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TECHNICAL PAPER

Table 3: Change in compressive strength (%) in the in specimens between the temperature range 30 and 600°C.
specimens exposed to elevated temperature The metakaolin mix exhibited the highest mass loss at about
15 %. The fly ash mix was found to have the highest temperature
MIX ID AFTER 300°C AFTER 600°C AFTER 900°C
resistance, from the least mass loss observed. Even though
P-FA +36.0 -28.1 -39.6
calcined clay was found to have a better performance than
P-CC +15.2 -16.4 -75.4
metakaolin with respect to mass changes, it could perform
P-FACC +16.8 -11.4 -55.6 comparable to fly ash geopolymer when in combination with
P-MK -37.9 -73.4 -92.7 fly ash (in the FACC mix). Until 100°C, metakaolin mix and fly
M-FA -4.8 -15.9 -22.5 ash mix have similar and appreciable thermal deterioration
predominantly due to the loss of free H 2 O, which is comparable
M-CC -4.6 -13.0 -39.8
with fly ash mixes. Metakaolin geopolymer faces a steep
M-FACC -4.4 -12.0 -33.2
mass loss between 100 and 150°C. The water molecules at
M-MK -14.9 -38.8 -63.8
the interstitial layers reported to be higher in mixes with
C-FA -3.0 -18.5 -33.2 metakaolin and calcined clay and hence, a higher mass loss was
C-CC -3.6 -15.8 -35.5 exhibited by clay based geopolymers at the above-mentioned
C-FACC -3.1 -14.4 -31.7 temperature range [23] . Fly ash and fly ash-calcined clay mixes
C-MK -8.8 -29.9 -55.2 perform better than the other two mixes in long temperature
range, especially when temperatures exceed 120°C.
from 300-900°C. FA, CC and FACC mixes showed similar values
of compressive strength and trend in mortar and concrete with Beyond 120°C, a gradual reduction in the weight was observed
insignificant differences. However, the variations among FA, with a relatively gentle slope until 200°C that corresponds to
CC and FACC were noticeable in case of paste. This could be the decomposition of amorphous phases in the geopolymer
because of the absence of interference of aggregates in the cementitious system. The amorphous phases present in the
governance over strength, thus providing more opportunity for geopolymer could be either NASH (Sodium aluminium silicate
the chemistry of the paste to control the strength of the matrix. hydrates) or CASH (Calcium aluminum silicate hydrates). The
All the test results conform to the good compressive strength of flattening of mass loss curve beyond 200°C indicates the
calcined clay, comparable with that of fly ash, and supports the unavailability of carbonate phases in the geopolymer matrix.
possibility of the fly ash-calcined clay combination. The results from TGA followed similar trends observed in
physical mass loss, and reduction in density and compressive
3.4 Thermogravimetric analysis (TGA)
strength. The temperature resistance of the geopolymer mixes
The percentage mass changes and differential mass loss with was highest for fly ash, followed by fly ash-calcined clay, calcined
varying temperature of heating for the various geopolymer clay and metakaolin. The enhanced performance of fly ash-
mixes from the thermogravimetric analysis are shown in calcined clay mix indicates the applicability of calcined clay in
Figure 5. Figure 5 documents the variations in mass observed geopolymer systems effectively by suitably combinations.

100 0.1

0.0
95

Weight (%) 90 dw/dT -0.1

-0.2
85 P-FA P-FA
P-CC -0.3 P-CC
P-FACC P-FACC
P-MK P-MK
80 -0.4
0 100 200 300 400 500 600 0 100 200 300 400 500 600
Temperature (°C) Temperature (°C)
(a) TG plot (b) DTA plot
Figure 5: Thermogravimetric analysis on different geopolymer paste specimens

THE INDIAN CONCRETE JOURNAL | MAY 2022 23