**2. Materials and Methods**

The main studies were performed using (EN 197-1):

Composite CEM V/A with a fly ash content of 27.4%, blast furnace slag—18.3%, clinker—45.7%, and gypsum—8.6%;

Portland cement clinker with mineralogical composition was used in the research: C3S—63.95%; C2S—15.15%; C3A—7.42%, C4AF—12.48%.

Fly ash and blast furnace granulated slag have the following chemical composition:


The Samples of composite cement for research were obtained by joint grinding of the components in the laboratory ball mill. To regulate the properties of the composite cement and the concretes made with the use thereof, the influence of naphthalene formaldehyde (SP-1) and polycarboxylate (Sika VS 225) superplasticizers [17] and grinding intensifierpropylene glycol (PG) [18] was used.

For mineral additives, the pozzolanic activity was determined by the CaO uptake method [19,20]. Additives (ash, slag, and their mixture) before the test were milled to the required specific surface area (Sssa) (Table 1).


**Table 1.** Pozzolanic activity of fly ash and ash of slag composition.

For the obtained composite cements, the grain composition (Table 2) was determined by the sedimentation method [21]. Also, the normal consistency, the change in the degree of hydration of the cement paste [22], and the standard strength of cement-sand samples over time were determined (Table 3) [23].

**Table 2.** Composite cements grain composition.




To analyze the influence of the composition of binders and concrete, as well as modifier additives under normal conditions of hardening and when subjected to heat treatment, experimental-statistical models were obtained using the method of mathematical planning [24–26]. When studying concrete mixtures and concretes, the three-level B4 plan was implemented [26]. According to plan B4, 24 series of experiments were performed. In each series, the water demand was determined until the specified slump was reached, and 6 concrete cubes 100 × 100 × 100 mm were made to determine the compressive strength [27]. To obtain concrete mixtures, quartz sand with fineness modulus Mf = 1.95

and crushed stone with Dmax = 20 mm were used. Samples were tested after 1 and 28 days. The conditions for planning the experiments are given in Table 4. After the implementation and statistical processing of experiments, mathematical models of the water consumption and the concrete compressive strength were obtained.

**Table 4.** Conditions for planning experiments when obtaining models of the concrete mixture water consumption and the concrete at the CC strength.


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

Experiments on laboratory ground cement showed that the pozzolanic activity of the ash-slag composition in the cement most significantly depends on the fineness of the grind, characterized by the specific surface area (Sssa), and, to a lesser extent, on the ash:slag ratio (Table 1). However, decreasing the ratio of ash to slag significantly affects the kinetics of CaO bonding, increasing the amount of bound CaO, especially in the period from 7 to 28 days.

Achieving an increased composite cement's specific surface area while minimizing energy consumption is possible with the use of grinding intensifier additives [28,29]. The propylene glycol (PG) has become widespread as such an additive. Along with the propylene glycol, the addition of superplasticizers and their compositions with PG have a certain influence on the grinding kinetics and grain composition of cement. The experiment results are shown in Figure 1 and Table 2.

**Figure 1.** The influence of additives on the specific surface area of CC at different durations of grinding. 1—PG—0.04%, Sika VC 225—0.5%; 2—PG—0.04%; 3—PG—0.02%, Sika VC 225—0.5%; 4— PG—0.02%; 5—SP-1—0.5%, PG—0.04%; 6—SP-1—0.5%; 7—Sika VC 225—0.5%; 8—without additives.

The addition of PG practically doubled the content of the cement's finest fraction. The composite additive PG + Sika VC 225 provides a grain composition of cement that is slightly different from the grain composition of cement with a single PG additive. The naphthalene-formaldehyde superplasticizer introduction into the cement also to some extent intensifies the grinding of the cement. This effect is significantly lower than the propylene glycol effect.

Enrichment of composite cement during grinding with the finest fractions significantly accelerates its hydration. This is evidenced by the results of experiments on determining the content of hydrated water in samples of cement stone with *W/C* = 0.3 at different durations of their hardening (Figure 2).

The hydration kinetics of composite cements ground with PG additives, superplasticizers SP-1 and Sika VC 225 can be approximated by a general equation [18]:

$$
\mathfrak{a} = k \lg \mathfrak{r} + B \tag{1}
$$

where *τ*—hydration duration; *k* i *B*—coefficients depending on the additives type and grain composition of cements.

