# Preliminary Study of New Sustainable, Alkali-Activated
## 3 Results
### AAC Characterization
#### 3.2.2 AAC Physical and Mechanical Characterization  
An in-depth physical and mechanical characterization was conducted. To perform a mechanical study of the different formulations depending on the L/S ratios, the apparent density (\(\rho_{app}\)) and the compressive strength (\(\sigma_{s}\)) were also evaluated. When analyzing the results performed by triplicate of the \(\rho_{app}\) exhibited in Figure 6, as expected, an increase of the L/S ratio (from 0.5 to 0.6) led to a decrease of the \(\rho_{app}\), due to the greater amount of water in the AAC formulations. Moreover, when comparing the results as a function of NaOH concentration (1 M, 4 M, and 8 M), it can be noticed that samples with the 4 M concentration have higher values, while samples with the 1 M concentration have lower values. On one hand, the 05 LS4M formulation was the densest (more compact), and on the other hand, the 05 LS1M was the least dense. Hence, regarding the density results, the 05 LS4M formulation would be expected that presents better behavior from a mechanical point of view. In this way, it was considered relevant to evaluate the mechanical properties concerning the compressive strength (\(\sigma_{s}\)), as constructive purposes were the main use for the AAC  
Figure 5: FTIR spectra results for the CSP precursor and the different AAC formulations obtained, depending on the L/S ratios (0.5 and 0.6) and the NaOH concentration. Figure 6: Apparent density results of the different AAC formulations obtained, depending on the L/S ratios (0.5 and 0.6) and the NaOH concentration. formulations. The results shown in Figure 7 demonstrate a decrease in the compressive strength when increasing the L/S ratio from 0.5 to 0.6, due to the greater amount of water needed in the formulations. Regarding the results as a function of NaOH concentration, it the highest values were observed in 4 M samples, and the lowest values in 1 M samples. Then the most compact and cohesive formulation had the highest compression strength values, and the most disaggregated formulation (see Figure 6) had the lowest compression strength results. Accordingly, the 05 LS4M formulation presented again better results from a mechanical point of view. In other words, the apparent density and compressive strength results are in agreement. The compressive strength obtained in this research is remarkable when compared to other studies. As mentioned, [PERSON] et al. obtained 70 MPa [25], but the samples were cured at 75 \({}^{\circ}\)C. [PERSON] et al. obtained around 49 MPa [26], but the glass waste was combined with fly ash (FA), which apports aluminum to the gel, and the samples were cured at 60 \({}^{\circ}\)C. [PERSON] et al. obtained around 30 MPa at 28 days [27]; in this case, the samples were cured at room temperature, but once again they used FA. These results are related to the abovementioned results of XRD and FTIR. On the one hand, XRD revealed the presence of CSH in 4 M formulations, and better evidence of sodium carbonate in 8 M formulations. As it is reported [24], when the activator concentration is high, it is difficult to produce the precipitation of hydrated calcium silicate. The presence of CSH in 4 M formulations improves mechanical properties. On the other hand, FTIR reveals slightly lower frequencies (peak between 1020 cm\({}^{-1}\) and 970 cm\({}^{-1}\)) for 4 M, which is related to NASH formation; this could be attributed to a better formation of NASH gel for 4 M formulations. In this manner, considering physical, mechanical, and microstructural results, it could be assumed that 4 M formulations must present more CSH and NASH phases than 1 M and 8 M formulations. The main aim for the SEM study in the present manuscript is focused on the evaluation of AAC cohesion. The AAC formulations for L/S = 0.5 were selected to be observed by SEM using secondary electrons, as is shown in Figure 8. Formulations with L/S = 0.5 were selected for the SEM evaluation due to the higher \(\sigma_{s}\) and \(\rho_{app}\) values compared with L/S = 0.6 formulations.