"Influence of Water Type and Water/Powder Ratio on the Strength and Hardness of Type III and IV Gypsum"

Abstract

Casts with a high surface hardness and abrasion resistance are essential for conventional dental prosthesis construction procedures. The water/powder ratio is a critical factor determining the physical and chemical properties of gypsum casts, and any modification to this ratio may alter these properties. This study evaluated the influence of water type and water-powder ratios on the Brinell hardness and the Compressive strength of type III & IV gypsum casts. Three groups of test samples were prepared using type III and type IV gypsum control group -recommended water/ powder ratio, and tests groups; 20% and 30% increase in water/powder ratio. The prepared samples were subjected to Compressive strength test and Brinell hardness test using the universal material testing machine. Two-way ANOVA and Tukey's test were used to analyse the data (α=5%). A significant reduction in Compressive strength and Brinell hardness was found in samples prepared with increased water-powder ratios using both the tap water and distilled water. Tap water samples showed significantly lower Compressive strength and Brinell hardness values than the distilled water samples. Chipping was observed in both type III and type IV gypsum samples prepared with an increase in the water powder ratio. Type III gypsum samples exhibited single fracture lines with the recommended water powder ratio whereas type IV gypsum samples had composite fractures. Distilled water samples exhibited higher Compressive strength and Brinell hardness values. Type III and IV gypsum samples prepared with increased water-powder ratio showed statistically significant reduction in the Compressive strength and Brinell hardness values.