29Si and 27Al MAS-NMR of NaOH-Activated Blast-Furnace Slag

b²⁹Si and ²⁷Al MAS-NMR were performed on NaOH-activated blast-furnace slag to better characterize the amorphous and poorly crystalline phases which occur in this system. The unreacted glass has a mainly dimeric silicate structure represented by a broad ²⁹Si peak (FWHM = 15 ppm) centered at –74.5 ppm [ Q ¹], with aluminum present exclusively in tetrahedral coordination. Upon reaction with 5M NaOH ( w/s = 0.4), three new ²⁹Si peaks with widths of ca. 2 ppm are formed at -78.5 Q ¹, –81.4 [ Q ²(1Al)J, and -84.3 [ Q ²]. Relative peak areas indicate a mostly dimeric silicate structure for the tobermorite-like C─S─H layers, with roughly a third of the bridging sites occupied by aluminum, and less than 10% by silicon. In addition to the tetrahedrally coordinated aluminum substituted in the C─S─H structure, ²⁷Al MAS-NMR reveals the presence of aluminum in octahedral sites, which is attributed to the aluminate phase (C,M)₄AH₁₃. ²⁹Si results indicate rapid initial consumption of the glass, with roughly a third of the glass reacting within the first day and another third consumed over the following 27 days.     

Density Measurements of Glasses in the Series xNa2S + (1 −x)B2S3

Measurements of the densities of glasses in the series x Na₂S + (1 − x )B₂S₃ are reported for the first time. As has been found in the corresponding oxyborate series x Na₂O + (1 − x )B₂O₃, the addition of Na₂S to B₂S₃ causes the density to increase, from 1.80 g/mL for pure B₂S₃ to 1.99 g/mL at the limit of the low-alkali sulfide glass-forming range of x = 0.25. These data provide evidence for the formation of tetrahedral boron units (BS₄⁻) as alkali is added. Volumes for the trigonal BS₃ and tetrahedral BS₄⁻ groups of 57.55 and 51.79 ų, respectively, were determined. As has also been found in the oxyborates, the tetrahedral boron group occupies a smaller volume than the trigonal boron group and causes the increase in density with added Na₂S.     

MAS NMR Studies of Partially Carbonated Portland Cement and Tricalcium Silicate Pastes

²⁹Si, ²⁷Al, and ¹H MAS NMR studies of partially carbonated mature ordinary Portland cement (OPC) and tricalcium silicate (C₃S) pastes have been carried out. The water-to-solid ratios ( W/S ) have been varied between 0 and 1 at hydration temperatures of 23^o and 90^oC. Various Q ⁿi units with n =0, 1,2,3, and 4, and a Q³ (1Al) group have been identified using ²⁹Si NMR. Cross-polarization experiments, in addition, have made it possible to assign the OH groups. Two types of fourfold- and one type sixfold-coordinated aluminum have been distinguished using ²⁷Al NMR. In C₃S pastes for w/s >0.7, progressive carbonation leads to a nearly perfect three-dimensional network consisting of Q³ and Q⁴only. In contrast, in OPC pasted only about 40% of the highly polymerized silicate units are formed, partially copolymerized with AlO₄ tetrahedra.     

Characterization by Multinuclear High-Resolution NMR of Hydration Products in Activated Blast-Furnace Slag Pastes

The effect of activators on the hydration of granulated blast-furnace slag (gbfs) was studied through compressive strength measurements, ²⁹Si, ²⁷Al, and ²³Na high-resolution nuclear magnetic resonance, and X-ray diffraction. Four different activations containing sodium hydroxide, sodium silicate, and/or calcium hydroxide (CH) were considered, at fixed amounts of alkali: 5% Na₂O, 5% Na₂O-2.5% CH, 5% Na₂O-7.5% SiO₂, and 5% Na₂O-2.5% CH-7.5% SiO₂. Silicate-activated gbfs cements have greater compressive strength than Portland cements over the whole period of study (1 yr). Also, silicate-free activated gbfs cements have poorer mechanical strength than silicate-activated cements. In fact, substantial structural differences were observed between hydration products in both kinds of activations. In silicate-activated pastes there exists an intimate mixture of C-S-H layers and AFm-like arrangements containing Al in octahedral sites bonded to the silicate layers, originated either from phase intergrowths or from a high density of Ca-Al incorporation in the interlayer spaces of C-S-H. In pastes obtained from silicate-free activation of gbfs there is a better chemical and structural definition among C-S-H and calcium aluminate hydrate domains (AFm and hydrogarnet).     

