## 4 Mechanical Properties of Geopolymer Concrete
### Concurrent Efficiency of Fly Ash and OPC on Mechanical Properties  
The use of FA in concrete mixtures has always been of interest to researchers for decades due to its ability to improve concrete durability and decrease CO\({}_{2}\) emissions. Hence, a large number of investigations were carried out to explore the role of FA in cement hydration and pozzolanic mechanism. Although FA does not participate in the early hydration stage, it can help to increase the cement hydration rate due to a filler effect promoting cement hydration. This occurs through seeding effects by preparing additional nucleation sites for hydration productions on filler surfaces, enhancing the efficient W/C ratio to ensure adequate space for the growth of hydration product, which results in the improvement of mechanical properties of this concrete type [35; 36]. Therefore, the achievement of this optimal combinative concrete comprised of geopolymeric and Portland cementitious materials is one of the main motivations of current investigations conducted in this field. A more comprehensive description can be found in the recent studies related to this topic. For instance, in research carried out by [PERSON] and [PERSON] [37], it was shown that the compressive strength can be enhanced with replacing the mix design of conventional concrete (OPC) with low calcium fly ash. Other observations also demonstrated that coexistence of hydrated products along with the structure of geopolymeric alumina silicate polymer can improve some mechanical properties, such as water absorption and porosity in this concrete mixture type. In an investigation conducted by [PERSON] et al. [38], the effect of physicochemical properties of fly ash on compressive strength of concrete with a high amount of fly ash was experimentally examined from another distinctive aspect. In this research, 50% of ordinary Portland cement weight was blended as the replacement of fly ash in order to measure the compressive strength of this mix design type containing a high value of fly ash and OPC. Fly ash is widely used as a complementary cementitious material which has been utilized in all kinds of concrete structures over a few decades due to its potential to enhance concrete durability and diminish CO\({}_{2}\) emissions. Therefore, many investigations have been carried out to distinguish the role of fly ash in hydration reactions and pozzolanic mechanisms in cement. The achieved findings from these studies indicate that fly ash would not react with water in the initial hydration stage, but it enables the cement to precipitate the rate of its hydration due to a filler effect in which the cement hydration can be accelerated by providing extra nucleation places for hydration products on filler surface and congruently leads to the enhancement of the water to cement ratio, confirming adequate space for the enhancement of hydration product. In a study by [PERSON] et al. [39], the effect of class F fly ash as the percentage of cement weight on sulphate resistance of type V Portland cement was investigated. The amount of fly ash considered was 15%, 20%, 25%, and 30% by cement weight for this experimental test. The laboratory observations exhibited that replacement of one portion of Portland cement with class F Fly ash led to the improvement of concrete resistance against sulphate attack, however this increment in concrete resistance was not as much as anticipated for class F fly ash.