In-situ DRIFT spectroscopic investigation on the chemical evolution of zinc phosphate acid-base cement

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy has bee n used to follow the chemical evolution of zinc phosphate acid-base cement (ZPC), which has been prepared from unmodified, as well as from aluminum- a nd zinc-modified orthophosphoric acid. For the first time, amorphous dizinc cyclotetraphosphate octahydrate, Zn2P4O12. 8H(2)O, although hydrolytically instable, has been observed as precursor phase prior to crystallization of alpha-hopeite, alpha-Zn-3(PO4)(2). 4H(2)O, in both systems. Within minutes from onset of mixing the infrared spectrum of unmodified ZPC is dominated by the characteristic vibrations of alpha-hopeite, due to matrix crystalliz ation. Setting of modified ZPC is strongly affected by the presence of init ially soluble aluminophosphate complexes, which form a hydrogel upon neutra lization. Steric hindrance plays the key role in the sluggish crystallizati on kinetics of modified ZPC. The high compressive strength of modified ZPC is brought about by a condensed anion structure. Conceptionally, setting of ZPC consists of initial Lewis acid-base reactions and subsequent stages of hydration. In this work, it is shown that the tendency for the Zn2P4O12. 8 H(2)O precursor phase to form is consistent with the Ostwald step rule of s uccessive reactions.