## 5 Discussion
It also can be realized that class F fly ash is quite effective on the permeability reduction of concrete, however, the optimum percentage of fly ash as the major factor should be considered to obtain more appropriate results [39]. It was noted that a 50% cement replacement ratio has the highest value of mechanical properties such as compressive strength comparable to other concrete mixtures produced by the replacement ratio of 0%, 25%, 75%, and 100% [45]. Furthermore, \"Does this cement replacement have similar effects on other concrete mixtures produced from other Portland types (Portland type 2)?\" is arguably an important question to be investigated, however other significant factors such as binder materials and activator solution ratio can be considered towards a more profound understanding of the chemical solution role in different concrete mixtures in the near future. Workability is typically attributed to liquid-to-solid ratio in most concrete mixtures such as natural pozzolan-based alkali-activated concrete. In laboratory schedule by [PERSON] et al. [46], very low slumps were measured for concrete mixtures composed of different binder content and alkaline material, which revealed a number of gaps and shortcoming in this case. Based on the outcomes of this research, it seems that the higher binder content is beneficial for the concrete mixtures containing the low ratio of water-to-solid compared to those with lower values of binder content and sufficient workability, which was achieved from the mixtures including high concrete binder in spite of the low ratio of water to solid. Generally, the improvement of compressive strength in the concrete mixtures thoroughly consisted of pozzolanic additives (100% natural pozzolan) as the replacement of cement Portland is associated with the following factors:  
* Binder content (fly ash quantity)
* Sodium silicate-to-sodium hydroxide (SS/SH) ratio
* Alkaline solution-to-binder ratio  
However, the secondary parameters such as concrete age and ambient condition, for instance heating at 60 \({}^{\circ}\)C, need to be considered as well. It is urgent to point out that each of the abovementioned factors can be effective for increasing compressive strength in a certain value range and beyond that, affected negatively on concrete strength. For example, the highest compressive strength belongs to the concrete mixture involving 400 kg binder content and SS/SH ratio of 2.5 with the strength more than 36 MPa at 7 days, compared to that with the value of 450 kg binder content. Furthermore, it was observed that the strength related to the concrete specimens with SS/SH ratio of 2.75 was marginally enhanced while increasing binder content from 400 to 450 kg/m\({}^{3}\), suggesting that the higher binder concrete needs a higher ratio of SS/SH. In the case of specimens cured for 28 days, similar results were recorded by the incremental value of 100 kg/m\({}^{3}\) from 350 to 450 kg/m\({}^{3}\) for binder content in spite of a marginal enhancement in the compressive strength while increasing from 400 to 450 kg/m\({}^{3}\). This proved the previous assumption in terms of the appropriate range of materials incorporated in the concrete mixtures [46]. As for the effect of alkaline solution-to-binder ratio, the quantity of Si and Na ions in the chemical combination of this concrete type is one of the central factors affecting the value of compressive strength, proving that it is essential to create a balance between total alkaline materials and binder content. This assures that all materials including aluminosilicate (Al-Si) are used in the alkaline activation process remaining very minimum unreacted particles in the mixture [46]. Noticeably, it can be justified that the presence of Si with high consumption in alkaline solution content led to the formation of Si-O-Si, in which this phenomenon probably happened in the experimental program conducted by [PERSON] et al. [46] with alkaline material-to-binder ratio of 0.525.