Frost-Resistant Rapid Hardening Concretes

This article presents the results of research conducted to determine the possibility of accelerating strength gain and enhancing the operational reliability of concrete. The study tested the effects of modern modifiers on concrete in order to select the composition of rapid hardening concrete (RHC) with better frost resistance characteristics. A basic composition of RHC grade C 25/30 was developed using traditional concrete calculations. Based on the analysis of previous studies by other authors, two basic modifiers (microsilica and calcium chloride (CaCl2)) and a chemical additive (a hyperplasticizer based on polycarboxylate esters) were selected. Then, a working hypothesis was adopted to find optimal and effective combinations of these components in the concrete composition. In the course of experiments, the most effective combination of additives for obtaining the best RHC composition was deduced by modeling the average strength values of samples in the early ages of curing. Further, RHC samples were tested for frost resistance in an aggressive environment at the ages of 3, 7, 28, 90, and 180 days to determine operational reliability and durability. The test results showed a real possibility of accelerating concrete hardening by 50% at the age of 2 days and achieving up to 25% strength gain by using both microsilica and calcium chloride (CaCl2). The best frost resistance indicators were observed in the RHC compositions with microsilica replacing part of the cement. The frost resistance indicators also improved with an increase in the amount of microsilica.


Introduction
Modern construction requires accelerated curing of concrete, especially in monolithic construction [1].
In monolithic house-building, the main economic indicator is the reduction of time needed for the construction of one floor to 3 days. This technical requirement is dictated by commercial necessity since the rapid attainment of the design strength of monolithic structures is necessary to increase the turnover of molds and formwork, make more efficient use of equipment, and enhance the productivity of construction activities. Such fast construction pace can be achieved by using concrete with accelerated strength gain, commonly known as rapid hardening concrete (RHC) [2,3]. The key advantage of RHC is its high strength in the early ages of curing, usually within the first few days. This type of concrete is suitable for situations that require the installation of a sturdy structure in a short time [4].
To accelerate the hydration of clinker minerals and intensify the concrete curing process, several technological approaches can be used. At low water-cement (W/C) ratios, there is an oversaturation of the aqueous medium by the products of hydration and hydrolysis, i.e., cement minerals, which causes accelerated strength gain [5]. The most effective strength gain occurs in well-compacted mixtures with low W/C ratios, where all the processes occur in thin layers of cement dough. Active mineral additives, such as microsilica, significantly affect the physical and technical properties of concrete, leading to improved characteristics and more efficient use of the chemical energy of clinker by creating additional centers of crystallization [6,7]. The chemical activity of these pozzolanic-based additives affects cement hydration processes (Ca(OH) 2 + SiO 2 + mH 2 O = CaO*SiO 2 *nH 2 O) [8,9].
To obtain high-strength RHC, the pore size can be reduced by adding ultradispersed active silica, a waste product of ferrosilicon production [10]. The use of polycarboxylate-based hyperplasticizer additives is also of interest in terms of accelerating concrete hardening. To ensure rapid hardening, an additive based on polycarboxylates should be used, as its molecules leave part of the cement surface vacant, providing free access to water by the cement grain and accelerating cement hydration [11]. In this case, an attempt can be made to simultaneously increase the amounts of polycarboxylate-based hyperplasticizer additive and cement in order to accelerate strength gain during the early ages of curing (2-7 days). Another option to accelerate concrete curing is to modify it with calcium chloride (CaCl 2 ), but it should be used with caution in reinforced concretes [12].
Obtaining modified RHCs of superior quality offers various benefits, including: -High technological properties of the obtained concrete mixtures by reducing their water content and increasing their homogeneity and resistance to stratification; -Acceleration of cement hydration rate at the early stage of curing, which reduces the construction time while ensuring high quality, as described by [13]; -Use of effective modifiers and hyperplasticizer additives, allowing the achievement of high rates of strength gain without reducing the operational properties.
The goal of this study was to select rapid hardening concrete compositions with over 70% compressive strength gain on day 2, equivalent to grade C 25/30 strength, and frost resistance of at least 180 cycles (where 1 cycle is 1 day of freeze-thaw).
