Page 259 - Handbook of Thermal Analysis of Construction Materials

240 Chapter 6 - Retarding and Water Reducing Admixtures

The retarding action of sugars is attributed to the formation of a
complex between the hexagonal phase and sugars. The formation of an
interlayer complex in the presence of Ca or Na gluconate is confirmed by
an endothermal peak at 80°C in the DTA thermogram. [28][29] This peak
cannot be assigned to the C AH because, under the drying conditions that
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4
were adopted, it would be transformed to C AH . Retardation of the
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formation of the ettringite phase in the presence of sugars is also evident
from DTA studies. For example, at 14 days of hydration the mixture
containing sucrose has 25% ettringite and that without it, 52%. Thus,
thermograms also reveal that the conversion of ettringite to the low
sulfoaluminate form is retarded in the presence of sugars. [27]
It has also been observed that a sugar, in certain concentrations,
may also act as an accelerator for the hydration of the tricalcium aluminate
systems. Thermograms indicate that at 0.025–0.05% sucrose dosage, peaks
due to cubic phases are intensified or those due to the hexagonal phases are
decreased. This would show that sucrose is an accelerator. At concentra-
tions of 2–5%, however, the number of hexagonal phases formed is
diminished indicating that the hydration of C A is retarded. [1]
3
The DTG, DTA, and XRD techniques have been applied to study
the retarding effect of sucrose on the hydration of C S. In Fig. 13, [10] the
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influence of 0.1% glucose (G), sodium gluconate (NG), and lignosulfonate
high in sugar (LSA) or low in sugar (LS) on C S hydration is compared
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using XRD. The addition of 0.1% glucose causes an initial delay in the
hydration of C S followed by a slight acceleration, as was found for
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lignosulfonate high in sugar acids. The induction period, however, is longer
with glucose than when lignosulfonate was used. The induction period is
longest with sodium gluconate (50 days). In the presence of 0.1% glucose
or 0.01% gluconate, amorphous calcium hydroxide was detected by DTA
at 7 days. At later ages, the crystalline form of calcium hydroxide was
identified by both XRD and DTA.

6.0 PHOSPHONATES

Phosphonic acid-based chemicals are known to form complexes
with many inorganic species, and the action of many retarders is based on
complex formation. Several phosphonic acids such as amino trimethylene
phosphonic acid (ATMP), 1-hydroxyethylidene-1, 1-diphosphonic acid