Rheology, Setting Time, and Compressive Strength of Class F Fly Ash-Based Geopolymer Binder Containing Ordinary Portland Cement

This paper investigates the incorporation of ordinary portland cement (OPC) to adjust/improve the workability, setting time, and compressive strength of Class F fly ash (FA)-based geopolymer binder. The geopolymer binder specimens were produced by mixing OPC with FA at a dosage of 0%, 5%, 10%, 15%, and 20% by weight of FA, respectively, and then mixing the mixture with a blended sodium silicate (SS) and sodium hydroxide (SH) solution at a SH concentration of 5 M and a SS/SH ratio of 1. A water-to-solid (W/S) ratio of 0.35, 0.40, 0.45, and 0.50, respectively, was used in preparing the specimens. The viscosity and setting time of the fresh geopolymer binder were measured by using a coaxial cylinder viscometer and a Vicat apparatus, respectively. The specimens were cured at 35°C in an oven for 7 days before tested to measure the unconfined compressive strength (UCS). The results show that the viscosity of the geopolymer binder increases with higher OPC content and lower W/S ratio. The addition of OPC reduces the setting time. The shortest initial and final setting times of 16 and 46 min, respectively, were obtained at W/S=0.35 and 20% by weight OPC. The incorporation of OPC increases the UCS of the geopolymer binder. The highest UCS of 42.4 MPa was obtained at W/S=0.35 and 20% by weight OPC. Microstructural and chemical analyses including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) were also carried out and the results indicate that the addition of OPC produces a denser microstructure by the formation of calcium silicate hydrate (CSH) gel along with geopolymer gel.