Calmetrix I-Cal 8000 User Manual page 29

(a) Heat flow
Arrhenius law applies only if a change in temperature merely changes the speed of reaction and not the
major path of reaction. This is usually the case for straight portland cement mixtures, which in turn makes
the foundation for the use of Maturity models for predicting degree of hydration and strength development.
However, a "major" change in the reaction path going from one temperature to another would invalidate this
concept. Cement-admixture incompatibility issues are sometimes induced by a temperature change such
that the major path of cement reaction has changed and therefore could be undetected by standard
laboratory room temperature tests. Figure 8 shows an example where an industrial concrete with 20% fly
ash and 0.3% low range water reducer by weight of cement hydrated normally at 10 and 23°C. The slower
hydration rate at 10°C is expected, however note the complete different path at 40°C as evident by both the
timing and abnormal shape of the main exotherm and the abnormal Arrhenius plot.
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Figure 2: Arrhenius plot based on the results in Figure 3 using
Eqv (1). The slope is the apparent activation energy for the
overall cement hydration process.
(b) Energy (integrated heat flow)