After more than 10 years of research and development is the Celitement system now fully understood or are there still unanswered questions?

Even the Portland cement system, of which several billion tonnes are produced every year, is still not fully understood. It would therefore be presumptuous to claim that we already understand all the scientific aspects of the Celitements. Very similar to the situation with Portland cement where a great deal of empirical knowledge serves as the basis for a thoroughly successful practical application, the purely scientific understanding of a building material is certainly extremely helpful and important. But, as with classical cement, incomplete knowledge of all the details is no obstacle to the development of an efficient binding agent. This is where the fine differentiation between “research” and “development” comes into play. Research serves to clarify the principles and development helps to put them into practice. However, the reaction products produced in the application of Celitements are known in principle from the research into cementitious building materials. This meant that we did not have to re-invent the wheel for some of the cement chemistry problems. However, clarification of the varied cause-effect mechanisms during the hydration as well as the amorphous structure of the intermediate and end products, which is difficult to characterize and also makes the classical cement system so complex, represent a great challenge for Celitements. The Karlsruhe Institute of Technology and Celitement GmbH have invested a great deal of work, time and money in research into this sector.
But, as with classical cement, a great many readily reproducible practical trials must then be carried out to be able to derive statistically secure information about relationships between structural and physical properties. This process has not ended either with cements or with Celitements. With such systems we tend to speak of “evolution” rather than “revolution”.

Are there any special occupational health and safety measures that apply when Celitements are used?

No, the same industrial safety measures apply when dealing with Celitement as for Portland cement.

Can Celitement be a 100 % replacement for Portland cement?

In principle yes, at least in almost all the applications that we have tested so far on a laboratory scale. However, complete replacement is not by any means the primary aim. Celitement is first and foremost a new type of special binding agent. The focus is on applications where pure C-S-H binding agents have a significant advantage over Portland cements.
It is, of course, always necessary to adapt and optimize the mix formulations. A simple 1:1 exchange of the two binding agents is often not particularly useful. We always advise innovators to reconsider their particular application ideas in the light of the specific properties of Celitements and then optimize them accordingly. This needs time but makes it possible to utilize the optimum performance of this new binding agent. The quantities of Portland cement produced worldwide and the plant capacities that this requires mean that no alterative binding agent, including Celitement, can suddenly claim to meet this still increasing demand for cement or to replace Portland cement. That would be overconfident. On the other hand, he who does not start will never get there. To start with we therefore want to show that the replacement of Portland cement is technically possible. The quantity and rate of substitution will then be determined by the market.

Can Celitement be processed like a normal cement?

In principle, yes. However, as a “cement” that already contains water (hydraulic calcium hydrosilicate hCHS) Celitement requires significantly less water than Portland cement to form C-S-H phases. The w/c ratio therefore has to be appropriately adjusted and lowered. This affects the workability, which must therefore be adjusted with, for example, PCEs.

How can one test the quality of Celitement? Can the procedures familiar from the quality assurance of Portland cement be used for this purpose?

It is necessary to differentiate here between the physical methods and the analytical ones. Just like the end product of the hydration of Celitement, namely the C-S-H, the autoclave intermediate product and the hCHS produced from it involve amorphous compounds that are structurally highly disordered but quite similar. In the context of production control this makes analytical characterization difficult. X-ray fluorescence (XRF) analysis established for production control in cement production can in fact be used for checking the chemical composition. However, if the “autoclave raw meal” consists only of the quite pure starting materials, namely hydrated lime (Ca(OH)2) and sand (SiO2), with their well known chemical compositions then the information that can be obtained from the XRF analysis (chemical) is relatively limited. The same also applies to the use of a diffractometer (XRD), which is chiefly important for identifying any incompletely converted initial phase or crystalline impurities. However, XRD alone is no sufficient for direct determination of the content of hCHS during the activation grinding. The use of vibrational spectroscopic methods for the characterization is preferable because of the amorphous character of all the intermediate and end products. These processes are still almost unknown from the quality assurance of Portland cement but have been further developed by us for the analysis of Celitements. Other established investigative methods are available in the form of the thermoanalytical and calorimetric methods that are best known from quality assurance in cement plants. They can also be used successfully for Celitements. One particular challenge is the particle size analysis that makes special demands because of the formation of agglomerates and conglomerates. On the other hand, the physical testing of Celitements hardly differs from the physical testing of conventional cements. This means that water demand, setting times and strengths are tested in the same way as for classical cements.

How does the intermediate product of the Celitement production process produced in the autoclave differ from the subsequent end product?

