Chemical, mineralogical and physical characteristics
During the hydration of Prompt natural cement, aluminates, formed at a low temperature, are responsible for quick setting (of the order of 2 to 3 minutes) and the first phase of increased strength during the first hours. Hydration of the belite brings the second phase of strength, which can be expressed over a number of months, as indicated in the figure below (Figure 1 hardening kinetics over 10 years).
Figure 1: hardening kinetics over 10 years
Prompt natural cement presents a characteristic of being usable across a very wide range of doses, between 10 and 50% of the weight of dry mortar, or even 100% in the case of slurry and grout.
The figure below (Figure 2: Hardening kinetics according to dose and water/cement ratio) gives an idea of the strength that can be achieved in the event of the cement being kept in water.
Two zones are particularly notable:
- the initial high strength zone, defined by a water/cement ratio of less than .05. These high performances affect operations such as sealing, quick masonry and waterproofing. High strengths imply low porosity. The performances of this zone, which meet the requirements of different structural stresses, are similar to those for artificial Portland cement.
- the low strength zone, with a water/cement ratio of more than 0.5, offers mechanical performances and doses which are similar to those of natural hydraulic lime mortars. The water absorption characteristics and porosity are also closer to those that you would find in 19th century Roman cements.
The chemical composition of Prompt natural cement belongs to the huge family of natural cements, also known as Roman cement.
Figure 3: The chemical composition of Prompt natural cement
The geological layer mined in underground quarries presents a very steady chemical composition (Table 1) with a carbonate titre (carbonate content of calcium and magnesium expressed in CaCO3) of 72 for 78 in the case of Portland clinkers. It is therefore close to a classic Portland clinker and natural hydraulic limes. As a result, in relation to all Roman cements that have been known over time, it is in the lower limit in terms of clay content, and relatively rich in CaO:
|PF 975 °C||SiO2||Al2O3||Fe2O3||CaO||MgO||SO3||K2O||Na2O|
Table 1: Mineralogical characteristics:
The originality of Prompt natural cement does lie in any particular chemical composition, but rather in the fact that it is fired at a low temperature and with a broad thermal spectrum of between 500 and 1,200°C (below malting point). This is slightly higher than that of hydraulic limes, which are a natural mixture of limestone and clay. This micron-sensitive mix is necessary to form minerals during firing, because the dissemination of atomic elements is low in a solid state.
The result is the formation of a wide range of minerals that are very different from those found in modern artificial Portland cement, but identical to those present in natural hydraulic limes, although in different proportions (Table 2):
- Part of the stone has not received sufficient temperature to be transformed. It is simply dehydrated. This would apply to blocks that are called unburnt.
- Another part of the stone is transformed to give amorphous or incompletely crystallized phases, including all the aluminates family (C4AF, C3A, C12A7, C4A3S et C2AS) responsible for setting and quick hardening during the first hours of hydration, as well as silicates in the form of belites (C2S). This gives increased strength over several months. A little alite (C3S) is present in small qualities, because this starts to form at around 1,200°C. This is the block that is referred to as burnt.
- There is very partial fusion (clinkerisation) in very local areas, forming a small quantity of alite. The fact that the chemical composition of the stone used as a raw material is close to that of Portland clinker allows the well-known formation of calcium silicate. Highly hydraulic, this adds strength after a few weeks: so-called over burnt blocks.
This latter point is very important, because the local fusion of materials is inevitable with a firing process in straight kilns. The minerals formed in this liquid phase must not change the natural cement during hydration. For example, a raw material with a higher clay content will in this liquid phase form more aluminal minerals, whose hydration is less well controlled. This can result in serious problems of durability.
This mixture of stones fired at different temperatures is what gives natural, traditional cement its characteristics.
|C3S||C2S||C3A||C4AF||C12A7||C4A3S||periclase||quicklime||calcite||sulphates||Others including amorphous phases|
|5-15%||40-60%||6 ± 2%||9 ± 2%||3 ± 1%||3 ± 1%||4 ± 1%||2 ± 2%||10 - 15%||3 ± 1%||10 -15%|
Table 2: Chemical and mineralogical composition of Prompt natural cement