Preparation of block CaO from carbide slag and its compressive strength improved by H3PO4

• H3PO4 improved compressive strength of block CaO from carbide slag calcination.
• Compressive strength of block CaO increased with increasing in molding pressure.
• Compressive strength of block CaO was better as the H3PO4 content is about 4%.
• Phosphate from carbide slag reacted with H3PO4 improved strength of block CaO.

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 3.2 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.