Complex additives, including grinding intensifier (PG) and superplasticizers SP-1 and Sika VC 225 can be considered polyfunctional cement modifiers (PFM):


The composite cements' main properties values, which were obtained by grinding in a laboratory ball mill, are given in Table 3. It follows from them that the strength of composite cement with a clinker content of 50% when adding PFM1 and Sssa = 450 m2/kg reaches 60 at the age of 28 days, Sssa = 550 m2/kg—70. When PFM2 is introduced, it is 55 and 60 MPa, respectively. At the same time, at the 3 days, the strength of cement with PFM1 reaches 50% of the 28-day strength.

The study of the concrete mixtures and concrete properties based on modified composite cement with the addition of PFM1 using mathematical planning of experiments (Table 4) made it possible to obtain mathematical models of the concrete mixture water demand (*W*) and the concrete compressive strength at the 1 (*f* <sup>1</sup> cm) and 28 days (*f* 28cm):

$$\begin{aligned} W &= 142.1 - 22X\_1 + 4.17X\_2 - 10.1X\_3 + 17.1X\_4 + 6X\_1^2 + 1.5X\_2^2 + 6.6X\_3^2 - 1.5X\_4^2 + 0.6X\_1X\_2 - 0.6X\_1X\_3 + 1.3X\_2X\_4 \\ &+ 1.3X\_2X\_3 - 0.3X\_2X\_4 + 0.4X\_3X\_4 \end{aligned} \tag{2}$$

$$f\_{\rm cm}^{\prime} = 33.1 + 1.3X\_1 + 8.9X\_2 - 7.5X\_3 - 1.2X\_1^2 - 4.1X\_2^2 + 2.8X\_3^2 + 0.3X\_1X\_2 - 0.3X\_1X\_3 - 2.5X\_2X\_3 \tag{3}$$

$$f\_{cm}^{28} = 70.9 - 0.3X\_1 + 9.8X\_2 - 13.8X\_3 - 2.5X\_1^2 - 4.28X\_2^2 + 3.78X\_3^2 + 3.9X\_2X\_3 \tag{4}$$

For the concrete compressive strength models, the influence of concrete mixture slump, provided that other varied factors are constant, was statistically insignificant. Factors can be arranged in the following sequence according to the decreasing influence on the studied properties:

$$\begin{aligned} W: \mathcal{X}\_1 &> \mathcal{X}\_4 > \mathcal{X}\_3 > \mathcal{X}\_2 \\ f'\_{\text{cm}}: \mathcal{X}\_2 &> \mathcal{X}\_3 > \mathcal{X}\_1 \\ f\_{\text{cm}}^{28}: \mathcal{X}\_3 &> \mathcal{X}\_2 > \mathcal{X}\_1 \end{aligned}$$

When analyzing models of water consumption of a concrete mixture (Figures 3 and 4), attention is drawn to the practically identical values of water consumption of a concrete mixture in the interval *W/C* = 0.45 − 0.35 and its significant increase at *W/C* < 0.35. this confirms the well-known [1] rule of water consumption constancy at *W/C* < *W/Ccritical*. At the same time, the results of the study show that the polycarboxylate superplasticizers addition makes it possible to shift the critical value *W/Ccritical* towards lower values. For concrete without superplasticizers, it is usually in the range of 0.43–0.4 [28]. The water demand model also reflects the increase in the concrete mixture water demand as the specific surface area of the cement increases.

**Figure 3.** The influence of *W/C* and PFM1 additives on the concrete mixture water consumption (Sssa = 450 m2/kg, Sl = 13 cm).

**Figure 4.** The influence of *W/C* and the specific surface area of the CC on the concrete mixture water consumption (PFM1 = 0.7%, Sl = 13 cm).

Analysis of the strength mathematical models (Figures 5–7) shows the unconditional leading value of the factors W/C and Sssa. The influence of the specific surface area of the CC increases significantly for the one-day strength of the concrete. With increased values of Spyt, the effect of increasing W/C on the early strength of concrete becomes less significant (Figure 5).

**Figure 5.** The influence of *W/C* and specific surface area on the concrete compressive strength at the 1 day (PFM1 = 0.7%).

**Figure 6.** The influence of *W/C* and PFM1 content on the concrete compressive strength at the 1 day (Sssa = 450 m2/kg).

**Figure 7.** The influence of *W/C* and specific surface area on the concrete compressive strength at 28 days (PFM1 = 0.7).

The models make it possible to design compositions of the concrete mixture on the CC with the PFM, specified workability, and strength.
