Analysis of MTA\u27s Setting Reaction: A Differential Scaning Calorimetry Study

Mineral Trioxide Aggregate, a common root-end filling material, was investigated using differential scanning calorimetry (DSC) to identify and characterize the formation of hydration products when the powder is mixed with water. Portland cement was used as a comparative material as it is the active component of MTA. Unmixed powders of both cements and individual components such as bismuth oxide, gypsum, and calcium hydroxide powders were also studied using DSC. The mixed pastes of MTA and Portland cement were studied at- immediate (0 hr), 2 hr, 4 hr, 12 hr, 24 hr, I week, 1 month, 3 months, and 1 year intervals after preparation. Except for the immediate sample, all samples were stored at 37°C and a relative humidity of 100%. The change in weight due to storage conditions and after DSC analysis was also studied. DSC thermogram peaks were identified and integrated to yield enthalpy to serve as an indicator of reaction products formed over time. The results showed water, calcium silicate hydrate, gypsum, and calcium hydroxide endothermic peaks in the thermograms obtained from the DSC analysis of MTA at various time intervals. Peaks at 90-110°C, 110-130°C, and 450-550°C were attributed to water, gypsum, and calcium hydroxide decomposition/dehydration, respectively. The size and the location of these peaks changed depending on the amount of phase present at that point of time. Mapping of the calcium hydroxide hydration product helped to study the hydration behavior and the progression of the reaction over time. Overall, similar endothermic peaks were seen in Portland cement pastes, but the size of the calcium hydroxide peak was generally smaller compared to MTA, although a large ettringite peak and small additional peaks were observed. The endothermic peaks from the various powders were helpful in corroborating the peaks formed in mixed pastes and also in differentiating the peaks formed due to hydration. There was initial loss of weight due to evaporation of loosely bound water in storage, followed by weight gain due to uptake of water by the cement paste in both MTA and Portland cement. The weight loss resulting from DSC due to water evaporation decreased as the water became more chemically bound as the time advanced. While the Portland cement samples showed a gradual decrease in weight loss, MTA showed an initial sharp decline in the first few hours followed by a small gain at three months. Based on the findings of this study, the formation of calcium hydroxide in mixed MTA increased till one month after which a decrease was seen, which may be due to a carbonation reaction. MTA\u27s hydration reaction yielded greater amounts of calcium hydroxide when compared to Portland cement