Effectiveness and Durability of Strengthening Agents Applied to Gravina Calcarenite: The Stone Material of the Sassi di Matera—UNESCO World Heritage Site

The conservation of the UNESCO World Heritage site of the Sassi di Matera requires an evaluation of consolidation strategies for Gravina calcarenite, a highly porous calcareous stone extensively used in the historic built environment. The pore system, characterized by a predominance of macropores combined with a significant microporous fraction, plays a key role in the performance and long-term behaviour of consolidating agents. Although consolidation treatments have been widely investigated for carbonate stones, studies specifically addressing Gravina calcarenite remain relatively limited. This study presents a comparative assessment of the performance and durability of four commercially available strengthening agents: an ethyl silicate-based product, two nanolime formulations, and a nanosilica-based treatment. The products were applied to Gravina calcarenite and evaluated using a multi-analytical approach integrating mercury intrusion porosimetry, water absorption assessed by the contact sponge method, ultrasonic pulse velocity (UPV), colorimetric measurements, and artificial aging tests. The results reveal distinct performance trends: the ethyl silicate-based product shows the most pronounced immediate reduction in water absorption, together with a consistent increase in UPV, although these effects are not retained after aging. The more concentrated suspension of Ca(OH)2 nanoparticles (50–250 nm) exhibits a moderate reduction in water absorption with limited chromatic impact, although a decrease in effectiveness emerges after aging. In contrast, the Ca(OH)2 nanoparticle suspension (100–300 nm), dispersed in isopropanol at a concentration of 5 g/L, shows more limited and variable changes across the investigated parameters. The nanosilica-based treatment shows a reduction in water absorption and the largest mean increase in UPV after application, although with greater variability among specimens, together with modifications in the finer and intermediate pore fractions. None of the investigated products provided a uniform and persistent improvement across all evaluated parameters. Overall, the evaluation of the investigated properties provides a basis for assessing treatment performance and supporting informed conservation strategies.