"I speak for the building, as the building has no voice"

Hot Mixed Lime Mortar

There is a growing interest in Hot Mixed Lime both in conservation and in new build. A Hot Mix is when quicklime is used to make mortar such that the lime slakes, and as part of that reaction, heats, during the making of the mortar. The resulting mortar can then be used either; hot, straight away, or later once it has cooled.

In Ireland today quicklime is only produced at Clogrennane Lime, near Carlow, but in the past it was produced all over the country in small neighbourhood kilns. The limestone used is very pure, so the resulting quicklime produces a non hydraulic lime that sets slowly in the presence of air and water. There is no initial set such as is found with imported Natural Hydraulic Limes such as St Astier or Otterbein.

It has become clear that in the past building lime was often transported as quicklime and mixed to a mortar without a separate dry slaking. The mixing was carried out either on the building site or sometimes at a builders merchant where it could be stored for later use. Inspection of old mortars often reveal large white lumps of lime with ragged edges typical of hot mix used immediately, caused by bits of the lime slaking some time after the mortar is placed and thus not blending into the mix.

Building lime is made by heating limestone to a temperature of around 800 degrees centigrade which causes it to give off carbon dioxide and turn into Quicklime. Limestone = Calcium Carbonate (CaCO3 or Calcium + Carbon + Oxygen x 3), Quicklime = Calcium Oxide (CaO or Calcium + Oxygen) with Carbon Dioxide given off to the atmosphere (CO2 or Carbon + Oxygen x 2). 

Quicklime is highly reactive with water, with the addition of which it changes from Calcium oxide to Calcium Hydroxide Ca(OH)2 or Calcium + Oxygen+ Hydrogen + another Oxygen + Hydrogen and emits a large amount of heat whilst approximately doubling in size. If just enough water is added to complete the slaking reaction the lime forms a dry hydrate, best known in Ireland as White Rhino. This can the used to make a non hydraulic mortar when fresh. However, calcium hydroxide gradually carbonates (reacts) in the presence of atmospheric water and carbon dioxide to form calcium carbonate…or in other words limestone, so if left un sealed it will gradually become a non reactive powdered limestone dust. 

Historic recipes for mortar often quote a mix of 3:1 sand to lime, this has been followed by users of Natural Hydraulic Lime and seems to make a workable mix. However when historic mixes are analysed they are often found to be much richer, nearer 2:1 sand : lime. This has been largely ignored in the creation of conservation replacement mortars.

When a mix is made using quicklime at 3:1, after the expansion of the lime the resultant mix is closer to 1.5:1 sand : lime, much closer to the historic mortars found in many buildings. The expansion of the quicklime into hydrated lime during slaking explains the anomaly between the 3:1 quoted in historic recipes and the 1.5:1 or 2:1 found in historic mortars. 

The resultant mix is completely different to work with as the high lime content makes it very sticky and gives a ‘doughy’ consistency which makes it ideal as a bedding mortar or for pointing. If used as a render however, there is a problem with late slaking lumps which can expand in the render causing popping. For rendering it is suggested the hot mix is left to ‘sour’ (left wrapped to keep it moist and away from air) for as long a period as possible to allow any late slaking to occur, this might be from hours to months, or even years. After storage the mortar must be knocked up, or remixed to enliven it again. This tends to combine any late slaked lumps into the mix making them less distinct in the hardened mortar.

When a hot mix is used without being soured the resultant mortar appears similar to historic mortars with white lumps of lime visible and the lime richness typical of old mortar. The choice of sand and coarser aggregates is still important and the usual discipline of mortar analysis of any historic mortar samples should be followed to clarify the grading .

Mortars made with quicklime will be essentially non hydraulic, meaning they will set slowly over time in the presence of carbon dioxide and moisture. The relative humidity of the mortar needs to be in the order of 30-80% for carbonation or setting to occur. This is most easily achieved by constant wetting and drying, but it is also important that the mortar surface should not dry excessively therefore it should be protected from wind and sun for the initial 1-2 weeks until the surface has carbonated somewhat. The mortar deeper in the wall will slowly stiffen over an extended period through a combination of drying and carbonation, in a thick wall this can be many years.

Quicklime can be sourced from agricultural suppliers such as Glanbia as ‘Growmax’ from Clogrennane Lime. It is used by farmers for increasing the ph of acidic soils. It should be noted that this lime does contain some lumps of under or over burnt lime together with the odd piece of other stones. These lumps can be inconvenient in fine work.

NOTE: If you are thinking of using quicklime in a mortar for the first time, it is strongly suggested that you either speak to me or first attend a Hot Mix Lime course such as those run from time to time by the Building Limes Forum Ireland (BLFI) http://www.buildinglimesforumireland.com , as quicklime is a hazardous material and can react on the surface of the body, both on skin, the membranes of the eyes and inside the respiratory duct causing severe burns. 

Safety sheets for quicklime can be down loaded here http://clogrennane.ie/wp-content/uploads/2016/07/q...

Further (mainly agricultural) information on GrowMax http://clogrennane.ie/wp-content/uploads/2015/09/g...

When working with lime it is advisable for all operatives to carry a personal eyewash, the most suitable one appears to be Diphoterine SIEW, which comes in a holster for attaching to your belt. It is manufactured by Prevor, France. http://www.prevor.com  and can be obtained from Innovection, tel: 01642 4211, mob: 087 600 2228.

This BLFI newsletter gives more information http://www.inhp.com/wp-content/uploads/2015/06/new...