Rehydroxylation of calcined swellable clay minerals at ambient conditions

Abstract

Thermal treatment of clay minerals is used to produce building materials like supplementary cementitious materials (SCM). New, environmentally friendly binders, that are alkali activated materials (AAM) or geopolymers can be produced with calcined clay minerals, too. Four typical reactions occur during thermal treatment of clay minerals, namely dehydration, dehydroxylation, breakdown of structure, and recrystallization of high temperature phases. Heating to temperatures below the breakdown of structure makes rehydration and rehydroxylation of the clay minerals possible, ass observed in ancient bricks and pottery. As swellable clay minerals or mixtures of different clay minerals come into focus as precursors for geopolymers or as SCM, it is important to pay attention to rehydroxylation in this context as well. This work studied the rehydration and rehydroxylation of three different homoionic (Na+, Ca2+, Mg2+) exchanged trans- and cis-vacant dioctahedral smectites at equal relative humidity (75% r.H.) at different temperatures (22, 40, and 60 °C) after calcination at 700 °C. Simultaneous thermal analysis was used to determine the amount of H2O uptake (after 4, 24, 48, 72, and 168 h) due to rehydroxylation. Rehydroxylation between 8.7% for a Na+ exchanged sample and up to 37.3% for a predominantly Mg2+ exchanged sample took place within 7 d. The process of rehydroxylation could be further verified by X-ray diffraction, these measurements should be part of future studies. The α-free isoconvertional approach resulted in Ea of rehydroxylation in the range 32–90 kJ/mol for Ca2+ exchanged samples.