Page 181 - Handbook of Thermal Analysis of Construction Materials

164 Chapter 4 - Introduction to Concrete Admixtures

Mode of Action. The common feature of all these admixtures is that
they are adsorbed on the solid-water interface. In a cement paste, opposing
charges on adjacent particles can exert electrostatic attraction, causing
flocculation. A considerable amount of water is imbibed in these agglom-
erates and adsorbed on the surface, leaving less water for increasing the
workability of paste or concrete. In the presence of a water-reducing
admixture, the surface charges become the same, hence the particles repel
each other and dispersion occurs. Thus, more water is available for reducing
the viscosity of the cement paste or concrete.
Several explanations have been offered to account for the retarding
action of water reducers. They are based on the following theories:
1. Retarders are adsorbed on the anhydrous cement particles
through ionic, hydrogen, or dipole bonding to prevent the
attraction of water.
2. An insoluble layer of calcium salt on the hydrating
particles is responsible for retardation.
3. The hydrated, rather than the unhydrated constituent,
adsorbs the retarder. The surface complex involving the
substrate of unhydrated cement and water followed by
adsorption is responsible for retardation.
4. The adsorption of retarder on the calcium hydroxide
nuclei poisons its future growth; the growth of calcium
hydroxide will not proceed until some level of supersatu-
ration is attained. Details of these theories are discussed
in a in Ref. 36.

3.2 Retarders

Retarders extend both the initial and final setting times of cement
paste and concrete (Table 7). [36] Higher amounts may enhance the retarda-
tion process. At equal dosages, sucrose is the most efficient retarder and,
hence, an accidental overdose of this admixture could create serious setting
problems.
The setting times depend on the type of cement, w/c ratio, tempera-
ture, and sequence of addition. Cements with low C A and alkali contents
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are retarded better than those containing large amounts of these constitu-
ents. For setting to occur, both C A and C S phases have to hydrate to some
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