| Summary: | Bismuth oxide (monoclinic α-Bi<sub>2</sub>O<sub>3</sub>) and zirconium oxide (monoclinic ZrO<sub>2</sub>) are the most popular radiopacifiers in commercial Portland cement-based endodontic restoratives, yet their effects on the setting and hydration reactions are not fully understood. This study compares the impact of 20 wt.% of Bi<sub>2</sub>O<sub>3</sub> or ZrO<sub>2</sub> on the early hydration reactions and C−S−H gel structure of white Portland cement (WPC). Cement paste samples were hydrated at 37.5 °C prior to analysis by <sup>29</sup>Si and <sup>27</sup>Al magic angle spinning nuclear magnetic resonance spectroscopy at 3 h and 24 h, and transmission electron microscopy at 3 h. Initial and final setting times were determined using a Vicat apparatus and reaction kinetics were monitored by isothermal conduction calorimetry. Bi<sub>2</sub>O<sub>3</sub> was found to prolong initial and final setting times and retard the degree of hydration by 32% at 24 h. Heat evolution during the acceleration and deceleration phases of the hydration process was reduced and the exotherm arising from renewed ettringite formation was delayed and diminished in the presence of Bi<sub>2</sub>O<sub>3</sub>. Conversely, ZrO<sub>2</sub> had no significant impact on either setting time; although, it accelerated hydration by 23% within 24 h. Increases in the mean silicate chain length and the extent of aluminum substitution in the C−S−H gel were observed in the presence of both radiopacifying agents after 24 h relative to those of the unblended WPC. The Bi<sub>2</sub>O<sub>3</sub> and ZrO<sub>2</sub> particles remained intact within the cement matrix and neither bismuth nor zirconium was chemically incorporated in the hydration products.
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