Examination of alkali-activated material nanostructure during thermal treatment

The key nanostructural changes occurring in a series of alkali-activated materials (AAM) based on blends of slag and fly ash precursors during exposure to temperatures up to 1000 °C are investigated. The main reaction product in each AAM is a crosslinked sodium- and aluminium-substituted calcium silicate hydrate (C-(N)-A-S-H)-type gel. Increased alkali content promotes the formation of an additional sodium aluminosilicate hydrate (N-A-S-(H)) gel reaction product due to the structural limitations on Al substitution within the C-(N)-A-S-H gel. Heating each AAM to 1000 °C results in the crystallisation of the disordered gels and formation of sodalite, nepheline and wollastonite. Increased formation of N-A-S-(H) reduces binder structural water content after thermal treatment and correlates closely with previous observations of improved strength retention and reduced microcracking in these AAM after heating to 1000 °C. This provides new insight into thermally induced changes to gel atomic structure and thermal durability of C-(N)-A-S-H/N-A-S-H gel blends which are fundamental for the development of new fire-resistant construction materials

Structural characterization of mullites synthesized by thermal decomposition of topaz

The structure of mullite obtained by thermal decomposition of topaz at 1340 °C and after additional thermal treatment at 1600 °C was characterized by 29Si and 27Al MAS NMR spectroscopy and x ray diffraction. No amorphous phase was detected by these techniques. The 29Si MAS NMR spectra of the mullite samples showed four resonance peaks at -81, -86, -90 and -94 ppm. The peak at -81 ppm corresponded to silicon near oxygen vacancies in the mullite structure. The peak at 86 ppm was related to a sillimanite-type site and was the major contribution, ranging from 42 to 50%. The two other peaks, at -90.0 ppm and -94 ppm, were interpreted as resulting from rearrangements of the sillimanite-type site by the replacement of AlO4 by SiO4 tetrahedra16. The 27Al MAS NMR data of the two samples exhibited three different peaks, two of which were attributed to tetrahedral sites at 40 -45 and 63 -67 ppm, while the third was assigned to an octahedral site at about -6 ppm. The ratio of tetrahedral to octahedral aluminum sites was found to depend on the temperature applied during the preparation of the mullite and was higher at 1600 °C

Atomic-scale structure of gel materials by solid-state NMR

The underlying principles of solid-state NMR spectroscopy are outlined with an emphasis on the physical origins of the interactions that affect NMR spectra so that an understanding of the structural information they convey is clearly understood. The fundamental components of the experimental approach are described. How the experimental data can be analyzed to provide structural characterization of sol-gel materials is illustrated through a series of examples from the literature. The short-range structural sensitivity of NMR means that it is an ideal probe of sol-gel materials since they are structurally disordered. Given the importance of silicates in sol-gel science, 29Si magic-angle spinning (MAS) NMR is a widely used nucleus in solid-state NMR studies of sol-gel materials. However, it is emphasized that to derive maximum benefit fromNMR characterization, a multinuclear approach is used, although each nucleus will have its own particular considerations which are presented. In this second edition, key advances in the experimental methodology (e.g., much higher applied magnetic fields, faster MAS rates, more sophisticated excitation approaches) since 2005 are outlined. The use of first-principles computational approaches to calculate NMR interaction parameters and hence better constrain structure provides an important additional dimension to the NMR approach. Materials where there has been a substantial expansion of sol-gel approaches since 2005 are included, with, for example, novel sol-gel schemes opening up preparation of phosphates where 31P MAS NMR is a sensitive structural probe. Another area where there has been substantial sol-gel activity since 2005 is in the preparation of bioactive calcium silicate-based materials, where multinuclear NMR is an ideal probe, including the use of 43Ca, a quadrupolar nucleus with a small magnetic moment, which has only really become readily accessible in recent years. © Springer International Publishing AG, part of Springer Nature 2018