Cement hydration, according to Papadakis , is a process in which water is added to powdered cement to create calcium silicate hydrate (C-S-H) gel (Chemical Reaction (1)). As a result of the cement hydration reaction, a specific hydroxyl group, namely calcium hydroxide, would be liberated.
(C3S; C2S) + H2O → C-S-H gel + Ca(OH)2 (1)
Calcium hydroxide is produced during the cement hydration process because of the presence of water, which might function as an activator for the pozzolanic reaction of fly ash. The pozzolanic reaction of fly ash produces calcium silicate hydrate (C-S-H) gel, which controls the cementitious characteristics of concrete (Chemical Reaction (2)) .
Ca(OH)2 + H4SiO4 (fly ash content) + H2O → (C-S-H) gel (2)
After 3 days of being combined with the cement mixture, the pozzolanic reaction of the fly ash would begin . The pozzolanic process, in comparison to the cement hydration reaction, is thought to take longer to complete [7, 8]. According to ASTM C618-19 , there are two types of fly ash: high calcium fly ash (Class C) and low calcium fly ash (Class A) (Class F). Table 1  shows the calcium concentration of fly ash for both Class C and Class F.
Calcium content in Class C and Class F fly Ash.
Types of Fly Ash
≥ 8% Calcium oxide (CaO)
< 8% Calcium oxide (CaO)
One of the most significant distinctions between Class C and Class F fly ash was that Class C had some cementitious characteristics before experiencing the pozzolanic reaction, but Class F had to rely exclusively on the pozzolanic reaction to get cementitious capabilities . It has also been demonstrated that concrete containing high calcium fly ash (Class C) reacts faster and develops higher early-age strength than concrete containing low calcium fly ash (Class F) . However, as compared to Class F fly ash, Class C fly ash has a weaker resistance to sulphate attacks and less suppressing capacity for alkali-silica growth [11, 12]. One of the primary benefits of HVFA concrete, according to Rashad  and Kim et al. , was its resistance to sulphate attack. The high calcium fly ash Class C, on the other hand have a negative impact on these HVFA concrete's most important characteristics. As a result, Class C fly ash should not be used in HVFA concrete casting.
Siddique  shown that increasing the fly ash replacement ratio from 40–50% resulted in a decrease in the compressive strength of HVFA concrete. It has also been observed that after one year of curing, the compressive strength of HVFA concrete cannot exceed that of OPC concrete. Padurangan et al.  conducted a similar experimental investigation on HVFA concrete with varied fly ash replacement percentages of 55%, 65%, 75%, and 85%. The results showed that HVFA concrete with a greater percentage of fly ash replacement have a slower compressive strength development than HVFA concrete with 55% and 65% fly ash replacement. At the 28th day, HVFA concrete with 75% and 85% fly ash replacement were able to surpass the compressive strength of OPC concrete, but HVFA concrete with 75% and 85% fly ash replacement were not. Both Siddique  and Padurangan et al.  found that increasing the fly ash replacement percentage causes the compressive strength of HVFA concrete to grow more slowly. When comparing the results of Siddique  and Padurangan et al. , however, there was a discrepancy. At the 28th day, HVFA concrete with 40% and 50% seen by Siddique  was unable to overcome the compressive strength of OPC concrete, but HVFA concrete with 55% and 65% observed by Padurangan et al.  was able to surpass the compressive strength of OPC concrete. This contradicts the conclusion drawn from both findings. One probable source of this discrepancy is the use of different binder materials (cement and fly ash) in concrete mixtures for both studies, which have distinct chemical reactions. Another explanation for this phenomenon might be the presence of an optimal fly ash replacement percentage, which is anywhere between 40% and 65%. As a result, more study should be focused on replacing fly ash in the range of 40–65%, while keeping the same quantity of binder material (cement and fly ash). This is to see if the inferred connection from the findings is correct or if there is an ideal proportion of fly ash substitution.
For the cement hydration process to be completely active, the water to binder ratio was found to be 0.22 to 0.25 . In the past, an excess amount of water was added to the concrete mix to make it workable. Wang and Park  and Sarika et al.  found that when the water to binder ratio surpasses a specific threshold value, which changes depending on the amount of fly ash replacement, the compressive strength of HVFA concrete cannot surpass that of OPC concrete on the 28th day. As lime water is a diluted form of calcium hydroxide, a low water to binder ratio was required to get a better compressive strength. With 50% ultra-fine fly ash substitution, the impact of utilizing saturated lime water in concrete casting was shown to be beneficial in improving the 28th day compressive strength of HVFA concrete . On HVFA concrete cast with raw fly ash, however, the increase in strength was less noticeable . According to Tahir and Chaudhry , mixing alkaline liquid with lime in fly ash concrete does not enhance the compressive strength of 25%, 35%, or 50% fly ash concretes, but rather decreases it. Heat and water curing was a typical curing process for HVFA concrete. Heat curing was shown by Yazici et al.  to be beneficial in increasing first-day compressive strength but decreasing long-term compressive strength of HVFA concrete. On the other hand, HVFA concrete that is water cured had a lower compressive strength on the first day compared to OPC concrete, but the difference became less noticeable by the third day. The cement hydration process in HVFA concrete is favored when the curing temperature is less than 35°C . When the curing temperature is approximately 50°C, however, the cement hydration process is inhibited and the pozzolanic reaction is favored within the HVFA concrete .
As a result, it was determined that additional study on HVFA concrete should be undertaken to fill up the information gaps. The discrepancy between Siddique  and Padurangan et al.  findings must be resolved via study. Aside from that, there is only a small amount of study on the influence of saturated lime water on the compressive strength development of HVFA concrete with a 40–60% fly ash replacement. As a result, the purpose of this study was to address the highlighted research gaps.