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

摘要

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 ~+) (HO2-) (H _2O) ~7 is provided by the modified Arrhenius expression ~(Kper)=7.329×10 ~(-161)× ~(T58.929)×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 _2O _2+HO2-→H _2O+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.