A eco-friendly acid fly ash geopolymer with a higher strength

Abstract

Phosphoric acid (PA)-based geopolymer is usually prepared with PA as activator. However, the PA-based geopolymer using fly ash as raw material has not received much attention, and its strength is characterized by instability because excessive or insufficient PA will produce a negative impact on the strength. As the curing time prolongs, the compressive strength of geopolymer with Al/P molar ratio ≠ 1 reduces. In this work, the fly ash (FA) was used as raw material, the aluminum dihydrogen phosphate (ADP) were used as activators to develop a new acid-based geopolymers (ADP-based geopolymer) at room temperature. The geopolymers microstructure and mechanism were investigated by X-ray Diffraction, Infrared Spectroscopy, Scanning Electron Microscopy, Nuclear Magnetic Resonance, and Mercury intrusion analysis. The ADP-based geopolymer mechanical performance were investigated by unconfined compressive strength test. The geopolymer pH, electrical conductivity (EC), setting time, and fluidity were also studied. Additionally, the thermogravimetric characteristics of geopolymer were explored by thermogravimetry analysis. Results show that the ADP-based geopolymer had a higher strength than the PA-based geopolymer, the strength raised with the ADP concentration increasing, and the strength would not be weakened by excessive ADP activator. As the curing time prolonged, the compressive strength increased, and a higher ADP concentration caused a longer setting time. As the ADP concentration increased, the geopolymer pH decreased but the EC increased. However, as the curing time prolonged, an opposite trend was observed. Microcosmic analysis indicated that the ADP-based geopolymer matrix were mainly AlPO4, CaHPO4 crystalline phase and amorphous matrix composed of Si–O–Al–O–P, Al–O–P, Si–O–P, Si–O–Si, and Si–O–P–O–Si units.