Inﬂuence of the Addition of Waste Glass and Microbiological Performance of Metakaolin-Based Geopolymers Cement

Glass recycling reduces the amount of waste to be treated or disposed in landﬁlls, allowing both to limit environmental damage and to save on the costs of transportation and disposal of waste. In this paper, an advantageous method for recycling glass containers (bottles, jars, jars for food, glasses, and cans for drinks, etc.) is presented. The glass is crushed and without being washed or separated from any foreign bodies it is safely incorporated into a metakaolin (MK)-based geopolymeric matrix. Pure MK and mixtures obtained by adding diﬀerent percentages (30–50 wt%) of glass cullet are consolidated via alkali activation at 50 ° C. Infrared spectroscopy is able to reveal the formation of bonds in the mixtures between the geopolymeric matrix and the glass. Leaching tests are carried out to evaluate the eventual release of toxic metals, while the antibacterial tests complete the environmental evaluation of the ﬁnal consolidated products that show how the mechanical performance are modiﬁed by adding diﬀerent amount of glass cullet.


Introduction
The waste glass (WG), or cullet, has been used simultaneously as a precursor and fine aggregates in the alkali activated fly ashslag mortar. [1]Other authors presented the addition of WG to metakaolin (MK)-based alkali activated formulations, as those presented in this work. [2,3]A number of advantages can be listed DOI: 10.1002/masy.202100290 when WG addition is operated to different alkali activated materials (AAMs) formulations, such as: (i) reduction in the drying shrinkage [4] and water absorption due to the nonabsorbent nature of glass [5] ; (ii) enhancement of the fresh paste properties [6] ; (iii) improvement the resistance to acid attack [7] ; and (iv) amelioration in high temperature exposure. [2]ith this contribution we intend to deep the knowledge of the reactivity of ground WG in the AAMs formulations.In particular, we tested how WG affects the reticulation of MK in the alkali activated final product in terms aluminosilicate networking (Fourier-transform infrared [FTIR]), cationic leaching (amount of covalent bonds over ionic bonds), and mechanical resistance to compression (3D reticulation).Since the leachate can affect the surrounding biological environment, thus we added tests on antibacterial properties.

Experimental Section
The precursor aluminosilicate powder used was MK, Argical-M1000, Imerys, France, added with 30, 40, and 50 wt% of WG bottles ground to <30 µm.The precursor was alkali activated with a mixture of sodium silicate SiO 2 /Na 2 O = 3 and NaOH 8 M solution. [2]The paste was poured into plastic molds and the setting phase carried out at 50°C for 1 d followed by the hardening step at room temperature for 28 d (Figure 1).
Prestige 21 Shimadzu FTIR instrument equipped with a DTGS detector was used to evaluate the degree of aluminosilicate reticulation.The samples were analyzed in KBr pelletized disks containing about 2 mg of each sample and about 198 mg of KBr.The spectra were collected over a wavenumber range of 4000-400 cm −1 with resolution of 2 cm −1 (60 scans).The FTIR spectra were processed by Prestige software (IR solution) and Origin 8.
The ability of leaching heavy metals of all samples was carried out according to the EN 12457 European standard.Mechanical performance of the cylindrical-shaped specimens (21 mm diameter, 32 mm height) was tested under compression (Instron 5567 Universal Testing Machine).The microbial growths of Escherichia coli (ATCC 25922) and Pseudomonas aeruginosa (ATCC 10145) were tested in the absence and the presence of the solifidied geopolymers.After plating E. coli on TBX Medium (Tryptone Bile X-Gluc) and P. aeruginosa on Pseudomonas CN Agar,  the samples, sterilized under UV light for 1 h, were plated in the middle of Petri plates.E. coli was incubated for 24 h at 44°C, while P. aeruginosa for 48 h at 36°C.The reduction of the microbial growths was evaluated by the bacterial viability as reported elsewhere in Dal Poggetto et al. ( 2021). [8]

Results
FTIR spectroscopy was used to evaluate the geopolymerization occurrence.As it is possible to see in Table 1, there are the shifts to lower wavenumbers (from 1080 cm −1 of MK to 1015-1008 cm −1 of the samples) of the density state of peak maximum (DOSPM), which refers to the asymmetric stretching of Si-O-T (T=Si or Al).These shifts confirm the formation of the 3D networks. [9]he leachate elemental analysis showed a very reduced number of heavy metal present in the aqueous solution after the immersion of the AAMs containing WG, indicating a good chemical stability and a low ionic degree in the reticulated aluminosilicate constituting the solid AAMs (Table 2).Additionally, the starting WG, even though not washed, has a very reduced amount of contaminants.In particulars, WG contains Fe, little Ba, and Pb typically from crystal glass (the elements increasing in the leachate with increasing WG%).The presence of WG with the grain size used in this work does not act as reinforcement for the solid AAMs, presenting an evident decrease in compressive strength with increase in WG content (Figure 2).These values are slightly lower than those reported in the literature, [3] thus reflecting a general trend of this type of waste whose reactivity in alkaline environment should be considered limited.The well-known chemical stability of container glass is reflected in its inertness within the geopolymer network with respect to the other amorphous aluminosilicate, such as MK.
The activity of each material, which was further compared to that exercised by GP100, was evaluated by measuring the formed zones of inhibition.For all the samples, the bacterial viability was lower in E. coli than in P. aeruginosa, which appeared to be mildly inhibited in its growth (Figure 3).

Conclusion
The very low level of leached heavy metals cations cannot be responsible for the low viability of the bacterial species, more E. coli than P. aeruginosa.[12] The efficiency in depleting the E. coli bacterial is to be considered an interesting aspect for the application of these geopolymers as coverings materials in interiors, where such bacteria are not welcomed.
The mechanical performances are evidently decreasing with WG increase, as reported in literature indicating the use of nonstructural building materials, as coverings, for the formulations at higher WG content.

Figure 1 .
Figure 1.Preparation scheme and sample labels.

Figure 2 .
Figure 2. Compressive strength of all the formulations tested in this work.

Figure 3 .
Figure 3. Antibacterial properties of all the formulations expressed as bacterial viability towards E. coli and P. aeruginosa.

Table 1 .
FTIR main bands and interpretation peak.

Table 2 .
Elements present in the leachate in amounts above the unit, as from ICP/MS results.