First Observation of α‐Ca2[SiO3(OH)](OH)–Ca6[Si2O7][SiO4](OH)2 Phase Transformation upon Thermal Treatment in Air

Abstract

Thermal treatment transforms α‐Ca2[SiO3(OH)](OH) (α‐C2SH) into hydraulic Ca2SiO4 polymorphs, which is a process of considerable interest to the cement chemistry community. The thermal behavior of differently synthesized samples of α‐C2SH 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‐C2SH samples heated to different temperatures between 300° and 500°C suggest coexistance of α‐C2SH and dellaite, Ca6[Si2O7][SiO4](OH)2. IR absorption bands typical for the initial α‐C2SH phase diminish over the course of thermal treatment, accompanied by the appearance of bands characteristic for dellaite. The partial polymerization of the isolated SiO3(OH) tetrahedra present in α‐C2SH to [Si2O7] 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‐Ca2SiO4, was observed as an intermediate product of the dehydration of α‐C2SH. Until now, dellaite was believed to be only a high‐pressure phase. Dellaite coexists with x‐Ca2SiO4 up to 560°–620°C and decomposes to x‐Ca2SiO4 at higher temperatures. Upon further heating, the latter phase transforms into α′l‐Ca2SiO4, and finally gives β‐Ca2SiO4 after cooling to room temperature. The thermal decomposition of α‐C2SH 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‐Ca2SiO4.