A basic alumina-spinel low cement castables (castables A) and another castables (castables B) with 5% addition of sodium hexametaphosphate were prepared and heat treated at 110 ℃, 900 ℃ and 1 400 ℃. It is shown that after heat treating at 110 ℃, cold crushing strength (CCS) of castables B is more than 3 times of castables A and apparent porosity (AP) is less than half of castables A. The presence of Mg2P2O7 and AlPO4?2H2O is the main difference. At 800-1 000 ℃ that hydraulic bond reverses to dehydrate condition and castables A becomes weak with high porosity, castables B shows a CCS more than 4 times of castables A. Needles of magnesium phosphate are responsible for reinforcing microstructure of castables B at 900 ℃. After firing at 1 400 ℃, castables B shows extra ordinary CCS of more than 100 MPa. Reasons were discussed with X ray diffraction and scanning electron microscopy.