To achieve the goal, several solutions were defined: -Calculation of RHC compositions using two modifiers (microsilica and CaCl 2 ) and laboratory testing of their strength characteristics according to [14]; -Reduction of the W/C ratio according to Abrams' law of concrete strength described in [15] and confirmation of this law using polycarboxylate ester-based highly waterreducing additives; -Study the durability of the obtained RHC compositions with over 70% compressive strength gain on day 2 by testing frost resistance using the method of alternate freezing and thawing for 180 days in an aggressive environment [16].
The results of these studies have significant implications for production in construction work. The rapid hardening concretes obtained in this study will be particularly useful in construction projects in which a quick turnaround time is required, while their notable frost resistance might be important for preventing damage to concrete structures in areas with harsh winter climates.

Materials and Methods
The research was conducted in 8 stages, each of which was aimed at solving specific tasks:  Below is a description and characteristics of the raw materials used. Table 1 shows the characteristics of the binder (cement), taken according to [17].  [17] gives the following decoding of CEM I 52.5 N: I-denotes the first type according to the content of gypsum (counted for SO 3 ), which should be at least 1.5% and not higher than 3.5-4.0% (for high-strength cement without additives); 52.5-denotes the strength class corresponding to the minimum standard value of compressive strength, MPa; N-denotes the normal hydration reaction. Table 2 shows the chemical composition of CEM I 52.5 N. The chemical composition is given according to the standard [19]. To confirm the compliance of the selected cement with the norms and requirements of [19], several tests were carried out. The methods given in standards [17] and [20] made it possible to determine the following parameters:

•
Grinding fineness: The cement used had a grinding fineness of 94.4%. • Normal density and setting time of cement dough: The cement used had a normal consistency of 27.30%. The beginning of setting occurred after 2 h 11 min, and the end of setting occurred 4 h 10 min from the moment of mixing. These values were consistent with [17,20].
The fine aggregate used was sand from the manufacturer Giyada, LLP (Almaty region, Kazakhstan). This aggregate met the requirements of the standard [21]. Table 3 shows the characteristics of the fine aggregate used. To obtain satisfactory characteristics of the concrete mixture and the final conglomerate of rapid hardening concrete, it is necessary to use sand with a number of dust-like inclusions, the content of which does not exceed 1.5% [21]. The content of dust and clay inclusions in the sand used in this study was 1.08%. The fineness modulus (M f ) of the sand used was 2.6 (shown in Figure 1). These values were acceptable according to [21] for using this aggregate in the concrete. To obtain satisfactory characteristics of the concrete mixture and the final conglomerate of rapid hardening concrete, it is necessary to use sand with a number of dust-like inclusions, the content of which does not exceed 1.5% [21]. The content of dust and clay inclusions in the sand used in this study was 1.08%. The fineness modulus (Mf) of the sand used was 2.6 (shown in Figure 1). These values were acceptable according to [21] for using this aggregate in the concrete. The crushed stone used as a coarse aggregate in this study had 5-10 and 10-20 mm fractions and was produced by Baltabay, LLP (Almaty, Kazakhstan). This aggregate was found to meet the requirements of the standard [21]. Table 4 shows the characteristics of the coarse aggregate (crushed stone) used. According to [22], to obtain satisfactory characteristics of the concrete mixture while sieving the coarse aggregate (crushed stone and gravel) fractions of 5-10, 10-15, 10-20, 15-20, 20-40, and 40-80 mm, as well as a combined fraction of 5-20 mm, the total passing on the reference sieves should correspond to those shown in Table 5 and Figure 2 Figure 1. Sieve analysis grading curve for fine aggregate [22].

Group of
The crushed stone used as a coarse aggregate in this study had 5-10 and 10-20 mm fractions and was produced by Baltabay, LLP (Almaty, Kazakhstan). This aggregate was found to meet the requirements of the standard [21]. Table 4 shows the characteristics of the coarse aggregate (crushed stone) used. According to [22], to obtain satisfactory characteristics of the concrete mixture while sieving the coarse aggregate (crushed stone and gravel) fractions of 5-10, 10-15, 10-20, 15-20, 20-40, and 40-80 mm, as well as a combined fraction of 5-20 mm, the total passing on the reference sieves should correspond to those shown in Table 5 and Figure 2 below, where d and D represent the smallest and largest nominal grain sizes in mm, respectively. Up to 0.5 * For the crushed stone and gravel fraction of 5-10 mm and combined fraction of 5-20 mm for which additionally lower sieves (2.5 or 1.25 mm) are used, the total passing must be from 95% to 100%. Upon agreement between the manufacturer and the consumer, producing crushed stone and gravel with passing on sieve 0.5(d+D) from 30% to 80% by mass is allowed [22].

