Mineralogy and Chemistry of Raw Materials

Chemistry and mineralogy of the raw material
Manufacturing and finished products chemical and mineralogical data

 St. 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).

Chemical and mineralogical analysis of the deposit.

Chemical Analysis	%
Loss at ignition	40
CaO	4>
SiO2	13
>MgO	0.6	The absence of clay infiltration and the consequent minimal presence of Al2O3, sulphates and alkalis ensures the production of hydraulic limes based almost totally on the combination of Calcium Oxide and reactive silica
Al2O3**	1.1
>Fe2O3**	0.32
SO3 **	0
Na2O **	0.04
K2O **	0.1
Others**	0.84

 

Corresponding mineralogical composition

H2O (moisture content)	8
CaCO3	75
SiO2 (soluble)	11 reactive/combinable	The soluble silica, available to be combined with the CaO produced in the burning of the CaCO3) determines the hydraulicity of the finished products
SiO2 (insoluble)	2 inert/un-combinable
MgCO3	1
Others (derivatives from items marked ** above)	3

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 St. 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 1800^oF (1000^oC). 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.

Composition CHEMICAL (%)				MINERALOGICAL (%)
	NHL 5	NHL 3.5	NHL 2		NHL 5	NHL 3.5	NHL 2
Loss @ Ignition	16	18	20
Calcimetry (CaO2)	10	11	6
Insoluble	5.6	9.6	8		5.6	9.6	8
CaO	59	56	63	Free Lime Ca(OH)2	22	25	58
				Calcium Carbonate CaCO3 UNBURNT	23	25	13
SiO2	15	12	6	Calcium Silicate
Combined
Al2O3	1.92	1.66	1.3	C2S	43	35	17
				C3A	0.7	0.5	0.4
				C2AS	1.3	1.0	0.8
Fe2O3	0.57	0.49	0.4	C4AF	0.7	0.5	0.4
SO3**	0.41	0.45	0.31	CaSO4	0.7	0.8	0.5
				Others
MgO	1.01	0.98	0.75	The quantities of these components are so small that their mineralogical presence is too minute to be relevant. Very significant for the alkalis (K2O/Na2O) which, even in small quantities (1.5/2% as in ordinary cement) can produce ALKALI SILICA reactions
MnO	0.02	0.01	>0.01
TiO2	0.18	0.16	0.12
K2O	0.21	0.16	0.12
Na2O	0.07	0.06	0.04

** 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.

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