In-Situ DRIFT Spectroscopic Investigation on the Chemical Evolution of Zinc Phosphate Acid−Base Cement

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy has been used to follow the chemical evolution of zinc phosphate acid−base cement (ZPC), which has been prepared from unmodified, as well as from aluminum- and zinc-modified orthophosphoric acid. For the first time, amorphous dizinc cyclotetraphosphate octahydrate, Zn 2 P 4 O 12 ·8H 2 O, although hydrolytically instable, has been observed as precursor phase prior to crystallization of α-hopeite, α-Zn 3 (PO 4 ) 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 α-hopeite, due to matrix crystallization. Setting of modified ZPC is strongly affected by the presence of initially soluble aluminophosphate complexes, which form a hydrogel upon neutralization. Steric hindrance plays the key role in the sluggish crystallization 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 Zn 2 P 4 O 12 ·8H 2 O precursor phase to form is consistent with the Ostwald step rule of successive reactions.