Microstructure and phase characterizations of fly ash cements by alkali activation

Abstract

© 2020, Akadémiai Kiadó, Budapest, Hungary. Microstructure and phase characterizations of fly ash cement by alkali activation were investigated. High calcium fly ash (FA) at 70%, 80%, 90% and 100% by mass of binders was used in combination with Portland cement (PC), thus producing alkali-activated fly ash cements with some part of Portland cement and geopolymer (at 100%FA). Alkali solutions (Na2SiO3 and NaOH) were used as activators at alkali liquid/binder of 0.65, and Na2SiO3/NaOH ratio used was 0.67. Samples were cured at 23 °C (55% RH) and 60 °C (95% RH). The results showed that curing temperature significantly affects the reacted products. By curing at higher temperature ≈ 60 °C, a denser structure due to high-temperature curing plays a crucial role in terms of producing more semi-crystalline (N–A–S–H) structure as characterized by X-ray diffraction. Moreover, higher-temperature curing gave higher compressive strength than curing at 23 °C in all mixes. Optimum compressive strength obtained at 23 °C and 60 °C curing samples was found in 80FA20PC and 100FA samples, respectively. Thermal analysis results showed that N–A–S–H/(N, C)–A–S–H was detected in all mixes. Scanning electron microscope and energy-dispersive X-ray showed elements belong to N–A–S–H and (N, C)–A–S–H phases.