Synthesis of a mineral trioxide aggregate by spray-pyrolysis

A mineral trioxide aggregate (MTA) was newly synthesized by spray-pyrolysis following a sol–gel reaction. The precursor solution of MTA was prepared by condensation following a hydrolysis reaction between Ca(NO3)2·4H2O, Si(OC2H5)4, Al(NO3)3·9H2O, and (NH4)2SO4 under nitric acid. The MTA powder was then synthesized with the precursor solution at 1500 °C at a carrier gas flow rate of 10 L/min after producing droplets by ultrasonic spray generator. The particle shape was spherical, with little clusters of smaller particles on the large particles, and their average size was about 0.8 μm. The phase analysis of the MTA powder by X-ray diffraction showed that it was composed of Ca3SiO5, Ca2SiO4, and Ca3Al2O6 with a small amount of CaO and amorphous phases. The electron probe microanalysis showed that Ca, Si, Al, and S were distributed homogeneously within the particles. The final setting time measured by modified Vicat under 95% relative humidity was about 14 min. The early stage of setting was mainly led by the rapid formation of fiber-like calcium–silicate–hydrates (C–S–H) gels and Ca(OH)2 crystals due to the rapid dissolution of calcium silicates. The late stage of setting was dominated by the formation of large columnar-structured Ca(OH)2 crystals and Ca3Al2O6·6H2O crystals with the continuous formation of fiber-like C–S–H gels. The compressive strengths were about 10 MPa after 1 h and 68 MPa after 7 d of setting. These results showed that an MTA powder with a high compressive strength and fast setting rate could be synthesized by spray-pyrolysis following a sol–gel reaction.