Si NMR Study of the Hydration of Ca3SiO5 and ß-Ca2SiO4 in the Presence of Silica Fume

²⁹Si magic-angle spinning nuclear magnetic resonance (MASNMR) was used to study the room-temperature hydration of C₃S, ß-C₂S, and reactive ß-C₂S mixed with different amounts of silica fume (SF) that had been hydrated up to nine months and longer. The overall CaO:SiO₂ molar ratios of the mixes were 0.12, 0.20, 0.35, 0.50, and 0.80. NMR spectroscopy was used to quantify the remaining starting materials and the resulting hydration products of different species. A broad peak assigned to Q³, appearing in both the fourier transform (FT) and the cross-polarization (CP) modes, increased in intensity with increased SF content and with age. This Q³ species was attributed to two sources: (1) the surface hydroxylation of SF and (2) the cross-linking of dreierketten (chains of silicate tetrahedra arranged in a repeating three-unit conformation) in the calcium silicate hydrate (C-S-H) structure. A Q⁴ species also appeared in the CP spectra of samples with large SF additions after extended hydration and was attributed to cross-polarization by adjacent hydroxylated Q³ species at the surface of amorphous SiO₂.     

Application of Selective 29Si Isotopic Enrichment to Studies of the Structure of Calcium Silicate Hydrate (C-S-H) Gels

Selective isotopic enrichment of SiO₂ with ²⁹Si in a mixture with tricalcium silicate (C₃S) has allowed the Si from this phase to be effectively labeled during the course of the hydration reaction, thus isolating its contribution to the reaction. A double Q₂ signal has been observed in ²⁹SI solid-state MAS NMR spectroscopy of C-S-H gels of relatively low Ca/Si ratio, prepared by hydration or by carbonation of a C₃S paste. The origin of the weaker, downfield peak is discussed and tentatively attributed to bridging tetrahedra of a dreierkette silicate chain structure.     

Hydration of Tricalcium Silicate Followed by 29Si NMR with Cross-Polarization

The use of cross-polarization (CP) NMR in conjunction with magic angle sample spinning (MASS) to examine the hydration reaction of tricalcium silicate (C₃S) is described. In particular the very early stages of the reaction both with and without admixtures has been studied as well as the hydration in a ball mill. The combination of CP and non-CP ²⁹Si NMR permits the distinction between silicate units associated with protons, i.e., in hydrated material, and those in anhydrous material. It has been found that in paste hydration there is steady formation of a small amount of hydrated monomeric silicate units during the induction period. In ball mill hydration the formation of the crystalline calcium silicate hydrate, afwillite, which contains only hydrated monomeric silicate species, can be monitored. These results are interpreted in terms of possible mechanisms for C₃S hydration.     

29Si MAS NMR Study of Dicalcium Silicate: The Structural Influence of Sulfate and Alumina Stabilizers

The effect on β-C₂S of two stabilizing agents, calcium sulfate and alumina, has been investigated using high-resolution ²⁹Si solid state NMR spectroscopy. Syntheses were achieved via the gel route, wet or dry processes. Room-temperature NMR spectra characteristics were analyzed as a function of the sintering temperature. The incorporation of Al³⁺ and S⁶⁺ ions, which finds expression in a noticeable line broadening, is shown to be effective above 1200°C. The ²⁹Si chemical shift is unchanged upon doping, suggesting a mean SiO₄ tetrahedra geometry identical to that in pure β-C₂S. General trends on the structure adopted by C₂S upon Al³⁺ and S⁶⁺ doping are also discussed.     

