CaCl2 as a hardening accelerator for air lime-metakaolin slurries: Effects of two different types of metakaolin on kinetics, rheology and microstructure

Lime–metakaolin (MK) slurries/mortars offer several advantages; however, their setting times are typically 5–7 times longer than those of Portland cement-based systems, which hinders practical application. This study evaluates calcium chloride (CaCl₂), a widely used accelerator in cementitious systems but not previously explored in air lime–MK pastes, focusing on hardening kinetics, early-age microstructure, and rheology. Two metakaolins (MK1 and MK2), differing in chemical composition (notably Al₂O₃ and Fe₂O₃ contents), amorphicity, particle morphology and size distribution, were investigated with CaCl₂ additions of 5% and 10% (by binder mass). Hardening kinetics were assessed by Vicat needle penetration and semi-adiabatic calorimetry, while fresh-state behavior was characterized by mini-slump and rotational rheometry; early-age phase assemblage and morphology were examined by XRD and SEM. Mortar compressive strength was also evaluated. CaCl₂ exhibited a metakaolin-dependent effect, accelerating hardening in lime–MK1 slurries but delaying hardening in lime–MK2 slurries. Reference lime–MK slurries generally followed the Herschel–Bulkley model, whereas CaCl₂ promoted a transition toward Bingham behavior and reduced shear stresses. XRD and SEM evidenced the formation of Friedel's salt in CaCl₂-containing pastes, particularly in lime–MK2, consistent with MK2 higher aluminum content; concomitantly, its presence was associated with a disruption in development of monocarboaluminate and strätlingite. Mortar compressive strength followed the same metakaolin-dependent trend upon CaCl₂ addition, increasing for MK1 and decreasing for MK2. These results demonstrate that CaCl₂ does not act as a universal accelerator in lime–MK systems and that the metakaolin characteristics govern the early-age kinetic and rheological response.