PSI - Issue 67

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Hwan Lee et al. / Procedia Structural Integrity 67 (2025) 107–114 Author name / Structural Integrity Procedia 00 (2024) 000 – 000

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(c) use of supplementary cementitious materials (SCMs) like fly ashes, slag, silica fume, and pozzolans (Rajabipour et al. 2015); and (d) addition of lithium admixtures (Folliard et al. 2006). Previous research suggested that SCMs are effective in mitigating ASR by reducing permeability and decreasing porosity (Demir et al. 2020). However, challenges exist, including but not limited to the following: various factors affecting the availability of class F fly ash (Al-Shmaisani et al. 2022), regional limitations in sourcing non-reactive aggregates (Rajabipour et al. 2015), and the limited effectiveness of lithium admixtures in mitigating ASR (Kawamura and Fuwa 2003). This has led to renewed interest in identifying a broader range of materials that can be used to mitigate ASR. Nanomaterials are materials that have one of their dimensions in the nanoscale (< 100 nm) (“ISO - ISO/TS 80004-1:2015 - Nanotechnologies — Vocabulary — Part 1: Core terms” n.d.) . In recent years, nanomaterials have shown promise in enhancing the durability of cementitious materials (Zhang et al. 2020). For example, studies have shown that nanosilica displayed good efficacy in reducing expansion due to ASR (Cai et al. 2019; Zeidan and Said 2017; Zhang et al. 2019). However, other nanomaterials like nano metakaolin (NMK) and carbon nanotubes (CNTs) have been relatively underexplored in this context. Nano metakaolin (NMK) is a finely ground metakaolin, which is a dehydroxylated form of kaolinite clay (Zhan et al. 2020). NMK contributes to the strength and durability of cementitious composites by participating in the pozzolanic reaction and forming additional calcium silicate hydrate (C-S-H). This reaction fills the pores in the cement matrix, reducing permeability and enhancing resistance to deleterious reactions like ASR and sulfate attack (Aquino et al. 2001). Also, the reduced calcium hydroxide (CH) through pozzolanic reaction could produce a dense microstructure which could be beneficial for ASR resistance. However, NMK contains alumina (18.9 to 42.3%) along with a silica component, which could potentially improve the hydration process when compared to just silica-based nanomaterials like nanosilica (Zhan et al. 2020). NMK is typically used to replace cement at 5-10% and shows significant improvement in strength, increased pozzolanic activities, and reduction in porosity (Zhan et al. 2020). Whereas, carbon nanotubes (CNTs) are renowned for their exceptional mechanical properties. Thus, when incorporated into a cement matrix, CNTs can enhance mechanical strength, promote better crack resistance, and improve the overall durability of the material (Shi et al. 2019). The high aspect ratio of CNTs was shown to bridge micro-cracks, impeding the progress of crack propagation and delaying the onset of structural failures (Ramezani et al. 2022). In addition, CNTs have shown that they can improve the mechanical properties of concrete at very small dosages of below 0.1wt% (Silvestro et al. 2020; Sobolkina et al. 2012). Due to the hydrophobic nature of CNTs (Konsta-Gdoutos et al. 2010), they tend to agglomerate when mixed in water, leading to inhomogeneous mixing in the concrete matrix (Zhao et al. 2020). This requires that the CNTs be dispersed in water before being added to the concrete. In addition, due to the size and precision requirements during their manufacture, typical multi-walled CNTs tend to be $0.9-15 per gram (Cheaptubes.com n.d.). However, for this research work, a low-cost CNT was used ($0.01/g). Despite the growing interest in nanomaterials, there is limited research focusing on their contributions specific to ASR resistance. Both NMK and CNTs have been shown to reduce porosity and improve the mechanical properties of concrete, which can in turn have a positive impact on ASR resistance. The objective of this study is to evaluate the effect of NMK and CNT on ASR expansion and verify their impact on strength and permeability. Materials and methods 1.1. Materials A cement conforming to ASTM C 595-Type IL (with 12% limestone) was used in this work (ASTM C 595 2003). The median particle size of cement was 15 μm as determined using a Malvern Mastersizer 2000 laser diffraction particle size analyzer. Sand from the Jobe pit in El Paso, Texas, a known high-reactivity fine aggregate, was chosen for the ASR test (Folliard et al. 2006). A river sand conforming to ASTM C33 was used for the remaining tests. Two powder-type nanomaterials were selected for the study, NMK and multi-walled CNTs. The particle size and morphology of these materials were analyzed using Apreo scanning electron microscope (SEM). The NMK had a white color and displayed a plate-like structure, with diameters between 500 and 800 nm and a height ranging from 30 to 70 nm, making it a 2D nanoplatelet with at least one outer dimension in the nanoscale (see Fig. 1a). The CNTs