# Hydration Characterization of Two Generations of MTA-Based Root Canal Sealers
## 4 Discussion
As described by the manufacturers, both MTA-based sealers are composed of calcium silicate (C\({}_{2}\)S and C\({}_{3}\)S), salicate resin, and bismuth oxide [10,14], while Adseal is composed of epoxy resin, calcium phosphate, calcium oxide zirconium oxide, bismuth subcarbonate, and ethylene glycol salicate [13]. The content of salicate resins seemed to affect the setting time. After the mixing of the base and catalyst paste, MTA reacts with salicating forming anionic polymer [43]. The raw FTIR spectra of MTA-Fillapex showed low intense salicate bands that may be responsible for its prolonged setting times. In general, MTA is a hydrophilic material, composed of tri- and di- calcium silicate [Alite (C\({}_{3}\)S) and belite (C\({}_{2}\)S)] that react with water to form calcium silicate hydrate (CSH) as a polymerized (cured) calcium silicate phase [45]. At the end of the setting reaction, the curing stage of MTA depends on the formation of CSH that is responsible for the material hardening and strength [6]. The presence of a high amount of unreacted alite and/or belite indicates the incomplete setting of the material. FTIR spectroscopy can offer reliable information to evaluate the chemical structures [48,49]. FTIR analysis explains the process of hydration reaction until set, which was confirmed by XRD analysis. In the current study, FTIR analysis revealed similar compositions of the raw pastes of both MTA-based sealers. Their spectra showed bands of salicylate resin, calcium hydroxide and/or carbonate, calcium silicate (C\({}_{3}\)S, C\({}_{2}\)S), phosphate (vPO\({}_{4}\)), and tricalcium aluminate (Figure 1). The finding related to MTA-Fillapex is similar to that in previous studies [13, 50]. The FTIR spectra of its raw pastes determined the low intense C-O bond (\(\approx\)1210 cm\({}^{-1}\)) of salicylate (catalyste) that remained prominent in spectra after three months. It seems to not be involved in the setting reaction and results in a prolonged setting time. Regarding MTA-Bioseal, there was no more information other than that described by the manufacturer [14]. When the MTA-sealer is mixed in the presence of a humid environment and during the carbonation process, the hydration reaction is characterized by the formation of Ca(OH)\({}_{2}\) and calcium carbonates (CaCO\({}_{3}\)) as primary by-products [51] that are responsible for its antimicrobial activity and biocompatibility [4, 11]. Calcite is added to the cement for the hydration reaction and improves the mechanical properties of the sealer [52]. The carbonated bands dipped as the hydration reaction progressed, indicating their consumption during the hydration reaction. During the hydration reaction, immediately after the mixing of the pastes of the base and catalyst, the bands of C\({}_{3}\)S (\(\approx\)850 and 590 cm\({}^{-1}\)) or C\({}_{2}\)S (\(\approx\)470 cm\({}^{-1}\)) were dipped/humped and were replaced by the band of polymerized calcium silicate and the formation of a CSH phase at 440 cm\({}^{-1}\)[22, 26]. In addition, as the hydration reaction progressed, the unhydrated calcium silicate (v\({}_{3}\)SiO\({}_{4}\)) band \(\approx\) 940 cm\({}^{-1}\) was shifted to a higher wave number \(\approx\)950 cm\({}^{-1}\) (Figure 1) [22, 51] synchronized with a dip/hump of v\({}_{3}\)SiO\({}_{4}\) and v\({}_{2}\)SiO\({}_{4}\) (Alite) \(\approx\) 522 and 450 cm\({}^{-1}\), respectively, and the prominent of a sharp band at 1083 cm\({}^{-1}\). This is indicative of silica polymerization and growth of CSH as a final cured by-product when the MTA-based sealer undergoes complete setting [22, 26]. Concerning the FTIR spectra of MTA-Bioseal, no further changes were detected after 24 h, which confirmed that the setting time did not exceed 24 h. In the spectra of MTA-Fillapex, the bands related to CSH \(\approx\) 950, 795, and 440 cm\({}^{-1}\) failed to be detected up to three months, indicating its incomplete setting.