The dental material, mineral trioxide aggregate (MTA) is a modified Portland cement (PC) with added bismuth oxide (Bi2O3) radiopacifying powder. The reported biocompatibility of MTA has led to its widespread use as a root filling material. However, MTA has a long initial setting time of 2 h 45 min (M. Torabinejad, C. U. Hong and F. McDonald: J. Endod., 1995, 21, 349–352), which is undesirable for its application, for both patient and dentist. This study showed that the initial setting time can be decreased to several minutes by adding 30 wt-% plaster of Paris (PoP) and also investigated the long term effects of PoP additions on the compressive strength (CS), porosity and the solubility (in terms of calcium ion release) of an MTA-like PC system. Additions of 30 wt-% to MTA-like cements led to an almost complete loss of CS after 12 months of storage in an aqueous environment. This could be explained first by PoP interfering with the PC setting reaction and second by the appearance of a gypsum structure within the cement. As excess PoP was no longer consumed in the setting reaction, it is likely to have formed a more soluble gypsum structure, which was then dissolved, causing the porosity to more than double from 25 wt-% at 10 days to 56 wt-% after 12 months. PoP addition should therefore be restricted to ∼10 wt-%, which caused no changes in strength or porosity, the latter used as an indicator of cement sealing ability.
