Page 45 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Modern Techniques 29

Many industrial waste products have been used in concrete with
success. Some examples include fly ash, slag, silica fume, rice husk ash,
phosphogypsum, etc. Concrete, containing these minerals will have im-
proved durability, workability, and other properties. Fly ash, a high surface
area byproduct obtained as a glassy material, is separated from flue gases
during the combustion of pulverized coal in thermal plants. Conduction
calorimetry is used with success to follow the rate of hydration of
cement containing various types of fly ashes. In one study, the rate of
hydration of cement-fly ash (30%) was studied by using fly ash (FA) of
2
surface areas of 200, 450, and 650 m /kg. [75] These mixtures are designated
PFA-0, PFA-1, and PFA-2 respectively in Fig. 12. The fly ash cements
exhibit sharper peaks at earlier times at 11–12 hrs compared to the
reference cement. These peaks become sharper as the surface area
increases. The peak effects are associated with the hydration of C A after
3
the depletion of gypsum in the system. The initial exothermal effect due to
the hydration of the calcium silicate phase in cement is retarded by the fly
ashes. Several important applications of conduction calorimetry in the
field of cements are to be found in the publications of Ramachandran and
co-workers. [74][77][78]

Figure 12. The influence of particle size of fly ash on the hydration of cement.