Non-destructive THz techniques for monitoring environmental degradation of marble in iconic structures

Marble, mostly consisting of calcite (calcium carbonate, CaCO₃) or dolomite (calcium magnesium carbonate, CaMg(CO₃)₂), is produced via the recrystallization of limestone under elevated pressure and temperature conditions. Recent studies indicate that air contaminants from industrialization, especially sulfur and nitrogen oxides, are causing faster destruction of iconic historic structures. These contaminants, introduced by wet (acid rain) and dry mechanisms, present a considerable risk to the integrity of marble. Our previous research demonstrated the ability of THz Time-Domain Spectroscopy (TDS) to qualitatively detect sulfur in acid-treated marble samples, as shown by consistent reductions in THz transmission peak-to-peak (pk-pk) values, validated through Energy-Dispersive X-ray (EDX) analysis. In this study, we integrate THz Time-Domain Imaging (TDI) with the highly sensitive THz-Laser Feedback Interferometry (THz-LFI) to analyze sulfur-induced damage and irregularities in marble structures featuring Pietra-dura motifs. By imaging marble samples treated with varying concentrations of sulfuric acid (H₂SO₄) to simulate acid rain with pH values ranging from 3.5 to 5, we demonstrate the unique strengths of each technique and the enhanced insights provided by their combination. These findings emphasize a promising non-contact, non-destructive approach for assessing and monitoring the environmental impact on marble structures.