Synthesis and characterization of fly ash based self-dispersing, self-sensing geopolymer

Abstract

Extensive studies have been carried out to use carbon nanotubes (CNTs) to reinforce cementitious materials because of the extraordinary strength of CNTs. More importantly, new functions such as self-sensing ability can be introduced to the materials due to the excellent electrical conductivity of CNTs. However, the application of CNTs in reinforcing materials is hampered by three major challenges: proper dispersion of the nanoscale additives, scale-up of laboratory results and implementation on larger scale, and a lowering of the cost benefit ratio. It is not easy to disperse CNTs into cementitious materials. Aiming to address all these three challenges simultaneously, this study proposes to produce CNTs reinforced cementitious materials through directly growing CNTs on fly ash particles using a novel Poptube method. Unlike any other exiting methods, Poptube method uses microwave irradiation as heating source, and a single chemical (e.g., ferrocene) to provide both the carbon source and the catalyst for CNTs’ growth. Compared with existing methods, the Poptube method is much more cost-effective and can be easily scaled-up for mass production. CNT reinforced geopolymer can be produced by mixing these CNTs grown fly ash particles with other ingredients. In this way, the time-consuming and difficult task of dispersion of CNTs is eliminated since CNTs are self-dispersed into the matrix by the fly ash particles on which CNTs were grown. To evaluate the effect of growing CNTs on the reactivity of fly ash particles, a series of tests were carried out, including dissolution testing, electric conductivity testing, and imaging with scanning electron microscopy (SEM) and Atomic Force Microscopy (AFM). Results show that growing CNTs on the surface of fly ash does not reduce the reactivity of the fly ash because of the seeding effect provided by the CNTs. The composite effect induced by the CNTs was confirmed by Raman Spectrometer, which shows that the D-band of the CNTs varies with the applied thermal stress, suggesting effectively stress transfer from the geopolymer matrix to CNTs. This finding suggests that stress in CNTs reinforced geopolymer can be sensed by a Roman Spectrometer in a non-contact fashion. The self-sensing function of the nanocomposite mortar is evaluated using a four-electrode-DC method. At early age, geopolymer mortar is piezoresistive because of its high electric conductivity. However, DC induced polarization effect is very serious at this age. This polarization effect reduces with reaction time and becomes negligible at 35d. Similar piezoresitivity was achieved by the geopolymeric nanocomposite produced by using fly ash grown with CNTs using Poptube method, which is three orders magnitude more sensitive than the geopolymer one without any CNTs.