Optical characterization and x-ray diffraction studies of synthetic Plaster of Paris

Abstract

Gypsum samples were produced by reaction of dilute hydrochloric acid with hydrated lime and the resulting calcium chloride solution was reacted with dilute sulphuric acid. Calcinations were done at a constant o temperature of 120 C for a period of four hours. The choice of the production parameters was guided by the findings of previous studies. Optical absorption spectra of the samples were measured with respect to a reference standard (imported) plaster sample. This helped to identify the material sample by determining absorption maxima. Four characteristic absorption peaks were notable in the spectrum for the synthetic sample. These occurred, in descending order of intensity, at 1149.7 Å, 3405.3 Å, 1631.1 Å and 2136.4 Å respectively. Similar peaks feature in the spectrum for the reference plaster, though with small shifts in wavelength. X-ray diffraction analysis and x-ray fluorescent spectrometry were carried out on the samples to determine crystal structures and lattice parameters, as well as the elemental  composition of the synthetic sample for impurity level assessment. The XRD analysis indicated the  presence of hannebachite, gypsum and predominant calcium sulphate hemihydrate in the synthetic plaster. Its crystal structure was revealed as orthorhombic with lattice parameters a, b, c as 9.82 Å, 10.67 Å and 6.50 Å respectively. The structure found in the synthetic plaster was essentially similar to those in the reference sample. The level of possible impurity in the synthetic sample was established as minimal, since XRF analysis did not reveal the presence of elements with atomic mass higher than calcium.

Keywords:  Optical Characterization, X-ray Diffraction, Synthetic Plaster of Paris.