Page 295 - Handbook of Thermal Analysis of Construction Materials

Section 5.0 - Cement 273

Not much work has been reported on the effect of superplasticizers
on the hydration of C S. It has been reported that substantial retardation
2
occurs in the hydration of C S at all dosages of SMF. [13]
2

5.0 CEMENT

The characteristics of cement pastes or concrete from the initial
slump to strength and durability are directly or indirectly related to the type,
the rate formation, and the amount of the hydration products. Several
factors have to be taken into consideration in the elucidation of the role of
superplasticizers on the hydration of cement. Some of the factors that affect
the rate of hydration include the type of cement, the characteristics of the
ions in the solution (sulfate, OH, and alkalis), the particle size/surface area
of cement, the relative amounts of the individual phases of cement, the
dosage of the superplasticizer, the cation associated with the superplasticizer,
the type of superplasticizer, its molecular weight, the dosage, the tempera-
ture of hydration, and the water:cement ratio.
In cements, the hydration of C S is retarded, but not to the same
3
extent as when it is used in a pure form. In cement, part of the superplasticizer
is adsorbed by the C A phase so that only smaller amounts are available to
3
retard the hydration of the silicate phase. The rate of hydration of cement
in the presence of superplasticizers has been studied by many thermal
techniques. The effect of SMF on the hydration of cement can be followed
by conduction calorimetry (Fig. 8). [4][5] The peak occurring at 5 hours in the
reference sample is reduced and appears at later times. Retardation of the
appearance of the peak is increased with the amount of SMF added. At 4%
SMF, the peak effect becomes very broad and extends over a longer period.
The influence of superplasticizers on the hydration of cement may
also be determined by TG by estimating the amount of calcium hydroxide
formed at different times. In Table 4, the amounts of calcium hydroxide
formed in the presence of 1 or 2% SMF are given. [5][19] At earlier times, the
hydration is significantly reduced by 2% SMF up to 24 hours. After 24
hours, the difference in values is marginal between the reference sample
and that containing SMF. The retardation effects of superplasticizers have
also been confirmed by XRD, hydration degree, setting times, and conduc-
tivity investigations. [20]–[24]