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CONCRETE TECHNOLOGY36 CPI %u2013 Concrete Plant International %u2013 5 | 2025 www.cpi-worldwide.comFor the development of CO2-optimized and resource-efficient concretes, it will become increasingly important to ensure the greatest possible reduction in clinker content by reducing the water content as much as possible, while maintaining adequate fresh concrete workability. For the design of such highly optimized concrete mixes, it is crucial to consider the quality of the individual constituent materials %u2013 cement and aggregates %u2013 with respect to water and superplasticizer demand. This paper investigates the influence of different granulometric properties of fine aggregates on the workability and water demand of fresh concrete, based on systematic workability studies. Based on the findings of this study, a concept for estimating the absolute minimum water content for a specific concrete mix is introduced, taking into account the various constituent materials and plasticizing admixtures.1 IntroductionIn modern concrete technology, there have been significant changes in the requirements for individual concrete mixes in recent years. With the growing number of possible additives and admixtures, concrete compositions are becoming increasingly complex. For economic reasons, the binder content in concrete, and thus the paste volume, has in some cases been significantly reduced. Ecological considerations, particularly in the design of CO2-optimized concretes, also increasingly require concrete mix compositions with the highest possible clinker reduction. Up to now, this has primarily been achieved by replacing Portland cement clinker with reactive or inert supplementary cementitious materials (SCMs) [1, 2]. Such SCMs, including ground granulated blast furnace slag, fly ash, or limestone powder, have been used in practice for decades to produce clinker-reduced concretes. Due to the lower reactivity, or even inert behaviour, of such SCMs, particularly limestone powder, reducing the clinker content to maintain key mechanical and especially durability-relevant properties is usually linked to a reduction in water content or a reduction in the water-to-cement ratio (w/c ratio) [3 - 5]. Studies by Palm et al. [4], for example, show that the carbonation resistance of such concretes with ternary cements (clinker factor ~0.5), using slag and a high proportion of limestone powder as main constituents in addition to clinker, can be significantly improved by reducing the w/c ratio from 0.50 to 0.35. Accordingly, the feasible replacement ratio depends both on the required performance of the concrete and, in particular, on the potential for water reduction while maintaining adequate workability of the fresh concrete. In this context, plasticizing admixtures play a decisive role in achieving workability, as they enable clinker reduction alongside water reduction without sacrificing strength. However, to ensure the stability of the fresh concrete and avoid segregation, it is necessary to retain a sufficiently high paste content [6].The workability of fresh concrete is determined both by the properties and the volume of the paste. A reduction in water content inherently leads to a reduction in paste volume. Therefore, particularly in concrete with low water contents (low-water concretes), it is crucial that the mixing water is fully available for dispersing and fluidizing the fresh concrete. Initial practical experience shows that it is possible to reduce the water content to values as low as 130 l/m%u00b3 [7]. When designing such concretes in the future, special attention must be given to the quality of the individual constituent materials (e.g., cement and aggregate) with regard to their water and superplasticizer demand. The reduction in the water content places extreme demands on the dispersing performance of the superplasticizer, so that the desired consistency and workability can be achieved.Concrete with reduced water contents %u2013 mix design principles considering the aggregate propertiesPaste content optimizationn Tobias Schack, Institute for Construction Materials, Leibniz Universit%u00e4t Hannover & Haist Schack Strybny Betoningenieure GmbH, GermanyDipl.-Ing. Sebastian Dittmar, Master Builders Solutions Deutschland GmbH, GermanyBastian Strybny, M.Sc., Institut f%u00fcr Baustoffe / Leibniz Universit%u00e4t Hannover & Haist Schack Strybny Betoningenieure GmbH, GermanyDipl.-Ing. Klaus-Dieter Kallweit, Holcim (Deutschland) GmbH, GermanyMichael Haist, Institute for Construction Materials, Leibniz Universit%u00e4t Hannover, Germany