In　oeder　to　develop　high　performance　basic　dam　block　for　tundish　vessel　in　steel－making　process， influences　of　raw　materials，binders　and　additives　on　hydration　and　thermal　characteristics　of magnesia　castable　refractories　were　studied．Crack　initiation　and propagation　in　cement　bonded magnesia　castables　due　to　slaking　of　magnesia　clinker　at drying　process　were　suppressed　by　using spinel　clinker　instead　of　magnesia　powder．In　case　of　Na20・2CaO・P2O5（NC2P） bonded　castable，crack　initiation　due　to　slaking　of　magnesia　clinker　did　not　occur　but　bending　strength　at　high temperature　was　low． Mechanical　properties　of　NC2P　bonded　castable　refractories　at　high temperature　were　improved　by　using　magnesia　clinker　that　contains　low　SiO2 contents．

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