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First Observation of α‐Ca 2 [SiO 3 (OH)](OH)–Ca 6 [Si 2 O 7 ][SiO 4 ](OH) 2 Phase Transformation upon Thermal Treatment in Air
Author(s) -
Garbev Krassimir,
Gasharova Biliana,
Beuchle Günter,
Kreisz Siegfried,
Stemmermann Peter
Publication year - 2008
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1551-2916.2007.02115.x
Subject(s) - analytical chemistry (journal) , chemistry , infrared spectroscopy , x ray crystallography , phase (matter) , powder diffraction , absorption (acoustics) , infrared , mineralogy , polymerization , diffraction , crystallography , materials science , organic chemistry , optics , physics , composite material , polymer
Thermal treatment transforms α‐Ca 2 [SiO 3 (OH)](OH) (α‐C 2 SH) into hydraulic Ca 2 SiO 4 polymorphs, which is a process of considerable interest to the cement chemistry community. The thermal behavior of differently synthesized samples of α‐C 2 SH in air at ambient pressure has been investigated in detail by high‐temperature powder X‐ray diffraction (XRD), synchrotron‐based infrared (IR) microspectroscopy of single crystals, trimethylsilylation method (TMSM), and thermal analysis. Powder XRD data on α‐C 2 SH samples heated to different temperatures between 300° and 500°C suggest coexistance of α‐C 2 SH and dellaite, Ca 6 [Si 2 O 7 ][SiO 4 ](OH) 2 . IR absorption bands typical for the initial α‐C 2 SH phase diminish over the course of thermal treatment, accompanied by the appearance of bands characteristic for dellaite. The partial polymerization of the isolated SiO 3 (OH) tetrahedra present in α‐C 2 SH to [Si 2 O 7 ] dimers, as expected for dellaite, was proven by the presence of an IR band at 1050 cm −1 and by TMSM. For the first time, dellaite, along with x ‐Ca 2 SiO 4 , was observed as an intermediate product of the dehydration of α‐C 2 SH. Until now, dellaite was believed to be only a high‐pressure phase. Dellaite coexists with x ‐Ca 2 SiO 4 up to 560°–620°C and decomposes to x ‐Ca 2 SiO 4 at higher temperatures. Upon further heating, the latter phase transforms into α′ l ‐Ca 2 SiO 4 , and finally gives β‐Ca 2 SiO 4 after cooling to room temperature. The thermal decomposition of α‐C 2 SH is a much more complicated process than reported previously. Different factors, such as the source of Ca and the presence of NaOH during synthesis, seem to control the mechanism of its dehydration. The presence of dellaite is shown to lower the temperature of formation of β‐Ca 2 SiO 4 .