Effect and mechanism of different polycarboxylate superplasticizers on rheological properties of ultra-high performance cementitious composites with nickel-plated carbon nanotubes

Nickel-plated multi-walled carbon nanotubes (Ni-MWCNTs) can effectively alleviate their agglomeration issue in cementitious matrix due to the excellent wettability of the surface nickel coating, thus providing an effective way to regulate rheological and mechanical properties of ultra-high performance cementitious composites (UHPCC). Understanding the effects of Ni-MWCNTs types/content and different polycarboxylate superplasticizers on rheological behavior is beneficial to predict the properties of designed UHPCC. This study investigated the effects of two superplasticizers PCE-330 and PCE-220, two types of Ni-MWCNTs (short and thick M7, long and thin M3) at two contents (0.25 wt%, 0.5 wt%) on the rheological behaviors of UHPCC. Fourier Transform Infrared (FTIR) spectroscopy was employed to characterize the structural differences between the two PCEs. The relationship between the rheological behavior and electrical conductivity of UHPCC and its conductivity was also analyzed. The results show that the yield stress of UHPCC using PCE-330 increases linearly with increasing Ni-MWCNTs content and aspect ratio, while the plastic viscosity decreases, exhibiting stable rheological properties. Because PCE-330 abundant carboxyl/ester groups, forms a thicker adsorption layer on the surfaces of cement particles and Ni-MWCNTs by providing more carboxylic acid monomers and molecular weight, thereby increasing the internal friction within slurry. In contrast, PCE-220, dominated by polyether side chains, exhibits lower plastic viscosity after yielding and better pumpability. Additionally, M7 (short and thick) demonstrates superior compatibility with PCE-220, as its lower aspect ratio facilitates uniform dispersion and prevents excessive entanglement, while its nickel-coated surface provides effective anchoring sites for PCE adsorption. Resistance testing reveals a strong correlation between rheological and electrical performance: Well-dispersed Ni-MWCNTs that form interconnected network structures increase the yield stress of UHPCC while simultaneously reducing electrical resistivity.