Consolidation treatments for marble sugaring: reinforced lime versus nanolime

Pentelic marble is considered a valuable material in architecture and was the material of choice in antiquity. Today, on account of the polluted atmosphere in urban areas and other environmental factors, granular disaggregation (sugaring) is noticed on the surfaces of many monuments and buildings constructed with pentelic marble. Consolidation of these surfaces is thus necessary to preserve our architectural heritage. To this purpose, naturally weathered pentelic marble specimens from the Acropolis area in Athens, exhibiting granular disaggregation, were selected and studied. Two different materials, a suspension of traditional lime (“reinforced lime”), and a novel alcohol-based nano-dispersion of calcium hydroxide, with and without the addition of sodium stearate, were applied and evaluated as possible consolidation materials. The specimens were examined prior and after the consolidation treatments, in terms of morphological, chemical and chromatic characteristics, through stereomicroscopy, scanning electron microscopy (SEM–EDX), infrared spectroscopy (FT-IR) and colorimetry. The mass gain of each specimen on account of the consolidation treatment was evaluated. The different consolidation treatments were further assessed through sonication tests, to investigate the degree of adhesion with the substrate and the consolidation efficiency, as well as through static contact angle measurements, aiming to investigate a possible modification of the marble surface. The use of nanolime seems to outperform the traditional application of reinforced lime, in terms of consolidation efficiency, as well as applicability of treatment (4 spray application in contrast to 40), while the addition of sodium stearate seems to reduce micro-cracking and results in a more homogenous marble surface.