Conception of an isothermal calorimeter for concrete— Determination of the apparent activation energy

Instrumentation was developed to perform calorimetric tests on concrete in isothermal conditions using 0.11× 0.22 m cylindrical samples. The purpose of this article is to study the validity of Arrhenius' law and determine the changes occurring in the apparent activation energy of concrete as a function of temperature, by using this technique. Heat of hydration measurements were made for various isothermal curing conditions (10, 20, 30 and 40°C), enabling the apparent activation energy to be determined over this range of temperature using two different methods: the method of “superposition” and the method of “rates”. This parameter was calculated over a range of heat varying from 10 J/g to 140 J/g (corresponding to about 50% of the final value). The two methods give relatively similar results (with deviations of the order of 3 kJ/mol). However, it is noted that the apparent activation energy varies as a function of the temperature range considered, which limits the use of this technique to thin elements (of less than 0.2 m) subjected to slight temperature rises. In the cases of the concrete studied here, it seems however possible to use a single apparent activation energy value over the entire temperature range.