Influence of multi-walled carbon nanotubes on the fracture response and phase distribution of metakaolin-based potassium geopolymers

Abstract: This research investigated the effects of multi-walled carbon nanotubes (MWCNTs) on the chemistry, microstructure, phase distribution, and fracture response of potassium-based metakaolin geopolymers at the microscopic scale. We formulated novel protocols to cast geopolymers reinforced with 0.3, 0.6, and 1.5 wt% MWCNTs. We studied the chemistry using XRD, FTIR, and solid state ²⁹Si NMR. We characterized the microstructure and dispersion state of MWCNTs geopolymers using microscopic imaging and high-resolution scanning electron microscopy. We assessed the fracture behavior and mechanical properties using scratch tests and indentation tests. We used cluster analysis of indentation results to study the phase distribution. MWCNTs were well dispersed with an average accumulated area less than 8.9 μm ². XRD showed that MWCNTs preserved the amorphous phase. NMR showed that the addition of MWCNTs decreased Q ⁴(Al2) fraction, but increased Q ⁴(Al3) fraction. We observed a densification of the microstructure and a reduction in porosity. The microstructure showed that MWCNTs acted as bridges for fracture surfaces and connections for pores. The addition of 0.6 wt% MWCNTs increased the strength by 3.2%, and stiffness by 11.1%. Meanwhile, the addition of 1.5 wt% MWCNTs addition increased the fracture toughness by 10.5%. An inner strengthening effect was observed as MWCNTs reduced the microporosity, resulting in an increase in the indentation modulus and hardness for the dominant microphase. Therefore, MWCNTs promote the geopolymerization reaction, strengthen the geopolymer skeleton, affect the pore structure, and improve mechanical characteristics. Graphical abstract: [Figure not available: see fulltext.].