## 3 Results
### Hardened State Mortar Properties  
#### 3.3.1 Compressive Strength  
The average compressive strength of the AAEAFS uncarbonated samples is presented in Figure 6, which shows how the change in parameters affects the mechanical performance of the samples. The values presented in the figure have coefficients of variation ranging between 0.5 and 7.5%. Mixes N4S0.5, N6S0.5, N10S0.5, and N12S0.5 were eliminated due to inconsistent values and unexpected outcomes during the mixing process.  
The alkaline solutions of mixes N10S0.5 and N12S0.5 showed precipitation, possibly due to the saturation of silica, during its 24 h of cooling before the mixing day. Thus, the solution must undergo different conditions to avoid any fluctuation in the results. Correspondingly, the alkali activator is the most important factor controlling compressive strength.  
Figure 5: Slump values for mortar mixes (WRA content of (**a**) 0.5% and; (**b**) 1.0%).  
Different studies on alkali-activated EAFS used at least one of the alkaline activators stated in this study; [PERSON] et al. [35], [PERSON] et al. [25], and [PERSON] et al. [50] used both sodium hydroxide and sodium silicate to achieve compressive strength values of 40.7 MPa, 22.0 MPa, and 16.0 MPa, respectively. [PERSON] et al. [51] used only sodium hydroxide and recorded 27.0 MPa. This study presented maximum average compressive strength for uncarbonated specimens of 9.61 MPa for mixes with 4% and 2.5 of Na\({}_{2}\)O/binder and SiO\({}_{2}\)/Na\({}_{2}\)O ratios, respectively. Mixes N10S0 and N12S0, which were specimens with insufficient stability, exhibited values close to zero and thus were not reported. This could be due to the excess amount of sodium hydroxide and the lack of sodium silicate. Increasing the amount of Na\({}_{2}\)O to a given percentage increases the strength, after which the performance starts to deteriorate. Nevertheless, excess amount of OH\({}^{+}\), due to the Ca(OH)\({}_{2}\) across the particles of EAFS, reduces the interaction of Ca\({}^{2+}\) ions from the surface of EAFS [34]. Therefore, it can be stated that the low strength results from the inadequate amount of C-S-H gels produced by the reaction of Ca\({}^{2+}\) with Si\({}^{4+}\).  
Figure 6: Average compressive strength: (**a**) 4% sodium concentration; (**b**) 6% sodium concentration; (**c**) 8% sodium concentration; (**d**) 10% and 12% sodium concentration (Mix code-C represents carbonated specimens).  
Sodium hydroxide and sodium silicate have a significant influence on the mechanical properties of AAEAFS. Si\({}^{4+}\) and Al\({}^{3+}\) ions within the EAFS dissolve much more with a high concentration of OH\({}^{-}\)[52]. [PERSON] et al. [53] observed that increasing the concentration of the alkali activator increases the reaction rate as a result of a high alkali medium. [PERSON] et al. [54] stated that sodium hydroxide and sodium silicate are directly proportional to compressive strength only to a given level (from 3-5% by weight of the mix). The authors also stated that, if the amount of sodium concentration increased over a given limit, it would cause efflorescence, as shown in Figure 7. This is due to the migration of Na\({}^{+}\) ions to the surface of the specimens, leading to the precipitation of sodium carbonate.  
Another parameter affecting the compressive strength for all mixes was the curing time. The specimens in this study experienced 7-day, 28-days, and 91-days of curing in a dry chamber at 23 \(\pm\) 2 \({}^{\circ}\)C and RH of 65%. Since all specimens were sealed, there was no exchange of humidity with the surrounding environment. The maximum compressive strength (9.61 MPa) reported for the uncarbonated specimen N4S2.5, was obtained at 91-days of curing age.