Silicate Anion Structure Research Articles

Structure and dynamic viscosity of CaO–SiO2 and CaO–SiO2–B2O3 model slag systems

Correlation dependencies between the dynamic viscosity of slag and its structural parameters were studied to determine an optimal basicity of silicon smelting slag under the addition of boron oxide to eliminate slagging of the bottom of ore-smelting furnaces. Experimental studies were conducted on CaO–SiO2 and CaO– SiO2–B2O3 model slags obtained at 1600°С. Raman spectroscopic analysis was carried out using a Horiba JobinYvon HR800UV analyzer (France). Theoretical calculations of slag viscosity were performed using Urbain and Mills models. During the experiments, the key structural parameters of slag systems varied within the following limits: the experimental Raman spectrum deconvolution function from 1.41 to 2.45 and optical basicity from 0.58 to 0.68. The obtained experimental and theoretical data were related by mathematical dependencies. It was found that the dynamic viscosity of slag can be promptly determined by Raman spectroscopy on the basis of mathematical models. The dependence obtained shows that slag viscosity decreases upon an increase in the number of bridging oxygen atoms in the silicate anion structure. Notably, this decrease in slag viscosity is observed up to the value of the experimental Raman spectrum deconvolution function of ~1.55-1.60 or slag optical basicity of 0.60–0.62. When B2O3 is added, the viscosity undergoes a further decrease. In practice, for CaO–SiO2 slag systems, the use of boroncontaining flux as a liquefying agent is reasonable at CaO/SiO2 = 0.61–0.63 while maintaining the content of B2O3 in the slag at a level of 1%. The two models (classical and modified) proposed by Urbain were established to be more suitable for theoretical calculation of viscosity in CaO–SiO2 and CaO–SiO2–B2O3 systems. Mills’ model is not suitable for these purposes, since the correlation coefficients in the corresponding mathematical model are not sufficiently large. Further research in this direction is required in order to establish appropriate dependencies of slag viscosity on its structural parameters at different temperatures.

Structure Control of Calcium Silicate Hydrate Gels for Dye Removal Applications

The structure of calcium silicate hydrate (C‐S‐H) gels was modified by hydrothermal reaction with aqueous acetic acid solvent, and then the C‐S‐H gels were used for dye removal from aqueous solution. With increasing acetic acid concentration, the Ca:Si molar ratio decreased and the length of the silicate anion chain structure of the C‐S‐H gels increased. The silicate anion chain length affects the number of available silanol groups on the surface of the C‐S‐H gel: the longer the silicate anion chain length, the greater the number of negative charges and the higher the surface potential. C‐S‐H gels with a long silicate anion structure exhibited higher adsorption capacity for methylene blue than gels with a short silicate anion structure. The enhanced adsorption capacity of the C‐S‐H gels is related to the higher number of silanol groups in the bridging silica tetrahedra of the intermediate anion chain structure compared with those in the end units of silica tetrahedra.

Composition, silicate anion structure and morphology of calcium silicate hydrates (C-S-H) synthesised by silica-lime reaction and by controlled hydration of tricalcium silicate (C3S)

The main product of Portland cement hydration is C-S-H. Despite constituting more than half of the volume of hydrated pastes and having an important role in strength development, very little is known about the factors that determine its morphology. To investigate the relationship between the chemical composition, silicate anion structure and morphology of C-S-H, samples were synthesised via silica-lime reactions and by the hydration of C3S under controlled lime concentrations and with/without accelerators. The silicate anion structure of the samples was studied by 29Si magic angle spinning nuclear magnetic resonance spectroscopy and the morphology and chemical composition by TEM and SEM. All samples prepared via silica-lime reactions with bulk Ca/Si up to 1.5 were foil-like. The hydration of C3S at fixed lime concentration yielded foil-like C-S-H for [CaO]<22 mmol L− and fibrillar C-S-H for [CaO]>22 mmol L−1. A relationship between the silicate anion structure and the morphology of C-S-H was found for the samples fabricated with accelerators.

Model structures for C-(A)-S-H(I).

C-(A)-S-H(I) is a calcium silicate hydrate that is studied extensively as a model for the main binding phase in concrete. It is a structurally imperfect form of 14 Å tobermorite that has variable composition and length of (alumino)silicate anions. New structural-chemical formulae are presented for single- and double-chain tobermorite-based phases and equations are provided that can be used to calculate a number of useful quantities from (29)Si NMR data. It is shown that there are no interlayer calcium ions when the silicate chains are of infinite length and that one is added for each tetrahedral `bridging' site that is vacant. Preparations that have Ca/Si greater than about 1.4 include an intermixed Ca-rich phase. It is not possible to generate a structural model for a dimer that is crystal-chemically consistent with known calcium silicate hydrates if the starting structure is an orthotobermorite, i.e. of the type that has been used in all previous studies. Crystal-chemically plausible models are developed that are based instead on clinotobermorite. A number of models that represent different mean chain lengths are developed using crystal-chemical and geometrical reasoning. The models account for experimental observations, including variations in Ca/Si, H2O/Si, (alumino)silicate anion structure and layer spacing.

