Lime, Hydraulic or Non-Hydraulic?
Hydraulic lime sets by hydrolysis whereas non-hydraulic lime sets by carbonation. As hydrolysis requires moisture, hydraulic lime tends to be more forgiving in wet weather which will impede the setting and curing of non-hydraulic lime.
In physical terms, the two materials are very different to work with. Hydraulic lime is available as a bagged powder whereas the non-hydraulic lime is a putty, hence the name fat lime putty. This makes working with hydraulic lime much easier for most builders as it's practically the same as working with cement.
Non-hydraulic lime is softer and sets much more slowly. The carbonation process is very slow and the material remains soft and flexible. This of course, can be extremely advantageous if that's what is required. The hydraulic lime is available in degrees of strength, is faster setting and more durable.
With these general differences in mind, let's consider the practical considerations.
If it's a new build then there's an incredibly strong case for hydraulic lime. If you're working to existing with maintenance or repair work, it's always best to replace like with like. Technically there's no reason that hydraulic and non-hydraulic can't co-exist but it's better conservation practice to keep it original.
What skills have you got on-site? Bricklayers or masons experienced in conservation work may be highly adept with either material. Less experienced workers are generally more comfortable working with hydraulic lime.
Non-hydraulic lime is considerably cheaper than hydraulic lime and could give you a good saving on large jobs. At the same time, hydraulic lime could save you a fortune on labour and lessons learned throughout the course of the project.
If you're on a tight schedule then hydraulic lime may be more appropriately suited. Hydraulic limes are faster setting, it is also generally accepted that non-hydraulic limes can require up to 40% more labour.
Limes & Cement in Comparison