Silicon-29 Magic Angle Spinning Nuclear Magnetic Resonance Study of Calcium Silicate Hydrates

The reaction products formed in a series of fully "equilibrated," roomtemperature-hydrated, fumed colloidal silica plus lime water mixtures were examined using ²⁹Si magic angle spinning nuclear magnetic resonance. The data suggest that two structurally distinct calcium silicate hydrate (C-S-H) phases exist in the system CaO–SiO₂–H₂O. The more silica-rich C-S-H (Ca/Si = 0.65 to 1.0) consists predominantly of long chains of silica tetrahedra (Q² middle units) similar to those found in 1.4-nm tobermorite. The studied more lime-rich C-S-H (Ca / Si = 1.1 to 1.3) consists of a mixture of dimer (Q¹) and shorter chains (Q¹ end units and Q² middle units) similar to that reported for synthetic jennite. No monomer units (Q⁰) were detected.     

Silicon Substitution for Aluminum in Calcium Silicate Hydrates

²⁷Al MAS and multiquantum (MQ) MASNMR (magic-angle spinning nuclear magnetic resonance) spectroscopy were used to study the substitution of silicon by aluminum in calcium silicate hydrates (C-S-H), which are the main component of hydrated portland cement. Synthetic C-S-H samples were prepared, and their chemical stability was studied. Two-dimensional 3Q-MASNMR spectra revealed the chemical shift and quadrupolar parameters (delta_iso, nu_Q) that labeled aluminum sites in the C-S-H. Tetrahedral aluminum was observed in the bridging and nonbridging sites of the silicate chains.     

Investigation of the Structural Characterization of Mesoporous Molecular Sieves MCM-41 from Sepiolite

Mesoporous molecular sieves, MCM-41, were synthesized from sepiolite using acid leaching, followed by hydrothermal reconstruction and then calcinations at 540°C for 5 h. The structures and the porosity of MCM-41 were investigated by means of small-angle X-ray diffraction patterns, Brunaer-Emmett-Teller (BET), ²⁹Si MAS NMR, Fourier transform infrared (FTIR), and high resolution transmission electron microscope (HRTEM) methods. The results showed that the hexagonal MCM-41 was formed in an alkaline solution of pH 12, when crystallization was carried out at 100°C for 24 h. The specific surface area, pore diameter, and pore volumes of MCM-41 from sepiolite were 1036 m²/g, 2.98 nm, and 1.06 cm³/g, respectively. ²⁹Si MAS NMR results revealed that amorphous silica decomposed into Si–O chains consisting of two layers of Si atoms, with Q ³ configurations resulting in an increase in the fraction of Q ³ configuration during the crystallization of post-Mg-extraction sepiolite. The IR results illustrated that the complex of ≡≡SiO⁻–CTA⁺ was formed during the synthesis of MCM-41 from post-Mg-extraction sepiolite.     

Hydration Products and Reactivity of Blast-Furnace Slag Activated by Various Alkalis

Pastes of blast-furnace slag were cured for up to 90 d using sodium silicate (waterglass), NaOH, and three different mixtures of Na₂CO₃–Na₂SO₄–Ca(OH)₂ to activate reactions. The highest slag reactivity was observed for NaOH activation and the least for waterglass, although nonevaporable water indicated similar amounts of hydration products formed. The main hydration products found using X-ray diffractometry in all systems were calcium silicate hydrate (C-S-H) and a hydrotalcite-type phase. Microanalysis was performed on pastes activated using 50% Na₂CO₃·25% Na₂SO₄·25% Ca(OH)₂, NaOH, and waterglass; the chemical composition of the C-S-H in the waterglass case was different relative to the other two alkalis. For all alkaline agents used, the C-S-H seemed finely intermixed with a hydrotalcite-type phase of Mg/Al = 1.82, on average.     

Conduction and Dielectric Loss Mechanisms in β-Alumina and Glass: A Discussion Based on the Paired Interstitialcy Model

