Derived from a natural deposit, PROMPT natural cement is manufactured using an unchanged process since the 19th century. Fired at moderate temperatures and finely ground, it is a material with numerous applications and advantages. Discover in detail its firing process.
Low-temperature firing, unique to PROMPT natural cement
After being crushed, the stones, obtained from underground quarries (link to deposit page) with a particle size of 0-150 mm, are mixed with coal and then introduced into traditional vertical kilns. The stones are fired at low temperatures, ranging from 500°C to 1,200°C, and with a wide thermal spectrum, allowing the specific mineral phases of PROMPT natural cement to form. These firing temperatures are well below those of artificial cement, which approach around 1,450°C. The variation in firing temperature is due to several factors. The first is the variable intensity of the airflow inside the kiln. This intensity changes according to the heterogeneous particle distribution of the stone, resulting in more or less intense combustion. The second factor is the uneven distribution of coal, the ashes of which may locally cause the fusion of stones. Lastly, the presence of organic matter in the stone, which, when calcined, can provide heat.
An unchanged firing process since the 19th century
The operator of the process has various parameters to control the kiln :
- Visual examination of the surface at the top of the kilns
- Flue gas temperature
- Adjustment of ventilation
- Visualization of the firebox through thermocouples
They can also adjust the kiln operation by manipulating :
- The number and volume of charges
- The coal content in the charge
- Modulation of ventilation
- Extraction time
It should be noted that PROMPT natural cement is manufactured in the same kilns as those used in the La Pérelle plant since its establishment in 1875..
A matter of proportions
After exiting the kiln, the stones are called "logs" and classified into three categories: underfired, fired, and overfired, depending on their level of firing. Quality natural cement distinguishes itself by a well-proportioned mixture of these different logs. With a modern firing process, such as the rotary kiln, it is also possible to fire the material at low temperatures, between 800°C and 900°C. The cement will be natural; however, its mineralogy will not be representative of the natural cements of the 19th century.
After firing, the stones are finely ground to a much higher fineness, about twice that of ordinary cement. From the deposit to the finished product, no additives are used, hence its designation as natural cement. Unlike ordinary cement, which results from the high-temperature firing of a mixture of limestone and clay, PROMPT natural cement is directly extracted from limestone clay, making it unique.
To learn more about the manufacturing process, discover our grinding method and our deposit.
