Hydraulic Lime
In the context of lime or cement, the term 'hydraulic' means to 'harden under water'. Hydraulic lime can be considered, in terms both of properties and manufacture, as part-way between non-hydraulic lime and OPC. The limestone used contains sufficient quantities of clay and/or silica. The resultant product will contain dicalcium silicate but unlike OPC not tricalcium silicate.
It is slaked enough to convert the calcium oxide to calcium hydroxide but not with sufficient water to react with the dicalcium silicate. It is this dicalcium silicate which in combination with water provides the setting properties of hydraulic lime.
Aluminium and magnesium also produce a hydraulic set, and some pozzolans contain these elements.
There are three strength grades for natural hydraulic lime, laid down in the European Norm EN459; NHL2, NHL3.5 and NHL5. These are similar to the old classification of feebly hydraulic, moderately hydraulic and eminently hydraulic, and although different, some people continue to refer to them interchangeably.
Read more about this topic: Lime Mortar
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