The intermediate product from the autoclave consists of CSH phases that have already reacted and are therefore not hydraulically active. They are stabilized by strong hydrogen bridges, which make them hydraulically unreactive. It is only in the course of the activation that these hydrogen bridges are destroyed, structural rearrangement and partial de-watering if the CSH phases take place and the desired reactivity of the hCHS end products is obtained. The transposition of the sequence of the letters CSH → CHS and the prefix of the h (for hydraulic) are intended to illustrate this principle.

How large are the primary particles of Celitement on average?

Electron microscope images normally show particle sizes of 200-500 nm for the primary particles of Celitement. However, these often coalesce to form larger agglomerates.

How long can Celitement be stored?

We have determined from long-term storage trials under different conditions that Celitements do not differ in this respect from finely ground Portland cements. As reactive hydraulic binding agents they react with ambient moisture and carbon dioxide. Under dry, air-tight, conditions they can be stored for several months to up to over a year. After that there is a drop in performance in the same way as with Portland cements.

Is it harder or easier to develop marketable building materials systems with Celitement than with Portland cement?

Not necessarily harder but it has been found that it is good to make an impartial reassessment of the entire system in which Celitements are to be used and adapt it to the special properties of Celitements. For example, the possibly slightly changed interaction with current construction chemicals should always be borne in mind.

The particle size distribution that we have determined for the Celitement sample does not agree with the information given to us by Celitement GmbH. Why is this?

Celitements consist of a group of extremely fine binding agents with a nano-scale microstructure. Determination of the “actual” particle size distribution requires a combination of the established methods, such as air jet sieving and laser diffraction, with the use of image evaluation microscope methods or even small-angle scattering.
The primary particles of the hCHS join together to form agglomerates or conglomerates. These have varying mechanical stability depending on the production conditions. The agglomerates (or conglomerates) can disintegrate in different ways depending on the dispersion method chosen for the particle size determination, the dispersion energy applied and the nature of the measuring method. The same method and dispersion techniques should, as far as possible, therefore always be used for the characterization. However, this makes it more difficult to compare the particle size determinations from different laboratories with different techniques. Determination of the specific BET surface area provides absolute values that are more comparable. However, the difficulty of comparing the particle size distribution data from different sources is also well known with cements.

Unlike Portland cement Celitement does not contain any gypsum agent so how are the workability properties controlled?

Depending on the Celitement variant the workability properties are best set with superplasticizers, such as PCEs. However, the addition of inorganic substances such as quicklime, limestone meal or ground granulated blastfurnace slag can also be used for setting the workability properties. In principle, it is of course possible to use just water as the plasticizer. However, it must then be borne in mind that Celitements have a substantially lower chemical water demand than Portland cement and too much water causes a disproportionate deterioration of the physical properties of the end product. The gypsum agent in conventional cement is added solely to retard the reactive aluminates at the start of cement hydration. This allows the cement to be processed for several hours. Celitement does not contain any aluminates so it is not necessary to add any sulfate agent.

What compressive strengths can be achieved with Celitements?

The compressive strengths that can be achieved with Celitements are of the same order of magnitude as with conventional cements (CEM I 42,5R to CEM I 52,5R). However, it should be noted that because of the low water demand of Celitement the water/binder ratio is lower than that of conventional cements. Ultimately, the familiar technical concrete and mortar correlations apply with respect to the compressive strengths. It is also possible to draw up a “Walz curve” for Celitements.

What is the highest early strength that has been achieved so far with Celitement in the mortar test?

We have already produced samples with strengths after 2 days of 44 N/mm2. Values of around 25 N/mm2 are normally achieved after 24 h with the standard variants.

What is the highest mortar compressive strength after 28 days that has been achieved so far with Celitement?

We have produced standard samples with mortar strengths of 70 N/mm2. These values were obtained with a w/c ratio of 0.40 that is usual for Celitements. When assessing this w/c ratio it should be borne in mind that, in contrast to Portland cements, this involves a binding agent that already contains water.

What is the most important difference between Portland cement and Celitement?

Unlike Portland cement, Celitements do not contain any secondary phases, such as aluminates or ferrites. Celitement consists almost entirely of hCHS. However, from the purely material aspect Celitement provides the same main strength phase as the phase best known from the hydration of classical Portland cement, namely the C-S-H phase. This is formed from the calcium silicates C3S and C¬2S while with Celitement it is formed from the hCHS. We expressed this earlier in the slogan “Celitement – reduced to the max”. This means that while the end products exhibit great similarities the starting formulation is quite different. Celitements are based on a formulation with substantially less lime than Portland cement. The C/S ratio can vary between 0.5 and 2.0. We have explained the basic principle on the homepage.

