Use of natural dyes to color metakaolin-based geopolymer materials

Over the years several colored ceramic-like materials have been prepared by using many synthetic dyes, which are toxic and polluting. Up to date, no studies have been reported on the combination of natural dyes within the geopolymers (also known as amorphous aluminosilicate polymers). In this scenario, natural dyes such as curcumin, chlorophyll, and a freeze-dried grape marc from the wine agrifood supply chain have been employed to investigate their effectiveness ability to color metakaolin-based geopolymers. The colored geopolymers, cured at 25 or 40 °C for 24 h, are characterized after 56 days of ageing time at room temperature by the ionic conductivity and pH measurements, Fourier-Transform Infrared spectroscopy, compressive strength, and color assessment by the color space analysis before and after soaking the samples in water. Eventually, the control geopolymer samples are also characterized for their potential antimicrobial and cytotoxicity properties. The main findings revealed that employing natural dyes to color geopolymers offers a promising alternative to toxic synthetic dyes. The samples retained their color for brief time when exposed to water thus suggesting indoor application. Moreover, all the samples exhibited good geopolymerization as confirmed by FTIR analysis. Notably, samples cured at 25°C showed better compressive strengths compared to those cured at 40°C. Finally, the bioimpact of these materials suggests that the synthesis in loco of geopolymers is safe for the operator.