with different electrolytic solutions, with and without superplasticizer, at different times of hydration. Furthermore, SEM images are collected to characterize the chemical composition of hydration products formed on the clinker surface. It is shown that surface reaction increases drastically with the increase of the pH, and that surface roughness changes occur mainly in the first minutes of hydration. Moreover, the formation of hydration products is reduced when the clinker is treated with solutions containing polycarboxylate ether-based superplasticizer. Additionally, AFM images collected in tapping mode revealed the presence of nano-structures on calcium silicate phase.

Clinkering Reactions During Firing of Recyclable Concrete

Thermal transformations occurring during firing of recyclable concrete and hydrated Portland cement paste were monitored quantitatively by in situ and ex situ X‐ray diffraction, thermal analysis, and clinker microscopy. Reactions below 1150°C were dominated by either the decarbonation of calcium carbonates of the limestone aggregate fraction in the recyclable concrete or the low temperature decomposition of cement hydrates of the cement paste. Continued heating of the cement paste showed a rapid and extensive reaction of the free lime to form intermediate calcium silicate and aluminate phases such as belite, gehlenite, mayenite, and ye'elimite. The combination of free lime was less rapid in the dominantly limestone‐rich recyclable concrete. Similarly, subsequent melt and clinker phase formation (1250°C and higher) occurred more rapidly in the hydrated cement paste, indicating a positive contribution of cement hydrates to raw meal burnability. Reactions below typical cement clinkering temperatures were enhanced (1) by the intimate mixing of lime, silicate, and aluminate components in the hydrated cement and (2) by the elevated sulfate content that acted as flux and belite mineralizer. Lime formed in the thermal decomposition of portlandite (±450°C) was partially carbonated to calcite at temperatures below the onset of calcite decarbonation (±650°C).

Understanding and Controlling the Reactivity of the Calcium Silicate phases from First Principles

First principles calculations are employed to provide a fundamental understanding of the relationship between the reactivity of synthetic calcium silicate phases and their electronic structure. Our aim is to shed light on the wide range of hydration kinetics observed in different phases of calcium silicate. For example, while the dicalcium silicate (Ca2SiO4) phase slowly reacts with water, the tricalcium silicate (Ca3SiO5) shows much faster hydration kinetics. We show that the high reactivity of Ca3SiO5 is mainly related to the reactive sites around its more ionic oxygen atoms. Ca2SiO4 does not contain these types of oxygen atoms, although experiments suggest that impurities may play a role in changing the reactivity of these materials. We analyze the electronic structure of a wide range of possible substitutions in both Ca3SiO5 and Ca2SiO4 and show that while the influence of different types of impurities on structural properties is similar, their effect on reactivity is very different. Our calculations ...

Changes to the Triaxial Composition of the Hydrated Phases (CaO/Al2O3/SiO2) in the Metakaolin/Lime System

This study examines the composition of certain hydrates (calcium silicate, aluminum silicate, and related phases) produced by the pozzolanic reaction of waste‐paper sludge that had previously been activated at different temperatures. It summarizes and compares the evolution of the oxide compounds, and records their stability intervals. Changes to their mineralogical composition were analyzed using X‐Ray Diffraction, scanning electron microscopy, and energy dispersive X‐ray spectroscopy. The hydration products from 1 to 360 days of curing time were CSH gels, hydrotalcite‐type compounds (LDH), and stratlingite (C2ASH8). CSH gels were employed as substrates for growing other materials and their morphologies were modified from fibrous to hexagonal layers. The composition of the LDH‐type compounds observed in the carbonate group varied with changes in curing time. Two LDH‐type compound types were identified: (a) with and (b) without magnesium. Stratlingite was the only stable material after long‐curing times.

Effect of particle size on the in vitro bioactivity, hydrophilicity and mechanical properties of bioactive glass-reinforced polycaprolactone composites

