Experimental Investigation on the Phase Evolution and Corrosion Resistance of a Graphene Oxide/Multiwalled Carbon Nanotubes-Doped Cementitious Matrix in a Coupled Chloride–Sulfate Attack

The durability of modern infrastructure in extreme environments faces critical challenges, particularly under a coupled chloride–sulfate attack. Unlike single-factor erosion, this coupled environment induces a complex synergistic deterioration mechanism involving chemical decalcification and physical crystallization pressure, which remains underexplored for nanomodified composites. To address this, this study systematically proposes a synergistic enhancement strategy for optimizing the dispersion of multiwalled carbon nanotubes (MWCNTs) by utilizing graphene oxide (GO) as a functional nanotemplate. The phase evolution and corrosion resistance of cementitious matrices incorporating varying dosages of GO (0–0.05 wt %) and MWCNTs (0–0.15 wt %) were rigorously investigated under this complex coupled chemical erosion. The results demonstrate that while the performance gains from single-doped MWCNTs are limited by agglomeration, the hybrid incorporation of GO and MWCNTs yields a robust synergistic effect. After 120 days of coupled erosion, the hybrid composites (e.g., C0.125-G0.05) maintained a compressive strength of 41.11 MPa and a flexural strength of 18.91 MPa, significantly outperforming both the control and single-doped groups. Microstructural analysis revealed that GO acts as an efficient nucleation template and dispersant, effectively debundling MWCNT aggregates and facilitating the formation of a uniform, interconnected collaborative nano-reinforcing framework. This framework not only provides multiscale crack bridging but also optimizes the pore structure, creating a highly tortuous, low-permeability path for ion transport. Crucially, this dual physical–chemical barrier significantly inhibits the synergistic intrusion of aggressive ions (SO 4 2–, Cl –, and Mg 2+ ) and retards the detrimental transformation of cementitious C–S–H into non-cementitious magnesium silicate hydrate (M–S–H) and brucite (Mg(OH) 2 ). By suppression of the formation of expansive products such as gypsum, this strategy effectively preserves the structural integrity of the C–S–H phase. This study provides essential theoretical guidance for the design of high-performance cement-based composites in extremely coupled corrosive environments.