Figure 2.
Sieve analysis grading curve for coarse aggregate [22]. Table 6 shows the characteristics of the modifying additive (microsilica), adopted according to [8]. MCU-95 contains spherical particles with a diameter of 0.1 microns. Its bulk density ranges from 150 to 250 kg/m 3 . According to its chemical composition, MCU-95 is represented mainly by non-crystalline silica with a content of about 97%. It has a specific surface area equal to 3980 cm 2 /g, according to [23]. Table 7 shows the chemical composition of MCU-95, taken according to [8].   Table 6 shows the characteristics of the modifying additive (microsilica), adopted according to [8]. MCU-95 contains spherical particles with a diameter of 0.1 microns. Its bulk density ranges from 150 to 250 kg/m 3 . According to its chemical composition, MCU-95 is represented mainly by non-crystalline silica with a content of about 97%. It has a specific surface area equal to 3980 cm 2 /g, according to [23]. Table 7 shows the chemical composition of MCU-95, taken according to [8]. Comparing the chemical composition presented by the manufacturer to the quality standard [5], it followed that the content of oxides in the composition of MCU-95 was sufficient. Table 8 shows the characteristics of the chemical additive (polycarboxylate hyperplasticizer) adopted, according to [25].  Table 9 shows the characteristics of calcium chloride (CaCl 2 ), taken according to [27]. The effectiveness of additives in regulating the strength gain kinetics of concretes and mortars was evaluated, according to [26], based on the change in the strength (∆R t , %) of the developed compositions compared to that of the reference composition at the curing ages of 1 and 28 days (Equation (1)).
and R re f t -the strengths (in MPa) of the developed concrete or mortars and the reference compositions at the curing age t, respectively.
Further studies aimed to calculate the composition of RHC with Portland cement for the first test mixes and approve the reference composition. After that, experimental confirmation was used to improve the compressive strength indicators [14] in the early curing ages (2 and 7 days) of the RHC reference composition after modifying it with accelerants (MCU-95 and CaCl 2 ) proposed in previous studies [29][30][31]. In this case, the mixing time of the components after adding the additives was increased to 5 min to enable even distribution by volume. The best compositions in terms of early strength gain were tested in the laboratory for frost resistance [16] within 3 to 180 days to confirm the operational resistance and durability of the selected RHC compositions. The sequence of the tests is given below.

Calculation and Selection of the RHC Composition
When selecting the RHC composition with Portland cement for the first test mixes and approval of the reference composition, Equations (2) and (3) [32] were used to define the strength of concrete at an age (t, day) depending on the type of cement ( f cm (t), MPa): where β cc (t)-a coefficient that depends on the age of the concrete, t; s-coefficient that depends on the type of cement (for CEM I 52.5 N, s = 0.2). Experimental verification and correction based on the results of the preliminary calculations of the concrete composition are mandatory [32], and the following conditions must be fulfilled: After that, test samples of concrete were made from the experimental compositions, which were tested for compressive strength at the ages of 2 and 7 days.

Determination of the Compressive Strength
As part of the compressive strength test, cubes with a rib length of 100 mm were molded using a mixture of the reference composition and each subsequent modified composition. Three samples were taken per test. After the samples had reached the ages of 2 and 7 days, tests were conducted at a relative humidity of at least 95% and a temperature of 20 ± 5 • C, in accordance with [14]. The compressive strength of the concrete (R, MPa) was calculated with an accuracy of 0.01 MPa using Equation (4).
where F-breaking load, N; A-the area of the working section of the sample, mm 2 ; αscaling factor for converting concrete strength to concrete strength in cubic samples with rib size of 100 mm. In addition, the microstructure of RHC compositions with high early strength characteristics (2 and 7 days) was studied using a microscope. After the microstructure analysis, the compositions were tested for frost resistance.

