# Replacement of Natural Sand with Expanded Vermiculite in Fly Ash-Based Geopolymer Mortars
## 3 Results and Discussion
### Apparent Porosity  
The apparent porosities of the geopolymer mortar are shown in Figure 5. It is shown in the figure that, for a given FA and vermiculite content, the 10 molar NaOH mixes developed a higher apparent porosity in comparison with the 15 molar NaOH mixes. This agrees with previous studies on FA-based geopolymer mortar [50], FA-based geopolymer concrete [37] and geopolymers containing zeolitic tuff and marble waste [51]. This behavior was because of the higher microstructure compactness of the mixes having a higher activator concentration in comparison with those having a lower activator concentration [51]. The 10 molar NaOH mixes with 50% and 70% FA exhibited 2-7% and 4-10% higher apparent porosities in comparison with the 15 molar NaOH mixes, respectively. As is also shown in the figure, for a given vermiculite content and NaOH concentration, increasing the FA content resulted in the increased apparent porosity of the geopolymer mixes. This behavior matches with previous studies on concrete with recycled concrete fine aggregate [52] and gypsum composite [53; 54], and it is due to the hollow structure of FA because of the cencosphere in its microstructure, causing increased porosity of the mix [55; 56]. For the 10 molar NaOH solution, 70% FA mixes containing 0%, 10% and 30% vermiculite exhibited 13%, 12% and 17% higher porosities, respectively, and for the 15 molar NaOH solution, they exhibited 6%, 10% and 14% higher porosities compared with the companion 50% FA mixes, respectively.  
Figure 4: Flowability of geopolymer mortar.  
It is shown in Figure 5 that, for a given FA content and NaOH concentration, replacing natural sand with expanded vermiculite caused increased apparent porosities in the mixes. It is also shown that increasing the vermiculite content caused increased porosities in the mixes. These behaviors are consistent with those reported in previous studies on conventional mortar [25; 26], metakaolin-based geopolymer concrete [57] and clay-based bricks [58; 59]. At 50% FA, the mixes containing 15% and 30% vermiculite had approximately 5% and 9% higher porosities, respectively, and at 70% FA, they exhibited approximately 6% and 16% higher porosities in comparison with the companion mixes without vermiculite, respectively.  
Figure 6 illustrates SEM images of fresh-cut surfaces of the S20F50V30A specimen. The lamellar-structured expanded vermiculite, which contained air between the vermiculite flakes, is visible in Figure 6a,b, and the FA particles are visible in Figure 6c. As can be seen in the figures, the mix with expanded vermiculite contained a large amount of porosities in its microstructure. In addition, the interfacial transition zone (ITZ) between the expanded vermiculite and the mortar mixture was porous, resulting in the weak bonding between the expanded vermiculite and the mortar mixture. These are the reason why geopolymer mixes containing larger expanded vermiculite contents exhibit higher porosities, leading to a lower dry unit weight, compressive strength, ultrasound pulse velocity and thermal conductivity, as well as more water absorption, as will be discussed in the following sections.  
Figure 5: Apparent porosities of the geopolymer mortar.  
Figure 6: (**a**–**c**) SEM images of fresh-cut surfaces of the S20F50V30A specimen with different magnifications.