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TECHNICAL PAPER

make a note of changes in colour and appearance of cracks on 2.2.4 Fourier transform infrared spectroscopy
the surface of the specimens. After visual observation, the mass (FTIR)
loss in the specimens were measured and then the specimens
were tested for their compressive strength and density. The FTIR spectra were collected for powdered samples
prepared as mentioned in the procedure for TGA experiments.
2.2.1 Compressive strength measurements The spectroscopy data was obtained between the frequency
range of 400 and 4000 cm at a resolution of 4 cm . The FTIR
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Cube specimens made with cementitious paste, mortar, and spectra can be useful in understanding the phase changes with
concrete were tested for compressive strength before and after respect to the change in temperature regime as the stability
exposure to elevated temperature. A compressive strength of the compounds vary with change in temperature. The
test frame of 3000 kN capacity was used and the loading asymmetrical stretching of the bonds in geopolymer phases can
rate was maintained at 140 kgf/cm /min, as mentioned in the result in producing varying spectrum absorption based upon
2
IS: 516 (2004). Crushed samples were collected from the core of dissolution of Si-O bonds.
paste cube specimens for further analysis using micro analytical
characterization techniques. 2.2.5 X-Ray diffraction (XRD)
2.2.2 Helium pycnometry Powder X-ray diffraction was carried out on the samples
prepared for each specimen in every binder type before and
The true density of the geopolymer systems were determined after thermal exposure. The powdered samples were loaded in
using helium pycnometer based on the gas displacement the sample holder by backloading process and placed inside the
principle that can accurately measure the skeletal volume of diffractometer. Inside the diffractometer, Cu Kα radiation was
the specimen. Samples from the geopolymer paste specimens focused on the sample. A step size of 0.0167º for a complete
scan from 5 to 75º was used to obtain the diffractograms. The
before and after exposure to elevated temperature were
collected and dried at 105°C before keeping them in the qualitative analysis of the diffractogram peaks were carried out
sample compartment of the pycnometer. However, preheating with the help of standard patterns as per joint committee on
powder diffraction standards (JCPDS) database.
is not necessary for the samples collected from specimens
after exposing them to elevated temperature. The volume of
the sample collected from different specimens was maintained 3. RESULTS AND DISCUSSION
as constant value of 1 cm in each test. The samples were 3.1 Visual observations and density
3
maintained under a constant temperature during the testing
and helium gas was purged into the sample compartment of Figure 1 shows samples of various mixes after being exposed
pycnometer. Helium gas with lowest atomic size was able to to different temperatures. Colour changes were observed
penetrate the pores in the samples. The pressure difference for all the specimens subjected to different temperatures.
between the sample compartment with only helium and with Macro-cracks were also formed and visible in all cubes, but
corresponding to different temperatures, there were variations
helium and sample was used to calculate the density of the
sample using gas law. in crack pattern, crack width and intensity of cracking. During
the heating process, the specimens would have undergone
2.2.3 Thermogravimetric analysis (TGA) transformations such as moisture evaporation, chemical
decomposition, and an increase in internal vapour pressure,
TGA experiments were carried out on an equipment thus resulted in the formation of cracks. Among the various
manufactured by Netzch Technologies (Model No. STA 409 TC). mixes, metakaolin possessed the most intensive and branched
The collected paste samples from compressive strength test cracking, even at 300°C exposure. Specimens with fly ash mix
were ground and sieved through 75 µm sieve. The fine powder has the formation of large and wide cracks, with relatively much
samples were then kept under isothermal condition for 10 mins less branching. Minimum cracking was observed in samples with
in the crucibles. After preheating at a constant temperature in fly ash-calcined clay blends.
Nitrogen (N 2 ) environment, the powder samples were exposed Figure 2 (a) shows density measurements in the samples
to 1000°C by increasing the temperature at a rate of 10°C per collected from specimens before exposing to temperature
minute. The amount of powder samples in the crucible for each using helium pycnometry. The density measurements between
specimen varied between 35 and 50 mg. The changes in amount the different binder composition are insignificant except for
of each phase before and after exposure were determined with the case of metakaolin. The density variation after elevated
the cumulative and the differential mass loss data obtained temperature exposure followed a decreasing trend with an
between 30 and 1000°C. increase in temperature irrespective of the mix compositsions

20 THE INDIAN CONCRETE JOURNAL | MAY 2022