Where can I get information about the technical data, chemical composition, mineralogy and physical properties of Celitement?

In the download section of our home page you will find links to a selection of widely differing publications on Celitement. These include scientific papers that contain information about the chemical and mineralogical composition of Celitement.

Which superplasticizer do you use when processing Celitement?

We normally use PCE superplasticizers. The products from almost all established producers can be used but for our standard mixes we normally use BASF Glenium ACE40.

Why do the core-shell representations shown initially (2009-2013) no longer appear in the new publications and lectures about Celitement?

There are various intermediate products from the autoclave that can be converted into Celitement by activation grinding. The initial variant was based on alpha-C2SH, which is ground together with silicate carriers. hCHS (Celitement) is formed on the surface of the silicate carrier during the activation grinding and a so-called core-shell structure is obtained. When we wanted to register this approach with the European chemicals agency ECHA for registration in accordance with chemical legislation we were informed that with core-shell compounds a separate REACH registration is necessary for each different core material, i.e. for each silicate that we actually only need as a carrier. However, if a separate REACH registration is required for sand, ground granulated blastfurnace slag, glass and every other conceivable silicate core material then such a building material is of no economic interest. We then decided to proceed at first only with pure Celitement, i.e. pure hCHS without the core of silicates. This means that we changed from the so-called C1 variant (core-shell) to the C2 variant (pure hCHS) only because of the European law governing chemicals. This is a great pity as the first approach was also very good. However, perhaps the legislation will change in this respect and we will start research into the C1 variant again.

Why do you speak of Celitement as a “virtually” monophase binding agent?

No production process for building materials achieves quantitative conversion of all the starting and intermediate materials so the end product always contains certain secondary phases in minor quantities. The main product is naturally the hCHS (Celitement) but the batches produced at the moment contain fractions of unconverted Ca(OH)2, residues of quartz or even feldspar from impurities in the natural sand raw material and crystalline or amorphous calcium carbonate. They may also, depending on the purity and origin of the limestone raw material, contain spurrite or phases that are produced during the industrial calcining process for the production of quicklime or calcium hydroxide (slaked lime).

Why is the analytical characterization of Celitements so difficult?

In contrast to the mineral phases in Portland cement clinker Celitements are almost entirely amorphous. Analytical characterization of amorphous substances is, in general, more demanding than that of properly crystalline substances. In addition, the intermediate product from the autoclave (a CSH phase), the end product formed from it after the tribochemical transformation (hydraulic calcium hydrosilicate: hCHS) and the C-S-H (calcium silicate hydrate) resulting from the hydration of Celitement are all very similar. As with many other reactions the formation or conversion of the individual intermediate and end products of the Celitement process is not quantitative (i.e. the conversion rate is not 100 %). So if, for example, the actual conversion rate of Celitement after an application is to be determined with a high degree of accuracy it is necessary, in principle, to distinguish between all three material groups (CSH, hCHS, C-S-H. This is a very demanding analytical task, as is shown by the many attempts at qualitative and quantitative determination just of the C-S-H phases formed during the reaction of Portland cement.

Will established brand-name products or applications that were formulated on the basis of Portland cements have to be completely redeveloped and formulated if Celitement is to be used for them?

Probably not completely redeveloped, but we do recommend that all developers should reassess the system with Celitement and not simply make a 1:1 exchange of the binding agents. This approach may be appropriate as a first exploratory trial but as soon as possible after that the attempt should be made to make truly optimal use of the special properties of this binding agent. Celitements are in fact very similar to Portland cements in many properties but there are definite differences, especially with respect to the lessons learned when optimizing the workability properties and the rheology. However, conventional cement can very easily and simply be replaced by Celitement for numerous applications, although slight adjustments to the mix formulations are sometimes necessary. Rather greater adjustments may, however, be appropriate for some other applications, e.g. in the construction chemistry sector and in plaster and mortar technology.

You state that there are different variants of Celitement. In what way do they differ? What are the differences between the various Celitement variants that can currently be produced?

Celitements can differ, in particular, in the calcium to silicon ratio (C/S). This ratio can vary between 0.5 and 2.0 and determines the properties to an important extent. Its usual value is C/S = 1.3. The Celitement variants also differ in their specific surface areas (e.g. BET), the degree of conversion to the hCHS and the nature and quantity of the impurities. Depending on how these parameters are adjusted it is possible to produce Celitements with very high early strengths that hardly exhibit any post-hardening, variants with hardening characteristics like Portland cements of the 42,5-52,5 classes or even very slow variants. In particular, the rheological workability properties can be varied over a wide range.

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