Magnesia–ammonium phosphate-bonded cordierite refractory castables: Phase evolution on heating and mechanical properties

Abstract

A cordierite refractory castable was developed using the MgO-NH 4H 2PO 4 reaction. This castable was made with cordierite–mullite aggregates from scrap refractory material and a cement paste based on magnesia, calcined alumina, silica fume, and ammonium dihydrogen phosphate, which forms cordierite (2MgO·2Al 2O 3·5SiO 2) during heating at high temperature. The mix with water was cast into steel molds; the cold setting occurs within 30 min. The set castables were thermally treated and the evolution of the phases was observed. Struvite (NH 4·MgPO 4·6H 2O) was identified at room temperature; between 110 and 750 °C, the present phosphates were amorphous to X-ray diffraction (XRD). At 1100 °C, magnesium orthophosphate (Mg 3(PO 4) 2) and aluminum orthophosphate (AlPO 4) were present. At 1350 °C, the main crystalline phases were cordierite and mullite. Cold and hot flexural strength, thermal shock resistance, and physical properties were measured. The properties of magnesia–phosphate-bonded cordierite castables were compared with cordierite material obtained by conventional slip-casting method from aggregates, clay, talc, and calcined alumina.