A comprehensive review of the manufacturing process and properties of natural hydraulic limes

Within the sub-group of hydraulic limes, natural hydraulic limes (NHL) are obtained by calcining limestone containing silica and alumina at temperatures of less than 1250 °C. The content of hydraulic phases generated at these temperatures depends on the raw material, the firing and cooling conditions inside the kiln, and the slaking method, giving rise to natural hydraulic limes with different hydraulic features. Despite these differences, the European standards classify NHL limes above all based on their free lime content and the compressive strength of the mortar after 28 days of curing. This means that there are natural hydraulic limes on the market today which despite having the same index number (NHL2, NHL3.5 or NHL5) have very different compositional and textural characteristics. The variations in the limes can in turn lead to mortars with different properties in both the fresh and hardened states. Although the standard establishes ranges for the compressive strength values for the different index numbers, these ranges overlap, so giving rise to ambiguities in the classification of these limes. This review paper aims to extend our knowledge as to how different aspects of the manufacturing process of NHLs influence their final properties, with the ultimate objective of achieving a more rigorous classification of these limes. If hydraulic limes with more precisely controlled compositional and textural characteristics were available on the market, this would have a positive impact, especially on architectural heritage conservation, as more suitable, more clearly defined and more compatible hydraulic mortars would be available to restorers.