Mineralogy & chemistry of raw materials and products
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 conform strictly to the NHL classifications established by French Norm NFP 15.311 and European Norm EN 459.
The limestone in the Saint-Astier® basin (approx. 40 Km2) derives from crustacean deposits (chalky limestone) infiltrated by silica but untouched by clay. The site has been exploited for thousands of years but industrial production begun in 1851. The quarries which extend over 30 hectares have been owned by the same group since modern manufacture began. Tests conducted by the French government show a unique uniformity in the composition of the deposits (up to 100 m. depth).
| Chemical and mineralogical analysis of the deposit. | ||
|---|---|---|
|
Chemical Analysis |
% |
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. |
|
Loss at ignition |
40 |
|
|
CaO |
44 |
|
|
SiO2 |
13 |
|
|
MgO |
0.6 |
|
|
Al2O3** |
1.1 |
|
|
Fe2O3** |
0.32 |
|
|
SO3 ** |
|
|
|
Na2O ** |
0.04 |
|
|
K2O ** |
0.1 |
|
|
Others** |
0.84 |
|
| Corresponding mineralogical composition | |||
|---|---|---|---|
|
H2O (moisture content) |
8 |
|
The soluble silica, available to be combined with the CaO produced in the burning of the CaCO3) determines the hydraulicity of the finished products. |
|
CaCO3 |
75 |
|
|
|
SiO2 (soluble) |
11 |
reactive/combinable |
|
|
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
It has essentially remained unchanged since ancient times: the limestone is burned and slaked. Saint-Astier® NHL products are therefore amongst the very few traditionally produced limes. The manufacturer’s scientific knowledge and modern quality control, however, guarantee the production of reliable materials with consistent performance.
The burning process
The method and energy used are the determining factors in the quantity of silica that combines with Calcium Oxide (CaO) to form Calcium Silicates (CS). It is the CS that produce the hydraulic performance of the finished products. Burning takes place in vertical kilns at temperatures not over 1,000oC. The fuel is anthracite coal, imported from Wales for its purity, as it produces the least residuals.
Continuous checks are made to measure the efficiency of the burning (CO2 tests). They are essential to regulate the hydration that follows.
Hydration (slaking)
The controlled hydration process is so precise that virtually no quick lime (<1%) is 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 (0.09mm). As shown below, the amount of potentially damaging components produced is so minute that adverse reactions, leading to material deterioration, are not possible.
| Composition | CHEMICAL (%) | MINERALOGICAL (%) | |||||
|---|---|---|---|---|---|---|---|
|
|
NHL5 |
NHL3.5 |
NHL2 |
|
NHL5 |
NHL3.5 |
NHL2 |
|
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 |
56 |
|
Calcium Carbonate CaCO3 UNBURNT |
23 |
25 |
13 |
||||
|
SiO2 |
15 |
12 |
6 |
Calcium Silicate |
|
|
|
|
Combined |
|
C2S |
45 |
35 |
17 |
||
|
|
C3A |
0.7 |
0.5 |
0.4 |
|||
|
Al2O3 |
1.92 |
1.66 |
1.3 |
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 |
|
|||
|
MnO |
0.02 |
0.01 |
>0.01 |
|
|||
|
TiO2 |
0.18 |
0.16 |
0.12 |
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. |
|||
|
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.
C3S can occur due to “high spots” in the furnace and also due to autectic reactions caused by the presence of alkalis which lowers the fusion point.