Metakaolin-based geopolymers filled with volcanic fly ashes: FT-IR, thermal characterization, and antibacterial property

2.1 Raw material preparation and characterization

MK (Argical™-M 1000, Imerys, France) was used as a GP precursor. Its chemical oxide composition and granulometry are reported elsewhere in (efficient addition). FAs were taken from Etna Volcano (Sicily, Italy) and used as filler in MK-based GP. The chemical oxide composition (Table 1) of FA was obtained by X-ray fluorescence analysis performed with a Shimadzu Spectrometer EDX-720 (GmbH, Duisburg, Germany) equipped with a 50 W Rh target X-ray tube, a high-energy resolution Si (Li) detector, and five primary X-ray filters. The measurement was performed in the ranges of Na-Sc and Ti-U. The composition suggests that FA belong to Class F because of the higher content of silica and alumina with respect to calcium oxide [36,37]. Before their addition as filler (20 wt% with respect to MK content) in GP pastes, sodium hydroxide 8 M (NaOH) and sodium silicate (Na₂SiO₃, with SiO₂ = 2% and Na₂O = 8%, ρ = 1.346 g/ml, and pH = 11.0–11.5) were used as alkali activator solutions, while acetone (C₃H₆O), KBr, both of analytical grade, and MilliQ water were used for the sample analyses. All the reagents were purchased from Sigma Aldrich (Merck KGaA, Darmstadt, Germany).

2.2 GP syntheses

Table 2 reports the sample formulations, whereas Figure 1 reports the scheme of the GP syntheses without (GP0 25 and 40°C samples) and with the FA filler (GP20FA 25 and 40°C samples). Initially, NaOH 8 M and Na₂SiO₃ solutions were mixed with MK powder under mechanical stirring (ca 10 min) until the mixture became viscous. Then, the fresh pastes were poured into the plastic cubic and cylinder moulds, and after the mechanical removal of the bubbles, the moulds were carefully closed and the samples were cured at fixed temperatures (25 or 40°C) for 24 h. After the curing, the samples (GP0 25 and GP0 40°C) were stored at RT and analysed at different ageing times. The GP samples with FA filler (GP20FA 25 and 40°C) were synthesized following the same procedure and mixing the FA powder with the fresh GP paste for 5 min.

Figure 1

Flow chart showing the procedure of the GP synthesis without (a) and with (b) FA filler.

2.3 GP characterization

Indirect measurements of the aluminosilicate 3D reticulation proposed in the literature [38,39] were used to test the chemical stability of the consolidated paste. To determine the efficiency of 3D reticulation, powders from the consolidated samples were immersed in water as described in the Supplementary Materials. In addition, weight loss was measured over 56 days after the preparation of the final GP at different curing times (see Supplementary Materials for more details on the experimental procedures).

FT-IR analysis was performed in the range of 400–4,000 cm⁻¹ using the Prestige21 Shimadzu system, equipped with a DTGS KBr (deuterated tri-glycine sulphate with potassium bromide windows) detector, resolution of 2 cm⁻¹ (60 scans). The analysis procedure uses KBr discs realized weighing 2 mg of sample and 198 mg of KBr. FT-IR spectra were elaborated by IR solution and Origin software. The analyses were performed on the samples aged at 7, 14 28, and 56 days at room temperature.

The thermal characterization of the prepared GP was carried out in a Mettler Thermogravimetric Analyzer TGA 1 Star System. A roughly amount of 6 × 10⁻³ g of GP was placed inside an open alumina crucible and degraded from 25 up to 800°C, by using a heating rate of 10°C min⁻¹. Thermogravimetric (TG) experiments were carried out in both inert (nitrogen with a flowing rate of 0.06 l min⁻¹) and oxidative (static air) atmospheres. TG equipment was calibrated following the suggestion of the company, by exploiting the magnetic properties change of three metal standards (Isatherm, Nickel-alloy, and Trafoperm 86) at their Curie points (148, 355, and 750°C, respectively). Furthermore, following the recommendations of the ICTAC kinetics committee [40], we provided to correct the error in the mass determination due to the reduction of the buoyancy force on increasing temperature, by performing a blank TG run with an empty pan and then subtracting it from those of samples, so obtaining corrected degradation TG curves. TG data were thus reported graphically, by plotting the percentage of undegraded sample, (1 − D)%, as a function of temperature, where D = (W _o − W)/W _o, and W _o and W were the masses at the starting point and during scanning.

