## 1 Introduction  
According to the PBL Netherlands Environmental Assessment Agency report [1], Portland cement clinker production is one of the key drivers of global CO2 emissions. Production of Ordinary Portland Cement (OPC), which consists of more than 95% Portland cement clinker, exceeded a level of 4 billion metric tons in 2019 and is expected to increase by 25% over in the next 10 years [2, 3]. Since the production of 1 ton of OPC generates nearly 900 kg of CO2 [4], the global trend is to reduce the amount of OPC in concrete mixes by using binary cements, as well as partially replacing it with supplementary cementitious materials (SCMs) such as Ground Granulated Blast Furnace Slag (GGBPS), Siliceous Fly Ash (SFA) and Silica Fume (SF) [5, 6, 7]. Slag cement CEM III/B (as per EN 197-1) is characterized by an embodied CO2 value of 232-359 kg CO2e/t, which is more than two times lower than OPC. For GGBPS and fly ash, these values are significantly lower: 79.6 kg CO2e/t and 0.1 kg CO2e/t, respectively [4]. The environmental impact of OPC production is an issue that cannot be ignored, especially in the design of High-Performance Concretes (HPC), which are not only characterized by high compressive strength (from 50-60 MPa to 100-120 MPa at 28 days) as a result of low water/binder ratio, but also by the high modulus of elasticity, high density and enhanced durability in comparison to normal strength concretes [8]. Based on the literature review, it can be stated that the above-mentioned criteria can be met using binary cements with a high GGBPS amount (e.g., CEM III/B). Thanks to the latent hydraulic properties of GGBFS, its activation in the presence of cement clinker occurs at an appropriate concentration of Ca\({}^{2+}\), OH\({}^{-}\), SO\({}_{4}\)\({}^{2-}\) ions and alkali in the solution. As reported by [PERSON] et al. [9], GGBFS C-S-H hydrates add density to the cement paste, as they are more gel-like than the OPC hydration products. The incorporation of GGBFS in normal, high-strength and ultra-high-strength concretes results in the improvement in the pore structure and durability properties of the composite, such as increased electrical resistivity and reduced chloride penetration, as stated by many researchers [10; 11; 12; 13; 14]. As reported by [PERSON] [15] and [PERSON] [16], in HPC design, particular attention should be paid to the qualitative and quantitative selection of SCMs due to the high sensitivity of durability to changes in mineral additives content and water/binder ratio. The use of GGBFS in low emission HPCs can also be potentially beneficial in terms of an effective reduction of the water/binder ratio. As reported in the review article of [PERSON] et al. [5], CEM III/B with 72% content of GGBFS presented higher sensitivity to polycarbonate-based superplasticizer dosage than OPC. These observations encouraged researchers to design low-clinker and high-GGBFS HPCs with the use of highly effective superplasticizers. However, the effectiveness of GGBFS to reduce superplasticizer demand is considered to decrease with an increase of GGBFS fineness and/or at low water/binder ratios (<0.30) [5]. As reported by [PERSON] et al. [12], the use of Ultra-Fine GGBFS (UFGGBFS) improves the mechanical and durability properties of High Strength Concretes (HSC). The researchers observed that the 30% replacement of OPC with UFGGBFS made it possible to increase the HSC compressive strength at a very early age and achieve a higher modulus of elasticity after 28 days of curing compared to mixes with 100% OPC. Moreover, the effectiveness of UFGGBFS was greater for mixtures with a lower water/cemetitious materials ratio. However, HPCs with a low water/binder ratio are especially prone to cracking due to the autogenous shrinkage under restrained conditions. This could be prevented by incorporating fibers into the cement matrix, i.e., polymer or steel fibers. The use of fibers significantly enhances the tensile strength and ductility of the cement-based materials and hence improves their durability [17].