The　carbonation　phenomena　of　the　light　weight　castable　with　alumina　cement　binder　was　investigated．The results　obtained　are　as　follows： 　1）The　reaction　speed　of　carbonation　is　faster　in　the　temperature　at 20 ℃ than　that　of 30 ℃．This　is　because the　carbonation　reaction　of　unstable　CAH10　which　is　mainly　generated　in　light　weight　castable　at 20 ℃ proceeds rapidly． 　2）The　increase　of　humidity　promotes　the　carbonation　reaction．This　is　because　the　carbonation　reaction　of C2AH8 which　has　the　higher　resistance　of　carbonation　than　CAHl10 proceeds　rapidly　in　high　humidity． 　3）　The　reaction　speed　of　carbonation　is　extremely　increased　when　light　weight　castable　is　partially　steeped in　water　and　we　observe　collapse　and　peeling　at　the　surface　of　castable，where　water　is　vaporized　and　alkali is　cncentrated．This　is　because　that　the　concentration　of　alkali　which　is　due　to　the　movement　of　water promote　the　carbonation　reaction． 　Therefore　the　most　effective　factor　of　the　carbonation　reaction　is　the　movement　of　water　in　light　weight castable． 　In　order　to　prevent　the　carbonation　reaction　it　is　important　to　stop　the　movement　of water，and　we　confirm　that the　water-proof-coating　on　the　surface　of　light　weight　castable　can　control　the　carbonation　reaction　of　alumina cement．

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