Time-dependent drying shrinkage model for alkali-activated slag/fly ash-based concrete modified with multi-walled carbon nanotubes

Abstract

This study systematically investigates the effectiveness of multi-walled carbon nanotubes (MWCNTs) in controlling the drying shrinkage of slag/fly ash-derived alkali-activated concrete (AAC). A comprehensive experimental program is conducted to evaluate the influence of varying MWCNT contents (0–0.2 wt%) on the binder hydration kinetics, workability, mechanical properties, and drying shrinkage of AAC. Advanced characterization techniques including scanning electron microscopy, X-ray diffraction analysis and nuclear magnetic resonance spectroscopy are employed to elucidate the modification mechanism of MWCNTs. The test results reveal that the incorporation of MWCNTs reduces the initial/final setting times by 18–32 % and decreases the slump flow value by 15–41 %, attributed to the accelerated binder hydration kinetics and increased matrix viscosity. The optimal enhancement efficiency is at 0.1 wt% MWCNT dosage, achieving the maximum strength increments of 10.04 % (compressive), 19.78 % (splitting tensile), and 21.23 % (flexural) through microfiber crack-bridging mechanism. Notably, MWCNT-modified specimens exhibit 72 % and 58 % reductions in 7 days and 56 days drying shrinkage respectively, when compared to pure AAC. The three synergistic mechanisms are identified as nanoscale fiber crack bridging, nucleation sites for hydration product formation, and pore structure refinement through physical filling. The thermal analysis results also confirm the enhanced cumulative heat release (23–35 %) with MWCNT addition, correlating with microstructural densification through reduced porosity (27–42 %) and microcrack density. However, diminished effectiveness at higher concentrations (≥0.15 wt%) is attributed to MWCNT agglomeration induced by van der Waals forces and hydrophilic interactions. Based on the experimental results, a time-dependent dual-phase drying shrinkage prediction model incorporating hydration kinetics and fiber reinforcement factors is developed, demonstrating high accuracy against experimental validation. These findings provide critical insights into nanomaterial-based shrinkage control in AAC, offering a promising pathway for the development of dimensionally stable alkali-activated composites.