From raw materials to production: an in depth look at St. Astier natural hydraulic lime.
Natural hydraulic lime is becoming increasingly important in Restoration, Conservation and New Build. It is however essential that the specifier is aware of the characteristics of the products available on the Market to ensure that the correct material is chosen. For this reason we take a look at one of Europe’s largest producers.
Louis Vicat in 1850 surveyed the limestone of the St. Astier basin situated in the Perigord area of Dordogne where building limes have been produced even before the evidence left by the Romans. Vicat concluded that the material was appropriate for the production of natural hydraulic lime. Industrial production begun in 1851. Today’s St. Astier production capacity is over 100.000 T per year.
The deposits extend between Montaceix and Neuvic sur l’Isle (approx. 10 km) and the layer is several hundred meters thick. It was formed during the Upper Cretaceous period (approx. 75 Mil. years ago) by marine sediment (mostly crustacean and corals). The sea in the basin was not subject to severe currents and this allowed the formation of a uniform and undisturbed layer of calcareous rock infiltrated mainly by silica with only insignificant traces of other elements. This characteristic of the rock is unique in Europe and is the reason why the St. Astier products are so reliable.
The purity of this calcareous rock makes the all important difference between producing materials with constant characteristics or being unsure of the final qualities as commonly experienced when clay beds or less pure limestone deposits are exploited.
The EU norms (EN 459.1/2/3) on natural hydraulic lime (NHL).
They state that natural hydraulic lime is the product of burning and slaking limestone (or clay such as Lias clay). It should not contain any additives such as pozzolans, gypsum, air entrainers, ash or cement. Where additions are present they will have to be qualified and the resulting products will have to be named as NHL-P or NHL-Z indicating additions of pozzolanic or hydraulic materials of up to 20%. The norm also allows up to 3% of SO3 content and between 3%-8% of free lime.
Although the distinction between pure NHL products and others, where additives have been introduced, is welcome, the opinion of many on the EU limits, however, is that in the case of SO3 the limit is considered too high and too low for free lime, again making possible the use of non suitable materials. For their customers peace of mind St. Astier Limes give the following figures for all their NHL products: no additives, only traces of SO3 (0.45%-0.54%) and between 15%-45% free lime.
Pozzolanic and hydraulic materials additions (such as cement) are clearly necessary in products derived from poor raw materials when hydraulic properties and other mechanical characteristics are not constant. The presence of these additives in some cases could be damaging and it will be adviceable to ask Suppliers to state whether or not their products contain additives and, specifically, what has been added.
The importance of the raw materials.
Limestone and clays will contain Silica but do contain also Sulphates, Alumina, Iron, Magnesium, Manganese, Potassium and others. Burning the limestone/clay will produce quick lime and will combine its chemical components in relation to the burning temperature.
The ideal result would be to obtain a product containing the required value of combined Silica with the lowest presence of potentially damaging other components such as Tricalcium Aluminate (C3A) and soluble Sulphates. Tricalcium Aluminate starts occurring when materials are burned at 9000C and increases at 10000 C and over. The highest values are found in ordinary cement (7%-15%). Obviously the lower is the amount of Alumina and sulphates contained in the raw material, the better will be the final product quality. St. Astier deposits are exceptionally low in Alumina and in Sulphates, the resulting products are therefore virtually free of these potentially damaging components.
Ordinary cement mortars and mortars made with lime where cement has been added are sure to contain Tricalcium Aluminate which in contact with Suplhates and water can produce Sulphate attack starting with efflorescence and progressing to badly damaging joints, brick and stone. The BS 5628 warns about this but does not indicate that a simple solution could be the use of a pure NHL mortar.
Manufacturing process and controll
The production process is also extremely important to ensure the final quality and performance of the products. St. Astier whole production, from rock extraction to packaging is fully automated and computer controlled.
