Influence of additives and alternative precursors on the properties of lime-activated slag systems

Lime-activated slag binders offer a low-carbon alternative to Portland cement but remain highly sensitive to precursor chemistry and additive strategy, especially when alternative or waste-derived materials are incorporated. This study investigates the combined effects of synthetic calcium silicate hydrate (C–S–H) seeding and colloidal nano silica on lime-activated slag systems containing bone ash and metakaolin as alternative precursors. Sixteen mixtures were evaluated using a constant water-to-precursor ratio and lime dosage to isolate chemical and microstructural effects. Mechanical performance was assessed through compressive and split tensile strength, while durability-related behaviour was examined using water absorption and porosity measurements. Microstructural evolution and chemical composition were analyzed through scanning electron microscopy and energy dispersive spectroscopy, including elemental mapping and atomic ratio analysis. The results from this study showed that compressive strength is governed primarily by localized densification of reaction products, with nano silica and C–S–H seeding significantly enhancing strength in slag-rich systems. However, these improvements do not always translate into improved tensile resistance or reduced transport properties. Mixtures incorporating bone ash and metakaolin exhibit pronounced strength–durability decoupling, characterized by high compressive strength alongside elevated porosity and water absorption. EDS analysis reveals that this behaviour arises from chemically competitive environments in which calcium is partitioned among silicate, aluminate, and phosphate domains, limiting the formation of a chemically continuous binding network. • Lime-activated slag was modified using nano additives and alternative precursors • Bone ash and metakaolin introduce calcium competition and network disruption • Strength, porosity and absorption were linked to microstructure and composition • EDS maps reveal that calcium partitioning governs strength–durability decoupling. • Tensile strength and transport depend on microstructural continuity