Urfa stone: Evaluating the durability of the oldest known building stone in history

Abstract This study investigates the durability of Urfa stone, which has shaped the architectural heritage of Şanlıurfa, Türkiye, including its iconic use at Göbeklitepe. Although widely used in historic and contemporary structures, its long-term performance under varying environmental conditions remains unclear. To investigate this uncertainty, an extensive experimental campaign was conducted on more than one hundred specimens. Petrographic and geochemical results indicate that Urfa stone is a biomicritic limestone composed mainly of calcite. The physico-mechanical tests show that the stone’s compressive strength (16.9 MPa dry and 10.6 MPa saturated), together with its moderate sonic velocity (3418 m per s), reflects the characteristics of a relatively low-strength limestone with a moderately cemented micritic matrix. In addition, its elevated porosity (19.9%) and high capillary water absorption (3.9 kg per m²√h) facilitate rapid moisture uptake, suggesting that the stone is highly susceptible to salt crystallization. This response aligns with its moderate resistance to freeze-thaw cycling and its pronounced vulnerability to salt weathering. The compressive strength decreases of 21% after 50 freeze–thaw cycles and 42.7% after 25 Na₂SO₄ cycles support this pattern and indicate that salt-related damage is the primary driver of mechanical weakening. Analyses further demonstrate that the visible wavy patterns on the stone surface intensify through salt accumulation, oxidation, and mineral precipitation, eventually developing into structural failure zones. By assessing the durability of Urfa stone, this study provides practical guidance for predicting its long-term performance and supporting its sustainable use across different architectural contexts.