BCD 2017

T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 7 37 PROFESSIONAL SERVICES 1 HOT-MIXED MORTARS Advantages and limitations ADAM BROWN ‘H OT MIX’, ‘hot limes’ and ‘hot-mixed mortars’ are all terms used frequently in the conservation industry. But what exactly are they? A hot-mixed lime mortar is one based on three components: quicklime, water and an aggregate such as sand. In a conventional mix, aggregate is mixed with lime which has previously been slaked. In this case, quicklime is mixed with the aggregate and then ‘slaked’ with water. The process generates heat, hence the terms. It may be used immediately as a hot mix or later when cool. QUICKLIME Quicklime is limestone (a rock rich in calcium carbonate) that has undergone a chemical change in a kiln, liberating it of all the carbon and water it holds (CaCO3 500–600 ° C CaO + CO2), creating a very unstable material (calcium oxide) which needs to hydrate. Quicklime will do so very energetically with any moisture it comes into contact with, resulting in a strong exothermic reaction and the production of lime (CaO + H2O – Ca(OH)2), calcium hydroxide. The addition of water , slaking, has the potential to produce temperatures exceeding 250 o C very quickly. However, when creating a mortar the quicklime can reach these high temperatures as hot spots only, and the overall temperature of the mortar should not exceed 100 o C – the temperature at which the water turns to steam. SANDS The general rule with mortars made with lime putty or naturally hydraulic lime (NHL) is that clean, washed sands and aggregates should be used. Dirtier sands tend to contain impurities such as clay minerals which absorb water and swell, so more water has to be added. This results in an increased risk of shrinkage in the mortar as it dries. With a hot mix, however, testing has shown that dirtier silica sands often work best as clean silica sands can make the mortar brittle. As the quicklime slakes it expands, so the original volume of lime increases significantly. A sand with a higher fines content (including clay minerals) can help combat the increase in binder due to the higher surface area of smaller particles. However, it is important that the fines content is not too high as excessive fines will still make the mortar prone to shrinkage. HISTORIC MORTARS Before modern cements took over in the early 20th century, lime was the principal binder used for making mortars and renders. It was produced by burning limestone in a kiln at temperatures of 800°C. For the core of the limestone to reach the required temperatures in a reasonable time, the temperature in the kiln had to reach 950°C. In early kilns the temperature varied throughout, leaving lumps of lime unconverted and some sections over-burned . The finished product could then be supplied to site as quicklime, or it could be slaked immediately and left to mature as lime putty, saturated with water. Today, it is generally accepted that lime putty was reserved for fine plasterwork, and that, for ordinary construction, mortars would have been hot-mixed on site with quicklime, water and sand/aggregate. Whether the material would have been used hot or stored and used cool is open for debate and various historic texts refer to both applications. Although a ratio of one part lime putty to three parts aggregate is widely specified for modern mixes, there is evidence that different mixes would have been used historically. This would have been down to several factors such as type of work, location, availability and cost. When using a hot-mixed mortar today, a mix ratio of 1:3 quicklime and aggregate will, after expansion, produce a mortar that is 1:1.4 or even 1:1. Many historic mortar samples that have been analysed show a similar ratio with a high binder content. Making such a rich mix with a lime putty would have been impractical as the mortar would have been sloppy. As our acceptance of their historic authenticity has gathered pace, more and more claims are being made about the benefits of using hot-mixed mortars and about the detrimental impacts of NHLs. While these claims may have been made with the best intentions, they are largely unsubstantiated by any scientific and real-life data. BENEFITS OF HOT-MIXED MORTARS A hot-mixed mortar offers a very workable material. Due to its high lime content, it is very sticky and this can help stop the mortar being washed out. It offers good water retention that can be controlled during the mixing process. And it can be a cost-effective option; the purchase price of quicklime is usually less than putty or NHL and cheaper, dirtier sands can be used. From testing it has also been proven that hot-mixed mortars offer extremely high vapour permeability. Under lab conditions a hot-mixed mortar at 1:3 can offer vapour permeability up to 1.36kg/m2sPa, whereas a comparable NHL 3.5 mortar at 1:3 will be around 0.69kg/ m2sPa. This has yet to be proven in the field or over long enough periods as carbonation is a very slow process. Nevertheless, it seems likely that a hot-mixed mortar could be twice as breathable as an NHL3.5 mortar, offering significant advantages to historic fabrics in many situations. WHICH TYPE OF QUICKLIME WILL WORK BEST? Quicklime is available in two forms: ‘kibbled’, which is granular, containing particles from 1mm up, and finely powdered. Both of these are chemically identical, but kibbled is more dense than an equal volume of powder, and as mortars are usually mixed by Hot mixing on site (Photo: Cornish Lime)