The calorimetric behaviour of the prepared GP was evaluated by using a Mettler DSC 1 Star System (Mettler Instrument, Greifensee, Swizterland). DSC scans were carried out in the range 25–500°C, by employing roughly 6.0 × 10⁻³ g of GP and a scanning rate of 10°C min⁻¹. The characteristic DSC parameters were determined by performing three scanning for each GP and reporting the averaged value, the deviation remain within ±0.5%. Before the experiments, DSC equipment was calibrated, in both enthalpy and temperature, following the company’s suggestions consolidated by our previous study [41].

The antibacterial properties of the GP were assessed against Escherichia coli (ATCC 25922) and Pseudomonas aeruginosa (ATCC27853) as Gram-negative bacteria, Staphylococcus aureus (ATCC 25923) and Enterococcus faecalis (ATTC29212) as Gram-positive bacteria performing the Kirby-Baure method [42]. The whole procedure follows the one reported in [43] and it is summarized in Figure 2. After the incubation times, the diameter of inhibition halos (IDs) in relation to Petri plate diameter (D) (6 cm) was calculated. Four measures for each sample were carried out to determine the mean standard deviation. Results were expressed as bacterial viability (%):

Figure 2

Flowchart procedure for the evaluation of the antibacterial properties.

Bacteria viabilities without samples are expressed as 100% bacterial viability. The mean Standard Deviation is expressed as relative standard deviation.

3.1 Sample observations

GP0 and GP20FA were removed from the moulds at RT after 7, 14, 28, and 56 days (Figure 3). From 7 to 56 days of aging time, GP0 at 25°C changes its colour from dark beige to light beige, suggesting the occurrence of the efflorescence phenomenon [44,45]. However, the samples were hard under finger pressure, and smooth with only a few bubbles visible on their surfaces. GP0 samples cured at 40°C were similar to GP = samples cured at 25°C, but with dryer surfaces. GP20FA cured at 25 and 40°C was dried and smooth. All these samples at different ageing times possess few bubbles and black dots on their surfaces due to the presence of the filler. The presence of white colour on GP specimens suggests the setting of the efflorescence phenomenon also for these samples. This phenomenon was also observed for GP samples prepared via alkali activation of FA and MK in [46].

Figure 3

Images of the samples after extraction at different ageing times.

IC and pH analyses were conducted on GP0 25°C, GP0 40°C, GP20FA 25°C, and GP20FA 40°C samples to obtain indirect information about the stability of the synthesized samples [47]. All these samples showed a similar trend over time, as shown in Figure S1a–d. For all samples, within 48 h, there is an increase in pH and IC. pH values were stable at 11.5 for all the samples aged up to 56 days, whereas regarding the IC values, GP0 25 and 40°C possess values at 150–170 mS/m after 56 days of ageing times, while GP20FA 25 and 40°C possess values at 200 mS/m.

The integrity test was carried out on the samples after 7, 14, 28, and 56 days. The sample images after the test, reported in Figure S2, show that all the samples overcame the test. Indeed, none of them released their pieces in the waters, which appeared always very clean and transparent. The pH values of the water in which the samples were immersed slightly differ based on the ageing time of the samples. The values ranged from 10.73–10.95 (samples aged 7 days) to 10.10–9.93 (samples aged 56 days).

