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TECHNICAL PAPER

Table 1: Chemical composition of binders used

OXIDE COMPOSITION (%) CaO SiO 2 Al 2 O 3 Fe 2 O 3 MgO Na 2 O LOI (%)
Calcined clay 0.5 49.5 41.5 1.9 0.4 0.2 2.4
Fly ash 1.3 59.3 29.9 4.3 0.6 1.1 1.8
Metakaolin 0.02 54.8 41.6 0.6 0.3 0.2 0.4

12 M was prepared by mixing sodium hydroxide pellets with concrete specimens were evaluated for compressive strength
distilled water. The alkaline activator was prepared by mixing loss, mass loss and reduction in density after exposing them
the Na 2 SiO 3 and 12 M NaOH solution such that the sodium at elevated temperature (between 300°C and 900°C). The
silicate to NaOH ratio was fixed at 1.5 for all the mixes. However, fractured pieces collected as samples from paste specimens
the activator to binder ratio for different binder combinations were used to characterize the microstructural changes in the
was varying between 0.60 and 1.35. The activator solution was geopolymer system before and after exposing them to the
prepared 24 hours prior to the casting of specimens and must elevated temperatures. The characterization techniques such
be used before 36 hours from the time of preparation. as X-ray diffraction, Thermogravimetric analysis, and Fourier
transform infrared spectroscopy were used to understand
The nomenclature for the paste (P), mortar (M) and concrete the abovementioned microstructural changes in geopolymer
(C) specimens was proposed to have symbol for the type of systems at the elevated temperatures. Even though the physical,
specimen followed with the acronym for type of SCMs used mechanical and microstructural changes in the specimens before
in the study, for example, P-FA represents geopolymer paste and after exposing them to elevated temperature were carried
specimens made with fly ash. The paste specimens of each
binder type were mixed and cast as cubes using a Hobart mixer out in paste samples, concrete and mortar samples studies were
with the activator to binder (a/b) ratio mentioned in Table 2. limited to physical and mechanical characterization alone due to
Similarly, the mortar specimens were mixed at the ratio of 1:2.75 the difficulty in complete removal of aggregate phases that can
influence the micro analytical study outputs.
(binder-to-aggregate) with an a/b ratio suitable to achieve a
minimum workability of 150 mm flow as per ASTM C1437.
2.2 Exposure to elevated temperatures and
Freshly cast specimens were removed from mould after 3 subsequent testing of physicomechanical
hours and exposed to high temperature curing at 70°C for properties
24 hours. The size of the cubical paste and mortar specimens
were 50 × 50 × 50 mm. After curing, the specimens were kept The geopolymer specimens after curing were exposed to
at ambient lab temperature. In addition to paste and mortar, elevated temperatures such as 300, 600 and 900°C. The
concrete cubes were cast with a common binder content of specimens were placed into an air-filled muffle furnace and
350 kg/m for all 4 binder combinations and the a/b ratios heated at constant rate of 0.60°C/s. After achieving the desired
3
were fixed to achieve a minimum initial slump of 50 mm after temperature level, the geopolymer specimens were exposed to
casting. The homogeneity of the cast specimens was ensured that temperature for 1 h and then were left for cooling inside the
by proper vibration of the fresh concrete/mortar in the moulds furnace to ambient temperature. At the end of each elevated
by keeping them on a table vibrator. The paste, mortar, and temperature exposure, the specimens were visually observed to

Table 2: Mixes used in the study
BINDER TYPE CLASS F FLY ASH CALCINED CLAY 50 % CC+50 % FA METAKAOLIN

Molarity of NaOH (M) 12 12 12 12
Activator/binder ratio for paste 0.40 0.60 0.50 1.00
Activator/binder ratio for mortar 0.60 0.75 0.70 1.20
Activator/binder ratio for concrete 0.75 0.90 0.80 1.35
Sodium silicate/NaOH 1.5 1.5 1.5 1.5
Paste (P) nomenclature P-FA P-CC P-FACC P-MK
Mortar (M) nomenclature M-FA M-CC M-FACC M-MK
Concrete (C) nomenclature C-FA C-CC C-FACC C-MK
Note: FA–Class F fly ash; CC–Calcined clay, FACC–Fly ash + Calcined clay; MK–Metakaolin

THE INDIAN CONCRETE JOURNAL | MAY 2022 19