## 1 Introduction
[PERSON] and [PERSON] [29] studied, inter alia, the effect of 0.15-0.30% volume content of PP fibres on the rheological and mechanical properties of SCC. Compressive strength decreased with increasing fibre content in all SCC mixtures. The highest compressive strengths for PP fibre reinforced concrete were achieved with a 0.15% fibre fraction. A noteworthy improvement in the flexural strength of all SCCs was obtained with the increase in fibre content, compared to plain concrete. [PERSON] and [PERSON] et al. [30] reported that 0.3% by volume of PP fibres increased the flexural strength of concrete by approximately 11.5%. [PERSON] et al. [31] studied the strength of self-compacting lightweight concrete with the addition of PP fibre and found that the use of 0.1%, 0.2% and 0.3% PP fibres provided flexural strength increases of 5%, 8.5% and 10.5 % respectively compared to the reference concrete. [PERSON] et al. [32] established that the hybrid effect of PP and polyolefin fibres improved compressive and tensile strength and reduced corrosion, but their excessive content can already cause the opposite effect. [PERSON] et al. [33] studied the mechanical properties of HPC reinforced with polypropylene fibres. Test results indicated that, the strength of polypropylene fibres reinforced HPC increased with the increase of the fibre volume fraction. However, the compressive strength increased slightly, while the splitting tensile strength and flexural strength were improved significantly. The research also showed that the chemical adhesion between the PP fibre and the matrix is low, which may be influenced, among others, by the smooth surface of this fibre [34]. In addition, a water film forms at the interface between the fibre and the concrete matrix, which has a negative impact on the bond strength, because portlandite crystals increase their volume in the water environment and consequently increase the porosity in the contact zones [35]. In order to use the fibre strength and improve the overall properties of the composite, it is necessary to strengthen the interfacial bond strength of the PP fibres to the HPC matrix. [PERSON] and [PERSON] [36] found that silica fume added to the RPC matrix significantly improves the bonding of the steel fibre to the matrix due to the interfacial hardening effect during fibre sliding. [PERSON] et al. [37] established that GGBFS can be used as an alternative silica source in RPC. [PERSON] and [PERSON] [1] stated that cement replacement with the optimum value of 35% GGBS gave the highest 28 day compressive strength in the dosing range from 10 to 80%. On the other hand, [PERSON] [38] reported that the optimum cement replacement level is about 50% and sometimes as high as 70% and 80%. Therefore, it is possible that the physical and chemical interactions of the GGBS particles may be effective in improving this adhesion. Many studies have shown that GGBS and PP fibres can be advantageously used in concrete applications. To the best of the author's knowledge, there are no research reports on the hybrid influence of PP fibres and steel industry by-products, i.e., GGBS, on the rheological and mechanical characteristics of high-performance self-compacting concrete (HPSCC). In this study, PP fibres were added to HPSCC to control the microcracks development, and GGBS was added to enhance the interfacial bond strength between fibres and the matrix and modify its microstructure. Firstly, the filling and passing ability of GGBS-modified, fresh, PP-fibre-reinforced HPSCC was studied by slump flow and L-box tests. Secondly, the impact of PP fibre content on compressive, tensile splitting, and flexural strength in HPSCC was investigated by using strength tests. Thirdly, the strength models are proposed to predict compressive, splitting tensile and flexural strength based on experimental results. The objective of this research was to evaluate fibre content, rheological characteristics, mechanical properties and strength models of HPSCC for construction applications.