Abstract: The rupture strength and surface hardness of set gypsum was studied by doping in additive agents as calcium hydroxide、melamine、polyvinyl alcohol、reactant addition agent. The dose of each additive reagent was investigated. The rupture strength was slightly reduced while surface hardness was increased during the increasing dose of calcium hydroxide varied from 0 to 10%. The rupture strength was the highest when the dose of melamine was 2%. The surface hardness was best when the dose of melamine was 4%. The appropriate dose of polyvinyl alcohol was 0.8%. It was demonstrated that the trend of rupture strength and surface hardness of set gypsum was opposite when the dose of reactant addition agent increased from 3%. The SEM shows that the morphology and arrangement of the dihydrate gypsum crystal have been apparently changed by the influence of additive agents. The shape of the micro-crystal was almost needle-like and cylindrical. The rigid net structure has been formed by crossed micro-crystal and the arrangement of crystal has been in order. The gas-pocket structure has been dispersed homogeneously. Accordingly, high rupture strength and surface hardness of the set gypsum have been obtained.

Key words: set gypsum%rupture strength%surface hardness