Can durable concretes be produced with Celitement?

Yes, extensive durability trials have already been carried out on Celitement concretes. Bearing in mind the parameters that are important for Celitement (suitable PCE superplasticizer, water/binder ratio) it is possible to achieve very good properties with respect, for example, to carbonation, freeze-thaw resistance, chloride migration and sulfate resistance or ASR. However, as with all concrete mix formulations, the durability does not just depend on the binding agent used. The term durability is also very diverse. It ranges from freeze-thaw resistance, resistance to freeze-thaw with de-icing salt, carbonation and depth of chloride penetration through sulfate resistance, alkali-silica resistance and resistance to chemical attack to wear resistance. However, if the concrete mix formulations prepared with Celitements are well thought through and adapted to suit the particular requirements then, in principle, there is no problem in producing durable concretes for practically all exposure classes. Irrefutable proof can, however, ultimately only be provided by long-term experience in structures and unfortunately this is not yet available. So far the experience of durability has been based on accelerated durability tests taken from standards and regulations that also form the basis of building authority approval tests. The evidence cannot always be provided immediately with the first test mix formulation. Just as with concretes based on Portland cement those made with Celitements also have to be optimized to meet the demands of different exposure conditions. ASR performance was tested in: “Investigation of alkali-silica reaction on mortars with alternative binder systems: Alkali activated Slags and Celitement. J.T. Sonntag et. al. Cement 13, 2023, 100078.”

Celitement forms pure C-S-H phases. Is this not a problem with respect to creep and shrinkage problems?

It is well known that the C-S-H phases are particularly responsible for creep and shrinkage and Celitement actually exhibits greater shrinkage and creep deformation than conventional cement. However, the shrinkage and creep deformation is also defined by a number of other parameters, such as porosity, grading curve of the aggregate, etc., and small differences in the shrinkage and creep deformation play only a secondary role in many applications. It is also possible to offset shrinkage and creep deformation by suitable additives. As a consequence this aspect does indeed have to be considered with the very high levels of C-S-H in pure Celitement systems but it is not a fundamental problem. With the concrete mix formulations that we investigated it was interesting that the differences from Portland cement mix formulations were not as significant as had been thought. The classical measures for reducing shrinkage can also be used successfully with Celitements.

Do concretes produced with Celitement require a special type of curing?

The principles for curing normal concretes also apply for those made with Celitement. However, the chemical water demand of Celitements is lower than with Portland cement so adjustments are needed. The well-known principles for producing qualitatively high-grade durable concretes also apply when using Celitement. To this extent it is possible to build on familiar concrete technology experience.

Is the low pH of the pore solution in Celitement not a problem for passivating the reinforcing steel?

We believe not, as the pH of the pore solution in Celitements lies between pH 11.5 and 12.8. It is therefore on average lower by a power of ten than in Portland cement but is adequate for passivating steel reinforcement. The pH value fluctuates somewhat depending on the amount of incompletely converted portlandite Ca(OH)2. It could, if desired, be raised to any required pH value by the simple addition of quicklime or alkali hydroxide. However, we don’t think that this is at all necessary. The carbonation responsible for the depassivation of reinforcing steel is a concrete technology durability factor. If a concrete is dense enough and therefore almost impermeable to gases and water then no CO2 can penetrate through to the reinforcing steel. However, extremely dense and very durable concretes can be produced with Celitements. A pure C-S-H phase is formed that is only slightly distorted by other phases with different morphologies so the pore system can become very impermeable. We can therefore see absolutely no limitations for its use in reinforced concrete. On the contrary, a low pH can definitely be a positive factor for alternative reinforcement systems. Wood- and cellulose-based reinforcement, glass fibre reinforcement and other systems that possibly could not otherwise be used because of a very high pH (>13) could perhaps even be implemented more simply with Celitements. For more details, e.g. the performance of Celitement in comparison to other alternative binder concepts in electric resistance measurement of steel reinforcement, we refer to „R. Achenbach, M. Raupach, B.I.E. Kraft, H.-M. Ludwig, Comparative study on the electric resistance of mortars made of low carbon binders, Materials Today: Proceedings, 2023,ISSN 2214-7853,” Another important Publication: “Passivation of Steel Reinforcement in Low Carbon Concrete R. Achenbach, M. Raupach, Buildings 2024, 14, 895.” With a mean value of 971 Ωm the Celitement series showed the highest resistivity of all binder concepts in the naturally water-saturated state.

Is there yet any long-term experience of the durability of building elements or structures produced with Celitement?

No, because the small amounts produced by the pilot plant are not yet sufficient for the production of complete structures. So far we have only produced test pieces for classical durability tests. These simulate the ageing of building materials in practical use. According to the tests carried out so far the concretes and mortars made with Celitement have similar durability to those made with conventional cement.

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