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Thermal decomposition of boehmite and bayerite has been studied by X-ray diffraction and by NMR and IR spectroscopies. Coordination and site distortion of Al polyhedra have been estimated by 27Al NMR spectroscopy while IR optical properties of transitional aluminas, η-, γ-, δ-, and θ-Al2O3 were derived from IR near-normal specular reflectance technique. In the low-temperature phases, aluminum vacancies are located in tetrahedral positions in γ-Al2O3 while in η-Al2O3 they are distributed at random between tetrahedral and octahedral positions. In addition, a small amount (5%) of pentahedrally coordinated aluminum was found in γ-alumina, which has been tentatively assigned to Al at the external surface of alumina particles. At 800 °C a notable increase in structural ordering was detected in both phases; in the case of η-Al2O3, this change accelerates the continuous transformation into θ-Al2O3 which is totally achieved at 1000 °C. On the contrary, γ-Al2O3, does not show any sign of transformation until 900 °C; at 950 °C pentahedral cations disappear and formation of θ-Al2O3 is detected by IR and NMR techniques. From 900 to 1200 °C, γ-, θ-, and α-Al2O3 coexist in samples obtained from boehmite. Above 1200 °C corundum is the only thermodynamically stable phase in both series of samples.

Reaction products formed during alkaline activation, with 8M NaOH solutions, of fly ashes have been characterized with 29Si and 27Al magic‐angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. In particular, the influence of curing conditions (time and temperature of reaction) has been analyzed. NMR results show the formation of amorphous tecto‐silicates in which the amount of aluminum decreases in the two consecutive formed phases. The Si/Al ratio of zeolite precursor obtained at 85°C changes from 0.95 to 1.86 when curing time is increased from 5 h to 1 week. The evolution of the mechanical properties of prepared cements has also been discussed in terms of the phases formed and texture and morphology of samples.

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Structural features responsible for lithium conductivity in Li 1+x Ti 2−x Al x (PO 4 ) 3 (x = 0, 0.2, and 0.4) samples have been investigated by Rietveld analysis of high-resolution neutron diffraction (ND) patterns. From structural analysis, variation of the Li site occupancies and atomic thermal factors have been deduced as a function of aluminum doping in the temperature range 100−500 K. Fourier map differences deduced from ND patterns revealed that Li ions occupy M1 sites and, to a lower extent, M3 sites, disposed around ternary axes. The occupation of M1 sites by Li ions is responsible for the preferential expansion of the rhombohedral R3̅ c unit cell along the c axis with temperature. The occupation of less symmetric M3 sites decreases electrostatic repulsions among Li cations, favoring ion conductivity in Li 1+x Ti 2−x Al x (PO 4 ) 3 compounds. The variations detected on long-range lithium motions have been related to variations of the oxygen thermal factors with temperature. The information deduced by ND explains two lithium motion regimes deduced previously by 7 Li NMR and impedance spectroscopy.Article pubs.acs.org/IC

The influence of the alkaline activator (NaOH, waterglass, or Na 2 CO 3 ) on the structure of the hydrated calcium silicate formed in alkali-activated slag (AAS) cement pastes has been investigated by FTIR, 29 Si and 27 Al 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 Na 2 O 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. † Q slag ϭ Q 0 /Q 1 (slag); ‡ Q Total ϭ ⌺Q n where Q n stands for Q 1 , Q 2 , and Q 3 units

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The "Al and "Si magic-angle spinning nuclear magnetic resonance (MAS-NMR) study of the kaolinite-mullite transformation has shown the presence of A1 in tetru-and pentacoordination in dehydroxylated kaolinite. The 29Si NMR signal analysis of samples heated above 400 "C demonstrates that the tetrahedral sheet of kaolinite begins to break down near 600°C and continues to do so to 900°C. From the 27Al NMR signal evolution, it can be deduced that the exothermic peak at 980°C in DTA curves is associated with the mod$ication of the coordination of Al, which changes from the tetra-or pentacoordination to the more stable octahedral coordination. Heating the sample at 880°C ,for 36 h produces the same transformation in the coordination of A1 ions and the elimination of the exothermic peak at 980°C in the DTA diagram. After this transformation, all spectra show two tetrahedral lines characteristic of mullite, indicating that nuclei of mullite with low crystalliniv are generated during the exothermic process which are not detected by XRD. At higher temperatures tetrahedral NMR peaks increase in intensity, yielding, at 1200"C, the 3:2 mullite NMR spectrum.