Structure of Calcium Silicate Hydrates Formed in Alkaline-Activated Slag: Influence of the Type of Alkaline Activator

The influence of the alkaline activator (NaOH, waterglass, or Na₂CO₃) on the structure of the hydrated calcium silicate formed in alkali-activated slag (AAS) cement pastes has been investigated by FTIR, ²⁹Si and ²⁷Al magic-angle scattering nuclear magnetic resonance, and TEM/EDX techniques. In all cases, the main product formed after 7 d of activation, with activators giving an Na₂O concentration of 4%, is a semicrystalline calcium silicate hydrate with a dreierkette-type anion. In these structures, linear finite chains of silicate tetrahedra ( Q ² units) are linked to central Ca-O layers, and tetrahedral aluminum occupies bridging positions in the chains. The main chain length and the amount of aluminum incorporated in the tetrahedral chains depend on the activator used. The detection of Q ³ silicon entities in alkaline-activated slags is discussed in relation to the possible formation of cross-linked structures that may be responsible for increased flexural and compressive strengths in AAS mortars.     

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.     

Structural Investigations of Alkali Silicate Gels

Investigation of a series of synthetic alkali silicate gels and gels produced by the alkali silica reaction (ASR) in field concrete using ²⁹Si NMR spectroscopy, X-ray diffraction, and bulk chemical analysis shows that local structures of the synthetic and field gels are quite similar. The most abundant Si sites for the field and synthetic gels with similar compositions have Q³ polymerization, and the number of non-bridging oxygens per Si is similar for these samples. These samples also yield a basal X-ray diffraction peak near 8–12 Å, suggesting that the structure is dominated by sheet-like units, consistent with the dominant Q³ polymerization. Calculations based on the relative site abundances of the sites observed by ²⁹Si NMR and the bulk chemical compositions indicate that there is insufficient alkali to charge-balance all the non-bridging oxygens and that there is a significant concentration of Si–OH linkages. The results provide strong support for the basic structural concepts of the so-called kanemite model for ASR gel proposed by Wieker and coworkers, although the overall gel structure is likely to be more complex.     

Influence of Minor Ions on the Stability and Hydration Rates of β-Dicalcium Silicate

The effects of Al³⁺, B³⁺, P⁵⁺, Fe³⁺, S⁶⁺, and K⁺ ions on the stability of the β-phase and its hydration rate were studied in reactive dicalcium silicate (C₂S, Ca₂SiO₄) synthesized using the Pechini process. In particular, the dependences of the phase stability and degree of hydration on the calcination temperature (i.e., particle size) and the concentration of the stabilizing ions were investigated. The phase evolution in doped C₂S was determined using XRD, and the degree of hydration was estimated by the peak intensity ratio of the hydrates to the nonhydrates in ²⁹Si MAS NMR spectra. The stabilizing ability of the ions varied significantly, and the B³⁺ ions were quite effective in stabilizing the β-phase over a wide range of doping concentrations. The hydration results indicated that differently stabilized β-C₂S hydrated at different rates, and Al³⁺- and B³⁺-doped C₂S exhibited increased degree of hydration for all doping concentration ranges investigated. The effect of the doping concentration on degree of hydration was strongly dependent on the stabilizing ions.     

Early-Stage Thermal Oxidation of Carbothermal β-Sialon Powder

Early-stage thermal oxidation (below 1100°C) of carbothermally synthesized β-sialon powder was monitored by X-ray powder diffraction, solid-state ²⁹Si and ²⁷Al MAS NMR spectroscopy, and thermogravimetry. No crystalline oxidation products were detected by XRD but ²⁹Si and ²⁷Al MAS NMR indicated the early formation of amorphous silica, followed by the formation of an amorphous aluminosilicate with an atomic environment similar to that of mullite. The initial oxidation was described by a linear kinetic law with an activation energy of 170 kJmol⁻¹, suggesting the rate-limiting step to be due to dissolution of O₂ in an amorphous silica surface layer on the β-sialon particles.     

Highly Reactive β-Dicalcium Silicate: II, Hydration Behavior at 25°C Followed by 29Si Nuclear Magnetic Resonance

The hydration behavior at 25°C of highly reactive β-dicalcium silicate synthesized from hillebrandite (Ca₂(SiO₃)(OH)₂) was studied over a period of 7 to 224 d using ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS NMR). The hydration product, C-S-H, contains Q² and Q¹ silicate tetrahedra, the chemical shifts of which are independent of the water/solid (w/s) ratio and curing time. Until the reaction is completed, the amounts of Q¹ and Q² formed are independent of the w/s ratio, being determined only by the degree of reaction. The ratio Q²/Q¹ increases as the reaction progresses and as the curing time becomes longer. From the values of Q²/Q¹, it appears that the hydrate is a mixture of dimers and short single-chain polymers. The Ca/Si ratio of the hydrate is high, taking values close to 2.0, but the Ca/Si ratio does not influence the Q²/Q¹ ratio. It was also found that the NMR peak intensities allow quantitative assessment similar to XRD.     

