Limestone-rich binders beyond clinker reduction: durability as the sustainability boundary—a critical review

Lowering the clinker factor with limestone—through Portland-limestone cements, high-limestone fillers, limestone calcined clay cement (LC3), and limestone–SCM combinations—is among the most scalable routes to decarbonizing concrete, but production-stage CO2 savings alone are not a sustainability outcome. This critical narrative review, using a structured, question-driven search, reassesses limestone-rich binders through a durability-bounded framework linking phase assemblage, alkalinity reserve, transport, exposure class and structural service life. Synthesizing evidence on carbonation, chloride binding, the coupled carbonation–chloride corrosion trap, combined and sequential exposures, conditional ternary synergy, and implementation burdens, it shows that the decisive limit is not a fixed limestone content but the point at which dilution, reduced buffering, or field penalties negate the initial gain. These findings are consolidated into a multi-gate decision framework classifying binders as genuinely sustainable, conditionally sustainable, insufficiently evidenced, or superficially low-carbon. Limestone-rich binders deliver durable decarbonization only when clinker reduction, exposure-specific durability, constructability and service-life performance remain aligned.