Effects of additives on properties of soluble gypsum core filled with kaolin

The effect of kaolin filler's addition on the properties of gypsum mixture was investigated, and the appropriate ratio of gypsum to kaolin was determined. MgSO ₄ and NaCl was added in the gypsum mixture respectively to improve the flexural strength and collapsibility of the core after roasting, and their effects on the property of gypsum core were compared. The effect of Al(NO ₃) ₃'s decomposition behavior on the dimension variation of gypsum core during roasting was also investigated. The fracture surface morphology of the samples after roasting with different composition was observed by SEM, and the effects of additives on the microstructure and macroscopic property of core were analyzed accordingly. Experimental results show that, the strength and dimension variation of gypsum mixture is reduced by the adding of kaolin filler. When the content of kaolin was 50%, the strength and dimension variation of gypsum mixture have a good balance. The flexural strength and collapsibility of the core after roasting can be improved by adding MgSO ₄ and NaCl. It is also discovered that the flexural strength and collapsibility of core are better with the additive of MgSO ₄, and the dimension variation property of core is better with the additive of NaCl. Moreover, the absolute value of dimension variation of the core during roasting can be effectively decreased by the adding of 9% Al(NO ₃) ₃.