AN X-RAY INVESTIGATION OF THERMAL DECOMPOSITION OF PORTLANDITE

Thermal decomposition of portlandite, Ca(OH)**2, has been studied in air and in vacuum using x-ray single- crystal techniques. In air, the crystals were decomposed in situ on the goniometer arcs while x- ray reflections were simultaneously recorded. The transformation to CaO was not accompanied by topotaxy; this is attributed to the high nucleation rate of CaO crystallites in air. When Ca(OH)**2 single crystals were decomposed under vacuum (and subsequently exposed to x-rays), some orientation of the CaO crystallites occurred.