Page 322 - Handbook of Thermal Analysis of Construction Materials

Section 2.0 - Fly Ash 299

cement pastes containing different amounts of fly ash were hydrated, and
the hydration products were subjected to TG analysis. The amounts of non-
evaporable water and Ca(OH) were estimated up to 90 days. Up to 7 days,
2
Ca(OH) contents in high volume fly ash cements started to decrease. In
2
untreated cements, the Ca(OH) was about 11–12% compared to only 2–3%
2
in fly ash cements. These results and other tests indicated that in high
volume fly ash cements a relatively more homogeneous product resulted
with low lime contents, and the formation of a low C/S ratio C-S-H product
produced a stronger body.
Many fly ashes cannot be used as supplementary materials or
mineral admixtures because they possess low pozzolanic activity. Some
activators may be added to make them hydraulic. The addition of Na SiO
2 3
and Ca(OH) to fly ash cement mixtures results in the acceleration of the
2
hydration of cement as evidenced by increased intensities of the endother-
mal peak (calcium hydroxide decomposition) in thermal analysis tech-
niques. Strengths are also increased. [12] Fly ash can also be activated by
mixing it with phosphogypsum and wet hydrated lime. The estimation of
ettringite and lime may be carried out by DTA, and the results may be used
to explain the strength development in such systems. [13]
Bonding occurs when fly ash and lime are hydrothermally treated.
The products formed in such mixtures are advantageously identified by
DTA. Thermal analysis indicates that initially C-S-H (I) forms and is then
converted to 11 Å tobermorite. Carbonation also can be followed by
[6]
DTA. By a hydrothermal process, a cementing material was synthesized
from a mixture of fly ash, lime, and water. The resulting product was
analyzed by DTA, TG, and XRD. [14]
Atmospheric fluidized bed coal combustion (AFBC) at a tempera-
ture of about 850°C with calcite as a sorbent, produces solid residue of
different chemical composition from that produced at higher temperatures.
The AFBC residue possesses cementing properties. It consists of anhydrite,
free lime, quartz, and aluminosilicates. DTA and TG techniques have been
utilized to determine melting intervals, oxidation of unburnt coal, lime-
stone, and anhydrite. [15]
The presence of unburnt carbon in fly ashes has many adverse
effects on concrete including the appearance of grayish blackness in
mortars and concrete, need for higher w/c+f ratio to obtain required
consistency, and decreased air entertainment. Thermogravimetric analysis
has been developed to estimate hydrated lime, calcium carbonate, and
unburned carbon in fly ashes. [16] The methodology is as follows: