Enhancing Flash-Setting Behavior by Incorporating Various Retarders and Their Effect on Mechanical and Microstructural Properties of Hybrid Alkali-Activated Mortar

This study laid an effort to address the flash-setting behavior of hybrid alkali-activated paste made with 25% OPC and 75% fly ash by incorporating sodium tetraborate (BR) and sucrose (S) as retarders at dosages of 2%, 4%, 7% and 10% each. The effect of these retarders on fresh and mechanical and microstructure properties of hybrid alkali-activated mortar (HAAM) was investigated. The microstructure of HAAM was evaluated through X-ray diffraction, thermo-gravimetry analysis, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy analyses. The addition of sodium tetraborate outperforms sucrose in enhancing the fresh and mechanical properties. The incorporation of 10% BR eliminates the flash-setting and also assisted in attaining highest mechanical strength. The B─ O─ Si bond formed in the presence of sodium tetraborate and subsequently changed to B─ O─ Al─ O─ Si bond, thus taking more time than the usual formation of Si─ O─ Al─ O bond and assisting in prolonging the setting time. The B─ O─ Al─ O─ Si bond is known as boroaluminosilicate bond having needle-shaped structure as observed from morphology analysis. Further, the contribution of bridging oxygen in O 1s spectra increased with the dosage of BR and also assisted in broadening the aluminosilicate network through crosslinking of bridging oxygen.