Studying the Microstructure of Concrete
To study the microstructure of concrete, small pieces of concrete were cut from the area of interest and prepared for microscopy. First, any loose debris or dirt from the surface of the concrete was removed. Then, a diamond saw was used to cut small pieces of concrete. Next, progressively finer grits of sandpaper were used to polish the surface of the concrete until it was smooth and flat. Finally, a metallographic polishing machine was used to polish the surface to a mirror finish.
The polished concrete samples were then mounted onto a glass slide using a small amount of adhesive, ensuring that the area of interest was in the center of the slide.
To enhance the contrast of the concrete microstructure, the sample was stained using a 1% aqueous solution of methylene blue. The stain was applied to the sample and allowed to sit for a few minutes before being rinsed off with water.
The stained concrete sample was then placed onto the stage of the ZEISS Axio Vert.A1 microscope (JSC ZEISS, Oberkochen, Baden-Württemberg, Germany). The objective lens was selected for magnification at ×500 to visualize the microstructure of the concrete. The illumination and focus settings were adjusted to obtain clear images of the microstructure, and imaging software was used to capture and analyze the images.
The images were analyzed to identify and quantify the various components of the concrete microstructure, such as the cement paste, aggregate particles, and any voids or cracks.
After examining the microstructure of the RHC samples, the compositions with the best characteristics were tested for frost resistance.

Determining the Frost Resistance of Concrete
The test to determine frost resistance was carried out using the second accelerated method of alternate freezing and thawing [16]. The essence of the test is that during freezing, the liquid volume expands by more than 9% [9]. In NaCl salt solution, concrete is destroyed more quickly. Therefore, instead of a 5% solution, a 10% saturated NaCl salt solution was used as a reagent to identify possible hidden defects in the concrete structure due to rapid formation of the concrete matrix skeleton.
Tests were conducted on samples (6 per cycle, i.e., ages of 3, 7, 28, 90, and 180 days) using an LGPv laboratory freezer with the Profi electronic system (JSC Liebherr, Bulle, Switzerland). The samples were saturated in 10% NaCl solution and then thawed in a 10% NaCl solution. The accelerated tests by the second method were carried out according to the conditions and modes in Tables 10 and 11, respectively, as specified in [16]. During the frost resistance tests, the main indicator was the mass loss of the samples. Samples that lost more than 2% of their original weight were considered to have failed the frost resistance test and were not further tested [16].
After obtaining the data from the frost resistance tests, the dependence of the effect of MCU-95 on the frost resistance at early ages (3,7,28,90, and 180 days) and the compressive strength of RHC at the ages of 28 and 180 days was determined. Based on the tests performed, comparative calculations were made, and the composition of RHC with the best performance characteristics was determined. Table 12 shows the test composition of RHC with pure cement, obtained by calculation according to [32] (further addressed as reference composition).  Table 12 demonstrates that the primary factor contributing to strength gain following the calculation was the low W/C ratio of 0.37, which corresponded to the proportions of the standard [35]. Table 13 displays the early curing strength characteristics of the reference composition with pure cement (No. 1 *) and compositions where part of the cement was replaced by MCU-95 (No. 2-6).  Table 13 demonstrates a general tendency towards increased early strength gain when replacing part of the cement with MCU-95. Moreover, the effect increased with the amount of MCU-95, which was consistent with the results outlined in [36]. The slight strength reduction of composition No. 5 when the MCU-95 content was 1.6% could likely be attributed to human error during testing. However, it is worth noting that despite this deviation, it achieved the desired strength target of 70% on day 2 of hardening. Figure 3 displays an image of the microstructure of the fracture of the sample of RHC containing MCU-95, captured with a ZEISS Axio Vert.A1 microscope via reflection shooting at a magnification of ×500.  