Effect of early-hydration on paste rheological properties for ultrahigh performance geopolymer concrete(UHPGC)

The development of ultrahigh-performance geopolymer concrete (UHPGC) has attracted considerable attention owing to its low carbon emissions and sustainable development. However, its rheological properties are affected by complex factors, which are difficult to control, limiting its engineering applications. Therefore, improving the rheological properties of UHPGC is critical. In this study, the effects of granulated blast furnace slag (GBFS), alkali activator modulus (AAM), and water-to-geopolymer binder (W/B) and their interactions on both rheological and mechanical properties were investigated. The results showed that response surface methodology (RSM) was suitable for multi-objective optimization of UHPGC. Additionally, GBFS and AAM and their interaction significantly influenced the plastic viscosity and yield stress of UHPGC. At 60.9% GBFS, W/B=0.33, and AAM=1.44, plastic viscosity and yield stress decreased to 3.15 and 20.60, respectively, while compressive and flexural strengths rose to 128.5 MPa and 16.9 MPa. The precipitation of primary C-S-H gel on the surface of undissolved particles increased the internal friction of the paste, thereby obstructing and obstructed the flow of the paste. However, the reduced dissolution lowered the early hydration rate of the slurry and improved its rheological property by decreasing the GBSF content and increasing AAM. The decrease in GBFS content and the increase in the AAM and W/B reduced the concentration of leached Ca ions in the paste, retarding the hydration and reducing the content of primary C-S-H gel precipitated on the surface of undissolved particles. This change weakened the internal friction within the paste, reduced the plastic viscosity and yield stress.