Hydraulic Lime Mortars (N.H.L.)

The use of natural hydraulic lime mortars is now widespread for conservation work in the U.K. N.H.L. mortars with hydraulic properties (literally, the ability to set under water) were used in the U.K. in the past, but are modern N.H.L. mortars suitable for replicating historic lime mortars or maintaining the historic fabric of a building? While it is true that hydraulic lime mortars can produce mortars with similar physical and mechanical properties to those used historically. Several factors must be considered when choosing a hydraulic mortar for conservation work. The first is the mortar’s compatibility with the building’s existing fabric. N.H.L. mortars have more compressive strength than traditional lime mortars. Higher compressive strength can lead to increased deterioration rates of some types of stone.

Currently, there is no standard for pre-mixed hydraulic lime mortars. Pre-mixed N.H.L. mortars can often contain additives that can affect the functionality and appearance of the finished mortar. Choosing a hydraulic lime that will match the existing historic mortar as closely as possible is vital. Another factor to consider is the porosity of N.H.L. mortars. Hydraulic motors are less porous than traditional lime mortars. Hydraulic lime mortars can trap moisture within the masonry, leading to dampness and mould growth problems.

Caring for N.H.L . Mortars

Unlike non-hydraulic limes, hydraulic limes harden when they come into contact with water. Having this ability makes them ideal for use in wet conditions. However, they must be protected from the elements, as they can quickly freeze if exposed to cold temperatures. For this reason, hydraulic limes are not recommended for use during the winter months. In addition, using them during drought can also be problematic because they need to be hydrated to cure correctly. As a result, it is essential to carefully consider the climate conditions before using hydraulic limes in your construction project.

Notably, N.H.L. mortars require greater care and attention than traditional lime mortars. In particular, they must be regularly cared for during and after application to prevent them from drying out too quickly. Hydrating them regularly over a minimum of two weeks is vital to create the hydraulic set. Protecting the work from direct sunlight and frost is essential; this is often done with hessian. Before selecting an N.H.L. mortar, it is important to consult with a qualified conservation professional. Consultations and mortar analysis will ensure it is the right choice for the repair.

Preparing N.H.L, Mortars

Most people aren’t aware that Hydraulic lime mortars should be weighed into their mixes. To make a mortar with natural hydraulic lime, you must obtain the manufacturer’s R.B.D (Relative Bulk Density). The R.B.D is required to fill the void particle ratio between the sand. To determine the weight of the hydraulic lime necessary. Firstly you would decide on the amount of mortar to be made in litres.

When you have decided on the amount of mortar you will make, you will divide this amount by the ratio of sand used. The amount is then times by the R.B.D. from your chosen hydraulic lime manufacturer.

When you times the ratio of sand with the R.B.D., this gives you the weight of lime needed. You round up the amount and convert the total into Kilos. The calculation is then used to measure the amount of N.H.L. used in the mix.

It is vital to weigh N.H.Ls because bucket measures are inconsistent, and each manufacturer has a different R.B.D. Unfortunately, suppliers rarely make purchasers aware of how difficult caring for and achieving a compatible hydraulic mortar can be.

Traditional Hydraulic Lime Mortar

Whilst more feebly hydraulic blue lias, lime was used locally to its source. The potential of this lime, in combination with pozzolans, for waterworks was first recognised by John Smeaton. He searched for materials to use when planning the construction of the Eddystone Lighthouse in the mid-1750s. He sourced blue lias lime from Watchet in Somerset. Pozzolans were necessary because of the typically high free lime content of the lias – This was significantly much higher than in currently available N.H.L.s.

Later, blue lias lime became the preferred binder for ‘hot mixed’ concrete footings and floors. Blue lias was also used for some below-ground civil engineering construction. Hydraulic lime was never generally used for above-ground structures, for which fat limes were usually preferred, and most certainly when the crafts retained control over mortar design. Blue lias was considered the ‘strongest’ water lime in the U.K. but delivered mortars of significantly lower strength and density – when used without pozzolan – and greater capillarity than currently available N.H.L.’s. A revival in its appropriate use would be very welcome.

Air Lime Mortars

Lime has been a binding material for construction purposes since prehistoric times. It is a relatively soft and easily worked stone which can be scratched with a knife and is composed mainly of calcium carbonate (CaCO3). When calcined at around 820°C, this changes to calcium oxide (CaO), known as quicklime. If water is added to quicklime, it undergoes an exothermic reaction and becomes hydrated lime [Ca(OH)2]. However, if you add enough water, you can create calcium hydroxide, which is lime putty,

Historically Lime putty was made by slaking quicklime over water, resulting in a mixture of calcium hydroxide and calcium oxide suspended in water to allow the lumps of lime to break down. This process would take several weeks, and the resultant putty would be used as a delicate finishing plaster. It has an infinite shelf life providing it is kept covered and wet. Consequently, it was often made before needed and left to mature before use.

