*2.3. Samples Preparation*

The samples were prepared by manually dispensing the dry ingredients and mixing the whole using an automatic mixer at a constant mixing time and speed. The whole dry mix was then poured into water and mixed with water according to the procedure in the standard [2]. The first mixing with water was done using an automatic mixer for 30 s. After this time, the mortar was mixed by hand for about one minute and again in the automatic mixer for one minute. After waiting for a mortar maturation time of 5 min for all samples, the mortar was mixed again in the mixer for 15 s. The consistency of the mortar was measured on a shaking table, 15 strokes per 15 s. Two perpendicular diameters of the cone spread were measured and the average of the two measurements expressed in mm was calculated.

The spread of the reference sample - without additives—was taken as the reference consistency. The amount of superplasticiser in subsequent samples depended on the consistency of the mortar—a spread of up to 10 mm more or less than that of the reference sample

was considered acceptable. Once the amount of superplasticiser had been established, the whole mixture was mixed again in an identical manner. Once mixing with water was complete, samples were prepared for compressive and flexural strength measurements in the shape of a 40 mm × 40 mm × 160 mm cuboid according to [1]. After 28 days of seasoning under conditions according to [1], tests were performed to measure the bending tensile strength Rg and compressive strength Rc.

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

Flexural and compressive strength tests of the seasoned specimens were carried out using a laboratory testing machine according to the guidelines in the standard [1]. Three bending strength measurements and five compression strength measurements were made for each modification. The averaged results obtained are presented graphically in Figures 3–5.

**Figure 3.** (**a**) Dependence of average flexural tensile strength Rg, and (**b**) compressive strength Rc on the simultaneous change of cement and fly ash.

**Figure 4.** (**a**) Dependence of mean flexural tensile strength, Rg, and (**b**) compressive strength, Rc, on the simultaneous change of cement and limestone powder.

**Figure 5.** (**a**) Dependence of mean flexural tensile strength, Rg, and (**b**) compressive strength, Rc on the simultaneous change of cement and granulated blast furnace slag.

Figure 3a shows the effect of the simultaneous change of cement and fly ash on the bending tensile strength, Rg. By analysing the result, in the analysed range of changes, the addition of the silica fly ash while reducing the proportion of cement reduces the strength, Rg. However, this change is small and did not exceed 5% when replacing cement with fly ash at 10%. When analysing the effect of these changes on the strength of Rc, it was found that the gradual substitution of cement with silica fly ash improves the strength of Rc. The observed trend of change can be explained by the properties of silica fly ash. Silica fly ash is classified as a type II additive. It consists mainly of fine spherical vitrified grains obtained from the combustion of coal dust, with pozzolanic properties, containing mainly SiO2, Al2O3. The results obtained are consistent with those of Giergiczny in [5], as well as those of Rudkowska et al. [14].

The main substitutions that the use of fly ash entails are a change in water requirements and workability. In the case studied, the replacement of cement with silica fly ash was associated with the need to use a plasticizer to maintain a constant consistency (Figure 5a).

Figure 4a shows the effect of the simultaneous change of cement and limestone powder on the flexural tensile strength, Rg. By analysing the result, it can be seen that, in the range of changes studied, replacing the cement with limestone powder reduces the strength of Rg. However, this change is small and did not exceed 5% when replacing cement with limestone powder at 10%. A similar trend was observed when analysing the effect of these changes on the strength of Rc. Again, the gradual replacement of cement with limestone decreased the Rc strength. The decrease in Rc strength in this case was proportional to the amount of limestone powder and was approximately 10% when replacing the cement at 10% with limestone powder.

In the case studied, the replacement of cement with limestone powder was associated with the need to use a plasticizer in order to maintain a constant consistency (Figure 6b).

Figure 5a shows the effect of the simultaneous change of cement and granulated blast furnace slag on the bending tensile strength, Rg. By analysing the result, it can be concluded that, in the analysed range of changes, the replacement of cement with granulated blast furnace slag reduces the strength Rg. When replacing cement with 10% blast furnace slag, the change does not exceed 10%. When analysing the effect of replacing cement with blast furnace slag on Rc, it was found to be uneven. At slag amounts up to 6%, the strength increases slightly. However, a 6% substitution appears to be a limiting amount, as further increases in the proportion of slag in the binder composition are associated with a decrease in Rc strength. At a slag proportion of 10%, this decrease does not exceed 5%. It can therefore be assumed that an increase in the proportion of this additive in the

binder slightly alters the strength properties of the mortars. Granulated blast furnace slag is a type II concrete additive alongside silica fly ash and silica dust. Granulated blast furnace slag is classified as a material with latent hydraulic properties and is therefore a basic component of multi-component cements. The similar compressive strength values of mortars containing up to 10% slag in the binder composition are also related to its specific surface area. In the case analysed, it was 3700 cm2/g and was similar to the specific surface area of cement. The results obtained are consistent with those obtained in [12].

**Figure 6.** The proportion of plasticizer as a percentage of cement to maintain consistency in mortars modified with (**a**) silica fly ash and (**b**) limestone powder.

In the analyzed case, in order to maintain a constant consistency, replacing cement with ground blast furnace slag did not require the use of a plasticizer in order to maintain a constant consistency. This was caused by the similar value of specific surfaces of cement and slag.
