The effects of H3PO4 on the compressive strength of block CaO, phase composition and morphology of calcined carbide slag were investigated. The results indicated that block CaO could be prepared from carbide slag through molding and calcination. H3PO4 had a promotive effect on the compressive strength of block CaO, which reached the maximum value at 32 MPa with a H3PO4 content of 4%. Meanwhile, the compressive strength of block CaO increased with an increase of molding pressure. Moreover, the P-containing phase in carbide slag shifted from hydro-phosphate to phosphate after calcination. The close connection on the surface of particles was enhanced by the formation of phosphate. (C) 2014 Elsevier B.V. All rights reserved.