Novel coloring of geopolymer products using a copper solution immersion method

•Novel coloring of geopolymer products by immersion into a copper chloride solution has been performed.•It was clarified that there were two coloring processes in green.•One was the precipitation of atacamite crystals.•The other was the diffusion of copper ions into geopolymer products.

Geopolymer products were fabricated from a slurry composed of metakaoline, potassium silicate solution, and potassium hydroxide with a mass ratio of 57.1:34.3:8.6, which was cured at 60 °C and 100% relative humidity for 1 week. The hardened geopolymer products were immersed in a 0.1 mol/L copper chloride solution at 25 °C for 1–168 h. The copper chloride solution to geopolymer product mass ratio was maintained at 2.5. According to visual observations, the spread of color from the surface to the interior of the geopolymer product seemed to stop once the immersion time reached 24 h. After immersion for 168 h, atacamite (Cu2Cl(OH)3) crystals were partially formed on the geopolymer product surface and at depths of 150–300 μm. These precipitated atacamite crystals contributed to a green coloration of the sample. However, the observed green coloration, which reached a depth of approximately 900 μm in the geopolymer product after an immersion time of 168 h, was mostly caused by the presence of isolated copper ions due to the substitution of potassium ions in the geopolymer product by copper ions from the solution. Thus, two coloring mechanisms were active in this system.