Thermal treatment transforms alpha-Ca-2[SiO3(OH)](OH) (alpha-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 alpha-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 alpha-C2SH samples heated to different temperatures between 300 degrees and 500 degrees C suggest coexistance of alpha-C2SH and dellaite, Ca-6[Si2O7][SiO4](OH)(2). IR absorption bands typical for the initial alpha-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 alpha-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 alpha-C2SH. Until now, dellaite was believed to be only a high-pressure phase. Dellaite coexists with x-Ca2SiO4 up to 560 degrees-620 degrees C and decomposes to x-Ca2SiO4 at higher temperatures. Upon further heating, the latter phase transforms into alpha'L-Ca2SiO4, and finally gives beta-Ca2SiO4 after cooling to room temperature. The thermal decomposition of alpha-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 beta-Ca2SiO4.