3.3. Raw materials characterization
3.3.1. Natural pozzolan
The pozzolan fragments used in this study show a low bulk density (Fig. 4a), high porosity, and high water absorption of around 1100 kg/m3, 54%, and 13%, respectively. From a chemico-mineralogical point of view, the chemical analysis of the pozzolan shows the composition of a basic rock (SiO2 = 37%), moderately aluminous (Al2O3 = 15%), strongly ferromagnesium-titanium-bearing (Fe2O3 + MgO + TiO2 = 28%), and more calcareous than alkaline (CaO/ (Na2O+K2O) = 6.62) (Fig. 3a). Microscopic analysis of thin-section of pozzolan fragments under a polarizing microscope showed a vacuolar texture (V) (Fig. 4c), appearing in white and a vitreous matrix (M), showing in black, punctuated with microlites of feldspathic plagioclases (PL), diopside (Px), amphibole (Am), and opaque micro grains (hematite). Mineralogical analysis by X-ray diffraction corroborates the observation results of the thin-section and showed the presence of small amounts of amorphous phase as well as the crystalline phase well expressed in peaks of varying intensity (Fig. 5), corresponding to microlites of plagioclase (anorthite), calco-magnesian pyroxene (augite), iron oxides (hematite), and olivine (forsterite). FT-IR analysis also confirmed the mineral phases revealed by the XRD (anorthite, augite), by the manifestation of the absorption bands around 537 cm-1 and 460 cm-1 (Fig. 6); this reflects the presence of Si-O-Al and Si-O-Si bending vibrations, and the absorption band around 1042 cm-1, which corresponds to the asymmetrical and symmetrical stretching vibrations of the Si-O-Si and Si-O-Al links.
3.3.2 Natural perlite
The centimetric perlite fragments had a high bulk density of around 2300 kg/m3 (Fig. 4b), a low porosity varying about 1.5%, and a low water absorption rate of 0.66%. The chemical composition of the perlite, determined by XRF, showed that it is an acidic siliceous rock (SiO2= 73.46%), moderately aluminous (Al2O3=13.15%), and with a sodi-potassic character (Na2O + K2O = 3.33%) (Fig. 3b). The observation of thin-segments of the perlite under the polarizing microscope had shown a hyaline texture (amorphous) (Fig. 4d), punctuated by a few tiny grains and microlites of plagioclase, which corroborates the XRD results (Fig. 5), revealing the amorphous texture of the perlite, attested by the presence of an amorphous phase halo with a few peaks corresponding to the microcrystals of anorthite and quartz. The pozzolan infrared spectrum (Fig. 6), conducted by FTIR analysis, showed the presence of bands around 1000, 94 cm-1 and 780 cm-1, corresponding respectively to the Si-O-Si and Si-O-Al bonds’ stretching vibrations, and a band at 460 cm-1 attributed to bending vibration of the Si-O-Si bond. The bands displayed at 3456 cm-1 and 1639 cm-1 are assigned respectively to the bending vibration of the H-O-H bond and the stretching vibration of the -OH bond.
3.4. Characterization of geopolymer products
3.4.1. Macroscopic appearance
Cylindrical samples of the synthesized geopolymers are presented in Fig. 7. Macroscopic observation of pozzolan-perlite-based geopolymers shows satisfactory solidification without surface cracking or efflorescence, except for the G50 geopolymer which shows a very cracked matrix. Their colors vary from light yellow to light grey as the percentage of perlite increases. As the addition of 50% perlite to the pozzolan resulted in cracked specimens, the perlite addition has been limited to 50%.
