The ASTM Standard practice for measuring Kinetics of hydraulic cementitious mixtures using isothermal calorimetry describes a method for measuring the relative differences in hydration kinetics of hydrating cementitiuous mixtures (ASTM, 2009), either of paste or mortars. The isothermal calorimeter used in this practice consists of a heat sink with a thermostat, two heat flow sensor and a sample vial holder which is attached to each senor. One vial contains a fresh sample mixture and the second vial holds a thermally inert mixture of sand and water with similar thermal properties as the active sample. The heat produce by the hydration of the active sample is transferred and passes across the heat flow sensor. The output from the calorimeter is calculated from the difference between the outputs of the heat flow sensors from the active and inert samples. The heat is allowed to flow away from the sample therefore the measurement will be taken at an approximately constant temperature. This characteristic defines the calorimeter as isothermal.The thermal power curves that are developed can be used to evaluate the isothermal hydration kinetics of the combined mixtures of various materials during the early stages after water addition. These hydration profiles can be used to evaluate the following:

 

• Relative setting characteristics (based on final time of set)

• Material compatibility

• Sulphate balance

• Early strength development

• Effects related to composition, proportions and time of addition of materials

• Curing temperature

 

Isothermal calorimetry is limited by sample size  (5 g to 100 g) and as a result only cement pastes and mortars are measured. Therefore adiabatic calorimetry is prefered for concrete analysis. 

 

An 'off the shelf' isothermal calorimeter produced by ()()) can be seen in the figure below.