Figure S3 reports the weight loss test on the GP samples. The addition of 20 wt% of FA results in a reduction of weight loss at 7 days, indeed both GP20FA 25 and 40°C values are lower than the values of the sample obtained without the FA. Moreover, Figure S3 points out that the thermal treatment had an effect in terms of stability, indeed during the ageing times, the weight loss values of samples treated at 40°C are slightly lower than the ones obtained from the samples cured at 25°C.

3.2 FT-IR analysis

Figure 4a and b show the FT-IR spectra of GP0 cured at 25 and 40°C compared at different ageing times from 7 up to 56 days, while Figure 4c and d shows the FT-IR spectra of GP20FA cured at 25 and 40°C. In MK, GP0 and GP20FA spectra are present the bands at 3,450 cm⁻¹ (strong) and 1,650 cm⁻¹ (medium) are due to hydration water [16].

Figure 4

FT-IR spectra (a) GP0 at 25°C, (b) GP0 at 40°C, (c) GP20FA at 25°C, and (d) GP20FA at 40°C at different ageing times.

MK shows a typical strong band at 1,080 cm⁻¹ due to asymmetric stretching vibration of Si–T–O (T = Al or Si). Moreover, the band at 460 and around 800 cm⁻¹ was assigned to the symmetric bending of T–O and Al(iv)–O, respectively [48,49,50,51].

All the GP spectra cured at 25 or 40°C and aged up to 56 days reveal the increase of the bands at 720 and 870–860 cm⁻¹ due to the formation of new AlO_X polyhedral species formed during the synthesis [52]. Indeed, the band at 870 cm⁻¹ is due to the high amount of Al(v)–O species [49,52]. In addition, in the GP samples aged from 7 to 56 days, the strong band at 1,080 cm⁻¹ of MK is displaced up to 1,016–1,012 cm⁻¹, suggesting a new aluminosilicate network formation due to the increased substitution of Al in the tetrahedral silicate network [50]. This displacement was also observed in [31] by in situ attenuated total reflectance FT-IR on GP flash pastes made up of volcanic FA and MK. The broad bands in the range 1,435–1,385 cm⁻¹ could be related to the efflorescence phenomenon (formation of sodium carbonate on GP surfaces) mainly for the GP0 samples and also for the calcium presence in GP20FA samples [53].

3.3 Thermal analysis

GP20FA samples cured at 25 and 40°C were degraded in a thermobalance, in dynamic heating conditions (RT – 800°C), to verify how the presence of volcanic ash and the different conditioning post-preparation affect their thermal behaviour. GP0, cured at 25 and 40°C, was also degraded in the same conditions for comparison. TG curves for the prepared GP are reported in Figures 5 and 6, in inert and oxidative atmosphere, respectively. TG curves, as expected, show the highest mass loss for the GP without volcanic ash added and treated at 25°C, while both the experiment in inert and oxidative environments clearly show a reduction in the mass loss when the volcanic ash is added in the GP formulation. If the oxidative atmosphere seems to flatten the influence of preventive heat treatment on the GP mass loss (Figure 5), the degradations (Figure 6) in an inert environment restore hierarchies, confirming that the presence of volcanic ash has a positive effect on the GP stabilization and that the curing treatment at 40°C reduces the weight loss. At 800°C, all the studied GP showed a stable solid residue ranging from 81 to 87% in static air atmosphere and 83 to 94% in nitrogen flow.

Figure 5

TG degradation curves, in static air atmosphere, for GP0 at 25°C, GP0 at 40°C, GP20FA at 25°C, and GP20FA at 40°C.

Figure 6

TG degradation curves, in nitrogen flow, for GP0 at 25°C, GP0 at 40°C, GP20FA at 25°C, and GP20FA at 40°C.

