*2.2. Composition of Cement Mixtures and Measured Properties*

In the study, three cement mixtures were prepared, and their compositions are shown in Table 2. The proportions between the components were selected to achieve the compositions of the cement types CEM II/C-M (S-V) and CEM VI (S-V) included in the PN-EN 197-5 standard [11]. Measurements included the period of development of standard strength (up to 28 days) and concerned the compressive strength, linear change, and phase composition of the cement mixture specimens. Evaluation of the microstructure of the cement mortars was carried out by SEM.

**Figure 1.** XRD patterns of PC, GBFS and SFA.

**Table 2.** Compositions of the cement mixtures studied.


Compressive strength tests of the cement mortars were carried out on prismatic samples of 40 mm height, 40 mm width, and 160 mm length after 2, 7, and 28 days according to the procedure described in the PN-EN 196-1 standard [12].

Linear change measurements were performed on cement paste samples prepared with a water-to-cement ratio of 0.33. Cement pastes were molded into prismatic samples of 25 mm height, 25 mm width, and 100 mm length. Molded samples were stored for 24 h in a high-moisture atmosphere and at a temperature of 20 ◦C. Samples were then removed from the molds and stored under water at a temperature of 20 ◦C until testing. Linear changes in cement paste prisms were investigated in the Grauf–Kaufman apparatus every day for up to 7 days.

The identification of the phase composition of the cement paste samples prepared with a water-to-cement ratio of 0.33 was determined using the X-ray diffraction (XRD) technique. A Philips X'Pert Pro MD diffractometer (Cu Kα1 line monochromatized with a Ge(111) monochromator) was used. The standard Bragg–Brentano geometry with a θ-2θ setup was applied (0.008◦ step size and 5–90◦2θ range). Studies were performed for cement pastes 60PC-20GBFS-15SFA and 40PC-40GBFS-15SFA after 2, 7, 14, and 28 days.

The microstructure of the cement paste was observed with the FEI Nova NanoSEM 200 scanning electron microscope equipped with an EDS microanalyzer. Polished crosssections of pastes were covered with a thin layer of carbon to avoid charging. Studies were made for the cement paste 60PC-20GBFS-15SFA after 2 and 28 days.

#### **3. Results and Discussion**

#### *3.1. Compressive Strength of the Cement Mortar Samples*

The compressive strength measurements were performed for all cement mixtures studied. Results are given in Table 3.


**Table 3.** Compressive strength of the cement mortar samples.

According to Table 3, after 2 days the strength of the 60PC-20GBFS-15SFA mixture was 13.73 MPa and achieved a minimum of 10 MPa as required by the PN-EN 197-1 standard [4]. After the same time period the 50PC-30GBFS-15SFA mixture represented a strength of 9.85 MPa, a drop of 28% compared to the 60PC-20GBFS-15SFA mixture. In the case of the 40PC-40GBFS-15SFA mixture, the strength was only 6.34 MPa and about two times lower compared to the 60PC-20GBFS-15SFA mixture. The factor determining the drop in the early strength of the 50PC-30GBFS-15SFA and 40PC-40GBFS-15SFA mixtures is lower content of CEM I 42.5R in the mixtures, forcing a lower content of alite (C3S). The lower content of C3S gives a lower increase in the calcium silicate hydrate phase (C-S-H) after 2 days of hydration, especially in the case of the 40PC-40GBFS-15SFA mixture.

From Table 3, after 7 days, the strength of the 50PC-30GBFS-15SFA and 40PC-40GBFS-15SFA mixtures was still lower than that of the 60PC-20GBFS-15SFA mixture, but the difference between the strength of these cement mixtures was less. After 7 days the strength of the 40PC-40GBFS-15SFA mortar was 23.49 MPa, while the strength of the 50PC-30GBFS-15SFA and 60PC-20GBFS-15SFA mixtures was 19.75 MPa and 17.69 MPa, respectively. The high increase in the strength of the 40PC-40GBFS-15SFA mortar in the period from 2 to 7 days, two and half times compared to the strength after 2 days, results from the highest content of granulated blast furnace slag of laten hydraulic properties in this mixture. After 7 days the strength of the 40PC-40GBFS-15SFA mortar met the required 16 MPa according to the PN-EN 197-1 standard [4].

As is shown in Table 3, after 28 days, the strength of the 60PC-20GBFS-15SFA, 50PC-30GBFS-15SFA, and 40PC-40GBFS-15SFA mortars was 41.73 MPa, 40.13 MPa, and 37.68 MPa, respectively, and achieved a minimum of 32.5 MPa as required by the PN-EN 197-1 standard [4]. After 28 days the drop in strength of the 40PC-40GBFS-15SFA mixture was only 4% compared to the 60PC-20GBFS-15SFA mixture.

According to requirements of the PN-EN 197-1 standard [4], the 60PC-20GBFS-15SFA and 50PC-30GBFS-15SFA mixtures can be classified as CEM II/C-M (S-V) cements the 32.5R and 32.5N strength classes, respectively, while the 40PC-40GBFS-15SFA mixture corresponds to CEM VI (S-V) cement of the 32.5N strength class.
