Effect of hydration on the biaxial flexural strength of a glass ionomer cement.

OBJECTIVES: The purpose of this study was to test the hypothesis that hydration during setting enhances the strength of glass ionomer cements.
METHODS: Thirty-four groups of eight disks each were prepared and sealed in nonpermeable bags to age for 30 min to 360 d before biaxial flexure testing. At each time interval, one group was tested immediately, while the second group was immersed in water for an additional 48 h prior to the same testing. Six of each group were used for strength testing and the remaining two were placed in a desiccator for determining evaporative water content with respect to initial weight.
RESULTS: A major portion of the disk strength developed within the first 2 wk and continued to improve at a much slower pace with time. Tukey's multiple range test indicated that additional 48 h immersion caused significant strength reduction for disks aged less than 30 d. The amount of weight loss in the desiccator decreased with aging time. The profiles of biaxial flexure strengths with respect to weight loss are similar for both groups. It infers that additional water storage introduces surface flaws that result in the strength reduction. SIGNIFICANCE: Glass ionomer cement must be kept from direct water contact as long as possible to reach an acceptable level of strength.