Reverse-diffusion phenomenon of high ductility fly ash-based geopolymer against carbonation

Abstract

Geopolymer has been a research hot spot in the field of building materials because of its superior performances, less pollution, low energy consumption and wide range of raw materials. In this study, the carbonation resistance of high ductility fly ash-based geopolymer was researched by means of accelerated carbonation experiment. The compressive strength, carbonation depth and pH value were adopted to macroscopically evaluate the carbonation enhancing influence of modified geopolymer. Meanwhile, the mercury intrusion porosimetry (MIP), Fourier transforms infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) were adopted to evaluate the micropore structures, geopolymerization process and mineralogical phases, respectively. The experimental results showed that under rapid carbonization condition, the carbonization resistance of control geopolymer was obviously lower than that of cement mortar, and the incorporation of modified MWCNTs and PVA fibes could enhance the carbonization resistance of high ductility geopolymer from micron scale and nano scale. Importantly, under natural carbonization condition, a reverse-diffusion phenomenon of geopolymer was observed (the complete carbonization zone became the uncarbonization zone), and the pH value of complete carbonization zone was improved ranging from the 8.1 to 11.5, manifesting that the carbonization resistance could be improved significantly. The micro-analysis indicated that the main reason of causing reverse-diffusion phenomenon was the reverse migration of large amount of OH−, which was from the dissolution of magadiite by means of the physical phase transform from solid to liquid under low CO2 concentration.