Cementitious Calorimetry
An investigation into the thermal properties of Corex slag and the relationship between thermal profiles and setting times
Kyle Cullen van der Westhuizen - 4th year Dissertaion - CIV4044S
The temperature profiles of the three experimental mixtures, as well as the corresponding inert* profiles, where plotted directly from the data obtained from the semi-adiabatic system and can be seen in Figure below. The temperature profiles were similar in shape to examples presented in the ASTM draft standard (2012) for mixtures of similar mass and insulation.
TEMPERATURE PROFILES
*Note: The inert sample were non-reactive samples of sand and water
The heat evolved during the 24 h curing period was calculated by using the following equation:
qt = Cp x (Tt – To ) x ms/mc
Where:
qt - total heat per unit mass of binder
Cp - specific heat capacity
Tt - temperature of the sample at time t
To - temperature of sample at the beginning of the test
ms/mc - ratio of sample mass to binder mass
HEAT PROFILES
The specific heat capacity used in heat calculations was determined by the weighted average specific heat of the constituent materials. The specific heat capacity of cement and cement extenders was taken as 880 J/kg.K and a value of 4187 J/kg.K was used for water. The temperature change was determined by subtracting the temperature of the inert sample from the active sample at the equivalent time. The calculated heat profiles are displayed in the figure below.
The heat rate profiles were developed by means of numerical differentiation with respect to time of the heat curves and can be seen in the figure below. The rate at which heat is liberated directly indicates the rate at which the exothermic reaction is taking place.
HEAT RATE PROFILES
The setting times of the mixtures determined by SANS 50196-3:2006 were compared to the corresponding thermal profile in order to determine any relationships between the main peak response (MPR) and initial and final setting times. After comparing the results no clear correlation between the setting times and a specific fractions of the MPR for the cement pastes were found. The ASTM draft standard (2012) recommends fractions of 20% and 50% for initial and final setting times respectively, but indicates that this proportioning is for mortars that are wet sieved from a concrete mixture.
Both the temperature and heat rate profiles were used to evaluate setting time relationships by the fractions method and are displayed below.
THERMAL PROFILES AND SETTING TIME (FRACTIONS METHOD)
PROPORTIONS OF TEMPERATURE PROFILE
PROPORTIONS OF HEAT RATE PROFILE