This study investigated the effects of phosphate on the hydration characteristics of three eco-cement clinkers made utilizing water purification sludge ash, sewage sludge ash and industry sludge ash. Analytical results demonstrate that the eco-cement A (ECO-A) pastes had a similar setting times, final setting times, compressive strengths and degree of hydration as ordinary Portland cement (OPC) pastes. Analytical results also show no damage to the hydration existed during the clinkerization process when adding up to 20% sludge. Increasing the P2O5 content in the investigated clinker resulted in the formation of alpha-C2S. Compressive strength, degree of hydration and delay in setting time observed in the ECO-B and ECO-C pastes may be attributed to large amounts of alpha-C2S. When the amount of phosphate in ECO-C exceeded 0.46%, the amount of C3S in the clinker decreased, setting time increased and the strength of the eco-cement decreased. (C) 2009 Elsevier B.V. All rights reserved.