The importance of the compostion St. Astier
raw material and manufacturing method in the pure and natural hydraulic
|Addition of pozzolanic and hydraulic material (cement being
the most common) is clearly necessary in products derived from poor raw
materials when hydraulic properties and other mechanical characteristics
are not constant. The presence of these additions in some cases could be
damaging and it is advisable to ask suppliers to state whether or not their
products contain additions and, specifically, what has been added.
In the case of SO3, the limit in the norm is considered too high whilst the limit for free lime is considered too low, 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 additions, only traces of SO3 (0.45%-0.54%) and between 15% to over 50% free lime.
Most important of all, the St. Astier NHL products are totally compatible and will not react with old mortars because they do not contain reagent components. Salts, which might be contained in the structure or have been introduced by previous unsympathetic interventions, will be allowed to migrate out of the structure without affecting the soundness of an NHL mortar. Furthermore interventions with NHL mortars are not irreversible and materials recyclability is ensured.
The importance of the raw materials.
Limestone and argillaceous limestone that contains silica will also contain sulphates, alumina, iron, magnesium, manganese, potassium and other compounds. Burning the limestone at temperatures above 800oC will combine the above components with the calcium carbonate forming calcium silicates, aluminates and sulphates. The ideal result would be to obtain a product containing the required value of combined silica with the lowest possible presence of potentially damaging other components such as tricalcium aluminate (C3A) and soluble sulphates.
Tricalcium aluminate starts occurring when materials are burned at 1650F and increases at 1850F and over. The highest values are found in ordinary cement (sometimes over 10%). Obviously the lower the amount of alumina and sulphates contained in the raw material, the better the final product quality. St. Astier deposits are exceptionally low in alumina and in sulphates, the resulting products are therefore virtually free of these components.
Ordinary cement mortars and mortars made with lime where cement has been added are sure to contain high quantity of tricalcium aluminate which in contact with suplhates and water can produce sulphate attack starting with efflorescence and progressing to damaging joints, bricks 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.
High presence of gypsum is also to be avoided. Its sulphate content can be disastrous.
Manufacturing process and quality control.
St. Astier NHL products are used worldwide for their quality and reliability and in the most varied climatic conditions. Innumerable buildings of all ages have been preserved and restored with these materials over the last 150 years.
The St. Astier range of NHL products is available in UK solely through a network of specialist companies involved in conservation, restoration and new build. Part of their responsibility is to assist in the task of selecting the most suitable materials, mortar design, colour matching, analysis of existing mortars, training and choosing the correct aggregates to use in mortar mixes, so essential to ensure optimal performance.