Microstructure and pozzolanic properties of water treatment sludge calcined for 3 h at 600-900 degrees C were investigated by XRD, FTIR, SEM, and TGA techniques and Chapelle test. Calcination of alum sludge was accompanied with sequence of chemical reactions including; dehydration of colloidal Al(OH)3, formation of Al2O3, and crystallization of amorphous silica. Calcination conditions greatly affect the microstructure and pozzolanic activity of calcined alum sludge. Under the hydration at high temperature of calcined alum sludge with lime, boehmite (AlOOH), and amorphous silica react with lime forming amorphous calcium aluminate hydrates and calcium silicate hydrates. The amount and density of hydrates increased with calcination temperature up to 800 degrees C then decreased after that. Calcination of alum sludge at 800 degrees C is enough to improve its pozzolanic properties. (C) 2014 Elsevier Ltd. All rights reserved.