Development of glass-ceramics from Soda lime silica glass waste with addition of kaolin and coloring oxide for Turquoise’s imitation

Turquoise, a hydrated phosphate of copper and aluminum, is an opaque, blue-to-green mineral with the chemical formula CuAl6(PO4)4(OH)80.4 H2O. Owing to its unique hue, turquoise is rare and valuable, and it has been appraised as a gemstone for thousands of years. However, with high porosity, low specific gravity, fracturable, and low hardness of just under six or slightly more than window glass, treatments are required before use as jewelry. Nowadays, imitations, and synthetics from glass, plastic, or pressed turquoise powder bonded with resin, are generally found in the market. In our present study, we aim to investigate an alternative method to form glass-ceramic for turquoise imitation by using a direct sintering method based on the soda-lime-silica glass waste with kaolin mixer and coloring oxide additives. In the preparation, the pre-mixed powders were poured into a stainless-steel die, then, the die was brought to the pressing machine for sample forming. A conventional furnace was applied for heating the samples to ∼1000 °C with a soaking time of 4 h. The obtained ingots were characterized by several techniques. The chemical compositions were determined by the Energy Dispersive X-ray Fluorescence (ED-XRF), and their chemical structures were evaluated by the X-ray diffractometer. Meanwhile, their chemical fingerprints were obtained by Raman spectroscopy. As for comparison, the natural turquoise, as well as the commercial imitation samples, selected from the local market were undergone the same characterization. We have found that with the proper content of the metal oxide additives, the products in unique blue and brown-turquoise appearances can be formed and cut to decorate the jewelry set for the niche market. Thus, an alternative method for recycling soda lime-silica waste glass based on a single-step sintering process at relatively low temperature enabled jewelry applications.