Moessbauer Study of the Effect of Calcium Content on Iron Oxide Transformations in Fired Clays

Abstract

Sixteen clays were studied by Moessbauer measurements in their unfired and fired forms. Thirteen of these clays were collected near ancient Greek sites and the remaining three in England and the United States. They were fired at 700°, 900°, and 1080°C to investigate the physicochemical transformations of the clay components in this temperature range. The results show distinctly different responses of the clays, according to their CaO content. Firing of the calcareous clays (>5 % CaO content) at 700°C produces small oxide particles (mainly hematite). This phase remains stable on further firing up to 1080°C, where it decreases slightly. Contrary to this behavior, the noncalcareous clays display a continuous increase of the amount and size of the oxide particles when fired up to 1080°C. This difference is attributed to stable calcium aluminosilicate phases which are formed in the calcareous clays as opposed to the continuous vitrification which occurs for the noncalcareous clays by firing up to 1080°C. The variation of the quadrupole splitting of the central doublet with firing temperature also differs for the two types of clays. The quadrupole splitting values among the clays span a range of 1.1 to 1.6 mm/s for a firing temperature of 700°C. Above this temperature, a distinct separation of the two groups occurs with the quadrupole splitting of calcareous clays lying higher.