The Building Limes Forum (BLF) is at the heart of a debate about natural hydraulic lime (NHL) and hot-mixed mortars. It has hosted a number of events in the UK to encourage discussion of this hottest of topics. In September it took the debate to Norway, where Nidaros Cathedral, Trondheim, had invited it to hold a ‘Rediscovering Traditional Mortars’ conference. It felt like a milestone along the way to a better understanding of historic lime mortars, writes Lewis Proudfoot, Stone Section Manager at Cliveden Conservation in Taplow, Berkshire.

The Nidaros conference was a culmination of some excellent research over the past few years; research that is both scientific in its depth and historic in its reach, with anecdotal evidence and case studies from projects around Europe.

It has caused us at Cliveden Conservation to review our own work and reconsider the mortars we specify and recommend on a daily basis. We have improved our in-house mortar analysis, adding an informed element of interpretation to complement the chemical analysis we already carry out.

Ongoing research by Historic England and Historic Environment Scotland is showing that while the classification system of NHLs based on compressive strength after 28 days is fairly accurate to begin with, hydraulic limes continue to cure and harden for many months.

Importantly, NHL2 is stronger than the ‘feebly hydraulic’ limes identified in many historic mortars and there is an overlap between each classification. At 28 days NHL2 could be anywhere between 2N/mm² and 7N/mm². NHL3.5 can be between 3.5 and 10N/mm² and NHL5 anywhere between 5 and 15N/mm² (www.stastier.co.uk/nhl/info/hydraul.htm).

This blurring of boundaries arising from continued hardening over time means the apparently simple choice between NHL 2, 3.5 and 5 might not be so straightforward after all. An NHL2 could start off as hard as an NHL5 and continue to harden for months.

This research has informed many a discussion and the ‘rediscovery’ of the conference title is apt as people are using new findings to frame existing knowledge.

For example, we know the Romans used pozzolans to make cement-hard hydraulic mortars for marine environments, yet we seemed to have forgotten this, often using hard ‘water limes’ when non-hydraulic ‘air limes’ are more appropriate, a differentiation the Historic England research brings back into focus.

Nigel Copsey’s comprehensive look at historic texts and contemporary case studies from across Europe and North America provides the compelling narrative of the hot-lime revival in 21st century England.

Traditional mortars were mixed by masons who had a clear understanding of the requirements of mortar. Time and again texts and images describe the hot mix method, whereby lime and sands are mixed and slaked together, often at the site of the buildings in large pits. Some of these pits are still visible today through archeological exploration.

The reasons for this are obvious, yet we have moved away from this simple craft of mortar making towards a prescriptive, off-the-shelf, ready mix method. In doing so we are putting our faith in an unreliable system, forgetting the knowledge and losing the skills that once put the craftsperson at the heart of the heritage we all work so hard to maintain.

In a presentation to the BLF in Coventry in 2016 (Traditional Lime Mortars and Masonry Preservation, BLF Vol 24, 2017), David Wiggins lays out the characteristics of effective lime mortars in protecting masonry buildings from water ingress and highlights the differences between modern NHL and traditional hot-mixes.

The key is the amount of carbonated free lime (calcite) in traditional mortar mixes – often in ratios of 1:1.5 (lime to aggregate).

This provides the active porosity required to draw water away from masonry via poultice mechanics while providing a well bonded structure. Having a harder mortar than the surrounding masonry leads to the kind of decay we are now familiar with from hard cement pointing.

A basic understanding of lime mortar will tell us that a mix of 1:1.5 using NHL will be hard and brittle and the same with putty will be too wet. The quality of mortar that the historic texts point to is not one of strength or hardness, but one that is workable and tenacious.

Only a hot mixed mortar can achieve this high level of carbonated free lime and maintain the characteristics of a good mortar. This is because the quicklime used in hot mixes doubles in lime-yield during slaking (from 1:3 to 1:1.5).

Wiggins, Copsey and the ongoing Historic England research are given practical credibility by the work of our hosts at this conference, the masons at the Nidaros Cathedral.

Since 2000, the cathedral has been trying to replicate the medieval mortars used to build the soapstone facades of Nidaros. Of all the various mortars used over the years of restoration at the Cathedral (and elsewhere throughout the world) it is the medieval mortars that have been most successful. (Chris Pennock, Restoration Mortars at Nidaros Cathedral, BLF Vol 24 2017).

The catalogue of repair mortars at Nidaros loosely follows the trends of mortars over the years. There are cement mortars from the 19th through to the 20^th century, putties gauged with Portland cement in the 2000s, and NHLs most recently.

The failures of each of these mortars are visible and the durability of the medieval mortars clear. It was Nidaros’s aim to produce an authentic, compatible mortar – lime-rich, feebly hydraulic, with added pozzolans to provide additional hydraulicity and durability. Almost two decades of tests have shown that hot mixing using local limestone burnt on site is the best way of replicating the medieval mortars.

The Nidaros experience encapsulates all that the conference set out to promote. It demonstrates that by using modern research methods, scientific knowledge and an understanding of the traditional methods, we can replicate historic mortars far more authentically than by using any modern materials.

NHL mortars are not equivalent to traditional mortars, nor do they behave in the same way. Natural Hydraulic Lime in its modern form simply did not exist in medieval times, so why should we use it now?