## 4 Conclusions  
Based on the research conducted, the following conclusions were drawn:  
* It is possible to obtain High-Performance Concrete while greatly reducing the amount of Portland cement clinker in the concrete mix (64-114 kg/m\({}^{3}\)). The designed HPCs reached compressive strength of 76.1-91.7 MPa after 28 days of curing and 88.0-99.9 MPa after 90 days of curing.
* Low-clinker eco-efficient HPCs were characterized by a very slow to slow rate of compressive strength development within the first 28 days of curing according to EN \(206\,f_{cm2}/f_{cm28}\) ratio. However, the differences between 2-day and 28-day compressive strength values were the highest (from 420% to 957%) compared with the further curing periods: 28-day-90-day (9-16%) and 90-day-730-day (0.8-1.4%).
* Commonly used compressive strength development models insufficiently described the properties of low-clinker HPC, greatly overestimating the early age compressive strength. However, the differences between the estimated and tested values decreases in the long curing period.
* None of the recommended elastic modulus development models made it possible to estimate HPC modulus of elasticity after long curing period with acceptable precision. Basic variants of the proposed models underestimated the elastic modulus, while the corrected Eurocode 2 model overestimated the value of modulus of elasticity.
* All the designed HPCs, due to incorporating high GGBFS content, SCMs and maintaining low water/binder ratio, were frost resistant after 56 days of curing without the addition of air-entraining admixtures.
* The high density and homogeneity of the HPC binder matrix have been confirmed by means of SEM microstructure analysis and tests of depth of water penetration under pressure.
* The microstructure of the HPC was dependent on the binder composition. Replacement of the 45% of cement by the GGBFS resulted in a significantly higher amount of unreacted slag grains in the microstructure. The higher content of SCMs in the binder was associated with the smaller amount of calcite crystals as an effect of carbonation at the same depth. A lot of microcracks were observed for the binder matrix with higher amounts of Portland cement clinker. Still, it was impossible to determine with certainty whether there was a relation between the amount of Portland cement clinker and the number of microcracks.
* pH value tests confirmed that the low-clinker HPCs retained their protective properties in relation to steel reinforcement.  
Conceptualization, W.J.[PERSON] and K.K.; methodology, K.K., W.J.[PERSON] and K.C.; software, K.K. and K.C.; validation, W.J.[PERSON] and K.C.; formal analysis, W.J.-R.; investigation, K.K., W.J.[PERSON] and K.C.; resources, W.J.-R.; data curation, K.K. and K.C.; writing--original draft preparation, K.K.; writing--review and editing, K.C. and W.J.-R.; visualization, K.K. and K.C.; supervision, W.J.-R.; project administration, W.J.-R.; funding acquisition, W.J.-R. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding. The APC was funded by the Strategic Research Project of the Warsaw University of Technology: \"Circular Economy\".  
Not applicable.  
**Acknowledgments:** The authors would like to thank Gorazdze Cement S.A. for providing CEM III/B and GGBFS samples and Warbud-Beton Sp. z o.o. for providing SFA and SF samples as well as for the technical support.  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.