Influence of carbon nanotubes on properties of cement mortars subjected to alkali-silica reaction

This research investigates the impact of carbon nanotubes (CNTs) on the expansion, mechanical properties, and microstructure of cement mortar subjected to alkali-silica reaction (ASR). A total of eight compositions were cast to investigate the influence of CNT surface condition, concentration and aspect ratio, and ultrasonication energy. Two types of CNTs were used as reinforcement: pristine CNTs with an aspect ratio of 800 (Type I) and COOH-functionalized CNTs with an aspect ratio of 2500 (Type II). CNTs were incorporated at concentrations of 0.1c-wt.% and 0.3c-wt.%. The results indicated that certain combinations of mix proportions could significantly mitigate the ASR-induced damages in the mortar bar test (ASTM C1260). Specimens reinforced with 0.1c-wt.% of Type I CNTs exhibited the highest resistance to alkali-silica reaction. While specimens reinforced with 0.3c-wt.% of Type II CNTs had marginal improvements. More specifically, specimens reinforced with 0.1c-wt.% of Type I CNTs reduced the 14-day ASR expansion by 73% against control, and increased the 28-day compressive strength, flexural strength, static and dynamic elastic moduli by 22%, 36%, 54%, and 22%, respectively. Finally, scanning electron microscopy and X-ray energy-dispersive spectroscopy revealed that CNTs resist ASR with three main chemo-mechanical mechanisms: 1) restrain crack propagation, 2) refine the pore structure of the composite, and 3) reduce the alkalinity of the pore solution by altering the ASR gel compositions.