Figure 3 demonstrates that the cement stone with the addition of MCU-95 had a more homogeneous structure than the cement stone without additives [37]. The addition of active SiO2 created conditions for the formation of a structure with the densest packing of crystals, consisting mainly of low-base calcium silicate hydroxide (Ca5(OH)2Si6O16-  Figure 3 demonstrates that the cement stone with the addition of MCU-95 had a more homogeneous structure than the cement stone without additives [37]. The addition of active SiO 2 created conditions for the formation of a structure with the densest packing of crystals, consisting mainly of low-base calcium silicate hydroxide (Ca 5 (OH) 2 Si 6 O 16 -4H 2 O ⇒ (CaO/SiO 2 < 1.5)), which confirmed the theory presented in [38]. Figure 4 below illustrates the effect of MCU-95 on the kinetics of strength gain of RHC during the early ages of curing, where SD stands for standard deviation.  Figure 3 demonstrates that the cement stone with the addition of MCU-95 had a more homogeneous structure than the cement stone without additives [37]. The addition of active SiO2 created conditions for the formation of a structure with the densest packing of crystals, consisting mainly of low-base calcium silicate hydroxide (Ca5(OH)2Si6O16-4H2O(СаО/SiO2˂1.5)), which confirmed the theory presented in [38]. Figure 4 below illustrates the effect of MCU-95 on the kinetics of strength gain of RHC during the early ages of curing, where SD stands for standard deviation.  Figure 4 shows that replacing part of the cement with MCU-95 up to 50 kg in testing RHC of grade C 25/30 allowed for an increase in strength gain of 16% at the age of 2 days and 26% at the age of 7 days while maintaining all of the strength characteristics specified in [39]. It appeared that microsilica, mainly consisting of active SiO2, significantly affected the fundamental physical and technical properties of concrete, resulting in improved characteristics. Specifically, microsilica contributed to a more efficient use of the chemical energy of clinker by creating additional centers of crystallization. Additionally, an increase in the amount of microsilica in the composition led to an increase in the amount of stable low-base calcium silicate hydroxide (Ca5(OH)2Si6O16*4H2O(CaO/SiO2˂1.5)), which reduced the most unstable component of cement stone-Ca(OH)2 crystals. This is important for producing dense and durable concrete [29].  Figure 4 shows that replacing part of the cement with MCU-95 up to 50 kg in testing RHC of grade C 25/30 allowed for an increase in strength gain of 16% at the age of 2 days and 26% at the age of 7 days while maintaining all of the strength characteristics specified in [39]. It appeared that microsilica, mainly consisting of active SiO 2 , significantly affected the fundamental physical and technical properties of concrete, resulting in improved characteristics. Specifically, microsilica contributed to a more efficient use of the chemical energy of clinker by creating additional centers of crystallization. Additionally, an increase in the amount of microsilica in the composition led to an increase in the amount of stable low-base calcium silicate hydroxide (Ca 5 (OH) 2 Si 6 O 16 *4H 2 O ⇒ (CaO/SiO 2 < 1.5)), which reduced the most unstable component of cement stone-Ca(OH) 2 crystals. This is important for producing dense and durable concrete [29]. Table 14 presents the strength characteristics of RHC during the early ages of curing for the reference composition (No. 1 *) and compositions with increased cement content in the concrete (No. 2-7). Table 14 indicates that increasing the amount of cement to 15% only resulted in a 1% increase in strength gain at the age of 2 days and a 2% increase at the age of 7 days. These were relatively low indicators compared to the data presented in Table 13, which reflected the strength data on the 2nd and 7th days of curing of the RHC compositions containing MCU-95.    Table 14 indicates that increasing the amount of cement to 15% only resulted in increase in strength gain at the age of 2 days and a 2% increase at the age of 7 days. T were relatively low indicators compared to the data presented in Table 13, which refle the strength data on the 2nd and 7th days of curing of the RHC compositions contai MCU-95. Figure 5 illustrates an image of the microstructure of a fracture of a sample of made with clean cement. The image was obtained with a ZEISS Axio Vert.A1 micros via reflection shooting at a magnification of ×500.  Figure 5 shows that the structure of cement stone without additives was heterog ous, had a block character, and was represented by weakly crystallized interlayers of h base calcium hydro silicates (2Ca(OH)2SiO2-H2O(СаО/SiO2 ˃1.5)), which was consi with the opinion of the authors of [6]. Figure 6 shows the relationship between the strength gain of RHC and the cem consumption, with SD standing for standard deviation.  