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One α-tricalcium phosphate (α-TCP) powder was either calcined at 500°C to obtain fully crystalline α-TCP or milled for different durations to obtain α-TCP powders containing various amounts of X-ray amorphous tricalcium phosphate (ATCP). These powders containing between 0 and 71wt.% ATCP and up to 2.0±0.1wt.% β-TCP as minor phase were then hydrated in 0.1M Na2HPO4 aqueous solution and the resulting heat flows were measured by isothermal calorimetry. Additionally, the evolution of the phase composition during hydration was determined by in situ XRD combined with the G-factor method, an external standard method which facilitates the indirect quantification of amorphous phases. Maximum ATCP hydration was reached after about 1h, while that of crystalline α-TCP hydration occurred between 4 and 11h, depending on the ATCP content. An enthalpy of formation of -4065±6kJ/mol (T=23°C) was calculated for ATCP (Ca3(PO4)2), while for crystalline α-TCP (α-Ca3(PO4)2) a value of -4113±6kJ/mol (T=23°C) was determined., (Copyright © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.)

Abstract: This study was performed to understand the influence of a critical amount of CA on the kinetics of C3S hydration. For this purpose, monoclinic C3S was blended with 15 wt.% CA and investigated by heat flow calorimetry and in situ XRD at 23°C at a water to cement ratio of 0.5. The binary mixture shows 3 distinct heat flow maxima where the underlying C3S dissolution is proceeding stepwise. The C3S dissolution rates during the 3 steps are varying strongly, depending on the hydrate phase precipitated during the respective reaction step. Comparison of these dissolution rates with a pure C3S reference sample allows the conclusion that the dissolution rate of pure C3S after the heat flow maximum might be governed by either the remaining available reactive surface of C3S or a diffusion‐controlled process, which would both be influenced by the respective hydrate phase precipitating on the C3S surface. [ABSTRACT FROM AUTHOR]

As it was recently shown that β-tricalcium phosphate (β-TCP), similar to α-TCP, can be mechanically activated by prolonged ball milling, partially amorphized β-TCP is promising for the development of new bone cement formulations. Hence in the present study the hydration of partially amorphized β-TCP with different contents of an amorphous phase (amorphous tricalcium phosphate, ATCP) was investigated by isothermal calorimetry and quantitative X-ray diffraction (XRD) to obtain the hydration enthalpies of β-TCP and ATCP. Measurements were conducted at 23 °C, a 0.1 M Na2HPO4 aqueous solution was used as mixing liquid. For the hydration enthalpy of crystalline β-TCP, a value of ΔHR(β-TCP → CDHA) = -23.9 kJ/molTCP can be calculated from literature data. In this study ΔHR(ATCP → CDHA) = -83 ± 6 kJ/molTCP was determined from new measurement results for the hydration of amorphized β-TCP to calcium-deficient hydroxyapatite (CDHA). Hence the hydration enthalpy was shown to be increased by a factor of 3.5 due to amorphization. Additionally, the hydration enthalpy of ATCP resulting from amorphization of β-TCP was compared and demonstrated to be not significantly different to that of ATCP resulting from α-TCP amorphization, which was -78 ± 2 kJ/molTCP. This indicates that amorphization of α-TCP and β-TCP results in an amorphous phase with the same properties. [ABSTRACT FROM AUTHOR]

Abstract: XRD measurements of the hydration of synthetical cement (SyCem) were used to calculate the resulting heat flow from changes in the phase content. Calculations were performed by application of thermodynamic data. The comparison with data recorded from heat flow calorimetry was in good agreement with the calculated heat flow. The initial maximum of heat flow mainly is caused by the aluminate reaction. During the entire main period the silicate reaction dominates hydration with a high and long first maximum of heat flow. The second but less intense heat flow maximum – only visible as a shoulder in most of the technical cements – can be attributed to an acceleration of the aluminate reaction with the enhanced dissolution of C3A and the final formation of ettringite. Moreover, the investigation showed that the dissolution process of C3A is directly controlled by the availability of the calcium sulfate phases. [ABSTRACT FROM AUTHOR]