Pore structural changes in hardened cement pastes, subjected to drying and wetting/drying cycles, were studied at micrometer and nanometer levels. Characterization techniques included Nuclear Magnetic Resonance (NMR), nitrogen and water vapor adsorption, mercury intrusion porosimetry (MIP) and under-water weighing. Coarsening of pore structure was observed with MIP and increase in the true density of C-S-H was suggested by the result of under-water weighing. Decrease in specific surface area due to drying was observed with nitrogen adsorption, and water vapor adsorption associated with Excess Surface Work (ESW) analysis suggested a development of cohesive structure in C-S-H. NMR confirmed polymerization of silicate anion chains. The drying-induced coarsening of pore structure is probably attributed to polymerization of silicate anion chains and development of cohesive structure in C-S-H.
The layered double hydroxide (LDH) A12Li(OH)6[NiC14]I/2 was synthesized by anion exchange of A12Li(OH)6(NO3) and the coordination structure of [NiCI4] 2-in the interlayers was investigated. The LDH had a hexagonal cell with a = 5.06 and c = 23.06 .~. Since the basal spacing of one layer was 7.7 .~, the interlayer distance was calculated to be 2.9 ,~ and was too narrow for the usual tetrahedral NiCI4 structure. From extended X-ray absorptionofine structure (EXAFS) measurements, the first neighbour Ni-C1 distances in the LDH were 2.10 A which was compatible with four-fold coordination. Since the electron spin resonance (ESR) spectrum of the LDH showed no absorption, the [NiCI4] 2-structure was considered to be square planar, rather than the usual tetrahedral structure in the interlayers of LDH. et al., 1982). The LDHs with a combination of M3+-M '+ are very limited, however, and only a combination of A1-Li is reported (Chisem & Jones, 1994). Since the M '+ ions are distributed in ordered positions in the octahedral sheets, the interlayer anions are expected to occupy ordered positions in the interlayers. This ordered configuration of interlayer anions may lead to materials suitable as precursors for the preparation of nano-composite materials.Recently, Lopez-Salinas & Ono (1993) synthesized A12Li(OH)6[NiC14]I/2 by an anion exchange method. This LDH is considered to be an interesting candidate as a nano-composite since the uniform and fine Ni metal particles are well dispersed and may show attractive new chemical and/or magnetic properties. The structure of the [NiC14] 2-ion in the interlayers of the LDH was reported to be tetrahedral but the interlayer spacing reported was geometrically incompatible with a tetrahedral structure. In this paper, preparation of an A1-Li LDH with [NiC14] 2-ion in the interlayers, and the structure of the [NiC14] 2-ion therein, are described.
The microstructure of AAC changes during carbonation, resulting in degradation such as cracking. AAC panels under working conditions (Field-AAC) aged 5 to 33 years and AAC blocks carbonated under accelerated conditions (Labo-AAC) were subjected to microstructural analysis. The degree of carbonation increased with time and approached 60% both under working and accelerated conditions. Changes of micro-level structures such as surface and crystal structures were more significant under accelerated conditions than under working conditions. Meso-level structures such as the interparticle pore volume were similar regardless of the carbonation conditions. The increase in drying shrinkage was more significant under accelerated conditions than under working conditions because it originated not only from the moisture characteristics but also the micro-level structures.
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