Hydraulic Lime or Natural Hydraulic Lime (NHL)
Natural hydraulic lime (NHL) is also made from limestone. NHL is produced from limestone containing clay or other silica impurities. The limestone is burnt and then slaked. A harder set is obtained because calcium silicates and aluminates form in the presence of water as well as calcium carbonate from carbonation. Therefore, it can set under water hence the term hydraulic. NHL is graded by strength into three types 2, 3.5 and 5.
Since there are two types of natural lime that are suitable for repair of old buildings it has to be decided whether to use non-hydraulic lime or NHL. The more hydraulic a lime is the greater its strength but is less permeable and flexible. The right balance for the job needs to be struck. For example, slow setting non-hydraulic lime is good for internal plasterwork and for use on soft backgrounds such as cob, soft stone or friable material.
At the other end of the scale, rendering an exposed stone wall or chimney then a stronger NHL would be beneficial 3.5 or if very exposed for instance a granite church tower then the strongest, NHL 5 would be used.