Sustainable Recycling of Red Tomato Waste Through Metakaolin-Based Geopolymers

Abstract

The strong need to reduce waste disposal and pollution has led to an increase in the development of sustainable waste-based materials. One recent target is the reduction in the use of Portland cement, a construction material, because of the high water and energy consumption, as well as the high CO₂ release linked to greenhouse gas emissions and global warming. Geopolymers, or amorphous aluminosilicate polymers, are sustainable candidates for Portland cement substitution. In this study, metakaolin-based geopolymers have been synthesized at different curing temperatures (25, 40, and 60 °C), and the effect of the addition of 10 wt.% of red tomato waste has been investigated. The synthesized geopolymers are characterized through Fourier-transform infrared spectroscopy (FTIR), integrity test, chemical oxygen demand (COD), and antimicrobial activity. FTIR analysis reveals that both the curing temperature and the organic matter do not interfere with geopolymerization reactions. The integrity tests suggest that the samples are macroscopically well-hardened. However, the presence of yellow color from integrity test water leachates underlines the release of organic molecules. These compounds are not released after 4 days as also confirmed by COD analyses carried on up to 9 days. Eventually, all the samples showed antimicrobial activity against both Gram-positive and -negative strains.