# Replacement of Natural Sand with Expanded Vermiculite in Fly Ash-Based Geopolymer Mortars
## 3 Results and Discussion
### Thermal Conductivity  
The thermal conductivities of the different mixes are shown in Figure 11. It is shown in the figure that, for a given FA and vermiculite content, the mixes with the 10 molar NaOH solution developed lower thermal conductivities compared with those with the 15 molar NaOH solution. The lower thermal conductivities of the mixes having lower concentrations of the activator agree with previous research on metakaolin-based geopolymer paste [75] and geopolymer with marble waste and zeolitic tuff [51], and this is because the mixes with lower activator concentrations have higher porosities, which consequently results in increased thermal insulations in comparison with those having higher concentrations of the activator [51]. The 10 molar NaOH mixes with 50% and 70% FA exhibited 1-4% and 4-8% lower thermal conductivities compared with the 15 molar NaOH mixes, respectively. As is also observed in the figure, for a given vermiculite content and NaOH concentration, the mixes containing larger amounts of FA had lower thermal conductivities in comparison with those containing smaller amounts of FA. This agrees with previous studies on cement mortar [76], cement concrete [77], gypsum composite [78] and lightweight concrete [79], and this is due to the increase in the porosities of the mixes with increasing contents of FA, causing increased thermal insulations in the mixes [79]. For the 10 molar NaOH solution, the 70% FA mixes containing 0%, 10% and 30% vermiculite exhibited 27%, 36% and 36% less thermal conductivity, respectively, and for the 15 molar NaOH solution, they exhibited 23%, 34% and 34% less thermal conductivity in comparison with the companion 50% FA mixes, respectively.  
As can be seen in Figure 11, replacing the sand with expanded vermiculite led to a decrease in thermal conductivity in the mixes for a given FA content and NaOH concentration. It can also be seen from the figure that an increased amount of expanded vermiculite resulted in decreased thermal conductivities in the mixes. This behavior is similar with that found in previous research on conventional mortar [25; 26], metakaolin-based geopolymer concrete [57], slag-based geopolymer mortar [65], gypsum composites [58] and clay-based bricks [59]. As was reported previously [80; 81; 82], the thermal conductivity has a direct relationship with the dry unit weight; a decreased dry unit weight leads to a decreased thermal conductivity. The same trend was observed in this study, showing that an increased vermiculite content caused a decreased unit weight, which consequently led to a decreased  
Figure 11: Thermal conductivities of the geopolymer mortar.  
thermal conductivity of the geopolymer mortar. This is due to the lamellar structure of the expanded vermiculite, which contains large amount of air between the vermiculite flakes, helping to decrease the thermal insulation of the mixes [83]. At 50% FA, the mixes containing 15% and 30% vermiculite exhibited approximately 12% and 24% lower thermal conductivities, and at 70% FA, they exhibited approximately 24% and 34% lower thermal conductivity than the companion mixes without vermiculite, respectively. As was reported by [PERSON] and [PERSON] [84], concrete mixes containing 100% expanded polystyrene developed a thermal conductivity of 0.60 W/mK. However, the mortar mixes of the present study containing only 30% expanded vermiculite developed a similar or lower conductivity in comparison with the 100% expanded polystyrene mix of that study. It was also reported by [PERSON] et al. [21] that the thermal conductivity of the mortar mixes containing 40% expanded perlite was 0.53 W/mK, which was higher than the thermal conductivity of the 70% FA mixes in the present study containing 15% expanded vermiculite. As was reported by [PERSON] et al. [85], FA-based foamed geopolymer pastes containing 30% microsphere, 70% FA and 7% hydrogen peroxide as a foaming agent had a thermal conductivity of 0.095 W/mK and a 28 day compressive strength of 1.9 MPa, which are lower than those found in the present study.