Composition and expansion of rapid-burnout gypsum-bonded investment

Abstract

This study investigated the expansion behavior of the rapid burnout and conventional types gypsum-bonded investments. Composition, setting expansion and thermal expansion of gypsum-bonded investment of 32 products were examined. The content of the refractory material ranged from 49.3〜78.5wt%: silica was the main component except in one product mainly containing alumina. The calcium sulfate hemihydrate concentration estimated to range from 19.9〜43.0wt%. The setting expansion at 120 minutes was 0.19〜2.29%, and the rapid burnout type investments tended to expand earlier and greater than the conventional ones. The rapid burnout type of investment showed a three-step expansion below 100℃, around 300 and at 550℃; and these expansions were respectively defined setting expansion, transformation of cristobalite and that of quartz.