A spent fluid catalytic cracking catalyst (SFCC) from a Colombian petroleum company is used as the geopolymer precursor in this study. Before chemical activation, the SFCC was milled for 5 hours, using a ball mill. The resulting powder had specific gravity of 2630 kg/m³ and a mean diameter D (4, 3) of 16.8 µm. The particle size distribution, determined by laser diffraction using a Mastersizer 200 of Malvern, was: 10% below 2.46 µm (D10), 50% below 13.25 µm (D50) and 90% below 37.44 µm (D90). Its chemical composition, determined by X-Ray Fluorescence (XRF) using a Philips MagixPro PW–2440 spectrometer fitted with a rhodium tube, is shown in Table 1.

The X-ray diffractogram of the SFCC (Fig. 1) shows that this material contains an amorphous component, as well as the crystalline zeolite phases faujasite (Na₂Al₂Si₁₀O₂₄·nH₂O, Powder Diffraction File (PDF) # 012-0228) and analcime (NaAl₂Si₂O₆·H₂O, PDF # 003-0740), along with the aluminosilicate minerals andalusite (Al₂SiO₅, PDF # 039-0376), mullite (2Al₂O₃·SiO₂, PDF # 089-2645), sillimanite (Al₂O_3·SiO₂, PDF # 089-0888), kyanite (Al₂SiO₅, PDF #00-011-046) and quartz (SiO₂, PDF # 046-1045).

The alkali activator used was a commercial sodium silicate solution with 29.1 wt.% SiO₂ and 10.2 wt.% Na₂O and 60.0 wt.% H₂O, and solid analytical grade NaOH pellets. This was dissolved in the mix water and allowed to cool until reaching room temperature prior to preparation of the specimens. All the activating solutions reach a pH higher than 14.

For the preparation of the geopolymer specimens, the alkaline activator was formulated to obtain overall (activator + solid precursor) SiO₂/Al₂O₃ molar ratios of 2.0, 2.2, 2.4, 2.6, 2.8 and 3.0, and overall Na₂O/SiO₂ molar ratios of 0.20, 0.25 and 0.30. The amount of water in the alkali activator was adjusted to achieve a total H₂O/Na₂O ratio of 11 in all samples (Table 2). The fresh pastes were mixed for 12 minutes using a Hobart mixer, poured into cylindrical molds, and vibrated for 5 minutes to reach a homogeneous distribution in the mold and remove entrained air. Samples were kept in sealed molds at ambient temperature (25±5 °C) for 24 hours, then demolded and stored in a high humidity container (relative humidity >90%), also at ambient temperature, for 7 days.

The compressive strength was assessed for cylindrical paste samples of 30 mm (diameter) × 60 mm (height), using a universal testing instrument (Instron) at a displacement rate of 1 mm/min. Sample ends were flattened and made parallel using coarse sandpaper before testing. Each reported value corresponds to the average of 5 measurements. Powdered pastes were analyzed through:

The formulation of the specimens has a marked effect on strength. The highest mechanical strengths are obtained (Fig. 2) in samples formulated with a Na₂O/SiO₂ ratio of 0.25 and a SiO₂/Al₂O₃ ratio of 2.4, reporting values of up to 67 MPa. In activated SFCC pastes formulated with a low Na₂O/SiO₂ ratio (0.20), a significant decrease in the mechanical strength is identified at higher SiO₂/Al₂O₃ ratios beyond the optimum. This effect is least pronounced at higher Na₂O/SiO₂ ratio (0.30), where activated spent catalyst samples formulated with SiO₂/Al₂O₃ ratios between 2.4 and 3.0 do not show significant changes in the compressive strength as a function of the formulation conditions. Specimens formulated with a Na₂O/SiO₂ molar ratio of 0.20 and SiO₂/Al₂O₃ of 2.0 and 2.2 were not tested due to the low degree of compaction achieved in these specimens.

