3.5. Role of POFA-FA on the Development of Compressive Strength
The high content of pozzolanic silica and alumina in POFA and FA causes the silica based geopolymerization in presence of alkaline solution. The silica/alumina ratio and total content of silica, alumina, and Fe2O3 affect the strength development [11]. It is to be noted that the binders, POFA, and FA were kept at equal proportions for all the mixes. The silica/alumina ratio and the total contents of silica, alumina, and Fe2O3 in the POFA-FA mixture were found to be 3.95 and 82%, respectively, and Autef et al. [41] reported that high amount of silica content increased the rate of geopolymerization.
As seen from Table 1, 90% of particles of POFA and FA were found below 38.27 and 101.74 μm, respectively. The finer particles decrease the capillary pore effectively [42]. The angular and irregular shaped QD particles create voids within the mortar. The filler effect of the fine FA and POFA might reduce the voids and Ashtiani et al. [43] reported that higher packing factor enhanced the compressive strength.
The effect of oven curing on the strength development also plays significant role in geopolymer mortar as seen from the 3-day compressive strength; as seen from Table 6, the achievement of 76% of 28-day compressive strength at the age of 3-day is attributed to the heat curing in oven at 65°C temperature for 24 hours. However, the rate of strength increment from 3 days to 7 days was found between 12 and 16% and beyond 7 days, it is much lower as reported by Alexander and Mindess [44]. Similar finding on high early strength development due to heat curing at elevated temperature was reported elsewhere [11, 45]. The fineness of POFA and FA also has significant effect on the development of early age strength at 3 days. The finer particles have larger surface area that may increase the reactivity in the geopolymerization process [46].