Introduction to Portland Cement Concrete - Thermal Analysis of Construction Materials - page 43

It is generally thought that initially a reaction product forms on the
C 3 S surface that slows down the reaction. The renewed reaction is caused
by the disruption of the surface layer. According to Stein and Stevels, [6] the
first hydrate has a high C/S ratio of about 3, and it transforms into a product
of lower C/S ratio of about 0.8-1.5 through loss of calcium ions into
solution. The second product has the property of allowing ionic species to
pass through it, thus enabling a rapid reaction. The conversion of the first
to the second hydrate is thought to be a nucleation and growth process.
Although this theory is consistent with many observations, there are others
which do not conform to this theory.
The end of the induction period has been explained by the delayed
nucleation of CH. It does not explain the accelerated formation of C-S-H.
Tadros, et al., [7] found the zeta potential of the hydrating C 3 S to be positive,
indicating the possibility of the chemisorption of Ca ions on the surface
resulting in a layer that could serve as a barrier between C 3 S and water.
There are other mechanisms based on the delayed nucleation of C-S-H to
explain the end of this induction period. Other theories have been proposed
to fit most of the observations. Although they appear to be separate theories
they have many common features and have been reviewed by Pratt and
Jennings. [8] A detailed discussion of the mechanisms of hydration of
cement and C 3 S has been presented by Gartner and Gaidis. [9]

3.2 Dicalcium Silicate

-C 2 S proceeds in a similar way to that of C 3 S but
is much slower. Typically, 30% is reacted in 28 days and 90% in one year.
As the amount of heat liberated by C 2 S is very low compared to that of C 3 S,
the conduction calorimetric curve will not show well defined peaks as
indicated in Fig. 1. Accelerators will enhance the reaction rate of C 2 S.
Just as in the hydration process of C 3 S, there are uncertainties
involved in determining the stoichiometry of the C-S-H phase found in the
hydration of C 2 S. The hydration of dicalcium silicate phase can be repre-
sented by the equation.

The hydration of β

2 [2CaO•SiO 2 ] + 5H 2 O → 3CaO•2SiO 2 •4H 2 O + Ca(OH) 2

The amount of Ca(OH) 2 formed in this reaction is less than that
produced in the hydration of C 3 S. The dicalcium silicate phase hydrates
much more slowly than the tricalcium silicate phase.