**1. Introduction**

Currently, construction, regardless of the purpose of the buildings being created, is characterized by high requirements for the quality of materials. In the construction market, the leading position is occupied by cement concrete. Significant importance today is given to improving the durability of concrete, especially cement compositions. For various operating environments, the durability of concrete is achieved by increasing the entrained air content, reducing the W/C, and increasing the strength class of concrete, as well as cement consumption, using a limited number of types of cement and normalizing their mineralogical composition [1–3].

Concrete has the potential to be damaged if it is subjected to freeze–thaw cycles. Therefore, determining how to scientifically optimize the ratio of high-efficiency concrete raw materials and accurately predict the frost resistance of concrete to improve the durability of its application is of great scientific significance. The prediction accuracy of frost resistance is highest when the concrete mix proportion factors considered are the water binder ratio, cement content, fine aggregate dosage, coarse aggregate dosage, and compound superplasticizer dosage [4–6]. Seong-Tae [7] suggested a prediction method for minimum curing time based on the hydration of the cement. According to this method, the rate of the decrease of compressive strength increases when the onset time of frost damage is faster and the water–cement ratio is higher. Authors Zeng et.al revealed that graphene oxide can improve frost resistance and compressive strength of air-entrained mortars by 18.9% and 41.9%,

**Citation:** Tolegenova, A.; Skripkiunas, G.; Rishko, L.; Akmalaiuly, K. Both Plasticizing and Air-Entraining Effect on Cement-Based Material Porosity and Durability. *Materials* **2022**, *15*, 4382. https://doi.org/10.3390/ma15134382

Academic Editor: Krzysztof Schabowicz

Received: 19 May 2022 Accepted: 15 June 2022 Published: 21 June 2022

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respectively [8]. An important technical way to improve the frost durability of concrete is using air-entraining admixtures [9]. Air void structure is the key parameter that affects the frost resistance of concrete [10]. The entrained air void system in cement mortar and concrete directly affects both the fresh-state workability and the freeze–thaw durability of concrete pavements and structures [11,12]. The author Feng Yu [13] investigated the compounding use of AF and VMA which significantly improved the workability, air void structure, and the frost resistance of concrete. According to Feng Yu's results, at 0.0055% air-entraining agent (AIR) dosage, the combination of 0.15% AF and 0.015% viscosity modifying agent (VMA) reduced the number of large bubbles in concrete by 57.96%. Afterward, the number of fine bubbles was increased by 16.55% and the spacing factor was reduced by 18.09%. A decrease in spacing factor increases freeze–thaw resistance of concrete.

Air content is an important factor for achieving proper porosity of concrete. Zheng [14] and Rodríguez [15] presented test results that show that frost resistance increases with increasing air content, which makes the space parameter decrease in the hardened concrete. Moreover, the experimental results indicate that, in air-entraining concrete, total air content is not the only factor that affects the final properties of the concrete; air void structure parameters, including void size, shape, and distribution, are key factors as well [16]. The research results of Lazniewska-Piekarczyk et al. [17] on the influence of air-entraining admixture type have proved that a greater amount of micro pores in concrete is the effect of a synthetic air-entraining admixture. The combination effect of air-entraining admixtures on freeze–thaw resistance of cement mortar was also considered in [18].

The porosity of cement mortar is one of the most important microstructural features, which manifests at different length scales [19]. Capillary pores have the biggest influence on the resistance of concrete. Gel pores and water inside them have no influence on the frost resistance of concrete. In fact, water, due to its greater density, does not freeze in those pores. Closed air pores perform the role of reserve pores and increase the frost resistance of concrete [20]. Zhang's [16] analyses concluded that as the number of freeze–thaw cycles increases, the repeated action of periodic freezing and expansion forces and hydrostatic pressure on the pores inside the concrete cause the pores inside the concrete to gradually expand, penetrate, and form connected pores. Shinichi [21] investigated pore structure in high-strength concrete at early ages via the BSE imaging technique assuming the Powers model. At a very early age, most of the capillary pores in ordinary concrete are so large that their pore size distribution was discontinuous.

Moreover, frost resistance of concrete depends both on open porosity (the amount of capillary pores) and closed porosity (air content in the mixture), and quantitatively can be determined by the frost resistance factor KF, which is derived from the equation [22]:

$$\mathbf{K\_{F}} = \frac{\mathbf{P\_{c}}}{0.09 \cdot \mathbf{P\_{o}}'} \tag{1}$$

where *Pc*—closed porosity of hardened cement mortar (air pores) and *Po*—open porosity of hardened cement mortar (capillary pores).

The authors Funk [23] and Setzer [24] performed a comprehensive testing program before, during, and after standardized freeze–thaw weathering (CDF). Freeze–thaw cycling caused considerable deteriorations which were significantly modulated by the different admixtures via changes in cement micromorphology. According to the authors of [25,26], the frost resistance of concrete is determined by its porosity because water can only penetrate open pores. Capillary pores have the greatest influence on the conglomerate's frost resistance. They are open and simple to fill with water [27]. As for the air pores, as opposed to capillary pores, they increase the conglomerate's frost resistance. During the immersion process the air pores are closed and no water enters them [28].