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.     

Location of Aluminum in Substituted Calcium Silicate Hydrate (C-S-H) Gels as Determined by 29Si and 27Al NMR and EELS

Solid-state ²⁷Al and ²⁹Si magic angle spinning NMR spectroscopy has been combined with electron energy loss spectroscopy carried out in the transmission electron microscope to determine the location of Al substituting in a semicrystalline C-S-H gel present in a hydrated synthetic slag glass. The gel is found to contain mainly pentameric silicate chains in which the central silicon is substituted by aluminum.     

Thermal Reactions of Pyrophyllite Studied by High-Resolution Solid-state 27Al and 29Si Nuclear Magnetic Resonance Spectroscopy

Water is lost in two overlapping steps from well-crystallized pyrophyllite from Coromandel, New Zealand. The pyrophyllite structure survives the loss of the first 30% of the total water content, but the loss of a further 60% water in the second step results in the formation of pyrophyllite dehydroxylate, with corresponding changes in both the ²⁹Si and ²⁷Al solid-state NMR spectra. Detailed analysis of the ²⁹Si chemical shift of the dehydroxylate has allowed the silicate layer structure of this phase to be refined. A similarly detailed interpretation of the ²⁷A1 spectra is not possible because of electric field gradient effects which result in the loss of ∼90% of the A1 spectral intensity due to the formation of five-coordinate A1 on dehydroxylation. The loss of further water from the dehydroxylate on further heating results in the formation of mullite and cristobalite and is accompanied by changes in the ²⁹Si and ²⁷Al spectra which can be accounted for in terms of coordination changes in the structural regions which contained the residual hydroxyls.     

Characterization of Synthetic and Naturally Occurring Clays by 27Al and 29Si Magic-Angle Spinning NMR Spectroscopy

Magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy (²⁷Al) detected the coordination of Al in clays containing as little as 0.26% Al₂O₃. The ²⁹Si MASNMR of fluorphlogopite showed three distinct Si chemical environments which suggested short-range ordering. Synthetic laponite and mica-montmorillonite showed broad ²⁹Si resonances indicative of short-range disorder. A saponite showed four ²⁹Si resonances. Considerable insight into the short-range ordering of clays can be gained by ²⁷Al and ²⁹Si MASNMR.     

Progressive Changes in the Structure of Hardened C3S Cement Pastes due to Carbonation

The structures of partially carbonated hardened C₃S cement pastes have been investigated by a combination of ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy and analytical transmission electron microscopy, supported by X-ray diffraction and thermogravimetric analysis. Progressive changes in structure are reported for thin slices for a paste carbonated in pure CO₂ for times from 1 to 16 h, and the results are compared with those for a paste carbonated for 2 months in air. C-S-H gel of reduced Ca:Si ratio and increased silicate polymerization was formed during the early stages of carbonation. The morphology of the original C-S-H was, in the main, retained. A cross-linked silica-rich gel formed at later times in paste carbonated in CO₂ but not up to the time of 2 months in air. Calcium carbonate took the form of microcrystals of vaterite and calcite which formed dense masses between gel fibrils and around partially reacted CH crystals, possibly accounting for the observed slowing in the rate of reaction of CH with time.     

