Synthesis, thermal, and mechanical characterisation of metakaolin-based geopolymers coloured with grape marc extract

Over the years, several materials have been used for restoration purposes, with different types of dyes and colour hues. Recently, some researchers have proposed geopolymers (GPs) or amorphous aluminosilicate polymers for these purposes. In this work, an alcohol-based grape marc extract (GME, obtained via dark maceration assisted with ultrasound) was used as a natural dyeing agent for metakaolin-based GPs. The geopolymerisation occurrence was assessed by Fourier-transform infrared (FT-IR) spectroscopy and X-ray diffraction analysis, while the colour of the resulting material was determined through the colorimetric analysis in the L*a*b* colour space. Additionally, the thermal stability of GME and GPs was investigated by thermogravimetry coupled with FT-IR spectroscopy. The microstructure, the reticulation stability, and the antimicrobial activity of GPs were examined through the scanning electron microscopy, the pH and ionic conductivity measurements, integrity, and mass loss tests. Overall, a coloured geopolymer with suitable thermal, antimicrobial, and mechanical properties was obtained, justifying its potential use in restoration or, more generally, in the construction field.