Mechanical properties and electrical resistivity of multiwall carbon nanotubes incorporated into high calcium fly ash geopolymer

High calcium fly ash (HCF) is a pozzolan material and is available in large quantity in Thailand due to the existence of coal-based electrical power plants. It is used as a supplemental material to partially replace cement content in concrete as a movement toward concrete sustainability. In order to lift the sustainability level, a cementitious material without Portland cement called ‘geopolymer’ was introduced. Geopolymer can be produced from raw materials containing high alumina and silica, for example fly ash, blast furnace slag, and metakaolin. For high calcium fly ash geopolymer (HCFG), the unique properties include fast setting, and high early strength. In this study, in order to enhance the properties of HCF geopolymer, multiwall carbon nanotubes (MWCNTs) were introduced into the matrix. In addition to the investigation into basic properties, the effect of MWCNT on electrical resistivity was also investigated to determine its potential use in piezoelectric sensor applications. The results showed that the addition of MWCNTs improved the mechanical properties of HCFG. The maximum compressive and flexural strengths were obtained with a mix containing 0.2% MWCNTs. The EDS test also indicated the increase in geopolymerization and hydration products with the addition of MWCNTs. To investigate the piezoelectricity potential, the electrical resistivity under different levels of compression loads was investigated. The resistivity decreased with the increasing load level up to the first crack, and then decreased. The changes in electrical resistivity indicated the potential use of HCFG incorporated MWCNTs in self-sensing for structural health monitoring.