Fig. 9 shows the compressive strength of the hardened geopolymer
specimens. As can be seen the FA based geopolymer mortar
gains about 70% and 98% of its 28-day compressive strength at
the 3 and 7 days, respectively. After 7 days, there is no tangible
strength gain in the ET1 mix. The high initial strength might be
attributed to low SiO2/Al2O3 in this geopolymeric system. The existence
of relatively lower Si component in ET1 to ET5 mixes with
SiO2/Al2O3 = 4.07–5.38, make it possible that more ALðOHÞ4
4 species
were available for condensation in these materials at early
stages. Further, the Al component tends to dissolve easier than
the silicon components, and this enables a higher rate of condensation
between silicate and aluminate species than the condensation
between just silicate species [45], resulting in high initial compressive
strength. However, initial SiO2/Al2O3 ratio will not be constant
throughout the geopolymerization process. The SiO2/Al2O3 increases
during different stages of geopolymerization [16]. Changes
in the Si/Al ratio in the original particles, the reactive ones, and the
reacted product during the reaction process affects the trend of
compressive strength development [46]. Fig. 9 shows that geopolymerization
was almost complete after 7 days and the strength gain
beyond this period was found insignificant