Surface carbonation of synthetic C-S-H samples: A comparison between fresh and aged C-S-H using X-ray photoelectron spectroscopy

This paper presents a continuation of studies into silicate anion structure using X-ray photoelectron spectroscopy (XPS). A series of C-S-H samples have been prepared mechanochemically, and then stored under ambient conditions for six months. Storage led to surface carbonation, the extent of which was dependent upon the calcium/silicon ratio of the fresh sample. Carbonation arose through decalcification of the C-S-H, leading to increased silicate polymerisation. The surfaces of the most calcium-rich phases (C/S = 1.33 and 1.50) underwent complete decalcification to yield silica (possibly containing some silanol groups) and calcium carbonate. Carbonation, and hence changes in silicate anion structure, was minimal for the C-S-H phases with C/S = 0.67 and 0.75.

The Structure of Silicate Anions in Aqueous Alkaline Solutions

Molecular hieroglyphics: 29Si COSY NMR spectroscopy reveals the fascinating and beautiful array of aqueous silicate anions that exist in dynamic equilibrium under alkaline conditions. Six of the most abundant oligomers are shown above, with each line representing a Si-O-Si siloxane linkage.

The Structure of Silicate Anions in Aqueous Alkaline Solutions

The Structure of Silicate Anions in Aqueous Alkaline Solutions

乾湿繰り返しが及ぼすコンクリートの耐凍害性への影響とその劣化メカニズムに関する研究

Effect of drying and wetting cycles on frost resistance of concrete was evaluated by the ASTM C666 A and the GIF. Both methods showed that drying and wetting cycles deteriorated the frost resistance of non-AE high performance concrete. Though the micro cracks were observed, the effect on the frost damage was not proven. Change in microstructure of hardening cement paste by drying and wetting cycles was also studied by means of mercury intrusion porosimetry and NMR. Micro pores around 50-100nm in diameter increased. The NMR spectra identified to Q_0 and Q_1 decreased, while Q_2 increased, showing the condensation of silicate anion structure.

Effect of Silicate Anion Structure on Fixation of Chloride Ions by Calcium Silicate Hydrates

Calcium silicate hydrates, CaO–SiO2-H2O (C-S-H), were studied as a chloride fixation material. C-S-H of two different CaO/SiO2 ratios were synthesized and burned with calcium chloride in a temperature range from 600° to 1000°C. Minerals with a chemical composition of CaO·SiO2·CaCl2 and 9CaO·6SiO2·CaCl2 were identified by X-ray diffraction analysis. Comparing the diffraction intensity, it was found that the most efficient chloride fixation was attained when burned at 800°C. Changes in the morphology of silicate anion associated with burning and fixation of the chloride were studied in terms of chloride fixation capability using the trimethylsililation technique. It was confirmed that some silicate anions formed a glassy infinite chain where the chloride ions were fixed as a solid solution.

Calcium silicate structure and carbonation shrinkage of a tobermorite-based material

Carbonated autoclaved aerated concretes (AACs) show no shrinkage at a degree of carbonation approximately less than 20%. The 29Si MAS NMR spectrum showed that at a degree of carbonation less than 25%, the typical double-chain silicate anion structure of tobermorite-11Å was well maintained and interlayer Ca ions were exchanged with protons. This corresponded to the absence of carbonation shrinkage at a degree of carbonation less than 20%. When the degree of carbonation increased from 25% to 50% up to 60%, the double-chain silicate anion structure of tobermorite-11Å was decomposed and Ca ions in the Ca–O layers were dissolved, showing a possible mechanism of carbonation shrinkage.

DTA–TGA of unstirred autoclaved metakaolin–lime–quartz slurries. The formation of hydrogarnet

DTA–TGA was used to monitor the evolution of hydration products in unstirred autoclaved metakaolin–lime–quartz slurries with reaction time. Hydrogarnet was always one of the first phases formed at all metakaolin additions and invariably appeared before 11 Å tobermorite. These findings explain apparent inconsistencies in the literature because the continued existence of hydrogarnet depends on such factors as reaction time and bulk composition. DTA–TGA indicated that the lime–quartz reaction was retarded and inferred differences in the precursor silicate anion structure of calcium silicate hydrates with increasing metakaolin addition.

