Page 59 - Handbook of Thermal Analysis of Construction Materials

42 Chapter 2 - Introduction to Portland Cement Concrete

In spite of a large amount of work, even the mechanism of hydration
of C S, the major phase of cement, is not clear. Any mechanism proposed
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to explain the hydrating behavior of C S should take into account the
3
following steps through which the hydration proceeds. Five stages can be
discerned from the conduction calorimetric studies (Fig. 2). [31] In the first
stage, as soon as C S comes into contact with water there is a rapid evolution
3
of heat and this ceases within 15–20 minutes. This stage is called the pre-
induction period. In the second stage the reaction rate is very slow. It is
known as the dormant or induction period. This may extend for a few hours.
At this stage the cement remains plastic and is workable. In the third stage,
the reaction occurs actively and accelerates with time reaching a maximum
rate at the end of this accelerating period. Initial set occurs at about the time
when the rate of reaction becomes vigorous. The final set occurs before the
end of the third stage. In the fourth stage there is slow deceleration. At the
fifth stage, the reaction is slow and is diffusion controlled. An understand-
ing of the first two stages of the reaction has a very important bearing on the
subsequent hydration behavior of the sample. Admixtures can influence
these steps. Retarders such as sucrose, phosphonic acid, calcium gluconate
and sodium heptonate extend the induction period and also decrease the
amplitude of the acceleration peak whereas accelerators such as calcium
chloride, formate, and nitrite increase the amplitude of the peak.

Figure 2. Rate of heat development during the hydration of tricalcium silicate and portland
cement. (With permission, Noyes Publications, Concrete Admixtures Handbook, V. S.
nd
Ramachandran, ed., 2 Ed. 1995)