These results are comparable with the strengths of metakaolin-based geopolymers with the same curing duration and similar formulation conditions, including the reduction in strength at high SiO₂/Al₂O₃ ratios (16). This strength reduction is associated with the excess of silicates in the system supplied by the activator, as the extent of polymerization of the silica is high and its rate of reaction is slower than is required for optimal strength development (17). However, in the formulations with higher Na₂O/SiO₂ ratios, it is expected that the increased alkalinity promote a higher extent of dissolution of the spent catalysts, and therefore higher availability of Al-rich species, which are able to react with the additional silicate species that the activator can supply at higher SiO₂/Al₂O₃ ratios.

The X-ray diffractograms of the activated spent FCC pastes formulated with Na₂O/SiO₂ molar ratios of 0.20, 0.25 and 0.30 are shown in Fig. 3. All of the activated SFCC pastes show (Figs. 3A, 3B and 3C) an amorphous hump between 2θ values of 25° and 35°, attributed to the formation of an aluminosilicate type gel similar to those typically observed in metakaolin-based geopolymers (18). Mullite, sillimanite, kyanite and quartz, previously identified in the unreacted SFCC (Fig. 1), are also observed in the alkali-activated pastes, suggesting that these phases are not participating in the activation reaction to any notable extent. This is consistent with the observations in fly ash based geopolymers, where mullite and quartz are also identified as unreactive compounds (19), and the results of Rodríguez et al. (15) in alkali-activated specimens based on a Spanish SFCC material.

In samples formulated with an Na₂O/SiO₂ ratio of 0.20 (Fig. 3A), the intensities of the peaks assigned to faujasite and analcime are significantly reduced upon activation, when compared with the unreacted SFCC, along with the formation of zeolite Na-A (Na₁₂Al₁₂Si₁₂O₄₈·xH₂O, PDF # 039-0221), which has been identified as a reaction product in geopolymer materials based on metakaolin (20). The formation of multiple zeolite phases embedded in the geopolymer gel has been reported in several studies (18), and our findings are consistent with these results. The formation of ussingite (Na₂AlSi₃O₈(OH), PDF # 028-1037), which is a higher-silica secondary mineral often formed along with sodalite, is also observed under the activation conditions assessed.

Upon increasing the Na₂O/SiO₂ molar ratio to 0.25 (Fig. 3B), ussingite and zeolite Na-A are identified at the different SiO₂/Al₂O₃ ratios assessed. The intensity of the peaks assigned to faujasite is reduced upon activation. A similar trend is observed in the XRD patterns for the specimens formulated with the highest Na₂O/SiO₂ ratio (0.30) (Fig. 3C), where faujasite reflections are no longer observed. Considering this, and the fact that these samples have a more intense amorphous hump than specimens formulated with lower Na₂O/SiO₂ ratios, it is likely that faujasite is acting as a source of Al and Si species in the system, promoting the formation of an amorphous aluminosilicate type product. The excess hydroxide ion concentration, associated with higher Na₂O/SiO₂ ratios, is likely to influence the aluminosilicate gel precipitation at very early stages of reaction (21), promoting increased solubility, which affects the precipitation and the subsequent geopolymerization reactions. This is according with the results of compressive strength development presented in section 3.1.

The spectrum of unreacted SFCC (Fig. 4) shows bands at 912 cm⁻¹ corresponding to the OH deformation vibration (22), and the signals at 834, 612 and 526 cm⁻¹ attributed to asymmetric, double 6-ring, and bending vibration modes of the aluminosilicate framework in dealuminated faujasite (23). The band at 560 cm⁻¹ is assigned to the octahedral aluminum present in mullite, kyanite and sillimanite, as identified by XRD (Fig. 1), and the band at 460 cm⁻¹ corresponds to the symmetric bending modes of the Si-O-Si bonds (24).

All alkali-activated pastes display a main band between 1200 cm⁻¹ and 800 cm⁻¹ corresponding to the asymmetric stretching vibration mode of the Si-O-T linkage (where T may be Si or Al) (25). It is observed that this band shifts towards lower wavenumbers (∼1011 cm⁻¹) upon activation of the anhydrous SFCC precursor (1080 cm⁻¹). This displacement suggests a structural change of the SFCC upon partial dissolution in the alkaline environment, and the subsequent formation of an aluminosilicate type gel.

