Corrosion behaviour of basalt-boron fiber in alkaline and cementitious environments: Morphological and structural insights

• First study of Basalt-Boron Fiber ( BBF) corrosion in alkaline and cementitious environments. • BBFs resist Ca(OH)₂ but degrade in 4M NaOH, forming brittle corrosion shells. • Oil shale ash (OSA) with cement increases the density and complexity of the corrosion shell. • XRD and FTIR confirm fiber degradation and formation of hydrate products. • One-year exposure in OSA cement forms thick, ettringite-rich corrosion layers. Basalt boron fibers, known for their strength and radiation shielding properties, are increasingly considered for use in concrete structures, especially in nuclear waste management. However, their long-term durability in alkaline environments typical of cementitious systems remains poorly understood. This study investigates the corrosion mechanisms of BBF immersed in saturated Ca(OH) 2 solutions, 4M NaOH, and cement mortars – plain and modified with 30 % oil shale ash – for 60 days and one year. Structural and morphological changes in the fibers were analysed using XRD, FTIR and SEM-EDS. The results show minimal Ca(OH) 2 degradation, while 4M NaOH induces a distinct corrosion skin consisting of sodium/potassium oxides and carbonate-hydroxide compounds. Cement mortars form heterogeneous corrosion shells dominated by calcium carbonate, portlandite and ettringite, with OSA containing mixtures significantly increasing the deposit density and altering the shell morphology. FTIR analysis confirmed the formation of hydration and depolymerisation products of the network, particularly in cement environments. These results highlight the need to consider chemical stability in design, particularly in aggressive cement matrices.