The use of hot mixed mortars in England can be traced back centuries when they were first used to create lime ash floors by the Romans. Over time, they became increasingly popular for various applications, such as stone masonry, traditional brickwork, and lime plastering. One of the main advantages of using hot mixed mortars was the speed at which they stiffened up, allowing masons to build higher and faster. In addition, the heat helped to prevent the mortar from freezing in cold weather. Today, hot mixed mortars are still used in various applications, although modern methods and materials have largely replaced them. Nevertheless, they continue to play an essential role in the history of English construction.

The Differences Between Hydraulic and Air Lime Mortars

The key difference between hydraulic lime mortars (N.H.L.’s) and non-hydraulic limes is that N.H.L. relies on moisture to create the hydraulic set. Because hydraulic mortars need feeding moisture to enable them to harden, this means they require aftercare. The walls must be hydrated at least three times a day. Hydration must continue for at least 14 days after the hydraulic lime is applied to the wall.

Non-hydraulic lime relies on air to carbonate and needs little to no aftercare. N.H.L.’s contain materials such as clay and other additives, which become hydraulic when wet. The hydraulic reaction means they will set even when damp or submerged in water.

In contrast, non-hydraulic limes will only harden as they start to carbonate, a process driven by air movement. As a result, N.H.L. mortars are thought to be generally more robust than non-hydraulic lime mortars. They are therefore considered better suited to use where severe weathering may be an issue.

Although N.H.L. mortars have some advantages over non-hydraulic lime mortars, they should not be considered a panacea for all conservation projects. It is important to consider the specific needs of each task when deciding which type of lime mortar to use. Ultimately, the goal should be to create a mortar closely resembling the original mortar’s appearance, workability and performance.

Research Funded by Historic England

Historic England, working through the Building Limes Forum, funded a PhD at the University of Bath to assess the properties and performance of natural hydraulic lime mortars for conservation.

Mortar testing has been a long-standing source of controversy in the construction industry. A recent study by Christiano Figueiredo, Alison Henry and Stafford Holmes over two years found several problems with how mortars are tested for compliance with BS EN 459. That it does not give accurate data about “real” mortar made from them, producing test results that aren’t relevant to end users, these results ensure that European laboratories can still compare themselves against each other even if their materials don’t match up exactly.
The objective here (borrowed from Portland cement) isn’t geared towards helping you determine what type or quality works best; instead, they are looking at providing meaningful information on consistency levels.

For example, the standard specifies different curing conditions for N.H.L. 2 compared to NHL 3.5 and N.H.L. 5 mortars, which suppress the strength gain of the N.H.L. 2s. Furthermore, all testing is carried out at 28 days. Still, research has shown that unlike Portland cement (which gains 80% of its strength in the first 28 days). Hydraulic lime mortars have continued gaining strength for over two years and can ultimately reach compressive forces at least twice their 28-day strengths, sometimes considerably more.

Professionals and practitioners have widely reported the concern that N.H.L. mortars are too strong for most conservation applications. Reports also indicate a problem with the leaching of lime, migration of salts into adjacent masonry, and retaining moisture in walls where these compounds have been used, which causes further issues such as lack of durability due to increased risk factors like erosion or expansion and contraction from temperature changes.

Research Completed in 2017 on N.H.L Mortars

The research, completed in October 2017, involved the chemical and physical characterisation of 16 N.H.L.s on the market in 2014. Mortar samples were made using a representative selection of 11 of these. The aim of this was to learn more about the properties of ‘real’ mortars rather than the ‘standard’ mortars used to test compliance with BS EN 459

At the end of 28 days, the two mortars with the highest compressive strength were those made with two brands of NHL 3.5. All the NHL 3.5 mortars and even one of the N.H.L. 2 mortars were stronger than the weakest N.H.L. 5 mortar. The situation proved even more. To validate these results, ‘standard’ samples of mortars were made and tested at the university and by an independent commercial laboratory, strictly by the standard methodology required by BS EN 459. The binders were from the same manufacturer, but different batches were purchased from various suppliers.

The N.H.L. 2 samples tested exceeded the 2 MPa minimum compressive strength at 28 days required by the standard, although the values achieved by the university and the commercial laboratory were significantly different. While the values obtained for the NHL 3.5 were closer in both labs, the NHL 3.5 tested at the retail facility failed to meet the minimum 3.5 MPa at 28 days specified by BS EN 459, and the N.H.L. 5 tested in both labs fell far short of the required minimum strength. All three binders tested at the university achieved virtually the same compressive strength at 28 days when tested according to BS EN 459, regardless of the classification of the bag.

Things to Consider When Choosing N.H.L’s

These results throw up serious questions for people specifying and using N.H.L.s. There must be serious thought when choosing these mortars for conservation (or any other purpose). To what extent can you rely on the bag’s classification to understand an N.H.L. mortar’s potential long-term properties? Historic England research suggests not at all. If you buy a bag of N.H.L. 2, believing that this is the weakest N.H.L., there is a good chance that you could end up with mortar stronger than one made with an NHL.5

You can find out more about the research completed by: Christiano Figueiredo, Alison Henry and Stafford Holmes for Historic England below. Hydraulic lime production is coming full circle.

Properties and performance of lime mortars for conservation: the role of binder chemistry and curing regime

Figueiredo, C. (Author). 1 Jul 2018