evaluation of setting behavior in slag‑based geopolymer concrete using ultrasonic pulse velocity under varying mix proportions

In response to the growing environmental crisis and the urgent need to curb carbon emissions and improve waste management, slag‑based geopolymer concrete has emerged as a low‑energy, sustainable alternative to ordinary portland cement. this study investigates the initial and final setting behaviour of ground‑granulated blast‑furnace slag (ggbfs) geopolymer concrete using the ultrasonic pulse velocity (upv) technique alongside a freshcon mold system. key variables examined include the molarity and type of alkaline activator, the alkali‑to‑slag ratio, and the proportion of slag within the concrete matrix. upv traces delineate three distinct stages of induction (dormant), polymerization onset (activation), and network consolidation (hardening), each typified by its signature s‑shaped profile. in this study, increasing the concentration of the alkaline activator and the proportion of ground granulated blast-furnace slag (ggbfs) was found to dramatically shorten both initial and final setting times of slag‑based geopolymer mixtures; however, beyond an optimal activator concentration and solution-to-slag ratio, these accelerating effects attenuate and polymerization kinetics become disrupted. notably, whereas ordinary portland cement exhibits an induction period over 240 minutes, geopolymer samples activated at 6m complete their induction phase in under 30 minutes, a 6–8 fold increase in initial setting rate. such rapid setting, while advantageous for expediting placement cycles, poses challenges in mass concrete pours by heightening the risk of cold joints and imposing stringent formwork time constraints. conversely, this accelerated hardening can be leveraged as a significant asset in future investigations into precast concrete elements and tunnel‑lining shotcrete applications.