Page 181 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Water Reducers and Retarders 165

extent. The alkali in the cement is capable of interacting with the retarder
and destroying its capability to retard hydration. A small amount of
lignosulfonate (0.1%) slightly retards or accelerates the C S hydration,
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depending on the chemical composition and molecular weight of the
lignosulfonate. [71][72] Addition of Ca lignosulfonate to C S above a particu-
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lar dosage can indefinitely inhibit its hydration. When a retarder is added
to concrete 2–4 minutes after mixing, setting time is delayed 2–4 hours
beyond when the admixture is added with mix water. [36]

Table 7. Influence of Some Retarders on the Setting Times of Cement Paste

Admixture Amount Initial Set Time Final Set Time
(%) (h) (h)
None —— 4 6.5

Sucrose 0.10 14 24
0.25 144 360
Citric Acid 0.10 10 14
0.25 19 44
Ca Lignosulfonate 0.10 4.5 7.5
0.25 6.5 10

Addition of lignosulfonate retards both the C A hydration and the
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conversion of hexagonal hydrates to the cubic phase. [40] Ca lignosulfonate
retards C A hydration in the mix of C A + gypsum, but a more marked
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retardation occurs in the conversion of ettringite to monosulfonate.
In the hydration of C A or C S, the retarding effect appears to be
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in the decreasing order as follows: sugar acids > carbohydrates > lignosul-
fonate.
The hydration products of C AF with or without gypsum are very
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similar to those of C A and isomorphous with various compounds formed
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with C A hydrate. The rate of hydration of C AF is affected to a lesser
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extent than that of C A in the presence of retarders.
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