Calmetrix I-Cal 8000 User Manual page 30

Calmetrix I-Cal 8000 User Manual
(a) Heat flow
Figure 3: Example of temperature induced cement – admixture incompatibility
Cement-admixture incompatibility issues associated with hot weather concrete are often based on sulfate
imbalance, since higher temperatures generally make the aluminate reactions faster while the solubility of
calcium sulfate is lower compared to lower temperatures. The response (red curve) in Figure 3 is somewhat
typical. As the concrete was mixed at room temperatures before curing at three different temperatures inside
three separate isothermal calorimeter units, the initial positive or negative power readings represent non-
isothermal conditions and are not included in the calculation of activation energy, along with the initial heat
of dissolution. Note however the higher activity of the 40°C sample after having reached isothermal
conditions after some 60 minutes. The accelerating effect of temperature on aluminate hydration coupled
with a negative effect on gypsum solubility likely caused the mixture at 40°C to temporarily run out of soluble
calcium sulfate at early age, causing a poisoning effect on the strength-giving alite hydration that is often
observed as a result of poorly controlled aluminate hydration. The alumina activity then reached a stage of
lower activity which was low enough to allow for enough gypsum to dissolve to reach sulfate balance and
allow for some alite hydration until the mix most likely ran out of gypsum after about 18 hours as indicated by
the very steep exotherm. The steep slope and amount of heat released by this exotherm is typical of un-
controlled alumina hydration, which in turn effectively poisoned any further alite hydration during the test.
The sample cured at 40°C was soft without any appreciable mechanical strength at the end of the test,
which is also typical for the type of response seen by calorimetry at 40°C (red curve).
Not all cases of non-compliance with Arrhenius law are as clear-cut as the example shown in Figure 3.
Figure 4 shows an example of alkali activated fly ash concrete where the power plots have somewhat
different shape at 40°C relative to 23 and 10°C. The different shape seen by calorimetry at 40°C may
indicate a partially different reaction path or possibly an added reaction at high temperature, while the
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(b) Arrhenius plot
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