Page 317 - Handbook of Thermal Analysis of Construction Materials

294 Chapter 8 - Supplementary Cementing Materials

[3]
composite materials. In this chapter, these materials are referred to as
supplementary cementing materials. Also included in this chapter is a
discussion on other additions that are not treated in other chapters.
The main methods of investigating the effect of supplementary
materials and other additions on cement hydration include XRD, SEM,
NMR, Mossbauer spectroscopy, IR, and thermal analysis. Poorly crystal-
lized products that form in these materials are advantageously investigated
by TG, DTG, DTA, DSC, and conduction calorimetry. Thermal methods
have also been suggested for characterization of the supplementary and
other materials.

2.0 FLY ASH

Hydration of fly ash cement differs from pure cement in terms of
the hydration rates of the clinker phases, amount of calcium hydroxide
formed, composition of the clinker hydration products, and additional
[3]
hydration products from the reaction of the fly ash. Lower amounts of
lime are formed in the presence of fly ash because of the pozzolanic reaction
between the fly ash and lime formed in cement hydration. Fly ash generally
retards the reaction of alite in the early stages and accelerates the middle
stage reaction. The accelerated reaction is attributed to the existence of
nucleation sites on fly ash particles. The aluminate and ferrite phases
hydrate more rapidly in the presence of fly ash, and also there is a significant
difference in the hydration rate of the belite phase up to 28 days. Table 1
gives the relative hydration rates of cement compounds in the presence of
[4]
fly ash as derived from conduction calorimetry. It can be seen that the
earlier rates of hydration are generally retarded, and the later stage hydra-
tion rates are accelerated.
The reactivity of fly ash in cement depends on the glass content,
RH, temperature, alkali content of cement, particle size, and sulfate ion
concentration. High calcium fly ash is mainly glassy with some crystalline
phases and has a self-hardening property. In a three day old fly ash cement
paste, the composition of the hydrate formed from alite or belite is similar
to that formed from a normal portland cement paste, but at later stages the
CaO/SiO ratio is lower and Al O /CaO ratio, higher in a fly ash cement
2 2 3
paste. The silicate anion condensation occurs more rapidly in C S-fly ash
3
paste than in C S paste. [4]
3