Bond strength evolution of rendering mortars incorporating ladle furnace slag under natural outdoor exposure

Ladle furnace slag (LFS), a byproduct of steelmaking with slow-reacting phases, has emerged as a potential supplementary binder to partially replace Portland cement (PC), although its in-service behavior remains poorly understood. This study investigates how bond strength develops in rendering mortars incorporating LFS under natural outdoor conditions. Two mortar systems were analyzed: lime–cement mixed mortars and admixture-based mortars. Mechanical, physical, and microstructural analyses were used to elucidate bonding behavior in rendering mortars. Lime-based systems demonstrated stable long-term performance, with LFS showing a gradual increase in mortar–substrate bond strength of approximately 9–19% between 90 and 180 days, reaching values comparable to the reference mixtures (up to 0.293 MPa). In contrast, admixture-based mortars showed reductions in bond strength of up to 17% over time when LFS was incorporated, indicating greater sensitivity to environmental moisture exposure. Microstructural analyses confirmed limited participation of LFS in hydration, with results strongly influenced by the binder's chemical composition and environmental exposure. The findings indicate that LFS can be safely used in lime–cement rendering mortars, whereas its incorporation in admixture-based systems requires careful control of moisture transport properties to ensure long-term performance, supporting its potential as a sustainable supplementary binder for rendering mortars exposed to natural environmental conditions.