Lime, Hydraulic or Non-Hydraulic?

Hydraulic lime sets by hydrolysis whereas non-hydraulic lime sets by carbonation. The hydraulic lime can set underwater, that's because hydrolysis is a reaction caused by water. The non-hydraulic lime needs air to carbonate and thus set.

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.

Practical Considerations

With these general differences in mind, let's consider the practical considerations.

Project

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.

Experience

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.

Budget

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.

Timescale

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

Hydraulic Non-Hydraulic Cement (OPC)
Breathable Yes Yes No
Setting Speed Medium Low High
Flexibility Medium High No
Durability Medium Low High
Labour Low High Low
Cost High Medium Low
Shelf Life Medium High Medium