Impact of Bi₂O₃ and ZrO₂ Radiopacifiers on the Early Hydration and C–S–H Gel Structure of White Portland Cement

• Main Authors: Qiu Li, Nichola J. Coleman
• Format: Article
• Language: English
• Published: MDPI AG 2019-10-01
• Series: Journal of Functional Biomaterials
• Subjects: calcium silicate cement; endodontic bioceramic; bi₂o₃; zro₂; cement hydration; c–s–h gel; magic angle spinning nuclear magnetic resonance spectroscopy; calorimetry; filler effect
• Online Access: https://www.mdpi.com/2079-4983/10/4/46

Bismuth oxide (monoclinic α-Bi₂O₃) and zirconium oxide (monoclinic ZrO₂) 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 Bi₂O₃ or ZrO₂ 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 ²⁹Si and ²⁷Al 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. Bi₂O₃ 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 Bi₂O₃. Conversely, ZrO₂ 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 Bi₂O₃ and ZrO₂ particles remained intact within the cement matrix and neither bismuth nor zirconium was chemically incorporated in the hydration products.