In recent times, there has been a surge of interest surrounding Natural Hydraulic Lime (NHL), also known as hot lime mortar. As conservationists, we now exclusively recommend hot lime mortars due to their exceptional qualities and benefits in historical and listed buildings.
To understand the significance of hot lime mortar, it is important to delve into its historical roots. Lime production has been a time-honoured practice, dating back centuries. The process involves burning limestone, combining it with water, and mixing it with sand to create mortar. This concoction emits a fizzy, hissing sound and generates substantial heat. Gradually, the mixture solidifies into the magnificent, breathable lime mortars that are commonly observed in our heritage damp and timber surveys. This traditional approach required meticulous mixing, often incorporating generous amounts of animal hair such as horse hair, and sometimes allowing it to mature for several months. Adding brick dust, known as a pozzolan, would enhance the mortar’s strength.
More recently, hydraulic lime mortars have emerged as a convenient alternative, readily available for purchase under the NHL label, representing three common variations: 2.5, 3.5, and 5, denoting their set strength in Newtons. These lime mortars are user-friendly, resembling the simplicity of buying a bag of cement. They exhibit a quick initial set and gradually harden over weeks through a process known as carbonation, similar to cement. However, recent research indicates that these mortars tend to harden more than anticipated over time, compromising their flexibility and breathability, which are crucial qualities in construction.
To address this challenge, the solution lies in reverting to traditional hot lime mixes, which not only offer superior performance but are also cost-effective. It is important to handle the newly wetted powder with caution to avoid accidental contact, as it generates considerable heat. The recipe for hot lime mortar typically involves combining 2.5 parts of sharp, gritty sand with 1 part of lime, such as Calbux 90 or its equivalent.
Hot lime mortar presents numerous advantages over its hydraulic lime counterparts. Firstly, it ensures enhanced flexibility, allowing for natural movement and accommodating the inevitable shifts that occur within structures. This flexibility minimises the risk of cracks and fractures, safeguarding the integrity of the construction.
Furthermore, hot lime mortars offer superior breathability. Their porous nature enables effective moisture management by allowing water vapour to escape, preventing the accumulation of dampness and the subsequent deterioration of building materials. This breathability is especially valuable in historical or heritage structures, where the preservation of original materials and architectural integrity is paramount.
Furthermore, hot lime mortars exhibit exceptional compatibility with a wide range of traditional building materials, including brick, stone, and timber. Their compatibility facilitates seamless repairs and restoration work, ensuring harmonious integration with existing structures.
Lastly, the cost-effectiveness of hot lime mortar makes it an appealing choice. By reverting to traditional manufacturing methods, the expenses associated with production are significantly reduced, without compromising the quality or performance of the mortar.
In conclusion, hot lime mortar, or Natural Hydraulic Lime (NHL), provides a compelling alternative to hydraulic lime mortars. Its historical significance, along with its exceptional flexibility, breathability, compatibility, and cost-effectiveness, makes it a preferred choice for astute professionals in the construction industry. By embracing the traditional approach of hot lime mortar, we can ensure the longevity and resilience of our buildings while preserving their architectural heritage.