3.4.2. Compressive strength analysis
The results presented in Fig. 8 show that the compressive strengths of all synthesized geopolymers increased with the curing age (from 7 to 28 days of curing), with an average rate of increase of 27%. In detail, the 10% perlite addition to the pozzolan resulted in a relatively significant improvement in Rc from 12.56 MPa to 17 MPa at 7 days and from 14 MPa to 18.6 MPa at 28 days of curing. The progressive increase of the perlite content from 10% to 40% has considerably increased the Rc of the samples, which reached 50 MPa for 40% perlite after 28 days of curing. From 40% to 50% perlite, the Rc evolved negatively by a decrease of 18% (from 50 MPa to 41 MPa for 28 days of curing). It means that the G40 specimen is the mechanically optimal geopolymer. This indicates that the addition of natural perlite, at the 40% level, has contributed to the compressive strength development of the synthesized geopolymers, presumably related to the additional reaction resource obtained from the perlite. The increase in the compressive strength with the perlite incorporation was due to the relatively higher SiO2/Al2O3 molar ratio and the amorphous nature of perlite compared to pozzolan [1, 30, 31], which increases the content of reactive silica and alumina in the reaction medium and, consequently, the formation of more geopolymerization products, which could be mainly sodium aluminosilicate hydrate gel (N-A-S-H) due to the low calcium content [30, 32]. These results are in accordance with those found by Duan et al, , which show the increase in compressive strength of fly ash-based geopolymers with increasing silica fume content. It should be noted that the geopolymerization reaction of perlite could produce co-products of zeolitic nature. This has been observed in the works of Vance et al., , Taxiarcho et al., , Kozhukhova et al., , who detected the formation of the zeolitic phases, particularly phillipsite, in geopolymers based on natural perlite. Hence, the decrease in compressive strength observed in the present study beyond 40% perlite could be related to the appearance of zeolitic phases in the geopolymer matrix, leading to the formation of a heterogeneous structure and, consequently, the decrease in the mechanical properties [34–37].
3.4.3 Bulk density and porosity analysis
Table 1 shows the bulk density and porosity results according to the perlite content added to the pozzolan. Based on physical laws, bulk density varies inversely with porosity, which is explicitly shown in the case of this study. The bulk density and porosity variation of the geopolymers synthesized in this study is consistent with the trend observed from mechanical properties, which is justified by the high positive correlation between density and Rc (R2=0.99) (Fig. 9). The partial replacement of pozzolan by perlite, from 10 to 40%, resulted in an increase in bulk density from 1813 g/cm3 to 2055 g/cm3, correlated to a decrease in porosity from 14% to 9.71%. These results can be attributed to the formation of geopolymerisation products such as N-A-S-H gel as a function of the perlite addition (≤40%), hence the densification of the geopolymer matrix . The increase in the perlite content, from 40% to 50%, decreases the density, from 2055 g/cm3 to 1987 g/cm3, and increases the porosity, from 9.71% to 10.77%, which is linked, according to Nuaklong et al. , to the weak zones formation, thus affecting the mechanical properties of the synthesized geopolymers.
3.4.4 P-wave velocity analysis
The P-wave propagation velocities (Vp) measured at 28 days of curing are shown in Fig.10. The results show that the Vp velocity increases, from 3133 m/s to 4055 m/s, with the increase in perlite content, from 10% to 40%, while the addition of more than 40% perlite decreases the P-wave velocity, from 4055 m/s to 3615 m/s, a velocity lower than that of the G30 geopolymer. The increase in Vp shows that the internal structure becomes denser and more homogeneous by increasing the pearlite content to 40%, which explains the good P-wave propagation [29, 40]. The decrease in Vp observed by the addition of more than 40% of perlite is due, according to El Azhari and El Hassani  and Garnier et al., , to the existence of discontinuities in the geopolymers. These results are in agreement with macroscopic observations (Fig. 7), measurements of bulk density (Table 1), and compressive strength (Fig. 8), showing that the perlite addition up to a content of 40% leads to the formation of a dense and homogeneous matrix, which increases the compressive strength, and consequently, the increase of Vp in the synthesized geopolymers. Above 40% perlite, cracks in the G50 geopolymer matrix were well highlighted in Fig. 7, which is at the origin of the P-wave velocity decrease. Similarly, the linear regression results showed a positive correlation between Vp and Rc of the synthesized geopolymers, represented by the determination coefficient R2, which was of the order of 0.95 (Fig. 9).
