The effect of CaO properties on the early volume change behavior, compressive strength and microstructure of CaO-activated GGBFS composites

This study investigates the influence of calcium oxide (CaO) properties, specifically hydration reactivity, on early-age volumetric changes, compressive strength, and microstructure of CaO-activated ground granulated blast furnace slag (GGBFS) pastes. Four industrial quicklimes with distinct reactivities, characterized by t max values of 9, 19, 31, and 36 minutes, were tested at dosages of 5%, 9%, and 13% by binder mass. Reference mixtures activated with hydrated lime and Portland cement were included for comparison. Volumetric behavior was found to strongly depend on both the reactivity and dosage of quicklime: highly reactive limes (t max = 9–19 min) induced shrinkage up to 0.51 mm/m, whereas low-reactivity limes (t max = 31–36 min) led to expansion reaching 7.25 mm/m. In contrast, compressive strength at 28 and 120 days ranged from 11.4–17.2 MPa and 25.7–29.7 MPa, respectively, with no significant dependence on quicklime type or content. Microstructural analysis revealed a consistent amorphous C-S-H matrix across all samples, incorporating fine portlandite crystallites. The chemical composition of the C-S-H phase remained stable, with Ca/Si atomic ratios ranging from 0.92 to 1.45. Results demonstrate that careful selection of quicklime reactivity enables control over volumetric behavior without compromising mechanical performance, offering a viable strategy for tailoring CaO-activated slag-based binders for engineering applications.