A paired interstitialcy model is used as a basis for qualitative comparisons of conductivity and dielectric phenomena in β-alumina crystals and in glass. Thus, the increase in the conductivity of sodium silicate glasses with increasing Na₂O activity can be explained if the concentration of (Na₂*)²⁺ interstitial pairs increases with increased polarizability of O^2- ions, expressed in terms of the optical basicity parameter, Δ. Similarly, the occurrence of the pronounced minima in conductivity isotherms (the mixed-alkali effect in glass) is attributed to disappearance of mobile interstitial pairs, e.g. (Li₂*)²⁺ or (K₂*)²⁺, and the stabilization (by polarization interactions) of apparently immobile mixed-alkali pairs, (LiK*)²⁺. The phenomenon of coionic conduction in certain β-alumina crystals is an interesting departure from this general pattern. The orientation dependence of the electrical modulus spectrum of monocrys-talline β-alumina highlights the presence of a bimodal distribution of relaxation times, in which the low-frequency component ( v ₀=10¹¹ Hz) may arise from the rearrangement of interstitial pairs and the high-frequency component ( v ₀=2×10¹² Hz) may arise from less hindered ionic motions. It is suggested that the motions of interstitial pairs and surrounding cations are mutually catalytic and that some form of combined motion is responsible for both the electrical and mechanical relaxations in β-alumina and glass.     

Structural Equilibria in Silicate Glass Melts Investigated by Raman Spectroscopy

Structural equilibria of alkali silicate glasses and melts are investigated based on the Q ⁿ fractions ( n =number of bridging oxygen in SiO₄ tetrahedron) obtained from high-temperature Raman spectra. A mathematical treatment of the entropy of mixing, Δ S _mix, of structural groups of Q ⁿ is conducted and applied to the investigation of phase transformation and phase separation. Characteristics of Q ⁿ traces and the counter-map of Δ S _mix on Q ⁿ diagram are found to possess useful information to understand their mechanisms. An extended Q ⁿ diagram is also proposed to understand the melt structure and properties for the wide composition range.     

X-ray Photoelectron Spectroscopy Study on the Process of Apatite Formation on a Sodium Silicate Glass in Simulated Body Fluid

The process of apatite formation on the surface of Na₂O–SiO₂ glass in a body environment was investigated, mainly by X-ray photoelectron spectroscopy, as a function of soaking time in a simulated body fluid (SBF). The glass was found to release Na⁺ ions via exchange with H₃O⁺ ions in the SBF to form Si—OH groups on its surface. These Si—OH groups induced apatite formation indirectly, by forming calcium silicate and amorphous calcium phosphate. The formation of the calcium silicate and amorphous calcium phosphate is attributed to electrostatic interactions between the Si—OH groups on the glass surface and the calcium and phosphate ions in the SBF.     

Structures and Phase Relations of Aluminum-Substituted Calcium Silicate Hydrate

The structures of aluminum-substituted calcium silicate hydrate (C-S-H) forming in a series of aqueous suspensions formulated with colloidal silica, reactive alumina, and lime and aged for 1 year have been studied using ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy (with and without cross polarization (CP)), solution pH, electron microscopy, and X–ray diffraction. As in earlier work dealing with the nature of C-S-H in the system CaO-SiO₂H₂O₂ two aluminum-substituted C-S-H phases, having distinctly different anionic structures on the atomic level (Q₂ versus Q₁Q₂), were found to extend into the system CaO-Al₂O₃-SiO₂-H₂O₂ X-ray diffraction patterns of the two aluminum-containing C-S-H phases are nearly identical, suggesting that their intermediate-range order is very similar, but MASNMR spectra show that these two phases have markedly different silicate structures on the atomic-level scale. Both C-S-H structures can accommodate approximately 5 mol% of Al₂O₃ in tetrahedral and possibly octahedral coordination as well.     

Effect of Molybdenum on the Structure and on the Crystallization of SiO2–Na2O–CaO–B2O3 Glasses

Crystallization of the poorly durable Na₂MoO₄ phase able to incorporate radioactive cesium must be avoided in SiO₂–Al₂O₃–B₂O₃–Na₂O–CaO glasses developed for the immobilization of Mo-rich nuclear wastes. Increasing amounts of B₂O₃ and MoO₃ were added to a SiO₂–Na₂O–CaO glass, and crystallization tendency was studied. Na₂MoO₄ crystallization tendency decreased with the increase of B₂O₃ concentration whereas the tendency of CaMoO₄ to crystallize increased due to preferential charge compensation of BO₄⁻ entities by Na⁺ ions. ²⁹Si MAS NMR showed that molybdenum acts as a reticulating agent in glass structure. Trivalent actinides surrogate (Nd³⁺) were shown to enter into CaMoO₄ crystals formed in glasses.     

Optical Spectra of the Various Valence States of Vanadium in Na2 O.2SiO2 Glass

Optical absorption data are presented for Na₂O₂SiO₂ glass containing vanadium equilibrated in various states of oxidation. Spectra of 5⁺, 4⁺, and 3⁺ vanadium were derived. The variation in amounts of the three species is in accord with mass action considerations. The spectra are related to crystal field assignments.     