Synthesis of Calcium Silicate Hydrate with Ca/Si = 2 by Mechanochemical Treatment

An amorphous, C‐S‐H‐like phase with Ca/Si = 2 was synthesized from amorphous precipitated silica, calcium oxide, and water by mechanochemical treatment using a vibration mill at room temperature. The product was studied by XRD, 29Si MASNMR, TEM, analytical TEM, and TGA‐DTA. After 14 h of treatment, the starting materials react to form an amorphous phase (C‐S‐H‐like phase). The XRD pattern of C‐S‐H‐like phase resembles the C‐S‐H formed hydrothermally or by cement hydration except it has no reflection at 0.182 nm. The 29Si NMR results revealed a silicate anion structure of C‐S‐H‐like phase consisting mainly of a mixture of a monomer and a dimer. On heating, the C‐S‐H‐like phase decomposed into β‐dicalcium silicate below 1000°C.

高活性セメント鉱物β‐Ｃａ２ＳｉＯ４の低温合成と水和反応ならびに生成ジェル

This paper reviews the fields indicated by the title, which emphasizes the recent work by authors. It is possible to prepare pure β-dicalcium silicate (Ca2SiO4) without adding a stabilizer having a remarkably large specific surface area of about 7 m2 /g by decomposition of hillebrandite(Ca2(SiO3)(OH)2) at 600°C. The hillebrandite is hydrothermally synthe-sized using lime and quartz with Ca/Si=2.0 at 200-250°C. The hydration rate of the β-dicalcium silicate is extremely rapid. For water/solids ratio of 0.5, the hydration reaction is completed in 28 d cured at 25°C and 24h at 80°C. The silicate anion structure of C-S-H (Calcium Silicate Hydrate Gels) consists mainly of a dimer and a single-chain polymer. The C-S-H has an oriented fibrous structure and exhibits a 0.73-nm dreierketten in the longitudinal direction.

29Si and 27Al MASNMR Study of Stratlingite

Strätlingite (2CaO·Al2O3·SiO2·8H2O) is a complex calcium aluminosilicate hydrate commonly associated with the hydration of slag‐containing cements or other cements enriched in alumina. Strätlingite can coexist with the hydrogarnet solid solution [hydrogarnet (3CaO·Al2O3·6H2O)‐katoite (3CaO·Al2O3·SiO2·4H2O)] and calcium silicate hydrate (C‐S‐H). Since Strätlingite is present in many blended cements, the knowledge of strätlingite's characteristic silicate anion structure and how aluminum is accommodated by the structure is important. Phase pure Strätlingite samples have been synthesized from oxides in the presence of excess water and from metakaolinite, calcium aluminate cement, CaO, NaOH, and water. The samples were characterized using X‐ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) and then further examined using 29Si, with and without cross‐polarization (CP), and 27Al solid‐state magic angle nuclear magnetic resonance spectroscopy (MASNMR). For the most part, NMR data for these strätlingites corroborate structural information available in the literature. The aluminum atoms are both tetrahedrally and octahedrally coordinated, and the silicon atoms exist predominantly as Q2, Q2(1Al), and Q2(2Al) species. The presence of alkali affects the structure of strätlingite in subtle ways, significantly reducing the AlIV/A1VI ratio.

Behavior of Silicate Anions on the Formation Processes of Gyrolite Followed by &lt;sup&gt;29&lt;/sup&gt;Si NMR

Using lime and amorphous silica as the starting materials, gyrolite (Ca8(Si12O30) (OH)4⋅7H2O) was prepared hydrothermally with the Ca/Si molar ratio of 0.66 and 0.50 at 200°C for 0.5-128 h. The formation process was studied mainly by 29Si NMR. The initially formed C-S-H consisted mainly of long chains of silicate anions. For Ca/Si=0.66, the chain which had been formed was broken, and C-S-H changed into gyrolite and xonotlite. For Ca/Si=0.50, only gyrolite was formed from a long chain of the silicate anions. It is thought that the differences of the C-S-H structures, which were formed in the initial stage, greatly influenced the final products. In addition, for Ca/Si=0.50, the Z-phase was also formed as a precursor of gyrolite and its silicate anion structure was similar to that of the gyrolite.