The activation process also leads to a reduction in the intensities of the bands observed in the SFCC precursor at 460 cm⁻¹, 526 cm⁻¹ and 617 cm⁻¹, associated with the dissolution of the zeolite phases, consistent with the XRD results (Fig. 3). It is likely that the highly alkaline environment promotes the dissolution of the SFCC initially through the breaking of Al-O bonds, considering that this type of bonds has a lower energy (∼60 kcal/mol) than Si-O bonds (∼80 kcal/mol) (26).

Bands at 1630 cm⁻¹ and 3450 cm⁻¹ in the activated pastes are attributed to bending (H–O–H) and stretching vibrations (–OH) of water in the hydrated products, respectively (27). The higher intensity of these bands in the alkali activated pastes, compared with the SFCC precursor, is consistent with more adsorbed water groups (≡Si-OH … H₂O and ≡Al-OH … H₂O) in the aluminosilicate gel (28).

In samples with an Na₂O/SiO₂ molar ratio of 0.20, a lower SiO₂/Al₂O₃ ratio promotes shifting of the T-O-T band towards lower wavenumbers (1017 cm⁻¹ for SiO₂/Al₂O₃=2.4), suggesting the incorporation of higher contents of Al within the gel, compared with the samples activated with higher SiO₂/Al₂O₃ ratio that show slight variations in the T-O-T band position (1022 cm⁻¹ for SiO₂/Al₂O₃=3.0). These results are consistent with the formation of a more Si-rich gel in the samples formulated with the highest SiO₂/Al₂O₃ ratio. This is in agreement with the shifting of the band at 725 cm⁻¹ towards higher wavenumbers at increased SiO₂/Al₂O₃ ratios, as a consequence of the more Si-rich environment in the gel. The Si-O-T band is observed to be narrower and significantly more intense for the samples with SiO₂/Al₂O₃ ratios of 2.4 and 2.6, because of the higher extent of dissolution of the SFCC, contributing more silicate and aluminate species for geopolymer gel formation. Similar trends are observed in specimens formulated with higher Na₂O/SiO₂ ratios (0.25 and 0.30).

At higher alkalinity conditions, associated with higher Na₂O/SiO₂ ratios, the alkali content favors the initial dissolution of the aluminosilicate precursor and its consequent reaction to form the geopolymer product. This leads to the shifting of the main T-O-T band towards lower wavenumbers as identified in the activated specimens when compared with the unreacted SFCC. Conversely, at low alkalinity conditions a higher proportion of unreacted precursor is identified, as the T-O-T band of the SFCC does not exhibit variations in the activated samples.

Figure 5 shows the ²⁷Al MAS spectrum of the unreacted SFCC, with broad bands in the regions assigned to octahedrally (Al(VI)) (–10 to 30 ppm), tetrahedrally (Al(IV)) (50 to 80 ppm), and minor pentahedrally (Al(V)) (30 to 50 ppm) coordinated Al environments. The intense peak in the 10 ppm region is mainly attributed to the octahedral aluminum species present in kyanite and sillimanite phases (29), and extra-framework Al(VI) species in the dealuminated zeolites identified through XRD (Fig. 1). The resonance near 60 ppm is assigned to the tetrahedrally coordinated Al in the zeolites present in the SFCC (30). The intensity at 45 ppm is ascribed to a less ordered framework where Al atoms are present in imperfectly crystalline zeolites. The FCC catalyst suffers hydrothermal dealumination through its use in the catalytic cracking process where up to 90% of the tetrahedrally-coordinated Al in the zeolite framework is removed, and consequently the bonds involving Al are replaced by hydroxyl groups linked to silicon (31).

The ²⁹Si MAS spectrum of the unreacted SFCC (Fig. 6) shows a dominant resonance near –105 ppm, attributed to the Q⁴(0Al) sites of the highly dealuminated faujasite resulting from the different stages in the cracker unit (32), and also a contribution from quartz (–107 ppm) as identified via XRD (Fig. 1). The asymmetry of this band suggests that it overlaps with a wide range of resonances from different Si-containing phases identified in the SFCC, and therefore cannot be attributed to a single phase.