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 Al^IV/A1^VI ratio.     

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.     

Spectroscopic Properties of Er3+-Doped Na2O–La2O3–Al2O3–SiO2 Glasses

Er³⁺-doped sodium lanthanum aluminosilicate glasses with compositions of (90− x )(0.7SiO₂·0.3Al₂O₃)· x Na₂O·8.2La₂O₃· 0.6Er₂O₃·0.2Yb₂O₃·1Sb₂O₃ (in mol%) ( x = 12, 20, 24, 40, 60 mol%) were prepared and their spectroscopic properties were investigated. Judd–Ofelt analysis was used to calculate spectroscopic properties of all glasses. The Judd–Ofelt intensity parameter Ω _t ( t = 2, 4, 6) decreases with increasing Na₂O. Ω₂ decreases rapidly with increasing Na₂O while Ω₄ and Ω₆ decrease slowly. Both the fluorescent lifetime and the radiative transition rate increase with increasing Na₂O. Fluorescence spectra of the ⁴ I _13/2 to ⁴ I _15/2 transition have been measured and the change with Na₂O content is discussed. It is found that the full width at half-maximum decreases with increasing Na₂O.     

Redox Equilibria of Multivalent Ions in Silicate Glasses

Experimental studies were made on the compositional dependence of the redox equilibrium of Eu in synthetic silicate liquids, together with an empirical model describing the observed compositional dependence. Electron paramagnetic resonance (EPR) was used to measure the concentration ratio of Eu²⁺ to Eu³⁺ in various glasses formed by rapidly quenching silicate liquids. The compositional field studied comprised mixtures of SiO₂, TiO₂, Al₂O₃, CaO, MgO, and Na₂O. The proposed model describes the Eu²⁺/Eu³⁺ ratio over the entire compositional field in terms of parameters easily related to each glass composition. The general applicability and utility of the model is further demonstrated by its application to the Fe²⁺-Fe³⁺, Ce³⁺-Ce⁴⁺, and Cr³⁺-Cr⁶⁺ redox reactions in binary alkali oxide silicate glasses of Li, Na, and K.     

Aluminum K-Edge Absorption (XANES) Studies of Noncrystalline Mullite Precursors

Noncrystalline aluminum silicate precursors MP1 and MP3 with different ratios of Al in tetrahedral (^[4]A1), pentahedral (^[5]A1), and octahedral (^[6]A1) position were prepared from aluminum sec-butylate and tetraethoxysilane (Al/Si ratio: 3/1) making use of a slow and rapid hydrolysis process, respectively. The MP2 precursor was synthesized from aluminum sec-butylate and silicon tetrachloride (Al/Si ratio: 4/1) by rapid hydrolysis. All aluminum silicate gels were heat-treated to temperatures just prior to crystallization to mullite and γ-Al₂O₃, respectively (MP1 and MP3: 800°C; MP2: 300°C). Al K near-edge absorption spectra (XANES) of the mullite precursors and of several suitable crystalline reference materials were measured using monochromatic synchrotron radiation. The reference XANES spectra yielded a linear correlation between the energy position of the first inflection point of the white lines and the frequency of sixfold-coordinated Al (^[6]A1) in the samples. A similar though less straightforward correlation seems to hold also for the white line intensities. From these findings, the actual ^[61]A1 frequencies for the mullite precursors under investigation were derived to be <10% (MP1), =30% (MP3), and =60% (MP2). The ^[6]A1 frequency distribution in the mullite precursors displays a trend similar to that determined by ²⁷A1 NMR spectroscopy.     

The System Na2O-CaO-SiO2-H2O

A portion of the quaternary phase diagram for Na₂O-CaO-SiO₂-H₂O has been constructed. Plotting concentrations as their 10th roots allows compounds having solubilities which differ by several orders of magnitude to be represented on a single diagram. The compositional relationships among sodium-substituted calcium silicate hydrate, calcium-substituted sodium silicate hydrate, calcium bydroxide, a quaternary compound of approximate composition 0.25Na₂O · CaO · SiO₂· 3H₂O, sodium hydroxide monohydrate, and miscellaneous sodium silicate hydrates are presented. The quaternary diagram constructed shows the quaternary compound to exist in equilibrium with sodium-substituted calcium silicate hydrate and calcium hydroxide. Conditions in concrete pore solutions which favor the formation of this quaternary compound may also favor the occurrence of the alkali-silica reaction.