Development of Highly Reactive Metakaolin from Paper Sludge

The paper industry in Western Europe generates around 6 million tons/year of sludges, which contain about 60% dry matter mainly composed of cellulose fibers, kaolinite, and calcite. The present study deals with an original way of utilizing such wastes: the production of metakaolin by calcining paper sludge in the temperature range of 700°C to 800°C. After calcination, pastes containing 50% calcium hydroxide and 50% burnt sludge were hydrated and the lime consumption investigated by differential thermal analysis. The results show that a very reactive pozzolan is produced by calcining paper sludge at 700°C or 750°C for 2 or 5 hours. Despite a smaller kaolinite content, the burnt paper sludge exhibits more pozzolanic activity than commercially available metakaolins, especially at early ages. Thermodesorption analyses show that this higher activity is due to the presence of superficial defects that occur during the sludge calcination.