Found on http://www.buildingconservation.com “Hot-mixed Mortars: Advantages and Limitations”, Author: Adam Brown and “Lime: The Basics”, Author: Jonathan Taylor.
“Mortar” comes from Latin “Motarium” meaning crushed.
Modern day hot mixed-mortars are 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’ with water. The process generates heat through a chemical reaction and the mix may be used immediately as a hot mix or later when cooled.
Quicklime is limestone that has undergone a chemical change in a kiln, liberating it of all the carbon and water it holds, creating a very unstable material (calcium oxide) which needs to hydrate. Which it will do so very energetically with any moisture it comes into contact with, the result is a strong exothermic reaction which produces the lime.
A hot-mix mortar offers a very workable material, due to it’s high lime content it’s very sticky and this can help stop mortar being washed out. It offers good water retention that can be controlled during the mixing process, this can also be a cost effective option; often cheaper than putty or NHL!
Nobody knows when humans first discovered lime, what we do know is that in parts of the world evidence can be found back dating it’s use to over 16 000 years ago.
Before modern cements took over in the early 20th century, lime was the principle binder for making mortars and renders. It was produced by burning limestone in a kiln at temperature 800°C. In early kilns the temperature varied throughout, leaving lumps of lime unconverted and some sections over-burned.
KEY DATES OF LIME USAGE
The ancient people who lived in the area that is now Jordan, made a plaster from lime and unheated crushed limestone to cover walls, floors and hearths in their homes.
The Egyptians tanned their skin with lime and built one of the limestone wonders of the world; the 137 m high Cheops pyramid.
2800 B.C. – C. 1000 A.D.
Celts fertilised fields with lime. Lime colours were featured in Greek frescos.
The Chinese built the 2,500 km Great Wall by stabilising the soil with lime and used lime mortars to cement the stones together.
753 B.C. – 535 A.D.
Roman frescos and buildings featured different lime colours. Women coloured their hair to a light red hue with unslaked lime.
400 A.D. – C. 1100 A.D.
Alchemists discovered the caustic properties of lime and created a soap based on wood ash. The Lhoist logo recalls the alchemic symbol for lime.
1300 A.D. – C. 1800 A.D
Lime was widely utilised throughout Europe as a plaster and paint décor, and it served as a principal building material for homes.
The Scagliola technique came into fashion as an effective substitute for costly marble inlays. It comprised a pigmented mixture of lime dust, marble or scale. It was also used to create building facades, stucco columns, sculptures and other architectural elements that resemble marble.
18TH & 19TH CENTURIES
Black and Lavoisier described the chemical reaction of lime. Debray and Lechatelier discovered others qualities and applications. For example, limestone was included for the first time as an ingredient in toothpaste.
20TH AND 21ST CENTURIES
The proliferation of new innovations, especially the birth and rapid development of technologies, broadened the use of lime. Today, we encounter lime or the results of treatments involving lime in one form or another at just about every minute of the day.
The use of lime has evolved over time just as humans have done. Today, we use lime in our conservation work in a number of ways:
Lime is made by first burning chalk or limestone to form quick lime (calcium oxide) and then slaking the quicklime with water (forming calcium hydroxide). If no clay is present in the original limestone or chalk, the resulting lime is said to be ‘non-hydraulic’. This form stiffens and eventually hardens by reacting with carbon dioxide which is present in rainwater (in the form of a weak solution of carbonic acid) to form calcium carbonate once again; a process known as carbonation.
For conservation work, non-hydraulic lime is usually used in the saturated form known as ‘lime putty’. This is supplied to site covered by a thin film of water in air tight tubs, to minimize the risk of carbonation. It is made by slaking the lime with a slight excess of water. When matured (lime putty continues to mature for months), the result is the purest form of non-hydraulic lime, ideal for making fine plasterwork and limewash, but also widely used for pointing masonry and making render, daub and other lime-based mortars.
Dry slaked lime
To construct towns and cities at the rate required in the late 18th century, Gerard Lynch, the historic brickwork consultant, has convincingly argued that most lime must have been made on site and used immediately, without waiting for it to mature. Dry-slaking is ideal for this: lumps of fresh quicklime are slaked with a limited amount of water and then immediately covered over with damp sand; then, after screening to remove any remaining particles of unslaked quicklime, the mixture of sand and lime is knocked up with water ready for immediate use, although it was probably ‘banked’ to allow the lime to mature for a few days first.
Most builders merchants supply a dry form of non-hydraulic lime which can be used like lime putty if allowed to soak in water for a while. Known as ‘dry-hydrated’ lime or ‘bag lime’, it is generally considered to be inferior to lime putty, not least because an unknown proportion will have reacted with carbon dioxide by the time it reaches the site.
If the limestone contains particles of clay, after burning at 950-1200°C and slaking, the lime produced sets by reaction with water. Limestone containing the lowest proportion of clay (less than 12 per cent) results in a feebly hydraulic lime with properties close to non-hydraulic lime, which is relatively weak, permeable and porous. Higher proportions result in successively stronger and less permeable lime mortars. Because they react with water, hydraulic limes are usually supplied to site as dry powder. However, they can also be made by dry-slaking on site and may be knocked up with water and banked on site for a few days.
The hydraulic set takes place due to complex chemical changes involving the hydration of calcium silicates and aluminates in particular. A similar effect can be achieved by adding pozzolanic additives to non-hydraulic lime as these additives contain highly reactive silica and alumina. Pozzolanic additives include some types of brick dust, fired china clay (such as metakaolin and HTI/’high temperature insulation’), PFA/’pulverised fuel ash’, volcanic ash and pumice.
Mixtures of hydraulic and non-hydraulic lime were used in the past to create what English Heritage has termed ‘hybrid’ lime mortars (Historic Scotland describes them as ‘complex’ mortars). However, the performance of a hybrid mortar was called into question by English Heritage following a number of spectacular failures, after which it banned the use of these mixtures on grant-aided work. The results of a study by the Building Research Establishment and English Heritage, which are now being prepared for publication, show that the addition of a small amount of non-hydraulic lime (5-10 per cent) improves workability but anything above this level significantly impairs durability. Mixes containing 1:3:12 and 1:2:9 hydraulic lime:non-hydraulic lime:sand actually performed less well than a standard 1:3 non-hydraulic lime:sand mix in their tests.
Generally, mortars for conservation and repair work should include the same range and types of aggregate particles as the original mortar, as well as the same binder and any pozzolanic additives, unless any of these are actually harmful. This is to ensure that the new mortar performs in the same manner as the old and is similar in appearance. The original mix is best determined by analysis. Several companies offer mortar analysis services., often important to us when matching up aged stonework.