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A transmission electron microscopy study on the decomposition of synthetic hillebrandite (Ca2SiO4 · H2O)
Published online by Cambridge University Press: 03 March 2011
Abstract
The decomposition mechanism of hillebrandite (Ca2SiO4 · H2O) to larnite (β-Ca2SiO4) was studied by conventional and in situ hot-stage TEM methods. The hillebrandite structure is suggested to be a composition of parawollastonite (CaSiO3) and portlandite [Ca(OH)2] adjoining on {001} planes. The larnite fibers showed occasional bending and kinking as well as internal defects such as dislocations and domains. No transformation-related microstructures such as twinning were observed. The preferred orientations in the larnite fibers were not distinct. The decomposition mechanism of hillebrandite in air and in a vacuum may be different. In air, the CaO from the decomposed portlandite layer directly participates in the process of SiO4 chain breaking to form larnite. In the TEM this CaO is removed and apparently reprecipitated, so that hillebrandite converts directly to parawollastonite, A possible lattice correspondence between hillebrandite (C2SH) and larnite (β-C2S), based on crystallographic considerations, is proposed to be bC2SH ¶bβ-c2s, αC2SH ¶ ≍[102]β-C2S, and CC2SH ¶ ≍ [401]β-C2S.
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