At St. Astier the raw material is selected before burning (150 mm max.). The coal used for burning is the purest Anthracite coal, imported from Wales despite availability of local coal (high in sulphates and residuals). This is dosed in the required amount and mixed with the stone before introducing the charge into the furnace (vertical kilns of 200 T capacity). The temperature in the kilns is controllable. This is very important in the production of various types of hydraulic lime as variation in the temperature and in the burning time have an effect on the final strength of the product.
At the end of each burning all batches are checked for CO2. The amount of CO2 determines the efficiency of the burning and regulates the subsequent water addition during the slaking process.
Expansion tests (soundness) are also constantly made to ensure that no material with an expansion value over 2mm is processed further thus voiding the use of sub standard materials which would result in products subject to shrinkage and not resistant to rain and frost.
Laboratory tests are conducted continuously to comply with French mandatory tests (fineness, free water content, soundness, bulk density, ompressive strength, setting time, penetration, SO3 and available lime contents). These, combined to other tests ( tensile strength, elasticity moduli, workability, adhesion, whiteness), make available to the Customer the most comprehensive information allowing for the orrect choice of product to be safely made.
Apart from the absence of soluble Sulphates and Tricalcium Aluminate, directly due to the chemical composition of the natural stone, the sophisticated and controlled production process ensures the following:
1. That the correct amount of silica is combined in burning, to constantly achieve the required strength and hydraulic set with no need for additions of pozzolanic or other materials. This facilitate mortar design and ensures that mortars are quickly resistant to frost and rain.
Excessive humidity or rain water will not stop the hardening process as in putties or air limes. In rendering, if good working practice is applied, jobs are completed efficiently and speedily. Furthermore the application season is extended to nearly the whole year and frost rotection is necessary only for a short time. The production of various strength materials as NHL 5, 3.5 and 2 (commonly defined as eminently, moderately and feebly hydraulic), reduces the need for blending.
2. That efficient slaking produces the pulverisation of the burned lime to obtain very fine products (0.08 mm) with low bulk density.
Low density means large cost saving when mixes are made based on volume (on a 1:2 mix one needs only 305kg. of NHL 3.5 per m3 of sand versus 75 kg. of lime putty or 725 kg. of cement in an equivalent mix). That, with the correct sands, NHL mortars will have approx. 30% voids (versus 5% in cement based mortars). Calcite forming during carbonation will obstruct some of the voids but cannot obstruct them all. This is the reason why NHL mortars have excellent vapour exchange qualities (permeability) avoiding the risk of trapping condensation, so common when cement based mortars are used.
3. That NHL products retain a high percentage of available lime (15% to 45%). This explains their workability, their ability to accept re-working if required and the presence of self healing properties.
4. That the powders maintain the natural colour of the limestone. This, the absence of additives and the use of Anthracite coal which burns efficiently, gives St. Astier NHL limes a nearly white appearance (whiteness index from 67 to 76). This means that it will be easier to reproduce the natural colour of the sand used with the result that, in many cases, the mixing of different sands and other materials to obtain a colour can be avoided.
St. Astier NHL products are used worldwide for their quality and reliability. Recently the whole range has been introduced in UK and Ireland and has been successfully adopted in a number of projects, from historical buildings to new construction. The St. Astier range of NHL products is available solely through a network of specialist Companies involved in Conservation, Restoration and New Build. Part of their responsibilities is to assist the specifier in the task of selecting the most suitable materials including mortar design, colour matching, analysis of existing mortars, training sessions and assistance in choosing the correct aggregates to use in mortar mixes, so essential to ensure optimal performance.
TRADECO® Hemp renders
The qualities of TRADECO as a binder have proven very suitable to making Hemp mortars for renders, providing elasticity and good hardening properties.
Dosage: TRADECO 2 bags HEMP 100 litres Water addition: 55-65 litres.
Mortar mixing: add TRADECO to the water and mix for a few minutes to obtain a lime/water blend.Add the Hemp, suitably loosened and mix for about 10 minutes until an homogeneous consistence and plasticity is obtained (uniform colour and no lumps).