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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 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 mare than 100 MPa.Reasons were discussed with X-ray diffraction and scanning electron microscopy.