*3.2. Setting Time*

The setting time of cement or binder occurred when it loses its plasticity and slowly formed into hard rock type material. The initial setting time can be defined as the time taken by the paste to start stiffening; whereas the final setting time is when the paste begins to harden and able to carry some loads. The initial and final setting times for the AAFA paste through underwater placement in various water types are shown in Table 2. For the initial setting time, the AAFA specimens placed in seawater were recorded at the fastest time of 26 min, while the river water specimens recorded with the longest time of 30 min. For final setting time, the specimens in seawater and lake water recorded the same value of 35 min and for river water it recorded 37 min. The final setting time of Class C fly ash in dry condition (room temperature) is usually between 1–2 h, depending on the content of calcium (CaO) [38]. For the current study, the initial and final setting time of the AAFA specimens casted in dry condition reported 31 min and 40 min, respectively. This finding in agreement with past research where it was found that Class C fly ash recorded initial and final setting times of 32.15 min and 60.00 min for specimens casted in dry condition [39]. Source materials rich in Ca content have quick setting time in dry condition due to the higher dissolution rate of Ca2+ compared to Si4+ and Al3+. The reaction product of the source materials rich in calcium is expected to form of Ca-rich phases that will develop the fundamental skeleton of the AAFA network. According to previous research, reaction products such as calcium silicate hydrate (C-S-H), calcium aluminate silicate hydrate (C-A-S-H) and sodium calcium aluminate silicate hydrate (N, C-A-S-H) are expected to be present in Ca-rich phase [40].

**Table 2.** Setting time of AAFA cast underwater.


The quick setting time for underwater placement of the AAFA in seawater is due to Cl− ions which react with cations in the AAFA paste such as Na<sup>+</sup> and Ca2+. The formation of calcium chloride (CaCl2) is widely known as an accelerator for early strength development as well as a minimiser for the setting time [41]. Additionally, during underwater placement of the AAFA specimens, the existence of water helps to improve the properties of AAFA. According to Duxson et al. [42], water helps accelerate Si and Al dissolution process from the source materials by providing discontinuous gel nanopores to the paste, hence improved its performance. For practical application, it is suggested to use the retarding admixture to control the setting time, as it can delay the setting time and keep the AAFA concrete workable throughout the placing process.
