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TECHNICAL PAPER

1.1 Background (optimum of 600°C) is found highly reactive and has been
shown as an excellent addition (or partial replacement) with fly
Excellent resistance to extreme temperatures by geopolymers ash as a binder for geopolymers [21] . Past studies have proved
has been demonstrated in many works in the past [11-12,14] . It has the robustness and predictability of hardened properties of
been reported that geopolymers when fired at temperatures up calcined clay based geopolymers [22] . Up to 30 % increase in
to 1100°C have lost a maximum compressive strength of up to compressive strength is reported with replacement of traditional
60 %. However, all the geopolymer specimens tested were found fly ash with calcined clay. However, fire resistance being the
stronger than the specimens made of Portland cements post- major application domain for geopolymers is to be investigated
firing [15] . The high spalling resistance of geopolymers compared more for clay/metakaolin-based geopolymer systems. There are
to normal Portland cement concrete was demonstrated in past conflicts and lack of clarity in the understanding of the thermal
studies and was found attributed to the facilitation of release of deterioration mechanisms in various pozzolanic materials in
internal steam generated at high temperature due to the porous geopolymers. The current study evaluates the performance of
structure of geopolymer systems [16] . Another study has shown conventional geopolymer systems with variations of calcined
that residual compressive strength of OPC control specimens clay (CC) and with combination of calcined clay and fly ash as
turned to zero MPa on heating at temperature more than novel aluminosilicate binders and correlates the behavioral
800ºC, while those made with activated ash based geopolymers differences with respect to the microstructural properties of
retained the constant strength until 1000ºC [14] . Similar studies calcined clay. This research characterizes the damages in these
on bending strength showed a maximum decrease of 33 % in materials at various temperatures, and thereby explores the
the strength of geopolymers while OPC specimens decreased interaction of the geopolymer matrix at high temperatures using
to zero strength at temperatures > 600°C [17] . The mass loss advanced microanalytical tools such as FTIR (Fourier-transform
associated with geopolymers is reported to be the least infrared spectroscopy), TGA (Thermo-Gravimetric Analysis) and
(5 to 12 %) compared to other fire-resistant polymer XRD (X-ray Diffraction). This investigation would contribute
nanocomposites [18] . Also, geopolymers have the advantage of towards the effective deployment of abundantly available
not producing any toxic fumes at high temperatures of up to clay after calcination in geopolymers that require fire-resistant
1000°C [14] . The thermal stability of the geopolymer systems is characteristics.
well established. However, studies related to material-specific
alterations and the corresponding microstructural changes
are required to map the possible mechanisms that govern the 2. MATERIALS AND METHODS
performance.
2.1 Materials used
Even though calcined clays and metakaolin based geopolymers In this study, class F fly ash, calcined clay, and metakaolin
are studied for their hardened properties, much more were used as the binders. The alternative binders metakaolin
understanding is required with respect to their performance at and class F fly ash used were conforming to Indian standards
high temperatures. Khale and Chaudry (2007) suggested that IS: 16354 (2015) and IS: 3812-1 (2003), respectively. Calcined
metakaolin can reduce thermal conductivity in geopolymers clay was obtained from the local source, and the chemical
but is left unexplored further [19] . Kong et al. (2007) elucidated composition of the same conforms to the Indian standard
that the lack of sufficient pore distribution in metakaolin based IS: 1344 (1981). The calcined clay used for producing
geopolymers results in more damage at high temperatures, geopolymer in this study had a kaolinite content of 58 %.
whereas fly ash could help in releasing steam pressure due to The reactivity of the calcined clay depends on the kaolinite
pore connectivity [14] . Since the overall mechanism would depend content of the clay and lowgrade clays with kaolinite content
on the vapourising of moisture at temperatures above 100°C of 40 % and above shall be used to develop alternative binder
from these materials, and their further difficulty to escape out of systems such as geopolymers [13] . The oxide composition of
the system without causing any internal stress, the varying pore these primary binders is reported in Table 1. These primary
structure of different materials in a geopolymer system are very binders were activated using the alkaline solution prepared by
important to be unfolded.
mixing a laboratory grade sodium silicate (Na 2 SiO 3 ) and sodium
hydroxide solution (NaOH). The NaOH used was of laboratory
1.2 Research significance
grade in pelletized form with a specified purity of 98 %. As per
Low grade clays not suitable for cement production is abundant manufacturer’s data sheet, the major impurity phases in 98 %
in India with reasonable amount of kaolinite content pure NaOH were carbonate (1.0 % as Na 2 CO 3 ), silicate (0.01 %
(say > 40 %). Such clays after calcining can be a suitable as SiO 2 ), sulphate (0.003 % as SO 4 ), potassium (0.05 %) and zinc
alternative material to produce geopolymers in the places that (0.001 %). The chemical composition of sodium silicate solution
have shortage or non-availability of fly ash and GGBFS [20] . The (usually called as water glass) consists of Na 2 O = 7.5-8.5 %,
calcined clay obtained from treating kaolin at high temperature SiO 2 = 25-28 % and the remaining portion was water. NaOH of

18 THE INDIAN CONCRETE JOURNAL | MAY 2022