Mechanical and microscopic properties of fly ash phosphoric acid-based geopolymer paste: A comprehensive study

Abstract Fly ash (FA) is a common industrial by-product, which is usually used as raw materials of alkali-activated geopolymer or cement additive. However, there are few studies on phosphoric acid-based geopolymer cement synthesized with FA. In this study, FA phosphoric acid-based geopolymer (FAPG) was prepared by using low-calcium FA as raw material and phosphoric acid (PA) as reactant at room temperature. Microscopic properties and geopolymerization mechanism were studied by X-ray diffraction, scanning electron microscopy, mercury intrusion porosimetry, nuclear magnetic resonance instrument and Fourier transform infrared spectroscopy. The hydration characteristics were studied by isothermal calorimetry, and the thermal properties were explored by thermogravimetric analysis. Additionally, the setting time, compressive strength, pH and electrical conductivity of FAPG were measured. The results indicated that room-temperature cured FAPG paste activated with 50% PA and 0.3 liquid–solid mass ratio (L/S) yielded a maximum compressive strength during 0–90 days of curing. As curing time increased, the strength increased for FAPG with 50% PA, but increased at first and then scarcely increased for other FAPG pastes. The compressive strength decreased with L/S increasing. As curing time increased, the pH increased and electrical conductivity decreased. The initial and final setting time, and fluidity decreased with increasing PA concentration but increased with L/S increasing. The FAPG paste was a composite structure composed of crystalline and amorphous phase. The crystalline phase was composed of Brushite, Monetite and Berlinite, and the amorphous phase was composed of -Al-O-P-, -Si-O-P- and Si-O-Al-O-P- units. The average pore diameter of FAPG ranged from 37.38 to 64.44 nm. Heating to 1200 °C, FAPG paste presented four stages of mass loss. The total hydration heat increased with PA concentration increasing.