Limitations of Standard Salt Crystallization Tests for Compact Carbonate Heritage Stones: Evidence from Extended Testing on Portoro Limestone

Compact carbonate stones are widely used in architectural heritage for their aesthetic value and cultural significance, yet their long-term durability in saline environments remains insufficiently understood, particularly when assessed using standard salt crystallization tests developed primarily for porous lithotypes. This study investigates salt-induced deterioration in Portoro limestone, a compact ornamental carbonate extensively employed in historic architecture, considering four commercial varieties representative of heritage applications. Salt crystallization tests were performed using saturated sodium sulphate (Na2SO4) and sodium chloride (NaCl) solutions following the relevant European standard procedure, with the protocol extended to 45 cycles to capture delayed deterioration processes. Both untreated specimens and samples subjected to controlled thermal pre-conditioning at 300 °C and 500 °C were tested to activate latent microstructural weaknesses. Material decay was assessed through mass variation, porosity changes, surface observations, Leeb rebound hardness and ultrasonic pulse velocity measurements. Results demonstrate that deterioration is primarily controlled by salt type and microstructural characteristics rather than by total porosity. Sodium sulphate induced severe internal damage and abrupt structural failure associated with mirabilite crystallization, often following a prolonged phase of apparent stability. In contrast, sodium chloride causes mainly superficial effects with negligible mechanical impact. Thermal pre-conditioning accelerated damage development, while non-destructive techniques revealed internal deterioration well before visible damage occurred. These findings indicate that standard crystallization tests may be inadequate for low-porosity stones and that extended-cycle approaches provide a more reliable framework for durability assessment in saline environments.