Formation Processes of β‐C2S by the Decomposition of Hydrothermally Prepared C‐S‐H with Ca(OH)2

A mixture of CaO and silicic acid prepared with a Ca/Si ratio of 2.0 was hydrothermally synthesized at 80° to 200°C, and the thermal decomposition behavior of the products (C‐S‐H with Ca(OH)2) was analyzed using XRD, 29Si MAS NMR, and the trimethylsililation method (TMS). It was found that the main silicate anion structure of C‐S‐H was a mixture of a dimer and a single‐chain polymer (larger than Si5O16) and that polymerization advanced with an increase of the synthesizing temperature. On heating, the products decomposed to form β‐C2S. It was found that the decomposition was gradual and that the‐higher the temperature of hydrothermal synthesis, the lower was the temperature of the decomposition into β‐C2S. Although the decomposition proceeded to form a monomer (β‐C2S) from the polymer and dimer, this dimer was resistant to heat and did not decompose unless heated to above 400°C.

Characterization of C‐S‐H from Highly Reactive β‐Dicalcium Silicate Prepared from Hillebrandite

β‐dicalcium silicate synthesized by thermal dissociation of hydrothermally prepared hillebrandite (Ca2(SiO3)(OH)2) exhibits extremely high hydration activity. Characterization of the hydrates obtained and investigation of the hydration mechanism was carried out with the aid of trimethylsilylation analysis, 29Si magic angle spinning nuclear magnetic resonance, transmission electron microscopy selected area electron diffraction, and XRD. The silicate anion structure of C‐S‐H consisted mainly of a dimer and a single‐chain polymer. Polymerization advances with increasing curing temperature and curing time. The C‐S‐H has an oriented fibrous structure and exhibits a 0.73‐nm dreierketten in the longitudinal direction. On heating, the C‐S‐H dissociates to form β‐C2S. The temperature at which βC2S begins to form decreases with increasing chain length of the C‐S‐H or as the Ca/Si ratio becomes higher. The high activity of β‐C2S is due to its large specific surface area and the fact that the hydration is chemical‐reaction‐rate‐controlled until its completion. As a result, the hydration progresses in situ and C‐S‐H with a high Ca/Si ratio is formed.

Silicate Anion Structure and Dehydration Processes of Okenite

The silicate anion structure and thermal decomposition behavior of okenite were studied mainly using solid 29Si NMR. The silicate anion structure of okenite is very close to some parts of the sheet structure of gyrolite. Dehydration occurs at around 119 and 315°C which are caused by the decomposition of the double-chain and sheet structure, and the sheet structure, respectivly. Although, the dehydration is caused by the water in the interlayer of okenite, and the main silicate anion structure remains untill around 350°C. From 350 to 700°C, the silicate anion structure decompose gradually as the temperature increases and many kinds of silicate anions are formed from the double-chain and the sheet structure of okenite. By the thermal treatment above 800°C, wollastonite, pseudo-wollastonite and amorphous silica are formed and pseudo-wollastonite, cristobalite, quartz and silica glass are formed over 1200°C.

Dehydration Processes of Gyrolite

The thermal decomposition processes of natural and synthesized gyrolite, which have a sheet silicate anion structure with Ca/Si molar ratio of 0.66, were compared mainly by 29Si MAS NMR. In the two samples, the silicate anions has the same structure (protonated Q3), however their crystallinity was different. The protonated Q3 structure in both samples decomposed gradually below 800°C to form nonprotonated monomer (Q0) and chain silicate (Q1 and Q2). Above 900°C, the natural gyrolite decomposed to pseudo-wollastonite and silica glass, while the synthesized one changed to pseudo-wollastonite and cristobalite via wollastonite. These differences are believed to be caused by the differences in their crystallinity, decomposition rate and the flux effect resulting from the glassification of the impurities.

Influence of Starting Materials on the Formation of 1.1-nm-Tobermorite

The influence of SiO2 sources on the formation of 1.1nm-tobermorite (5CaO⋅6SiO2⋅5H2O) was studied by using quartz, quartz+silicic acid and silicic acid. The mixtures of lime and each SiO2 source with the Ca/Si molar ratio of 0.8 were autoclaved at 180°C for the spcified time under saturated steam pressure, and the variation of structure and composition of the products were examined by XRD, analytical-TEM and 29Si NMR. The reaction proceeds from the formation of calcium silicate gel (C-S-H) as a precursor to the tobermorite formation. The starting mixtures with quartz and quartz+silicic acid yielded tobermorite at the early stage of the reaction. However, the tobermorite formation delayed remarkably, when the silicic acid was used. This is attributable to the difference of the Ca/Si ratio among C-S-H. Tobermorite was easily formed, when the Ca/Si ratio of C-S-H was higher than 0.99. The silicate anion structure of C-S-H was influenced by the SiO2 sources. The C-S-H had single chain structure, when quartz was used. On the other hand, the silicic acid yielded further but partly cross linked chain structure and had strongly protonated chain end structure.