Effect of pristine and functionalized carbon nanotubes on microstructural, rheological, and mechanical behaviors of metakaolin-based geopolymer

This study presents microstructural, rheological, and mechanical behavior of metakaolin-based geopolymer nanocomposite reinforced with multi-walled carbon nanotubes (CNTs). Two concentrations and two types of CNTs were added directly to the activating solution, sonicated, and then mechanically mixed with metakaolin to form the geopolymer paste. The results indicated that the effect on the rheological properties was not significant, irrespective of the type and quantity of CNTs used. When compared to a reference paste, improvements of 3.8%, 13.2% and 28.7% were observed in the Young's modulus, compressive and flexural strengths, respectively, with the addition of 0.1 wt% pristine-CNTs, due to matrix porosity refinement. However, with the addition of 0.2 wt% there were decreases of 3.5%, 11.8% and 31.0%, for Young'g modulus, compressive and flexural strengths, respectively, as result of an increase of 10.2% in the matrix total porosity. However, with the addition of 0.1 and 0.2 wt% functionalized-CNTs (f-CNTs), notable mechanical property improvements were observed - up to 31.6% and 46.6% for Young's modulus and compressive strength with 0.2 wt% f-CNTs, and 66.0% for flexural strength with 0.1 wt% f-CNTs. These enhancements are explained by a reduction of 63.2 and 53.7%, for 0.1 and 0.2 wt % f-CNTs, respectively, in the macropore proportion of the matrix and a strong bond between the functionalized-CNTs and the geopolymer matrix.