Understanding the relationship between thermal analysis and geopolymer composition containing waste glass

The use of waste glass in geopolymer matrices represents a very important resource for recycling of inorganic materials [1]. The aim of this study is the investigation of the thermal and mechanical modification of the metakaolin-based geopolymer network due to the presence of a different amount of waste glass. The paste was prepared by mixing two homogenous powders, metakaolin and waste glass. No phase separation was observed since high viscosity was assured by a correct geopolymer formulation. The paste was poured into plastic molds and the setting phase carried out at 50 °C for 1 day, while the hardening phase was carried out at room temperature for 7, 14 and 28 days [2]. The chemical structure of the synthetized materials was investigated by Fourier Transform Infrared (FT-IR) spectroscopy, which showed the formation of bonds between the two components. We adopted FT-IR spectroscopy to highlight the reticulation degree of the final amorphous solid product, as accurately reported in a previous study [3]. The bands at 3462 and 1640 cm−1 were attributed to the -OH stretching and bending vibrations of water's hydration, respectively [4]. The band at 1640 cm−1 was preserved in waste glass containing geopolymers. Furthermore, in all the spectra the typical signals of the silica matrix are found. The asymmetrical Si-O-Si stretching vibrations were detectable in the spectral region 1080–1050 cm−1. Aluminosilicate’s typical signals, assigned to internal vibrations of Si-O-Si and Si-O-Al bonds, are found in the spectra of waste glass based geopolymers, whereas the frequency of the absorption bands seems to be approximately related to the Si/Al ratio in the aluminosilicate framework. In the present case of waste glass powder addition to MK-based geopolymer the peak at 1080 cm−1 shift to lower wavenumber (1008 cm−1), accompanied by an increased contribution of the Si-O-Al bond (950–960 cm−1), indicating the formation of a more efficient 3D geopolymer network. The spectrum of metakaolin also revealed sharp peaks ascribed to the stretching of hydroxyl groups and to Al(VI)-OH bonds in the kaolinite residues [4], whereas the Al(IV) absorption band at around 800 cm−1, characteristic of metakaolin, seems to be not found in the spectra of the corresponding geopolymers. Indeed, a band at around 690 cm−1, whose intensity decreases with the increase of waste glass content, is evident in geopolymers' spectra. The thermal behavior of the geopolymer samples was studied using a simultaneous Thermogravimetry/Differential Thermal Analysis (TG/DTA). The TG/DTA experiments were carried out from ambient temperature to 1500 K at a heating rate of 10 K/min under both argon and air atmosphere at 50 ml/min. The thermal characterization of the three mixtures (GPMK1, GPMK2 and GPMK3, respectively with 30, 40 and 50wt% of waste glass) revealed that the oxidizing atmosphere does not affect the thermal behavior of all the samples tested. The TG and DTA curves of GPMK and of the three waste glass-containing formulations under flowing Ar are reported in Fig. 1. Furthermore, they retained about 20 wt% of water, with lower amount for glass-rich geopolymers (GPMK2 and GPMK3 with 40 and 50 wt% of glass, respectively). Lower amounts of water were removed by dehydroxylation (in the range 573–973 K) in GPMK2 and GPMK3, since they have the lower content of geopolymer and, consequently, a lower number of hydroxyl groups to undergo condensation. The mechanical stability of the 3D geopolymeric network has been measured in terms of the compressive strength. After 28 days at room temperature, the samples synthesized using different percentages of waste glass showed a good geopolymerization, reaching the highest mechanical performance at 40 wt% of glass incorporated into MK. Preliminary biological data highlighted that cytotoxic and antimicrobial effects are significantly affected by the content of waste glass in the geopolymer matrix, due to the fact that the higher is the waste glass content the higher is the production of reactive oxygen species (ROS), responsible of both cytotoxic and antimicrobial effects. Figure 1. TG (up) and DTA (down) curves of GMPK and of the three mixtures at 10 K/min under Ar flowing atmosphere. Bibliography [1] M.R. El-Naggar and M.I. El-Dessouky, Constr. Build. Mater,132 (2017) 543-555 [2] M.A. Pereira, D.C.L. Vasconcelos and W.L. Vasconcelos, Mater. Res., 22 (2019) 2 [3] S. Luhar, T.-W. Cheng, D. Nicolaides, I. Luhar, D. Panias, K. Sakkas, Constr. Build. Mater, 222 (2019) 673-687 [4] M. Catauro, A. Dell’Era, S. Vecchio Ciprioti, Thermochim. Acta, 625 (2016) 20–27