Page 265 - Handbook of Thermal Analysis of Construction Materials

246 Chapter 6 - Retarding and Water Reducing Admixtures

mechanism of hydration, etc., in the hydration of cements and their
individual phases. The common techniques used for these investigations
include DTA, DSC, DTG, TG, chemical shrinkage measurements, XRD,
loss on ignition, IR, electron microscopy, and others. Most of these
techniques are applied to products that are hydrated to a specific length of
time. In the conduction calorimetric technique, the heat developed is
followed from the time water comes into contact with the cement or its
phase. The time at which the exothermic peak appears may be used to assess
the relative retarding actions of retarders. The time of termination of the
induction period provides an idea of the relative times of setting of cements
containing various admixtures. The total amount of heat produced at
different times may be used for determining the rate of hydration.
Ramachandran and Lowery [32] applied conduction calorimetry to
the study of the relative effects of various retarders on the hydration of
cement. The retarders used were calcium gluconate, glucose, glycolic acid,
molasses, sodium borate, sodium citrate, sodium heptonate, sodium
hexametaphosphate, sodium pyrophosphate, sugar-free lignosulfonate, and
sucrose. The dosage of the chemicals varied between 0.025 and 1.2%.
Gluconate was found to be a good retarder. Its action is possibly
related to its poisoning action on the hydration products of cement. It has
also been advocated for controlling slump loss in concrete. Conduction
calorimetric curves of cement with gluconate show large hump effects for
the hydration of the silicate phase (Fig. 18a). [32] At a dosage of 0.15%, the
hydration is retarded up to 54 hours. Compared to gluconate, sodium borate
is a mild retarder. Even at 0.7% the retardation effect lasts only for 24 hours
(Fig. 18b).
The most effective retarders, that need a dosage of about 0.15% or
less to achieve an induction period of 40 hours, include Ca gluconate,
sodium heptonate, and sucrose. The least effective retarders that require
more than 0.5% dosage for extending the induction period by 40 hours are
sugar-free lignosulfonate, Na pyrophosphate, Na hexametaphosphate, Na
borate, and glycolic acid. Glucose, molasses, and Na citrate are termed
moderate retarders requiring a dosage between 0.15 and 0.5%.
Conduction calorimetry has been used to obtain the induction
periods of cement containing different amounts of retarders. Table 3 [32]
provides data on the minimum concentration of a retarder required to
achieve different induction periods.