## 2 Materials and Methods
### Testing Methods  
The consistency of fresh concrete mixes was evaluated by conducting slump tests in accordance with EN 12350-2. The compressive strength of HPC was tested on 100 \(\times\) 100 \(\times\) 100 mm specimens after 2, 7, 14, 28, 56, and 90 days of curing following the EN 12390-3 standard. Compression tests were conducted on the three-sample series using a CONTROLS MC66 press. Specimens smaller than the standard 150 \(\times\) 150 \(\times\) 150 mm size were used due to the expected high concrete strengths and, consequently, high values of maximum force to be applied to samples. The modulus of elasticity was determined in compression tests on 2-year-old cylindrical specimens (d = 150 mm, h = 280 mm) according to the procedure described in EN 12390--13 Method A. Cylindrical specimens with a height of 300 mm were trimmed to 280 mm in order to obtain 2-cm-thick slice specimens for SEM/EDS testing. The compressed surface of samples was prepared by grinding. Three electronic compressometers (Figure 2) with a gauge length of 150 mm and 0.02 \(\upmu\)m sensitivity were used to measure longitudinal strains. Each test consisted of two preloading checks (wiring stability and specimen positioning check) and three loading cycles with a maximum loading equal to one-third of the estimated mix design compressive strength. The modulus of elasticity was calculated as the stabilized secant modulus of elasticity. Tests were performed on CONTROLS MC66 press on two-sample series. To estimate compressive strength, two cylindrical samples of each mix design were tested according to the EN 12390-3 procedure. Figure 2: Modulus of elasticity testing. Freeze-thaw resistance tests were carried out following the procedure presented in the Polish standard PN-B-06265 after 56 days of curing. Two six-sample series of the 100 mm concrete cubes were subjected to cyclic freezing and thawing. After saturation in the water at (+18 \(\pm\) 2) \({}^{\circ}\)C for 7 days, the first series of specimens was weighed, and then subjected to 150 cycles of freezing at (\(-\)18 \(\pm\) 2) \({}^{\circ}\)C for 4 h and thawing at (+18 \(\pm\) 2) \({}^{\circ}\)C for 2-4 h. The second series of six specimens was stored in water at a temperature of (+18 \(\pm\) 2) \({}^{\circ}\)C during the entire period of freeze-thaw resistance testing. After the last freeze-thaw cycle, the specimens from series 1 were again weighed, and their compressive strength was tested and compared to the compressive strength of the reference specimens. The test results were average mass loss and average percentage reduction in sample compressive strength after freezing and thawing. Concrete is regarded as being resistant to frost if the average mass loss is less or equal to 5% and the reduction of compressive strength is no greater than 20%. Depth of water penetration under pressure tests were conducted on two 150 mm cube samples after 56 days of curing according to EN 12390-8. After the application of water pressure to the specimen surface, the penetration depth was measured. A maximum water penetration depth of 30 mm was set as the limit value. The Scanning Electron Microscope (SEM) with Electron Dispersion Spectroscopy (EDS) method was used to evaluate the concrete microstructure and phase composition. FEI Nova NanoSEM 200 microscope equipped with EDAX Octane Elect EDS was used. The observations were made in a low vacuum mode (80 Pa), operating at 15 kV voltage using a vCD detector. Cross-sectional slice samples obtained from the upper 2 cm of the cylindrical specimens for the modulus of elasticity tests were used in the analysis. Before the SEM/EDS tests, the samples' surfaces were prepared by polishing using Struers Tegramin automatic polisher, and the samples were dried. The pH values of hardened concrete were measured after 56 days of curing on two samples for each mix design.