The ²⁷Al MAS spectra of selected SFCC-based geopolymers (Fig. 7) show changes in the line shape compared with unreacted SFCC (Fig. 5). The Al(V) resonance observed between 30 and 50 ppm (Fig. 7) in the unreacted precursor completely disappears upon activation, consistent with the high reactivity of Al(V) sites. In alkali activation of aluminosilicates, it has been shown that Al(V) and Al(VI) are converted to tetrahedral sites (Al(IV)) with an associated alkali cation to maintain electroneutrality (33).

A significant reduction of the Al(VI) band is identified in the activated SFCC, when compared with the unreacted SFCC, along with a significant increase in the intensity in the Al(IV) region of the spectra. The reduction in the intensity of the bands in the Al(VI) region is attributed to the dissolution of the dealuminated zeolite, leading to the formation of a highly crosslinked disordered aluminosilicate ‘geopolymer’ gel, as identified by FTIR.

Although the ²⁷Al MAS NMR spectra of the activated SFCC specimens are dominated by the resonance associated with Al(IV), a small resonance centered between 2 and 3 ppm is also observed in the spectra. This resonance corresponds to small amounts of Al(VI) from the residual mullite phase (34), identified in the XRD after the activation (Fig. 3). The amount of Al(IV) increases as the alkali content (Na₂O/SiO₂ ratio) increases from 0.20 to 0.30, in good agreement with the formation of a larger amount of aluminosilicate type gel, as observed via FTIR, and in good agreement with the decrease in intensity of the Al(VI) band remaining from unreacted material.

The peak at –105 ppm in the ²⁹Si MAS NMR spectrum of the unreacted SFCC (Fig. 8) is no longer identified in the geopolymer specimens, consistent with the dissolution of faujasite upon activation as observed via XRD (Fig. 3). The ²⁹Si MAS NMR spectra of the alkali activated SFCC binders (Fig. 8) show a broad band centered at –91 ppm assigned to Q⁴(3Al) sites, consistent with the incorporation of Al in the geopolymer gel. The spectrum of activated SFCC is within the region assigned to Q⁴(4Al), Q⁴(3Al), Q⁴(2Al) and Q⁴(1Al) sites, overlapping with the sites in the residual unreacted precursor, which means that it is difficult to identify particular sites from the unreacted spent catalyst and the newly forming phases.

Low alkali content (Na₂O/SiO₂ of 0.20) promotes the broadening of the ²⁹Si MAS NMR spectrum, along with a low intensity shoulder between –98 ppm and –120 ppm, corresponding to the Q⁴ sites of faujasite from the spent catalyst that are not completely consumed during the geopolymerisation process. Increasing the alkali content (higher Na₂O/SiO₂), the intensity of this shoulder is significantly reduced, confirming a higher extent of dissolution of faujasite from the SFCC at higher alkali content in the systems, as observed in the XRD results (Fig. 3). The main band also becomes slightly narrowed and more intense when the Na₂O/SiO₂ ratio is increased from 0.20 to 0.30, indicating a higher degree of ordering of the gel formed, in good agreement with the ²⁷Al NMR results (Fig. 7).

Figure 9 shows evident differences in the density, surface smoothness, and proportion of unreacted particles in activated SFCC as a function of formulation conditions, as a more homogeneous and dense microstructure is identified in specimens with increased SiO₂/Al₂O₃ ratio. The visibly low densities exhibited by samples formulated with SiO₂/Al₂O₃ of 2.0 and 2.2 are consistent with their low compressive strengths (Fig. 2), this can be associated with a low degree of reaction of the SFCC under these activation conditions. Although the highest mechanical strength is obtained in samples formulated with a SiO₂/Al₂O₃ ratio of 2.4, its microstructure (Fig. 9C) is not as dense as in specimens formulated with a SiO₂/Al₂O₃ ratio of 2.6 (Fig. 9D). This suggests that the materials might be more brittle at these activation conditions. Samples formulated with an SiO₂/Al₂O₃ ratio greater than 2.6 (Figs. 9E, F) show large cracks associated with severe shrinkage, which can lead to the reduced mechanical strength observed at these activation conditions.