Figure 5 shows that the structure of cement stone without additives was heterogeneous, had a block character, and was represented by weakly crystallized interlayers of high-base calcium hydro silicates (2Ca(OH) 2 SiO 2 -H 2 O ⇒ (CaO/SiO 2 > 1.5)), which was consistent with the opinion of the authors of [6]. Figure 6 shows the relationship between the strength gain of RHC and the cement consumption, with SD standing for standard deviation. Materials 2023, 16, x FOR PEER REVIEW 12 of 19 Figure 6. The effect of cement consumption on RHC early strength gain. Figure 6, showing the dependence of strength gain on cement consumption, revealed that a simple increase in the amount of cement to 15% aimed at improving the strength gain did not give a noticeable effect, as previously indicated in [29]. Table 15 shows the strength characteristics of RHC in the early ages of curing (2 and 7 days) for the reference composition (No. 1 *) and compositions with a simultaneous increase in the amounts of both cement and chemical additive AR Premium (No. 2-7).  Table 15 shows that increasing the amount of cement to 15% while simultaneously increasing the amount of AR Premium to 20% demonstrated good early strength gain dynamics of RHC. Thus, at the age of 2 days, the strength increased by 51%, and at the age of 7 days, it increased by 70%. This increase in early strength with the use of AR Premium was accompanied by a decrease in the W/C ratio, which was consistent with Abrams' law of concrete strength [15]. Figure 7 shows the dependence of strength gain on the amount of polycarboxylatebased hyperplasticizer AR Premium, where SD stands for standard deviation. 17 Figure 6, showing the dependence of strength gain on cement consumption, revealed that a simple increase in the amount of cement to 15% aimed at improving the strength gain did not give a noticeable effect, as previously indicated in [29]. Table 15 shows the strength characteristics of RHC in the early ages of curing (2 and 7 days) for the reference composition (No. 1 *) and compositions with a simultaneous increase in the amounts of both cement and chemical additive AR Premium (No. 2-7).  Table 15 shows that increasing the amount of cement to 15% while simultaneously increasing the amount of AR Premium to 20% demonstrated good early strength gain dynamics of RHC. Thus, at the age of 2 days, the strength increased by 51%, and at the age of 7 days, it increased by 70%. This increase in early strength with the use of AR Premium was accompanied by a decrease in the W/C ratio, which was consistent with Abrams' law of concrete strength [15]. Figure 7 shows the dependence of strength gain on the amount of polycarboxylatebased hyperplasticizer AR Premium, where SD stands for standard deviation.  Figure 7 shows that increasing the amount of chemical additive based on polycarboxylates by up to 20% accelerated early strength gain. This could be anticipated since the use of PCE decreased the W/C ratio in the mixture. Therefore, the fundamental law of concrete strength, which relates strength to the W/C ratio, was not violated in this case and operated similarly in RHC. Consequently, the reduction in W/C ratio led to an increase in strength characteristics, which was consistent with the findings of previous studies [11]. Table 16 shows the strength characteristics during the early ages of curing of RHC for the reference composition with pure cement (No. 1 *) and compositions with varying amounts of CaCl2 as a percentage of the cement (No. 2-5).  From Table 16, we see that adding CaCl2 to the concrete mixture led to a general tendency towards an increase in early strength gain, up to a certain amount equal to 2% of the weight of cement. Furthermore, the experiments conducted suggested that an increase in the amount of CaCl2 to 3% of the weight of cement and more was accompanied by the same dynamics of strength gain increase at the ages of 2 and 7 days as when the amount of CaCl2 was equal to 2%. This complemented information on the acceleration of hardening stated in [30]. Figure 8 shows an image of the microstructure of a fracture of the RHC sample containing CaCl2, obtained with the ZEISS Axio Vert.A1 microscope via reflection shooting at a magnification of ×500. 17 Figure 7 shows that increasing the amount of chemical additive based on polycarboxylates by up to 20% accelerated early strength gain. This could be anticipated since the use of PCE decreased the W/C ratio in the mixture. Therefore, the fundamental law of concrete strength, which relates strength to the W/C ratio, was not violated in this case and operated similarly in RHC. Consequently, the reduction in W/C ratio led to an increase in strength characteristics, which was consistent with the findings of previous studies [11]. Table 16 shows the strength characteristics during the early ages of curing of RHC for the reference composition with pure cement (No. 1 *) and compositions with varying amounts of CaCl 2 as a percentage of the cement (No. 2-5). From Table 16, we see that adding CaCl 2 to the concrete mixture led to a general tendency towards an increase in early strength gain, up to a certain amount equal to 2% of