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CONCRETE TECHNOLOGY42 CPI %u2013 Concrete Plant International %u2013 4 | 2025 www.cpi-worldwide.comIn the civil infrastructure sector, and particularly in tunnel construction, enormous quantities of materials are consumed due to complex soil-structure interactions and high durability requirements for the structures. The production of these materials is associated with significant CO2 emissions and resource consumption. Standard structural tunnel systems typically consist of concrete segments, known as tubbings, which are arranged into rings to form the tunnel lining. Currently, tubbings used in tunnel construction are generally produced using Portland cement, which results in high manufacturing-related CO2 emissions of approximately 700-1000 kg CO2 per m%u00b3 of concrete. This is largely due to the stringent requirements for early strength development in these precast elements. Achieving a compressive strength of 15 MPa within 8 hours is the standard in the production of tubbings.To increase early strength, various measures are available in practice, such as raising the fresh concrete temperature during hardening [1], increasing the fineness of the cement and thus the surface area of the reactive components to accelerate the hydration reaction [2], or promoting C-S-H nucleation by adding finely ground limestone powder [3, 4] or calcium-silicate-hydrate (C-S-H) nuclei [5].Increasing the fineness of the reactive binder components through extended grinding offers great potential for enhancing early strength development [6, 7]. In practical applications, even in the production of precast concrete using pure Portland cement, an increase in cement fineness is often pursued, for example by using CEM I 52.5 R. For Portland composite cements, especially those with ternary binder compositions (e.g., clinker, ground granulated blast furnace slag and limestone powder), the complex interactions between the individual components must be taken into account [8]. It was shown in [9] that the fineness of the individual binder components significantly influences the hydration kinetics and the resulting mechanical properties of the concrete. Systematic investigations demonstrated that the hydration of ultrafine slag (d50 < 2.0 %u03bcm) is 67% faster than that of conventionalslag (d50 < 10.0 %u03bcm) [7]. As the fineness of the slag increases,the phase composition changes and a more efficient microstructure is formed, with a greater volume of C-S-H phases [9]. Due to the increased fineness of the other reactive binder components, clinker can be replaced with alternative constituent materials such as limestone powder. For example, using very fine slag (d50 < 5.2 %u03bcm), the clinker content in standard structural concretes can be reduced to values as low as 30 wt.-% while maintaining a high limestone powder content (40 wt.-%) at a water-to-binder (w/b) ratio of 0.45 [10]. In addition to improved mechanical properties, concretes produced on this basis also show enhanced durability characteristics. However, it must be noted that the energy demand and associated costs of the grinding process, and thus the binder, rise significantly with increasing fineness of the constituent materials [11]. Therefore, both the ecological and economic boundary conditions must always be considered in conjunction with the technical performance of such highly substituted binder systems.Numerous studies in the literature also show that C-S-H nucleation can significantly enhance the hydration and early strength of concretes made with Portland cement [12, 13], as well as Portland composite cements with lower clinker content [14]. C-S-H nuclei promote the formation of new hydration products by increasing both the nucleation and precipitation of C-S-H. Since C-S-H is the primary hydration product responsible for strength development in concrete, accelerating C-S-H formation also improves strength development.Concrete technology optimisation approaches for resource-efficient and CO2-reduced tunnel construction High early strength despite low ecological footprint n Dr. Tobias Schack, Institute of Building Materials, Leibniz University Hanover, GermanyDr. Oliver Mazanec, Master Builders Solutions Deutschland GmbH, GermanyStefan Schubert, Dyckerhoff GmbH, GermanyNicolai Klein, Master Builders Solutions Deutschland GmbH, GermanyIngo Helbig, TPA GmbH, GermanyDr. Max Coenen, Institute of Building Materials, Leibniz University Hanover, GermanyDr. Peter-Michael Mayer, Ed. Z%u00fcblin AG, GermanyProf. Dr. Michael Haist, Institute of Building Materials, Leibniz University Hanover, Germany