Reuse of UK alum water treatment sludge in cement-based materials

Alum salts are commonly used as coagulants in the purification of surface water for potable supplies. The resultant waste alumina-rich sludge is currently landfilled in the UK. This study aimed to valorise sludge by using heat-treated alum sludge as an additive in cement. Alum sludge was calcined at 475–1100 °C and then characterized to correlate physical and mineralogical changes with cementitious activity and engineering performance. Alum sludge calcined at 825 °C transforms to poorly crystalline η-alumina (eta) and has cementitious activity. The calcined sludge rapidly reacts with gypsum to form ettringite which leads to a shortening of the induction period and onset of alite hydration. Gypsum depletion leads to undersulfated C3A hydration which consumes ettringite to form monosulfoaluminate thereby inhibiting further alite hydration. The poorly crystalline η-alumina is metastable, transforming to highly crystalline α-alumina at 1100 °C. In pastes containing more crystalline sludges, alite hydration is enhanced because the undersulfated C3A reactions are avoided, leading to improved performance.