the weight of cement. Furthermore, the experiments conducted suggested that an increase in the amount of CaCl 2 to 3% of the weight of cement and more was accompanied by the same dynamics of strength gain increase at the ages of 2 and 7 days as when the amount of CaCl 2 was equal to 2%. This complemented information on the acceleration of hardening stated in [30]. The calcium oxychloride compound CaCl2-3Ca(OH)2-12H2O shown in Figure 8 is obtained by the chemical reaction of Ca(OH)2 with CaCl2, and it can change the microstructure of concrete as well as the properties of its pore solution. The formation of calcium oxychloride (as an expansive phase) can alter the transport properties of concrete by causing pore blockage and severe failure due to internal expansion. The chemical reaction is described by the formation of a calcium oxychloride compound using the equation 3Ca(OH)2 + CaCl2 + 12H2O → CaCl2-3Ca(OH)2-12H2O. The reaction between calcium chloride and calcium hydroxide is rapid and can weaken the structure if formed in the concrete matrix due to internal hydraulic pressures that can occur. The calcium oxychloride compound is relatively unstable at room temperature and low levels of relative humidity. Calcium oxychloride can exist in different molar ratios, including CaCl2-3Ca(OH)2-12H2O, CaCl2-Ca(OH)2-xH2O, and CaCl2-Ca(OH)2. These phases can coexist and are interchangeable in the compound Ca(OH)2-CaCl2-H2O. Hence, the compound CaCl2-3Ca(OH)2-12H2O, as seen in Figure 8, is destructive and can change to CaCl2-Ca(OH)2 or CaCl2-Ca(OH)2-xH2O with changes in temperature or ambient humidity [12]. Figure 9 shows the dependence of the strength gain kinetics of RHC in the early ages of curing on the amount of CaCl2, where SD stands for standard deviation. The calcium oxychloride compound CaCl 2 -3Ca(OH) 2 -12H 2 O shown in Figure 8 is obtained by the chemical reaction of Ca(OH) 2 with CaCl 2 , and it can change the microstructure of concrete as well as the properties of its pore solution. The formation of calcium oxychloride (as an expansive phase) can alter the transport properties of concrete by causing pore blockage and severe failure due to internal expansion. The chemical reaction is described by the formation of a calcium oxychloride compound using the equation 3Ca(OH) 2 + CaCl 2 + 12H 2 O → CaCl 2 -3Ca(OH) 2 -12H 2 O. The reaction between calcium chloride and calcium hydroxide is rapid and can weaken the structure if formed in the concrete matrix due to internal hydraulic pressures that can occur. The calcium oxychloride compound is relatively unstable at room temperature and low levels of relative humidity. Calcium oxychloride can exist in different molar ratios, including CaCl 2 -3Ca(OH) 2 -12H 2 O, CaCl 2 -Ca(OH) 2 -xH2O, and CaCl 2 -Ca(OH) 2 . These phases can coexist and are interchangeable in the compound Ca(OH) 2 -CaCl 2 -H 2 O. Hence, the compound CaCl2-3Ca(OH)2-12H2O, as seen in Figure 8, is destructive and can change to CaCl 2 -Ca(OH) 2 or CaCl 2 -Ca(OH) 2 -xH 2 O with changes in temperature or ambient humidity [12]. Figure 9 shows the dependence of the strength gain kinetics of RHC in the early ages of curing on the amount of CaCl 2 , where SD stands for standard deviation. The calcium oxychloride compound CaCl2-3Ca(OH)2-12H2O shown in Figure 8 is obtained by the chemical reaction of Ca(OH)2 with CaCl2, and it can change the microstructure of concrete as well as the properties of its pore solution. The formation of calcium oxychloride (as an expansive phase) can alter the transport properties of concrete by causing pore blockage and severe failure due to internal expansion. The chemical reaction is described by the formation of a calcium oxychloride compound using the equation 3Ca(OH)2 + CaCl2 + 12H2O → CaCl2-3Ca(OH)2-12H2O. The reaction between calcium chloride and calcium hydroxide is rapid and can weaken the structure if formed in the concrete matrix due to internal hydraulic pressures that can occur. The calcium oxychloride compound is relatively unstable at room temperature and low levels of relative humidity. Calcium oxychloride can exist in different molar ratios, including CaCl2-3Ca(OH)2-12H2O, CaCl2-Ca(OH)2-xH2O, and CaCl2-Ca(OH)2. These phases can coexist and are interchangeable in the compound Ca(OH)2-CaCl2-H2O. Hence, the compound CaCl2-3Ca(OH)2-12H2O, as seen in Figure 8, is destructive and can change to CaCl2-Ca(OH)2 or CaCl2-Ca(OH)2-xH2O with changes in temperature or ambient humidity [12]. Figure 9 shows the dependence of the strength gain kinetics of RHC in the early ages of curing on the amount of CaCl2, where SD stands for standard deviation.  Figure 9 shows that the addition of CaCl 2 in the amount of 2% of the weight of cement affected the curing acceleration by 9.4 MPa at the early age of 2 days and by 7.5 MPa at the age of 7 days, which was consistent with [30,31].

