Pozzolanic potential of the calcined clay-lime system

Seven reference clays, 4 kaolinitic and 3 smectitic clays, were purified and thermally treated in order to estimate their pozzolanic potential in cementitious materials. The clays were calcined in a fixed-bed electrical furnace at temperatures ranging between 500°C and 900°C. Both raw and calcined clays were characterized by XRD, XRF and BET techniques. Their pozzolanic activity was evaluated using thermogravimetry (TGA) on pastes after 3, 7, 14 and 28 and 56 days. The results indicate that all kaolinitic clays are highly active at a broad range of firing temperatures (500-900°C), with no clear differences in activity. The crystallinity of the raw clay only influences the short-term activity of the calcined clay-lime reaction. The smectitic clays possess a clear optimal firing temperature at 800°C. Ca-rich smectites are proven to be somewhat more reactive than Na-rich smectite and hectorite, however, even at 800°C, it's activity is only mediocre compared to kaolinite.