Influence of carbon-based nanomaterials, silica fume and polypropylene fibres on the hygrothermal durability of repair mortars

Repair mortars exposed to environmental temperature variations must simultaneously ensure interfacial performance, durability, and dimensional stability, which remains a challenge for conventional cementitious systems. This study investigates the performance of repair mortars incorporating carbon-based nanomaterials, namely graphene and multi-walled carbon nanotubes (MWCNT), combined with silica fume (SF) and macro-polypropylene fibres (MPF), under hygrothermal exposure up to 80 °C. Mechanical, durability, electrical, and dimensional properties were assessed at 28 days and after 30 hygrothermal cycles. Scanning Electron Microscopy (SEM) provided complementary qualitative evidence to interpret the observed responses. Carbon-based nanomaterials increased substrate-repair bond strength by 89–153% relative to the reference mortar. MWCNT-containing mortars showed evidence consistent with nanoscale crack-bridging mechanisms. SF reduced water absorption by up to 49% and increased surface electrical resistivity (SER) by up to 169% at 28 days, consistent with reduced fluid transport. However, these initial transport-related improvements were not consistently retained after ageing. After 30 cycles, the MWCNT mortar showed the greatest retention of flexural strength and static elastic modulus among the investigated formulations. Overall, the results indicate that matrix densification and nanoscale reinforcement produced distinct benefits and trade-offs under hygrothermal exposure. The findings highlight that improved initial transport resistance does not necessarily translate into greater thermo-mechanical retention after ageing, providing relevant insights for the performance-based design of nano-modified repair mortars.