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Table 1 Compressive strength ranges for natural hydraulic lime mortar samples prepared in accordance with the current British Standard (BSI, 2010)

From: Specialist ‘restoration mortars’ for stone elements: a comparison of the physical properties of two stone repair materials

Classification 28 day compressive strength (MPa) Traditional terminology*
NHL 2 ≥2 to ≤7 Feebly hydraulic
NHL 3.5 ≥3.5 to ≤10 Moderately hydraulic
NHL 5 ≥5 to ≤15 Eminently hydraulic
  1. * Traditional terminology only loosely relates to modern classification. In reality, modern limes are much stronger than those produced historically.