Effect of Organic Carbon on the Slake Durability of Limestone Armorstone for Rubble-Mound Breakwaters

Slake durability is a critical parameter for evaluating rock resistance to moisture-induced degradation under cyclic wetting–drying, particularly for armorstone in rubble-mound breakwaters. While the existing literature has predominantly focused on clay mineralogy, porosity, and environmental factors, the explicit role of organic carbon (OC) in governing the slake durability of limestones remains largely unquantified. To address this knowledge gap, our study presents the first systematic comparative investigation of the slake durability behavior of one OC-free limestone (OCF) and five OC-bearing limestones (OCB1–OCB5) with varying OC contents (up to 1.44%). Extended slake durability tests were conducted up to 15 cycles, with the slake durability index (SDI) measured after each cycle. The results demonstrated that OC-bearing limestones exhibited markedly lower SDI than the OC-free limestone. A strong inverse correlation between SDI and OC content was established, driven by the OC-induced development of higher porosity and microstructural weakening within the mineral framework. Furthermore, the degradation constant (λ) derived from regression modeling effectively predicted long-term disintegration of the samples. These novel insights establish OC content as an essential, yet previously overlooked, criterion for geomaterial selection, suggesting that caution is warranted when considering OC-bearing limestones for high-energy armor layers, where OC-free varieties should be preferred.