Outstanding Problems in the Kaolinite-Mullite Reaction Sequence Investigated by 29Si and 27Al Solid-state Nuclear Magnetic Resonance: 11, High-Temperature Transformations of Metakaolinite

Solid-state ²⁹Si and ²⁷Al NMR spectra of kaolinite fired at 800° to 1450°C, interpreted in light of a newly proposed metakaolinite structure and complementary X-ray diffraction results, lead to the following conclusions about the hightemperature reactions: (1) Removal of the final residual hydroxyl radicals of metakaolinite at ∼9707deg;C triggers the separation of a considerable amount of amorphous free silica and the formation of poorly crystalline mullite and a spinel phase. (2) Mullite and spinel form in tandem, the former originating in the vicinity of AI-0 units of regular octahedral and tetrahedral symmetry randomly distributed throughout the metakaolinite structure. (3) The initially formed mullite is alumina-rich but at higher temperatures progressively gains silica, approaching the conventional 3Al₂O₃· 2SiO₂ composition. (4) The spinel phase contains insufficient Si to be detected by ²⁹Si NMR but has a ²⁷Al NMR spectrum consistent with γ-Al₂O₃. On further heating, the spinel is converted to mullite by reaction with some of the amorpholls silica, the balance of which eventually becomes cristobalite.     

Synthesis and Characterization of a SiAION Glass

Homogeneous SiAION glasses containing up to 1 wt% nitrogen were synthesized via a pressureless method with a controlled quench rate and structurally investigated using ²⁷Al and ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS NMR), Raman, and infrared (IR) spectroscopies. Minor changes occur with the incorporation of nitrogen into the aluminosilicate glass structure as evidenced by modifications to spectra of a nitrogen-free aluminosilicate glass. The ²⁷Al MAS NMR spectrum of the SiAION glass shows the existence of aluminum in 4-,5-, and 6-coordination to oxygen. The ²⁹Si MAS NMR spectra show a distribution of silicon sites in 4-coordination to oxygen. Raman and IR spectra of the SiAION glass show additional features due to incorporation of nitrogen in the structure compared with spectra of nitrogen-free aluminosilicate glasses.     

Solid-State 29Si NMR Analysis of Amorphous Silicon Nitride Powder

Solid-state ²⁹Si NMR techniques were used to characterize laser-synthesized silicon nitride powder prepared from the reaction of silane with ammonia. When the powder is exposed to water vapor, a hydrated layer rapidly forms at the surface. A comparison of ²⁹Si cross polarization (CP) and Bloch decay (BD)-MAS-NMR spectra revealed differences between surface and bulk compositions. CP-NMR identified Si-NH_x (x = 1, 2) species with a chemical shift of −45 ppm in the as-synthesized ( unexposed ) powder. In BD-NMR spectra, the nitride resonance is observed at −48 ppm. For the hydrated powder, CP-NMR identified additional =Si-OH ( Q³ ), =Si-(OH)₂ ( Q² ), and SiO₂ ( Q⁴ ) species present at the surface. The CP-NMR spectra were corrected for T_lpH relaxation effects and deconvoluted into individual components in order to extract quantitative measurements of the various species present.     

Solid-state NMR Evidence of 4-, 5, and 6-Fold Aluminum Sites in Roller-Quenched SiO2-A12O3 Glasses

The structures of roller-quenched SiO₂-Al₂O₃ glasses containing 10 to 50 wt% Al₂O₃ were investigated by TEM and high-resolution ²⁷Al and ²⁹Si magic angle sample spinning nuclear magnetic resonance (MASS-NMR) spectroscopy. The chemical shifts observed in the NMR spectra of these phase-separated (on the TEM scale) glasses provide evidence, for the existence of all three (4-, 5-, and 6-jold) Al-coordination units in these glasses and for the presence of A1 in both the Si-rich and Si-poor phases.     

31P and 29Si Magic-Angle Sample-Spinning NMR Investigation of the Structural Environment of Phosphorus in Alkaline-Earth Silicate Glasses

²⁹Si and ³¹P magic-angle sample-spinning NMR spectroscopy indicates that phosphorus added as P₂O₅ to alkaline-earth metasilicate glasses is present as monomeric (PO₄)^3– structural units and that incorporation of this phosphorus increases the average polymerization of the silicate portion of the glass. These results are consistent with published interpretations of Raman spectra of similar composition.