Durability Considerations in Replacing Blast Furnace Slag with Low-Grade Calcined Clay and Natural Pozzolan in Quaternary Cements

Belgium and the EU-27 face a shortage of suitable supplementary cementitious materials (SCMs) capable of supporting high levels of Portland cement substitution. To reduce CO₂ emissions from the cement industry, blended cements incorporating low-grade calcined clay, limestone, and lava (a natural pozzolan) are investigated. Calcined clay is combined with limestone to produce a limestone-calcined clay cement (LC3). The reactivity of these new blends is assessed using isothermal calorimetry and compared to a reference blend with ground-granulated blast-furnace slag (GGBFS). Results show that mixtures with calcined clay develop slightly lower 28-day strength than those with GGBFS, while blends with lava exhibit strength gains only at later ages due to delayed pozzolanic activity. Overall, concrete made with low-grade calcined clay and lava achieves comparable compressive strength to the reference (CEM III/A), but with higher capillary porosity, leading to increased water absorption, drying shrinkage, and reduced freeze-thaw resistance. Despite these durability limitations, the sustainability assessment reveals that the LC3 mix with low-grade clay and lava has a lower global warming potential per unit strength at 28 days than CEM III/A and is competitive with CEM III/B.