FESEM micrographs of specimens ET1, ET5, ET7 and ET8 cured
at a temperature of 65 C for 24 h and kept in ambient condition
for 28 days are shown in Fig. 7a–h. Two different magnifications
of FESEM micrographs are shown for each specimen. Fig. 7a shows
the micrograph of a specimen with 100% fly ash mortar (ET1). The
micrograph indicates that there is a strong bond between the unreacted
or partially reacted fly ash and polymerized binder with no
cracks detected along the interface. Some small fly ash particles
which have reacted with the alkali activator solution are observed
to co-exist with part of the remaining unreacted spheres and even
with some other particles partially covered with reaction products;
the Si content in the binder particles are substantially higher than
that of the geopolymer gel [16] and these unreacted/partially reacted
particles will react slower. The micrographs of mortar specimens
ET5 and ET7, which consist of 25% and 75% POFA,
respectively, show bonding similarities. Mortar specimens ET1
and ET5 which consist predominantly of FA show the presence of
a congested bulk of nanofibers produced on the surface of unreacted
fly ash particles as seen in Fig. 7a, b and d. Previous study
showed that hydrolysis reactions occur on the surface of the solid
particles [17]. Fly ash is known to contain a significant proportion
of particles with hollow spheres. When these hollow spherical particles
are partially dissolved they create porosity in the matrix containing
highly dispersed small sized pores. These un-reacted
particles were found in hollow cavities as seen in Fig. 7a–d [33].
Increasing percentages of POFA change the microstructure of
POFA/FA based geopolymer mortar. The FESEM results of sample
ET7 shows unreacted fly ash particles appeared in the porous form
(Fig. 7f). The microstructure of mortar ET8 which contains only
POFA (Fig. 7g and h) illustrate the porous structure of POFA particles
as well as increased dispersion of unreacted or partially reacted
particles. The unreacted POFA particles have ability to trap
air because of its inherent crumbled shape as can be clearly observed
in Fig. 3b. The density results also confirm the effect of porous
POFA as it produced lower density compared to the compact
image of FA based specimen ET1.