Hydraulic lime sets and hardens through a chemical reaction with water — unlike non-hydraulic lime, which requires atmospheric carbon dioxide — making it particularly valued in historic masonry restoration, damp environments, and specialty mortars where flexibility and breathability are required. Its classification is distinct from both Portland cement and ordinary slaked lime, reflecting its intermediate binding chemistry derived from naturally occurring silica and alumina impurities in the limestone feedstock. Demand in the US is largely driven by the historic preservation and specialty construction sectors, where European producers have traditionally supplied high-specification natural hydraulic lime (NHL) grades.

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