Types of lime for use in mortar, plasters and renders.
When I began learning about the uses of lime in heritage buildings I was absolutely baffled by all of the different types, until I realised that there were actually only two that are typically used and each has numerous names, hence the confusion!

The Two Main Types of Lime Used in Traditional Building
While lime comes in various forms, almost all heritage mortars, renders, and plasters rely on just two primary types. Each has distinct properties and ideal uses.

1. Lime Putty (Non-Hydraulic Lime)

• Made by slaking quicklime (calcium oxide) with excess water.
• Forms a soft, buttery material stored wet and matured over time.
• Sets slowly by carbonation – absorbing carbon dioxide from the air.
• Requires slow drying and is best suited to sheltered or internal work.
• Ideal for fine plastering, internal mortars, conservation work, and where flexibility and breathability are critical.
• Highly breathable and flexible – perfect for delicate historic masonry.

Note: Lime putty mortars are not suitable for exposed external work in harsh climates without modification (e.g., pozzolans or blending with NHL).

2. NHL – Natural Hydraulic Lime

• Made by burning limestone that naturally contains clay and other minerals.
• Sets by hydration (reaction with water) and carbonation.
• Supplied as a dry powder in grades:
• NHL 2 (Feebly hydraulic – softest)
• NHL 3.5 (Moderately hydraulic – general use)
• NHL 5 (Eminently hydraulic – hardest and fastest setting)
• Sets faster and harder than lime putty.
• Better suited to exterior, damp, or exposed conditions, such as chimneys, boundary walls, or foundations.
• Still ‘breathable’ and ‘flexible’, but less so than lime putty.

Does Lime Flex? Not Quite!
It’s a common misconception that lime “flexes” in the way modern materials might. In reality, lime doesn’t bend—it fractures at a microscopic level. But unlike cement, lime has a remarkable ability to self-heal. When minor movement occurs in a lime mortar or plaster, tiny cracks may form. These are often invisible to the eye, but over time, moisture carries dissolved lime into the cracks, where it re-carbonates—essentially refilling and sealing itself. This process is known as autogenous healing.

In contrast:
Cement is rigid and brittle.
Under stress, it resists movement until it reaches a failure point—then it cracks abruptly and severely.
These cracks do not heal and often widen over time, allowing moisture to penetrate and cause further damage.

Why this matters in heritage buildings:
Traditional buildings move subtly over time—due to moisture, settlement, thermal changes, or structural shifts.
Lime accommodates this movement through micro-fracture and self-repair.
Cement cannot do this, which is why its use in old buildings often leads to long-term damage.

Where to use each type of lime?
This is a topic that often ends in heated debate. How old is the building? Which type of masonry is it being applied to? How hard is that masonry? What is the exposure level of that location? What is the topography of the land where this mortar is being used? Is vapour permeability more important than water repellency? Working through these questions you quickly become aware that the choice of mortar may vary from location to location, not only with a country or county, but also within the grounds of a single dwelling.

The following guidance is solely for granite and slate built dwellings located in Cornwall, where almost every location is exposed to extreme winds and heavy rain.

Our preference for pointing mortar, external renders (not cob) and internal lime plasters in a typical, inland, Victorian Cornish cottage with moderate exposure is NHL 3.5.

The purists always push for a pure lime putty mortar, however, the performance of these mortars is evidenced to be extremely poor when faced with Cornwall’s sideways rain in 70 mile per hour winds that are driven off of the Atlantic Ocean.

As a starting point for consideration, hydraulic limes offer the perfect balance of water repellency and breathability, some will disagree and we fully respect and understand that their knowledge will be based from their own experience, however, monitoring buildings over a period of five to ten years often evidences the suitability of hydraulic mortars by the lack of dampness within a building and a reduction in failures within the mortars that do occur with non-hydraulic limes in Cornish buildings.