A Comparative Study on Crack-bridging and Microstructural Densification Mechanisms Governing the Mechanical Properties of Fly-ash-based M25 Concrete Modified with MWCNT AND SiO2 NPS

Abstract

This study examines the mechanical, microstructural, and hydration-related improvements in concrete modified with fly ash, multi-walled carbon nanotubes (MWCNTs), and nano silica. A comprehensive experimental program, comprising compressive, split tensile, and flexural strength tests along with SEM and XRD analyses, was conducted to evaluate the combined and individual effects of these additives. Fly ash improved the sustainability and long-term performance of concrete through its pozzolanic reaction, although its early-age strength contribution remained moderate. The inclusion of MWCNTs produced the most pronounced mechanical enhancements, with the 0.45% dosage yielding peak strength values due to efficient crack-bridging, improved stress transfer, and accelerated hydration. Nano silica also contributed to performance enhancement by refining pore structure and promoting the formation of a dense C–S–H matrix, with strength increasing progressively with higher dosages. The SEM images confirmed the development of a dense and compact microstructure, while XRD patterns displayed reduced portlandite peaks and enhanced amorphous C–S–H formation in nano-modified mixes. The findings demonstrate that nano-engineered additives, especially MWCNTs, significantly enhance the microstructure, mechanical strength, and durability of the concrete, highlighting their potential for producing high-performance and advanced composite materials.