Macroscopic and microscopic analyses on mechanical performance of metakaolin/fly ash based geopolymer mortar

Abstract

In this study, systematic macroscopic and microscopic analyses on the mechanical performance of metakaolin (MK)/fly ash (FA) based geopolymer mortar were carried out through compression tests, bending tests, and phase analysis, to illustrate the mechanisms of the interaction between fiber and nano additives and mortar leading to the improvement of mechanical properties of matrix. The detailed analyses on the microscopic mechanisms of mechanical performance enhancement due to the nano-SiO2 (NS) and polyvinyl alcohol (PVA) fibers were made based on the microscopic tests using field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectrometer (EDS), and X-ray diffractometer (XRD). The microscopic morphology and phase analyses demonstrated that an appropriate amount of NS promoted the geopolymerization and improved the mechanical properties of the mortar; however, after a certain value, the continued increase of the proportion of NS negatively affected mechanical properties of geopolymer mortar. Microstructural analyses and crystal identification indicated that in addition to its filling effect, NS formed more structurally stable (N, C)-A-S-H crystals that enhanced the adhesion between the matrix and PVA fiber surfaces. However, excessive NS negatively impacted the hydration of the FA and MK, thus inhibiting the formation of aluminosilicate, which resulted in a reduction in the strength of the geopolymer mortar. Through the above experiments, the microscopic mechanism of the macroscopic mechanical behavior changing caused by NS and PVA was revealed, aiming to provide reference for future improvement on mix proportion of MK/FA based geopolymers and in-depth research.