Polycaprolactone (PCL) composite films containing 5 wt.% bioactive glass (BG) particles of different sizes (6 μm, 250 nm, < 100 nm) were prepared by solvent casting methods. The ultra-fine BG particles were prepared by high-energy mechanical milling of commercial 45S5 Bioglass® particles. The characteristics of bioactive glass particles were studied by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS), and X-ray diffraction (XRD) methods. In vitro bioactivity of the PCL/BG composite films was evaluated through immersion in the simulated body fluid (SBF). The films were analyzed by FE-SEM, energy dispersive spectrometry (EDS), XRD, and atomic force microscopy (AFM). The mechanical properties of highly-porous PCL/BG composites were examined on cylindrical specimens under quasi-static compression load. It was found that partial crystallization of amorphous BG particles during a prolonged mechanical milling occurred and calcium silicate (CaSiO 3) and sodium calcium silicate (Na 2CaSiO 4) phases were formed. The introduction of submicron BG particles (250 nm) was shown to improve the bioactivity of PCL films. In contrast to BG microparticles, the submicron BG particles were distributed on the film surfaces, providing a high surface exposure to SBF with an improved nanotopography. A notable increase in the stiffness and elastic modulus of the composite was also obtained. As compared to submicron BG particles, lower bioactivity and elastic modulus were acquired for PCL/BG nanoparticles. It was also shown that in spite of high specific surface area of the nanoparticles, partial crystallization during mechanical milling and agglomeration of the nanoparticles during processing decrease the bioactivity, hydrophilicity and mechanical response of the BG-reinforced PCL composites.

Cement: A two thousand year old nano-colloid

Since Roman times, cement is one of the synthetic materials with the largest production and usage by mankind. Its properties allowed the expansion of the Roman Empire and the building of still fascinating works. In spite of the diverse use of cement and the abundant literature accumulated during a century of systematic scientific research on this material, the understanding of its properties is still far from complete. Several issues are still open, ranging from the understanding of the hydration kinetics and the influence of the modern industrial additives, to the deep comprehension of the atomic arrangement and nanostructure of disordered hydrated calcium silicate phase (C–S–H) formed by hydration. This feature article briefly summarizes recent results in the field, highlighting the necessity for a colloidal model of the cement microstructure that, combined with the layer-like structure of the colloidal units, is the most effective approach to fully describe the characteristics of this peculiar material.

Estudio arqueométrico y evaluación del deterioro de los materiales cerámicos de la fachada de la iglesia de Santa Maria del Carmine (Pavía, Italia)

We performed an archaeometric study of the ceramic materials from the facade of the church of Santa Maria del Carmine in Pavia (Italy). We obtained useful information about the firing temperatures, the provenance of raw materials and the type of oven used. The ceramics used in the facade are mineralogically rich in quartz, feldspar and phyllosilicates, and newly formed calcium silicate phases were detected in some samples. Microtextural observations revealed the presence of incipient reaction rims, phyllosilicate sintering and partial vitrification of the matrix. Ceramics show high porosity (32 to 45%) and pore sizes of between 0.4 and 1.3 µm. Our study of the different types of decay collected on the facade of the church (green and black patinas and efflorescences) revealed the presence of gypsum, whewellite, thenardite and moolooite.

Bio-inspired calcium silicate–gelatin bone grafts for load-bearing applications

Bio-inspired calcium silicate–gelatin composites with high strength were prepared through a simple pressing–hydrothermal method for load-bearing applications. The phase composition, morphology, and mechanical properties, including the in vitro fatigue behavior, were evaluated. Additionally, in vitro tests were conducted to investigate the biocompatibility of the composites. The results showed that gelatin was dispersed uniformly in the calcium silicate phase and that there was no phase separation between the two phases. Among the composites studied, the 10 wt% gelatin-containing calcium silicate not only exhibited the maximum value of the compressive strength (141.7 MPa), which is strong enough to be used in load-bearing sites of bone tissue, but also had a higher Weibull modulus, indicating a more uniform strength distribution and higher structural reliability. The stability of the composites was affected by in vitro cyclic loading. The presence of gelatin provided a favorable environment for osteoblast-like cells, resulting in a larger degree of cell proliferation, cell differentiation and mineralized tissue formation compared to the control without gelatin. Based on the above results, the bio-inspired calcium silicate–gelatin composites, in particular that containing 10 wt% gelatin, with high initial strength may be an acceptable bone graft substitute as a candidate for load-bearing tissue repair.

Modifying a dental ceramic by bioactive glass via the sol–gel route: Characterization and bioactivity investigation

The modification of a widely used dental ceramic by a bioactive glass via sol–gel method resulted in the fabrication of novel dental ceramic composites with bioactive behavior. The presence of leucite (Lt), apatite (Ap), various calcium silicate phases (CS) and a glassy aluminosilicate matrix were detected, while after sintering the predominance of wollastonite (W) among the other calcium silicate phases was observed, along with further crystallization of apatite. Concerning the bioactivity, the onset of the apatite formation was directly dependent on the bioactive glass amount, while a delay of the sintered specimens compared to the raw powders was also observed.

