Enhancing concrete's resistance to ASR by integrating metakaolin-carbon nanotube blends

Alkali-silica reaction (ASR) is a destructive reaction that occurs in concrete when reactive aggregates are involved and usually leads to premature loss of serviceability of the structure. Understanding the conditions in nano- and micro-scale that promote the generation of the ASR gel is crucial to control the rate of expansion and limit the course of the ASR. In this study, metakaolin (MK) blends reinforced with dispersed carbon nanotubes (CNTs) have been proposed for mitigating ASR expansion. Addition of up to 20 wt% MK is known to have a modest effect on reducing the ASR expansion and typically the 14-day expansion results exceed the ASTM C1567 0.1 % threshold. While the pozzolanic reaction of MK results in lower availability of calcium ions (Ca 2+ ) preventing this way the recycling of alkalis, addition of CNTs is shown to modify the calcium-to-silica (Ca/Si) ratio of Calcium-Silicate-Hydrates (C-S-H) at the interfacial transition zone (ITZ), further enhancing the alkali binding capacity of the cementitious matrix. Furthermore, the synergistic action of MK and CNTs is shown to reduce the rate of expansion with exposure time and keep the 14-day MK-CNT blend expansion value well below the 0.1 % threshold. Since ASR and the degree of expansion can notably affect flexural capacity and reduce compressive strength and Young's modulus, Linear Elastic Fracture Mechanics (LEFM) and uniaxial compression tests were conducted post-ASR to evaluate the effect of the MK-CNT blends on the retained modulus of elasticity and tensile strain energy absorption capacity.