A cordierite refractory castable was developed using the MgO-NH 4 H 2 PO 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 2 O 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 2 O) was identified at room temperature; between 110 and 750 o C, the present phosphates were amorphous to X-ray diffraction (XRD). At 1100 o C, magnesium orthophosphate (Mg 3 (PO 4 ) 2 ) and aluminum orthophosphate (AlPO 4 ) were present. At 1350 o 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.