The solubility constant of calcium peroxide octahydrate in relation to temperature; its influence on radiolysis in cement-based materials

Abstract

The solubility constant of calcium peroxide octahydrate ( K per) is determined in relation to temperature through an experimental method based on pH monitoring. The value of K per = (CaOH +) ( HO 2 - ) (H 2O) 7 is provided by the modified Arrhenius expression K per = 7.329 × 10 - 161 × T 58.929 × exp ( 5061.5 / T ) . At 25 °C, K per = (1.13 ± 0.01) × 10 −7. That expression takes into account the kinetics of the homogeneous decomposition of hydrogen peroxide according to the H 2 O 2 + HO 2 - → H 2 O + OH - + O 2 mechanism. The variation of the rate constant with temperature is achieved by completing the data taken from the literature by observing the kinetics at 65 °C and is expressed as k = A × exp (− E a / RT) where A = 600 M −1 s −1 and E a = 35 kJ/mol. At 25 °C, k = (4.5 ± 0.1) × 10 −4 M −1 s −1. In parallel, a new presentation of the available data on calcium equilibria in relation to temperature (complexation of Ca 2+ and of CaOH +, solubility of portlandite) is used to support the simulations made on the overall system (calcium + peroxide). By integrating the new data, it is possible to forecast a specific behaviour of γ radiolysis in a cement-based material as a function of the temperature. Below 25 °C, CaO 2·8H 2O precipitation acts in synergy with Allen’s chain-reaction and leads rapidly to a stationary state with a small amount of H 2. At 25 °C and above, precipitation stops, the stationary state is more difficult to achieve and the production of both H 2 and O 2 is significantly higher.