Multifunctional properties of carbon nanotube/fly ash geopolymer nanocomposites

Fly ash-based geopolymers are currently being considered as a viable replacement to ordinary Portland cement (OPC) due to multifold benefits such as cost efficiency, chemical stability, corrosion resistance, rapid strength gain rate, low shrinkage and freeze-thaw resistance. However, geopolymers tend to be more brittle than OPC and thus unsuitable for concrete structures due to safety concerns. Geopolymers with improved electrical properties can also be used as self-sensing materials capable of detect their own structural damage. Therefore, this paper is aimed at investigating the effect of multiwalled carbon nanotubes (MWCNTs) on the mechanical and electrical properties of fly ash (FA) geopolymeric composites. Geopolymeric matrices containing different MWCNTs concentrations (0.0%, 0.1%, 0.5% and 1.0% by weight) were synthesized and their mechanical properties (i.e., flexural strength, Young’s modulus, flexural toughness and fracture energy), electrical conductivity and piezoresistive response were determined. A scanning electron microscope (SEM) was employed to evaluate the distribution quality of MWCNTs within the matrix and determine their crack-bridging mechanism. The experimental results showed that the MWCNTs were uniformly distributed within the matrix at 0.1 and 0.5-wt% and they were poorly distributed and severely agglomerated within the matrix at 1-wt%. The experimental results also showed that the addition of MWCNTs increased the flexural strength, Young’s modulus and flexural toughness by as much as 160%, 109% and 275%, respectively. The MWCNTs also enhanced the fracture energy and increased the electrical conductivity by 194%. The geopolymeric nanocomposites exhibited a piezoresistive response with high sensitivity to micro-crack propagation.