## 2 Supplementary Cemertitious Material (SCM)  
Green buildings are essential elements for acquiring sustainability, and the concrete industry can move toward sustainability by introducing SCMs [13; 14], which can also lead to improved durability and mechanical properties of concrete [15; 16]. Industrial residues, such as SF and FA, are extensively employed as SCMs due to the high pozzolanic reactivity [16; 17; 18; 19]. Furthermore, continuous effort is made to introduce agriculturally sourced pozzolanic substances, such as rice husk ash (RHA) [20; 21], corn cob ash [22; 23], wood ash [24], natural zeolite [4; 25], bamboo leaf ash [26], and palm oil fuel ash [27; 28; 29; 30; 31]. As an example, when cement is replaced by micro-palm oil fuel ash (mPOFA) at certain levels, the compressive strength will be increased [32; 33]. Replacing 10% weight of cement with mPOFA increases compressive strength up to 33%. This could be caused by mPOFA occupying the space between the particles of cement and enhancing the creation of calcium-silicate-hydrate (C-S-H) gel [34]. Table 1 expresses the composition and properties of typical OPC and some common SCMs. The higher value of SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) and Fe\({}_{2}\)O\({}_{3}\) means the higher pozzolanic feature of SCM. According to ASTM C618, the value of all these three oxide compositions together should be more than 70%, as the requirement of material to be considered to be class N or F natural pozzolan. FA is known as one of the most common SCMs which can reduce GHG emission of concrete and enhance durability, fracture toughness, and compressive strength of this material [13; 42; 43]. On the other hand, it is noted in previous studies that although enhancement in durability and compressive strength was observed by using FA, extensive usage may lead to challenges such as higher carbonation and delayed hydration [43]. Moreover, SF is the other commonly used pozzolanic substance that can also enhance the strength and durability significantly due to the high purity of silica content with fine particle size increasing its reactivity [44; 35; 41]. However, the main challenge for SCMs which are by-products of other industries (such as SF and FA) is the limitation in global production [45]. Furthermore, the quality of by-products is the other issue. For example, over 66% of the accessible FA, which has one of the highest quantities between these by-products, is not appropriate for mixing with cement [46]. Calcined clay is a kind of artificial pozzolan and its pozzolanic activity is affected by parameters such as the quantity of calcined minerals, impurity measure, activation technique, and post-calcination. [47]. Calcined clays appear as a confident source of SCM, able to offer a considerable replacement of the Portland cement clinker in mixed cement [48]. Various types of clay minerals include illite, kaolinite, palygorskite, and montmorillonite [49; 50]. It is documented that among the different type of clay minerals, kaolinite has the highest pozzolanic activity [47; 51]. After calcining the kaolinite-containing clay, metkaolin is created that is an amorphous alumino-silicate (Al\({}_{2}\)Si\({}_{2}\)O\({}_{7}\)), which may make a reaction with calcium hydroxide to provide calcium-aluminate-silicate-hydrate (C-A-S-H) and aluminate hydrates [45]. Furthermore, carbo-aluminate hydrates could be produced as the reaction between the alumina and limestone [52]. The metkaolin (clay) is an abundant material and also its quality is further stable compared to FA and slag [53; 54; 55; 56]. [PERSON] and [PERSON] reported that by the presence of 20% metakaolin in the self-compacting concrete mixture, the tensile and compressive strengths (28 days) could be increased to 25% and 30%, respectively [57]. This indicates that calcined clay has the potential to show higher pozzolanic reactivity than FA. Increase in compressive strength of mortar blending at an early age was also observed by using metakaolin as SCM [48; 58; 59].