GP, thermally treated and not, were calorimetrically characterized by first carrying out DSC scans from 25 to 200°C, to investigate the temperature range of TG mass loss and then from room temperature up to 500°C. DSC curves for GP0 and GP20FA samples cured at 25 and 40°C are shown in Figures 7 and 8, respectively. DSC traces of the GP without volcanic ash are completely different with respect to those regarding GP20FA. GP0 samples, independently of the thermal treatment post-preparation, show a main double endothermic peak, with peak temperatures at about 75 and 105°C, followed by a less intense one at about 130°C (Figure 7). For the samples with volcanic ash, namely GP20FA 25 and GP20FA 40, only one endothermic peak is detectable at about 75°C, followed only by the other one at 130°C. Looking at the curing effect on the calorimetric behaviour, we noticed that the sample treated at 40°C, independently of the presence of volcanic ash, showed a lower value of enthalpy related to the endothermic phenomena (Figure 7).

Figure 7

DSC curves, from room temperature up to 250°C, for GP0 at 25°C, GP0 at 40°C, GP20FA at 25°C, and GP20FA at 40°C.

Figure 8

DSC curves, from room temperature up to 500°C, for GP0 at 25°C, GP0 at 40°C, GP20FA at 25°C, and GP20FA at 40°C.

Figure 8 reports DSC scans of all four GP from room temperature up to 500°C. We observed a shift of the endothermic peak for all the studied GP, with and without volcanic ash. From Figure 8, we also noticed a slight reduction in the enthalpy of melting for the GP containing volcanic ash. The calorimetric traces proceed in the same way for all GP up to 500°C, without highlighting other differences.

3.4 Antibacterial properties

The antibacterial properties of all samples were investigated at 56 days (Figures 9 and 10). Based on the observation of the plates, not all the bacteria assayed were sensitive to the presence of the samples. In particular, the inhibition halos were present only on the Petri plates with S. aureus, E. coli, and P. aeruginosa (Figure 9). Moreover, the inhibition halos values were more extensive against E. coli and P. aeruginosa, which are both Gram-negative bacteria, than S. aureus which is a Gram-positive bacterium. This behaviour is also confirmed by the value of the bacterial viability shown in Figure 10. A more accurate analysis of the IDs reveals that GP0 40°C and GP20FA 25 and 40°C in the presence of E. coli have increased antimicrobial activity. In comparison with the MK response, it can be noticed that the presence of FA as well as the heat treatment led to a slight increment in the antimicrobial activity. The reason why the heat treatment led to an increment of the antibacterial activity may be related to the fact that, as also demonstrated with the TGA analysis, this causes the water inside the geopolymeric system to be less. In addition, it is known that adding FA reduces the water demand in cementitious materials [54], leading to the samples containing FA, even less free water than the FA-free samples. This does not help the bacteria growth, thus leading to an increase in antibacterial activity of both the materials that underwent heat treatment and contained FA. Furthermore, another explanation of the antimicrobial activity of the GP20FA samples is related to the presence of FA. FA, as demonstrated by EDX analysis, are mainly composed of Fe₂O₃, SiO₂, and CaO. In literature, it is reported that Fe₂O₃ and SiO₂ have bactericidal and bacteriostatic activities against most Gram-positive and -negative bacteria [55]; however, the reason why no inhibition halos were present against E. faecalis is related to the fact that, to have a bacteriostatic effect, a minimum concentration of Fe₂O₃ is needed to be reached. Therefore, the use of only 20 wt% of FA suggests that it is not enough to reach this scope [56]. In addition, CaO could contribute, together with the GP environment itself, to form an alkaline environment that can successfully prevent the growth of both Gram-positive and -negative bacteria through inactivating ATP synthesis and generating oxidative stress [57]. However, comparing the result between GP0 40°C and GP20FA 25°C, it is possible to notice that the thermic treatment led to the same result of the addition of 20 wt% of FA without the thermal treatment.

Figure 9

Inhibition halo diameter for the investigated composites.

Figure 10

Bacterial viability for the investigated composites.