Investigating the effects of MWCNTs on sustainable alkali-activated mortar: physical, rheological, and mechanical analysis

Abstract

Alkali-activated material (AAM) is a potential sustainable substitute for normal Portland cement-based concrete, but the composition of the material and the design of the mixture has a significant impact on fresh and hardened properties. This research describes the formulation and mechanical behavior of a multi-walled carbon nanotube (MWCNT)-reinforced-based alkali-activated nanocomposite. MWCNTs were incorporated into the fly ash/hydrated lime-based alkali-activated material at concentrations of 0.1%, 0.2%, 0.3%, and 0.35% by weight of the binder. The significant dispersion of MWCNTs within the matrix was achieved in an aqueous solution of surfactant and superplasticizer by employing both magnetic stirring and sonication procedures. The fresh-state characteristics of the MWCNT-based AAM were evaluated in terms of setting time and rheology parameters. Notably, samples containing 0.3% and 0.35% MWCNTs exhibited setting times of 26 and 25 min, respectively. The rheological behavior of the nanocomposite adhered to Modified Bingham's model, displaying shear-thinning properties. Moreover, an increase in MWCNT content led to an augmentation in yield stress, with the highest enhancement of 11.1% and 13.1% in yield stress and plastic viscosity, respectively, observed for mortar samples with 0.3% MWCNTs. Mechanical assessments were performed at 7, 14, and 28 days to investigate the compressive and flexural strengths of the specimens. The addition of approximately 0.3% MWCNTs resulted in an impressive boost of up to 14% in compressive strength and an even more remarkable increase of 28% in flexural strength. Furthermore, after exposure to acid and sulfate attacks, the ACN3 samples (0.3% wt. of MWCNTs) exhibited minimal strength loss, with only 5.1% and 3.9% decrease in compressive strength after acid and sulfate attacks, respectively. According to these results, the MWCNTs can function as effective bridges to reduce and limit the progression of microcracks through the AAM-based nanocomposite under the circumstances of homogeneous distribution and good adhesion between both the MWCNTs and the adjacent AAM paste.