20 Peter Hood explains how to persuade building owners why failure of lime render and lime mortar during very cold weather is not a disaster COLDCOMFORTFORLIME INWINTER A great deal of ordinary lime mortar, and lime render, will seem to have failed this winter. I have in mind the sort of straightforward one-part-lime-to-three-partssharp-sand mortars fortunately much more in use again in older building repair and conservation. The worst problems will have occurred in mortars and renders applied after September last year. Owners of older houses will be asking why Portland cement wasn't added and what can now be done. It is worth considering causes, effects, remedies and better practice in future work. Ordinary lime mortars are unquestionably excellent servants of old historic buildings, but we have to understand their limitations and how to get the best performance out of them. The lime used today in ordinary mortars is usually calcium oxide (CaO), made by heating limestone (CaCO 3 ) just to the temperature at which the constituent carbon dioxide (CO2 ) is released as gas, and for long enough to release all the water (H?O) as vapour. The hydrating process, known as slaking or slacking, involves the immersion of the produced quicklime in clean water. The dehydrated lower burnt and more porous parts of the quick lime reabsorb the lost water very rapidly with a great deal of energy. At 45 °Cmost of the remaining lime will also hydrate. The temperature can continue to rise to more than 400°C and if insufficient water is present the hydrated lime will return to a harder-to-slake form of quick lime. Today's quick lime production operates at much higher temperatures and the process is much quicker, causing some overbum and leaving some underburn. Fortunately the more demanding production of lime for fluxing steel calls for a pure lime made in a sophisticated natural-gas-fired kiln which will answer the repair needs of older buildings, if handled appropriately. Unless you have a lot of experience of using freshly slaked lime, these highly reactive limes are best 'run' to a putty in large tanks and left for as long as possible. It is this dense white fat cheese-like lime putty that you add to clean dry sharp sand to make ordinary lime mortar. It must be realised that the largest part of mortar is the sand. This is not a matter of economics, though the sand is always far cheaper than the lime; it is a matter of practicality. TI1econtent ofthe sand can contribute chemical agents which may assist the lime to harden and bond the sand together, foffi1inga stonelike substance. The strength of the sand particles or the lime is hardly relevant in most conservation work; compressive strength in walls lower than 30 m is hardly an issue. We should be repairing and consolidating rather than rebuilding and restoring, after all. On its own sand will not 'hold up'. Clay will hold it together, but clay is easily washed out and easily affected by freezing as it holds a lot of water. Lime, on the other hand, is insoluble once carbonated and a really good lime can 'carry' a lot of sand if the sand is well graded. The range of particle size in the sharp sand wants to be from 0·5 mm to 3 mm and larger* in a continuous 'curve', varying for different purposes. It will be seen that the pores in that sand will be evenly distributed and equally graduated in ratio. The fines below 0·5 mm should have been washed out so that the lime may occupy that place and coat each particle. If the lime is added in a quantity equal to the water that can be held by the sand, the lime can be far less than a quarter part - as in a one part lime to three parts mortar - and therefore the retained water will be far less. Lime mortars need to be stiff, and only slightly more f1uid in the heat of SUll1ffier. If the lime has been 'run' to make a putty by long-term settlement, the amount of water retained will be far less than that in a young lime putty because the crystals of portlandite which form the putty grow outwards making a continuously denser mass, and the compression of weight of lime putty above further reduces the retained water content. Without carbonation, lime putty will hold water indefinitely. This needs to he remembered in massive reconstruction where only the surface mortar may evaporate enough water to absorb carbonic acid in rain. The practice of * The sand larger than 3 mm should be disrega,·ded in the calculation of ratio of lime to sand. making up heaps of mortar and leaving them to weather, turning them over by hand at intervals, has a lot to recommend it. The practice of using sloppy lime putty, wet sand and over-wetting joints and stones or bricks after September is foolish and problems must be expected after a hard winter following the application. There is always water in good lime mortars because they are porous once carbonated. The amount of water depends on the grading of the sand, the amount of water applied in that mixing, and the distribution of the sand and the lime if not thoroughly well integrated. Recently applied mortars will be wetter than older mortars, unless salts are present. On freezing allwater expands as it becomes ice. That expansion may break the bond between the lime and the sand, though inagoodmixthatis not aproblem. in Portland cement mortars it may break the chemical bond between the stone or brick and the mortar, irreversibly. The latter is a cause for concern because it is not renewable. It may also have taken some of the stone or brick with it. Where Portland cement mortars have been used extensively in repointing, the stone or brick itself may have spalled. In lime mortars the face may have been lost but for the large part the mortar will be all right, though weathered back. It isgenerally understood that Portland cement is a developed dense cement product of the mid-19th century and intended for engineering works. Used with a well graded sharp sand it makes a very lean unplastic mortar, which is why soft sand with lots of clay fines came to be substituted for the traditional sharp sands. In order to control the setting time of Portland cement, gypsum was incorporated. This made the mortar lean again, and so lime came to be added. This had the added function of retaining water and provide further plasticising by the wetting of the clay fines. Thus it was that the well known 1:1:6 and 1:2:9 mixes came about, with the lime supplied as a dry powder for adding with the cement at the point of mixing. These are not lime mortars, they are lime-plasticised Portland cement mortars. It was found that these new mixes could work well in the historic CONTEXT49
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