Characteristics of fly ash-based geopolymer concrete in the field for 4 years

Abstract

The long-term durability of geopolymer concrete (GPC) exposed to various working conditions is a critical issue for achieving potential applications in the field. This study focuses on the long-term mechanical properties of pure Class F fly ash-based geopolymer concrete (FGPC) pavement at field conditions in relation to the microstructure and chemical compositions of reaction products. The continuously increasing compressive strength over time is observed, obtaining 47.29 and 65.54 MPa at 28 d and 4th y, respectively. Scanning electron microscopy (SEM), low-field nuclear magnetic resonance (NMR), and capillary absorption test were conducted to investigate the change in the physical microstructure, while X-ray diffraction (XRD), energy disperse spectroscopy (EDS), and pH test for chemical composition. The long-term curing leads to the homogeneous and compact microstructure with low average porosity and secondary absorption rate falling to 16% and 0.0033 mm/s1/2. The steady internal alkaline environment in FGPC ensures and extends the development of geopolymerization, resulting in the increase of amorphous phase content (APC, up to 93%) and decrease of Si/Al and Na/Al molar ratios (down to 2.85 and 0.88). The change in the chemical composition indicates that the disordered geopolymer is gradually transforming into a more ordered structure over time. The strong correlations identified between mechanical properties and micro-characteristics during the long-term period provide guidance for adjusting the mechanical properties of FGPC via regulating and controlling the microscope properties.