The Importance of Sand and Lime
The really good thing about sand and lime is just how breathable and flexible it is and when your property moves throughout the year it doesn’t crack or break bricks as it does so. Remember old houses have stepped foundations which are shallow so movement is different depending on the ground they are built on.
Because period bricks and limestone are so porous the original sand and lime was weaker than the masonry around it.
This would allow mortar joints to act as route for moisture to escape after the wall had become wet and would easily evaporate in dryer weather. If moisture cannot escape due to strong cement mortar blocking its route it will force its self out through the face of your bricks or stonework. This will cause them to erode or shatter during wet and freezing weather.
Today a lot of properties have had their original pointing replaced with a modern cement mix. Properties made of soft bricks or limestone is suffering because cement is so hard and waterproof it isn’t flexible or breathable and is causing unseen long term problems.
The type of sand should be a sharp coarse and not soft building sand as this does not mix and bind well with lime it’s to compacted. There are different types of sharp sand which can be used to produce different colours mixes.
Sharpe sand is made up of tiny shards with irregular edges that do not interlock and this leaves voids between the sand particles. This will give sand and lime its flexible breathable qualities. The more coarse sharp sand used in the mix the more flexibility for movement it has and wont crack the pointing later.
Too much fine sand in a house with movement will just crack the new mortar beds later on. That’s why bold builders liked quite a coarse mix with large particles in it to stop this happening.
Soil Types and Seasonal Movement
The type of ground your house is built on will determine its potential for movement.
Rock and chalk are the best type of sub soil to build a house on. Sand and gravel also have quite good load bearing properties when it has been compacted. Although in areas were properties are built on heathland on steep hills movement can be common. Flexible clay is the most common type of sub soil houses are built on and it’s very prone to seasonal changes. Clay dries out in the summer and shrinks and in the winter rain and frost will cause the ground to swell up again.
Sand and Lime Mixes
In recent building-site history, mixing mortar has become a job for the general labourer, despite often being unqualified and poorly skilled. And yet the mortar is, and has always been, utterly central to masonry construction. Inappropriate mixes mar the appearance of the best-built walls and often compromise the integrity and durability of a structure.
Lime mortars were the norm for centuries, and the secret of the perfect mix for any given situation was passed from father to son and from craftsman to apprentice over generations; the techniques also varied considerably across the country to suit the nature and performance of predominantly locally-sourced materials. There were few textbooks and no formal training. It was a matter of tradition and instinct supplemented by generations of experiment and sound experience.
This chain of knowledge was severely interrupted by the First World War and the near-universal adoption thereafter of stronger, faster-setting and consistent (but not always appropriate) cement-based mortars. To a large extent, today's craftsmen have had to rebuild that knowledge base from scratch. But what if we have placed too much trust, and not enough understanding, in surviving texts, rather than analysing the sound evidence of centuries-old mortars?
Analysis of historic mortars reveals that the types of limes and sands and their mix ratios varied considerably. Richard Neve's book The City and Country Purchaser and Builder's Dictionary, which was published in 1762 (and in facsimile by David & Charles, 1969), illustrates this (see pp 198-199) with examples of varying mortar ratios used in and around London, often in different parts of the same building for the footings, inner and outer flank walls, and with the best reserved for the outer leaf of the facade. To a large degree, the type of lime and sand and the need to obtain a workable mix determined these ratios.
With the lime revival of the past 25 years (which for many years was primarily based on the use of pure, non-hydraulic lime prepared as a putty mixed with a well-graded aggregate) it is interesting to note that there has been an emphasis on the common use of a 1:3 lime: sand ratio based essentially on a measurement of the 'voids by volume' within a measure of dry sand. It is generally accepted that this measurement provides a good indication of the volume of lime binder required to ensure a coating of lime around every grain of sand, and technically it is quite correct.
The method used to measure the voids involves half-filling a graduated laboratory flask with an oven-dried sample of the specified sand, and then carefully pouring clean (potable) water into it from another identical graduated flask until all the voids are filled and the surface of the water rises level with the surface of the sand. The volume of water required to fill all the voids in this volume of sand can then be calculated by subtracting the volume of water left in the water flask from the volume it contained at the start, this being determined as the minimum volume of lime binder required for producing a good mortar. Typically this is found to be one-third of the original volume of the water and hence the ratio is determined as 1:3. But it is not correct to believe that this provides all the answers, and nor does it reflect the reasoning by which the 1:3 ratio was historically specified.
One Part Slaked Lime or One Part Quicklime?
It is vital to understand that, until the Second World War, a majority of limes were still prepared from freshly burnt quicklime delivered to site, as opposed to ready-to-use slaked putties, which would have been extremely heavy to transport, or bagged dry-hydrates. For general mortars the quicklime was then usually slaked to a crude powder (technically, a dry-hydrate) on site. One of the most popular methods to achieve this was to place a one-third measure of quicklime broken down to the size of nutmegs within a cubic yard of ringed sand, and then apply the minimum of water necessary to slake it, before quickly drawing the sand over it as it both heated and broke down in slaking. After slaking was completed the pile would be turned over dry to fully integrate the sand and lime. One option was then to add extra water to bring it to the working consistency of mortar ready for immediate use. Alternatively, the dry mix could then be thrown with the shovel through a large inclined 5mm (¼") meshed screen to remove large inclusions before mixing it with water, thus producing a top-quality 'front mortar' that was generally reserved for facade masonry.
The important thing to note here is that the lime used in the ratio of 1:3 was not prepared slaked lime (calcium hydroxide) but unslaked quicklime (calcium oxide), a fundamentally different substance in several respects, including volume. This vital point has frequently been overlooked and has led to misinterpretation of a great many historical mortar mixes based on original documents recording mortar ratios, or on those recorded within old craft books. A simple but very good example of this is to be found in an architect's private site book, for an entry dated 1927 on preparing lime mortar as follows: 'Mortar: Lime 1, Sand 3. Lime: slack [slake] with water and then cover with sand. After lime is thoroughly slack it is screened through upright screen and then mix with water to desired consistency'.
The proportions used by this architect for mixing quicklime with sand would not apply to a mix made with hydrated lime (whether hydraulic or non-hydraulic) because all quicklime’s increase in volume when they are slaked. The amount of increase varies according to the type and class of lime but typically this is between 60 and 100 per cent. Therefore the resultant lime: sand ratio for the finished mortar is always more lime-rich than the originally-stated ratio. That is why, under analysis, the majority of historic lime mortars are not commonly found to be 1:3 but typically vary between 1:1½ and 1:2, just as the original mortar makers and craftsmen intended. This is borne out by extensive analysis carried out over many years by The Scottish Lime Centre Trust. (At the last count the organisation has analysed around 4,500 historic mortar samples, approximately 80 per cent of which were from Scotland, 10 per cent from England with the remaining 10 per cent from various other countries.) The average lime: sand ratio on the organisation's entire database of historic mortar samples is around 1:1½.
The 1:3 quicklime: sand ratio suited most general building sands. However, sometimes builders had to use naturally fine and more uniform local sand, not the ideal well-graded building sand, but one that demands an increased lime
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