Calcium Silicate/Aluminate Cements: Phase Content Versus Fracture Toughness

Objectives: In dentistry, calcium-based cements for direct-, indirect-pulp capping and endodontic applications span over a wide range in the Tri-Calcium Aluminate (C3A) and Tri-Calcium Silicate (C3S) (C3A:C3S) compositional ratio. Here we aim to correlate relevant C3A:C3S phase ratios to their development in structural resistance (fracture toughness) over 1-month of hydration.
Methods: By means of solid-state reactions, crystalline Ca₃SiO₅ (C3S) and Ca₃Al₂O₆ (C3A) were synthesized, milled and partitioned into compositions to form mixtures in ratios C3A:C3S of 3:1, 1:1 and 1:10 containing fixed 10wt.% ZrO₂ and 25wt.% Calcite. Comparable paste viscosities were adjusted by addition of citric acid to water in concentrations of 1.4wt.%, 0.9wt.% and 0.4wt.%, respectively, for a water-to-cement ratio of 0.45. Heat flow calorimetry and In-situ X-Ray Diffraction were used to characterize the hydration kinetics and phase contents. The development of mechanical stability over 1-month hydration period was measured using a newly-adapted Chevron-Notch-Beam fracture toughness method using a silicone template, allowing demolding of specimens already at 30min from mixing. Specimens demolded at different time periods were tested in 3-point bending using a 20N highly-sensitivity load-cell under laser displacement tracking.
Results: The development rate of toughness was shown to increase with increase in C3A fraction, with 1/3 of the final toughness reached at ~1h, ~10h and ~48h for C3A:C3S compositional ratios of 3:1, 1:1 and 1:10, respectively, with final toughness at 1-month reaching 0.15 ± 0.01MPa√m for both 3:1 and 1:1 mixtures and a significantly lower value of 0.10 ± 0.01MPa√m for the 1:10 mixture. The time-evolution of toughness followed a power-law type-function that correlated well to the time-evolution of calcium aluminate carbonate hydrate phases.
Conclusions: The hydration rate of Tri-Calcium Aluminate/Silicate cements can be adjusted by changing the C3A:C3S-ratio, with C3A accelerating the hydration process and the development of toughness. The presence of C3A seems to increase the final mechanical properties of dental calcium-based cements.