3.4.5. XRD analysis
The X-ray diffractograms of the synthesized geopolymers as a function of perlite content are shown in Fig. 11. A large halo has been detected in all perlite-pozzolan-based geopolymer composites between 20°-40° (2θ) indicating the formation of a glassy phase in the system [1, 43, 44]. In detail, the amorphous phase halo increase is proportional to the perlite percentage added, which shows the contribution of perlite to the more formation of the binder phase (N-A-S-H) in composites elaborated by its siliceous and vitreous nature, which consequently leads to the increase of the mechanical properties of the synthesized geopolymers (Fig. 11a) [45, 46]. This tendency was well demonstrated in the study conducted by Zhuen et al.,  by integrating up to 15% of rice husk ash into the fly ash-based geopolymer. Besides, the partial replacement of pozzolan by perlite leads to the appearance and disappearance of crystalline phases. In the case of optimal geopolymer G40 (Fig. 11b), the decrease and disappearance of some peaks corresponding to anorthite, hematite, and augite, were detected after the geopolymerization reaction. According to Aziz et al., and Robayo-Salazar et al., [1, 29, 46], this behavior was related to the partial dissolution of these crystalline phases and their involvement in the formation of the N-A-S-H geopolymer phase. The appearance of the phillipsite-type zeolite phase (2q = 13.88°, 16.54°, 30.38°, 32.45°, 37.55°) has been observed by increasing the percentage of perlite up to 50%, which is also observed in the work of Aziz et al., and Moon et al., [29, 48]. Therefore, this zeolitic phase neo-formation is at the origin of the decrease in compressive strength exceeding 40% of the perlite (Fig. 8), favoring the formation of more zeolite phase in the geopolymer matrix (N-A-S-H) forming a heterogeneous microstructure, and consequently, the cracks appearance, which is observed in the macroscopic aspect of the G50 sample, hence the decrease in mechanical properties .
3.4.6 FT-IR analysis
The infrared spectra of the raw materials (pozzolan and perlite) and the mechanically optimal geopolymer G40 are presented in Fig. 12. From Fig. 12, it appears that the large absorption bands located at 3456 cm-1 and the less pronounced at 1639 cm-1 on the spectra of G40 and pozzolan, are assigned to the vibration modes of O-H bonds belonging to water molecules . The absorption bands situated between 1384 and 1440 cm-1, are linked to the C-O stretching of the CO32- group . These bonds are formed from a reaction between an unfixed Na+ into a geopolymer matrix with CO2 of the atmosphere [51, 52]. The absorption bands appearing around 1042 cm-1 in all spectra are linked to vibration modes of Si-O-Al bonds . This absorption band is higher than those found in literature based on other aluminosilicates such as metakaolin, fly ash, etc. which is near to 1000 cm-1 [54, 55]. Thus, the incorporation of 40% by weight of the reactive perlite in the pozzolan allowed the incorporation of high content of Si- species that made this band shifted, as indicated in Fig. 12, towards the lower wavenumbers compared to that of pozzolan. A comparable tendency was observed by the findings of Kaze et al.,  and Kamseu et al.,  where they used reactive silica from rice husk ash to improve the formation of iron silicate compounds from raw iron-rich laterites cured at 80 °C. Also, this shift has been well demonstrated in the geopolymers synthesized by Tawatchai et al., , with the increase of the glass waste powder percentage in their fly ash-based geopolymer. In the case of the G40 sample, this band is more pronounced and indicates the formation of the N-A-S-H geopolymer phase required to ensure better connectivity between different particles in the whole system. Moreover, it belongs to the geopolymer network as reported by other researchers justifying that the reaction has taken place , which corroborates the compressive strength results found (Fig. 8) that show the increase in mechanical strength with the perlite addition up to 40%.These results correlate with those obtained by XRD (Fig. 11).
3.4.7. SEM/EDS analysis
Fig. 13 represents the SEM micrographs and the corresponding EDS (Energy-Dispersive X-ray Spectroscopy) spectra of geopolymers (G40 and G50) containing high perlite contents. As shown in Fig. 13, the geopolymer with 40% perlite (G40) shows a homogeneous and dense microstructure with few pores and cracks, constituted mainly by the N-A-S-H type geopolymer gel. This was also confirmed by EDS analysis which showed the presence of silica, aluminum, and sodium as major elements with a molar ratio Na/Si=0.49, which was near to the optimum ratio (0.5) for the N-A-S H gel formation ; significantly, this contributes to the increase of the mechanical properties of geopolymers [30, 32]. As for the 50% perlite composite geopolymer, its microstructure seems more heterogeneous, porous, and fissured than that of the G40 geopolymer, composed of the N-A-S-H type geopolymer phase, with a molar ratio Na/Si=0.44, and the phillipsite-type zeolitic phase with a prismatic morphology. These results corroborate those found by the XRD (Fig. 11) that show the coexistence of the N-A-S-H phase and the zeolitic phase in synthesized geopolymer cements with a high perlite content (≥40 %) contributing to the formation of a heterogeneous matrix (Fig. 7), and consequently, to the decrease of the mechanical properties of the synthesized geopolymers (Fig. 8) .