Page 321 - Handbook of Thermal Analysis of Construction Materials

298 Chapter 8 - Supplementary Cementing Materials

The rate of hydration of cement-fly ash also depends on the
particle size of fly ash. Conduction calorimetric curves have been
obtained for cements containing 30% fly ash of surface areas of 200, 450,
2
and 650 m /kg. These mixtures are designated PFA-0, PFA-1, and PFA-2
[8]
respectively in Fig. 4. The fly ash cements exhibited sharper peaks at
earlier times at 11–12 hours compared to the reference cement. These peaks
become sharper as the surface area of fly ash increases. The peak effects are
associated with the hydration of C A after the depletion of gypsum in the
3
system. The initial exothermal effect due to alite hydration is retarded by the
fly ashes.
Grinding of high calcium fly ashes enhances their reactivity. The
DTA studies indicate that the intensity of peaks at about 150°C is increased
with highly ground fly ash additions. This is evidence that higher amounts
of ettringite are formed in the presence of finely ground fly ash. [9]

Figure 4. The influence of particle size of fly ash on the hydration of cement.

The Canada Center for Mineral and Energy Technology (CANMET)
developed a structural concrete incorporating high volumes (>50%) of low
calcium fly ash. Such concretes yield adequate early strengths, workability,
low temperature rise, and high later strengths. [10] The mechanism of
strength development in high volume fly ash concretes was investigated by
Feldman, et al., applying several techniques such as electron microscopy,
permeability, pore size distribution, and thermal analysis. [11] Fly ash