Are grinding aids used during the reaction grinding of Celitement?

This is not usually necessary but we have, of course, already carried out trials with a wide variety of grinding aids. These act in the way known from cement grinding and represent another parameter by which changes in the properties of the product can be brought about during the activation grinding.

Can Celitement also be produced from recycled raw materials or from secondary raw materials in general?

Yes, although the calcium component and the silicon component will have to be soluble and able to react with each other under the autoclave conditions. Preliminary laboratory trials into the use of recycled or secondary raw materials in the autoclave process have already taken place at the Karlsruhe Institute of Technology. At the moment, however, we are concentrating fully on reliable mastery of the processes and product quality when using very pure high-grade starting materials. We will only follow up any economic optimization by using secondary materials at some later date.

How much of the intermediate product from the autoclave is converted during the reaction grinding into actual end product?

As much as possible, naturally, as the degree of conversion determines the cost-effectiveness of the process to a very substantial extent. The more autoclave intermediate product is converted to hCHS during the activation grinding the more efficient is the Celitement variant. The degree of conversion depends not only on the grinding unit and the chosen operating conditions but also on the original mix formulation. It is a little like the strength classes of Portland cement. Starting from the same clinker it is possible, depending on the fineness of grinding of the clinker achieved, to produce CEM I 32,5 / 42,5 / 52,5 cements. With the activation grinding it is not, however, just the fineness (or better, coarseness) but mainly the degree of conversion that determines the efficiency. We normally aim at degrees of conversion > 80 %.

One now reads that sand has become a valuable raw material for concrete. Why does Celitement GmbH claim that the change from a lime-rich to a silica-rich (sand-rich) binding agent like Celitement is an advantage?

The sand that is much talked about in the context of scarcity of raw materials is one of the main components in concrete. For this purpose the sand must exhibit very specific particle shapes and physical properties. Desert sand, for example, just cannot be used for producing concrete. However, sand that is to be used in classical clinker production and also in the Celitement autoclave process is not subject to these requirements. It is completely dissolved during the production process and converted into entirely new compounds. This means that the scarcity of sand does not affect the production of Celitement as a binding agent but it does affect the production of concretes. In this case it is the same whether Portland cement or Celitement is involved.

Techniques for CO2 (carbon) capture in classical cement production are currently being tested. Can this technique also be used appropriately for Celitement?

Hardly any CO2 is released during the actual production of Celitement from hydrated lime and sand or during the activation grinding. However, CO2 is of course released from the raw materials and fuel during the production of the quicklime that serves as a raw material source for Ca(OH)2. The CO2 produced can be collected by carbon capture measures just as effectively as is the case with those measures currently being tested for the clinker production process (cf. Catch4Climate). This means that combinations of carbon capture technologies with the Celitement process are a definite possibility.

What is the most complicated or most complex sub-process in the production of Celitement?

This, by a large margin, is the second production step, the activation grinding. In contrast to plain comminution this even involves some coarsening. This paradoxical phenomenon for a grinding process can be explained by the fact that the CSH phases produced in the autoclave are very fine and exhibit extremely high surface areas. These tend to stick together during the activation grinding, which increases the average particle size. The mechanochemistry, also called tribochemistry, is a very complex process. Many different parameters have to be precisely controlled to obtain the desired end product. Sometimes we even talk about a “reactor” rather than a mill. When examined closely even the comminution of Portland cement clinker during classical cement grinding, which seems “simple” at first glance, is not as trivial as it seems at first. Here again, a wide variety of processes have to be considered. For example, the gypsum (a dihydrate) used in the cement grinding as a setting retarder becomes de-watered at the sometimes high temperatures in the cement mill and converted into other gypsum modifications with differing reactivities. This means that there is also some activation grinding in a classical cement plant. However, the activation grinding of Celitement is even more complex and therefore represents a key part of the expertise in our technology.

Why aren’t rotary kilns needed for the production of Celitement?

Celitement is a binding agent that already contains water. In contrast to the completely water-free calcium silicates from the Portland cement production process this structural water is really crucial for the properties of Celitement. A rotary kiln would evaporate this water and destroy the hCHS phase. Although no high temperature process is required for the production of Celitement itself the main raw material, i.e. calcium oxide or Ca(OH)2, must be obtained from limestone at about 900 °C. However, this is still substantially lower than the 1,450 °C temperature of the material in a rotary cement kiln.

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