According to Łazniewska-Piekarczyk [29], the influence of the type of admixtures on porosity and pore size distribution of high-performance self-compacting concrete (HPSCC) at a constant level of water were analyzed whilst considering the cement coefficient, the type and volume of the aggregate, and the volume of the cement mortar. Despite the fact

that the air content parameters differed from the standard recommendations, she found that HPSCC was frost resistant. The authors [30] investigated the freeze–thaw behavior of air-entraining cement mortars saturated with a NaCl solution at a concentration of 10 wt.% using experimental measurement approaches. The findings of these studies showed that airvoid entrainment tends to decrease thermal contractions but increase hydraulic expansion, ice nucleation expansion, and residual expansion. Some scholars modified cement with polypropylene (PP) fiber, such as Ping Jiang [31], and they found that with an increase of fiber from 0.25% to 1% the porosity was increased by approximately 6% due to the random distribution intersection and interleaving of (PP) fibers in concrete. The goal of this study is to investigate the technological properties and effects of air voids and porosity parameters of cement with different water–cement (W/C) ratios under the action of freezing–thawing cycles. The freeze–thaw (CDF) (test) technique is used to determine the surface scaling of the specimens. Strength of hardened cement mortar and porosity parameters are calculated in relation to freezing–thawing factors. In previous investigations, it has been noted that an important parameter for frost resistance is a decrease in capillary porosity using plasticizers and an increase in the amount of air in concrete using an air-entraining admixture. No research studies have investigated the combined effect of plasticizing and air-entraining admixtures on the freezing–thawing resistance of hardened cement.

### **2. Materials and Methods**


Portland cement without mineral admixtures CEM I 42.5 R conforming to EN 197-1 with water consumption of 26.6% was used. The mineral composition, physical properties, and mechanical properties of the cement are presented in Table 1, Table 2, and Table 3, respectively.

**Table 1.** Mineral and chemical composition of the cement.


**Table 2.** Chemical composition of the cement.


**Table 3.** Physical and mechanical properties of cement.


The results of the particle size distribution of cement are presented in Figure 1.

**Figure 1.** Particle size distribution of cement CEM I 42.5 R.

#### 2.1.2. Aggregate and Water

Natural river sand with a fraction of 0/2 was selected as fine aggregate. The grain-size distribution for the sand is shown in Figure 2.

Potable water was used for concrete mixtures. Water confirms the requirements of EN 1008.

2.1.3. Polycarboxylate Ether Superplasticizer (PCE)

Polycarboxylate ether is used as the superplasticizer admixture to improve the workability of cement mortar. The physical and chemical properties of PCE are shown in Table 4.

**Table 4.** Physical and chemical properties of the used PCE.


2.1.4. Air-Entraining Admixture (AIR)

SikaControl-10LPSA is a synthetic surfactant based on a brownish concrete admixture in liquid form. The physical and chemical properties of SikaControl-10LPSA are shown in Table 5.


**Table 5.** Physical and chemical properties of the used SikaControl-10LPSA.

#### 2.1.5. Antifoaming Agent (AF)

The polypropylene ether was used as a component that prevents foaming of the mortar. Antifoam is a chemical agent used to reduce and prevent the formation of foams during chemical mixing. Foams will cause serious problems in chemical processes and will also prevent utilization of the whole capacity of a container. Developing conveniently and rapidly on the surface of the foam is one of the main features of antifoams [32]. The physical and chemical properties of AF (A-316) are shown in Table 6.

**Table 6.** Composition and properties of the used antifoam agent.


#### 2.1.6. Mix Proportion

The composition, consistency (determined according to EN 1015-3), and density (determined according to EN 1015-6) of fresh cement mixtures are presented in Tables 7 and 8. The first five specimens have been mixed with a polycarboxylate ether and designated as PCE; the other three specimens have been mixed with an air-entraining admixture. In superplasticizer mortars, the main difference between mixtures is the different W/C ratio, which varies from 0.50 and 0.38. When changing the W/C ratio, the amount of cement and water remained the same. In the case of mortar with an air-entraining admixture, the W/C ratio was equal to 0.4 for all specimens.

**Table 7.** Composition, consistency, and density of the mixtures with plasticizing admixtures.



**Table 8.** Composition, consistency, and density of the mixture with plasticizing and air-entraining admixtures.

A Portland cement sample (sample C0) was prepared for the reference sample. Three chemical admixtures were used to create seven composite admixtures with the same mass ratio as follows: polycarboxylate ether in solid-state (symbol PCE), Anti-foaming admixture (symbol AF), and Air entraining admixture SikaControl-10LPSA (symbol AIR).

Variations of polycarboxylate ether content by mass were 0%, 0.1%, 0.2%, 0.3%, and 0.4%, whereas variations of Air entraining admixture Sika (AIR) content were 0.1%, 0.2%, and 0.3% for total cementitious materials. The control samples without admixture were prepared.

Dosage of anti-foaming admixture was constant with all specimens (10% from superplasticizer content).

Table 7 shows that with the subsequent increase in admixture (PCE and AF) in each receipt there was a decrease in the amount of water from 100% to 77%.
