Invisible near-infrared luminescent marker incorporating Egyptian blue in halloysite-based geopolymer mortars: Applications in covert tagging

Egyptian blue (EB), a calcium copper tetrasilicate known since antiquity as a pigment for its intense blue color, has been incorporated in minimal concentrations into halloysite clay-based geopolymeric mortars for use in covert optical tagging applications. The geopolymer matrix (GP) has been synthesized through alkaline activation of halloysite using 12 M sodium hydroxide, with EB added at variable concentration (0.2-0.4-0.6 wt%) with respect to the clay. Structural and thermal analyses (XRD, FTIR, TGA) have confirmed the successful geopolymerization and chemical stability of EB under highly alkaline conditions. Optical characterization via visible-induced luminescence (VIL) imaging and luminescence spectroscopy revealed that, despite the visually undetectable quantities, the embedded EB has retained its luminescent properties, emitting in the NIR range when excited in the red-visible region. Solid-state 27Al and 29Si NMR analyses have evidenced a reorganization a structural reorganization of the aluminosilicate framework upon EB incorporation, consistent with specific interactions between the pigment and the geopolymer network. A functional QR code has been successfully fabricated by stenciling GP_EB_0.6 mortar onto a GP background. Concealed under normal lighting and revealed via VIL imaging, the code granted access to a digital file with material specifications and synthesis protocols, demonstrating the system's potential as a covert, digitally retrievable tagging approach for secure applications.