Salt mixtures with various compositions in the NaBr-KBr-Na2CO3-K2CO3 system were made by the gravity casting process. Their fracture strength, fracture toughness, and dissolution rate in water were examined by bending test, bending test of precracked specimens, and dissolution of the salt mixtures in water, respectively. Salt mixtures with high fracture strength over 15MPa were found in the composition area in the primary carbonate crystal and K+ ion rich region. Such salt mixtures also showed high fracture toughness and only a few surface cracks. In the case of salt mixtures with low fracture strength, many cracks were observed on the surface and fracture toughness did not have a strong correlation with fracture strength. These findings suggest that the surface cracks formed during solidification lead to low fracture strength, and higher saturation solubility limits lead to higher dissolution rate of salt mixture in water. Salt mixtures of the NaBr-KBr-Na2CO3-K2CO3 system are expected to serve as core materials with good expendability as well as high strength, because the composition area showing higher fracture strength has a relatively high saturation solubility limit.
