Page 107 - Handbook of Thermal Analysis of Construction Materials

90 Chapter 3 - Formation and Hydration

that are not easily obtainable by some other methods. Thermal techniques
have been used to study kinetics of hydration, mechanism of hydration, the
influence of admixtures, identification of new compounds formed, estima-
tion of products, etc.
Conduction calorimetry of C S and cement shows five steps during
3
the hydration process (Fig. 13). In the first stage, as soon as C S or cement
3
comes into contact with water Ca and OH ions are released into the solution
phase. This is followed by a rapid release of heat that ceases within 10–12
minutes. This is called a pre-induction period. In the second stage, the
reaction rate is slow and it is known as the dormant or induction period. This
may be extended or shortened by a few hours by the use of admixtures. In
the third stage, the reaction proceeds rapidly and accelerates with time,
releasing a maximum amount of heat at the end of the acceleratory period.
At this stage, a rapid crystallization of CH and C-S-H occurs. In the fourth
stage, there is a slow deceleration. At the final stage, there is only limited
formation of products and at this stage the reaction is diffusion-controlled.
Conduction calorimetry permits determination of the rate of hydration as a
function temperature, water:cement ratio, type of admixture, particle size,
pH, etc.

Figure 13. Conduction calorimetric curves of hydrating tricalcium silicate and cement.