Thermal Transformation of Illitic-Chlorite Clay and Its Pozzolanic Activity

Abstract

Illite-chlorite clay from quarry located at Buenos Aires Province (Argentine) was characterized by XRD, FTIR and TG-DTA. Mineralogical transformations during clay firing under oxidizing conditions were studied from 100 to 1100 °C by XRD and FTIR. From select temperatures, calcined clay was ground (85% passing to 45 µm sieve) and the pozzolanic activity of blended cements (25% w/w) was evaluated by the Frattini test and the strength activity index (SAI). Finally, the hydration phase assemblage of blended cements was studied using XRD analysis.

The solid-state phase transformations of clay during thermal treatment involves: water loss at low temperature; the partial dehydroxylation of chlorite resulting in the “modified chlorite structure” between 500 and 600 °C and its collapse at 800 °C; the dehydroxilation of illite is completed and its structure collapsed up to at 900 °C. Up to 1000 °C, the clay minerals are collapsed thoroughly, with formation of amorphous compounds.

Results of Frattini tests indicate that materials present pozzolanic activity after 7 days when they are fired up to 900 °C, however the best SAI (~1.00) at 28 days occurs for clay calcined at 1100 °C. For blended cements, the hydration products assemblage is similar to plain Portland cement used at all ages.