Effects of Boehmite on the Calcination Shrinkage and Mechanical Properties of Gypsum-Bonded Molds

Calcination shrinkage is an essential factor in the evaluation of a gypsum-bonded mold. Typically, if traditional fillers are used in a gypsum-bonded mold, it is difficult to maintain sufficient strength due to the high dosages of fillers. Herein, a new filler system of gypsum, boehmite, is proposed to more efficiently compensate for shrinkage, while minimizing the impact on the strength. The effects of the fillers on the calcination shrinkages and mechanical properties of the gypsum-bonded molds are studied. The phase transition and thermal expansion behaviors of the gypsum-bonded molds are tested by thermal gravimetric analysis/differential scanning calorimetry (TG/DSC) and a thermal dilatometer. The microstructures of the fillers and gypsum-bonded molds are characterized by a scanning electron microscope (SEM). The results show that the dosage of boehmite for near-zero shrinkage is reduced to about 35 wt%, with the bending strength of the gypsum-bonded mold remaining above 2 MPa throughout the whole calcination process. The mechanism of calcination expansion is revealed and ultimately attributed to the splitting of boehmite particles. This research provides a novel gypsum-bonded mold for required high dimensional and mechanical properties, elevating the ability to form complex structures.