Sol–gel synthesis of bioglass-ceramics using rice husk ash as a source for silica and its characterization

SiO 2–Na 2O–CaO based bioglass-ceramics was synthesized through sol–gel route using rice husk ash as silica source. The decomposition behavior of gel was evaluated. Sodium–calcium–silicate phases were crystallized above 700 °C. Pellet made of glass-ceramics powder was sintered at 900 °C. In vitro bioactivity and biodegradability of glass-ceramics were investigated by incubation in simulated body fluid and Tris buffer solution respectively. Scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction were used to monitor the surface deposition on glass-ceramics during incubation. The material showed a good bioactivity with a formation of carbonated hydroxy apatite phase in 3 days of incubation. A quick degradation of the material was observed in Tris solution associated with an increase of pH due to the dissolution of Ca 2+ and Na + ionic species. All these results suggest the glass-ceramics, prepared utilizing rice husk ash, would be a low cost biomaterial for potential biomedical applications.

Incorporation of Phosphorus Guest Ions in the Calcium Silicate Phases of Portland Cement from 31P MAS NMR Spectroscopy

Portland cements may contain small quantities of phosphorus (typically below 0.5 wt % P(2)O(5)), originating from either the raw materials or alternative sources of fuel used to heat the cement kilns. This work reports the first (31)P MAS NMR study of anhydrous and hydrated Portland cements that focuses on the phase and site preferences of the (PO(4))(3-) guest ions in the main clinker phases and hydration products. The observed (31)P chemical shifts (10 to -2 ppm), the (31)P chemical shift anisotropy, and the resemblance of the lineshapes in the (31)P and (29)Si MAS NMR spectra strongly suggest that (PO(4))(3-) units are incorporated in the calcium silicate phases, alite (Ca(3)SiO(5)) and belite (Ca(2)SiO(4)), by substitution for (SiO(4))(4-) tetrahedra. This assignment is further supported by a determination of the spin-lattice relaxation times for (31)P in alite and belite, which exhibit the same ratio as observed for the corresponding (29)Si relaxation times. From simulations of the intensities, observed in inversion-recovery spectra for a white Portland cement, it is deduced that 1.3% and 2.1% of the Si sites in alite and belite, respectively, are replaced by phosphorus. Charge balance may potentially be achieved to some extent by a coupled substitution mechanism where Ca(2+) is replaced by Fe(3+) ions, which may account for the interaction of the (31)P spins with paramagnetic Fe(3+) ions as observed for the ordinary Portland cements. A minor fraction of phosphorus may also be present in the separate phase Ca(3)(PO(4))(2), as indicated by the observation of a narrow resonance at delta((31)P) = 3.0 ppm for two of the studied cements. (31)P{(1)H} CP/MAS NMR spectra following the hydration of a white Portland cement show that the resonances from the hydrous phosphate species fall in the same spectral range as observed for (PO(4))(3-) incorporated in alite. This similarity and the absence of a large (31)P chemical shift ansitropy indicate that the hydrous (PO(4))(3-) species are incorporated in the interlayers of the calcium-silicate-hydrate (C-S-H) phase, the principal phase formed upon hydration of alite and belite.

Effect of ZnO on phase emergence, microstructure and surface modifications of calcium phosphosilicate glass/glass–ceramics having iron oxide

The effect of ZnO on phase emergence and microstructure properties of glass and glass–ceramics with composition 25SiO 2–50CaO–15P 2O 5–(10 − x)Fe 2O 3– xZnO (where x = 0, 2, 5, 7 mol%) has been studied. They have been characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Surface modifications of these glass–ceramics in simulated body fluid have been studied using Fourier transform infrared reflection spectroscopy (FTIR), XPS and SEM. Results have shown a decrease in the fraction of non-bridging oxygen with increase in zinc oxide content. Emergence of crystalline phases in glass–ceramics at different heat treatment temperatures was studied using XRD. When glass is heat treated at 800 °C calcium phosphate, hematite and magnetite are developed as major phases in the glass–ceramics samples with ZnO up to 5 mol%. In addition to these, calcium silicate (Ca 3Si 2O 7) phase is also observed when glass is heat treated at 1000 °C. The microstructure of the glass–ceramics heat treated at 800 °C exhibits the formation of nano-size (40–50 nm) grains. On heat treatment at 1000 °C crystallites grow to above 50 nm size and more than one phase are observed in the microstructure. The formation of thin flake-like structure with coarse particles is observed at high zinc oxide concentration ( x = 7 mol%). In vitro studies have shown the surface modifications and formation of Ca–P-rich layer on the glass–ceramics when immersed in simulated body fluids (SBF) for different durations. The bioactive response was found to depend on ZnO content.