Mineralogy & Chemistry of Raw Materials

Manufacturing and finished products chemical and mineralogical data

Saint Astier® Natural Hydraulic Limes (NHL) are produced from the burning and slaking of a pure chalky limestone with siliceous content. No additions are made. They strictly conform to the French Norm NFP 15.311 and the European Norm EN 459 classifying NHL.

The limestone in the St. Astier basin (approx. 15 square miles) derives from crustacean deposits (chalky limestone) infiltrated by silica but untouched by clay. Exploited for thousands of years, industrial production begun in 1851. The quarries, owned by the same group from the industrial beginning, extend for 75 acres. Tests conducted by the French government show a unique uniformity in the composition of the deposits (up to 300′ (100 m) depth).

 
The production of different types of Natural Hydraulic Limes from the same raw material deposits proves that hydraulicity depends on the amount of silica combined and not on the total amount present. The theory that hydraulicity depends on the total amount of “clay (or silica)” in the raw material is fundamentally flawed.

The production method is essentially unchanged from the one used since ancient times: limestone burned and slaked. It is therefore correct to say that Saint Astier® NHL products are amongst the very few traditionally produced limes. The scientific knowledge of the manufacturer and modern quality control have, however, the favorable effect of producing reliable materials with constant performance.

The burning process: Its methods and the energy used are the determining factors in the quantity of silica that combines with Calcium Oxide (CaO) to form Calcium Silicates (CS) which produce the hydraulic performance of the finished products. Burning takes place in vertical kilns at temperatures not above 1800oF (1000oC). The fuel is anthracite coal, imported from Wales due to its purity, as it produces the least residuals.

Continuous checks are made to measure the efficiency of the burning (CO2 tests) which are essential to regulate the hydration that follows.

Hydration (slaking): The controlled hydration process is so precise that virtually no quick lime (<1%) will be present at the end. The efficiency of the slaking process is such that only a small percentage of the slaked material has to be milled to achieve the desired granulometry (#150 or 0.09mm). As shown below, the amount of potentially damaging components produced is so minute that adverse reactions, leading to materials deterioration, are not possible.

** The presence of SO3, absent in the raw material, is induced by the coal used in burning. The small level of it, however, is harmless. Higher gypsum (CaSO4) levels due to additions as in the case of ordinary cement or some other hydraulic binders can cause damage.