Durability performance of multi-walled carbon nanotube reinforced ordinary Portland/calcium sulfoaluminate cement composites to sulfuric acid attack at early stage

Exposure of cement-based structures to sulfuric acid environments can cause significant degradation. Therefore, this study focuses on the effect of multi-walled carbon nanotubes (MWCNTs) on the sulfuric acid resistance of ordinary Portland cement (OPC)-sulfoaluminate cement (SAC) composites (as repair material) at early stage. Samples were exposed to three concentrations of acid solution for 60 days. The degradation kinetics, chemistry, and microstructures of the composites were analyzed. The results of degradation kinetics (dimensional change, mass and strength loss) showed that the MWCNTs modified composites had a lower degradation rate compared to the composites without MWCNTs. XRD, TG, X-CT and SEM analysis showed that MWCNTs promoted hydration, improved structure compactness, interrupted the generation and propagation of microcracks through the bridge effect, and formed protective barriers. The degradation rate of the composite was reduced by the double optimization of chemistry and microstructure. However, diffusion-dissolution-precipitation is still the main mode of degradation in composites subjected to sulfuric acid attack, regardless of MWCNT content mixed into the composite system.