Results and Discussion
After comparing the data on strength during the early hardening period at 2 and 7 days, examining the microstructure of the obtained RHC samples, and studying previous works in this research area [10,11], it was decided to conduct further testing on the frost resistance of RHC samples. In this testing, some of the cement was replaced by MCU-95 microsilica. Three types of tested compositions, except for the specimens with pure cement, showed a compressive strength of 70% or more of concrete grade C 25/30 on the second day of hardening. However, only the samples with some cement replaced by microsilica had a more uniform structure compared to the others, as revealed by the microstructure study. Table 17 presents data on the strength and weight of RHC for the reference composition with pure cement (No. 1 *) and compositions with varying amounts of MCU-95 (No. [2][3][4][5][6] from Table 13 after being tested for frost resistance in an aggressive environment (i.e., in 10% NaCl solution) for 3, 7, 28, 90, and 180 days.  Table 17 shows a general tendency of improved frost resistance in RHC modified with MCU-95 (No. [2][3][4][5][6] in comparison to the reference composition made with conventional cement (No. 1 *). Furthermore, the better the performance, the higher the content of MCU-95. This indicated that the obtained results were in compliance with the theoretical assumptions regarding the creation of additional crystallization centers and reduction of pore space in the concrete body by using reactive pozzolanic additives (active SiO 2 ) [10]. Figure 10 shows the process of weight reduction of the RHC samples with different contents of MCU-95 and the RHC reference composition without MCU-95 during the test for frost resistance, where SD stands for standard deviation. Figure 10 shows a general trend of improvement in the frost resistance index of RHC compositions modified with MCU-95 (No. 2-6) compared to the reference composition with cement (No. 1), which failed the test for frost resistance after 180 days by losing over 2% in mass, which agreed with the opinion of the authors of [40]. Figure 11 shows the process of strength reduction in RHC compositions with different content of MCU-95 (No. 2-6) and the reference composition (No. 1) without MCU-95 at the age of 28 and 180 days during the test for frost resistance in 10% NaCl. Figure 11 shows a general trend towards improved frost resistance index in the RHC compositions modified with MCU-95 (with higher MCU-95 content leading to higher indicators) compared to the reference composition of conventional cement RHC. The reference composition failed the frost resistance test after 180 days, with a 10% loss in strength, which is consistent with the findings of [40]. Materials 2023, 16, x FOR PEER REVIEW 16 of 19 Figure 10. Weight reduction of RHC samples after frost resistance tests. * Reference composition. Figure 10 shows a general trend of improvement in the frost resistance index of RHC compositions modified with MCU-95 (No. 2-6) compared to the reference composition with cement (No. 1), which failed the test for frost resistance after 180 days by losing over 2% in mass, which agreed with the opinion of the authors of [40]. Figure 11 shows the process of strength reduction in RHC compositions with different content of MCU-95 (No. 2-6) and the reference composition (No. 1) without MCU-95 at the age of 28 and 180 days during the test for frost resistance in 10% NaCl.  Figure 11 shows a general trend towards improved frost resistance index in the RHC compositions modified with MCU-95 (with higher MCU-95 content leading to higher indicators) compared to the reference composition of conventional cement RHC. The Initial weight of samples, g:   Figure 10 shows a general trend of improvement in the frost resistance index of RHC compositions modified with MCU-95 (No. 2-6) compared to the reference composition with cement (No. 1), which failed the test for frost resistance after 180 days by losing over 2% in mass, which agreed with the opinion of the authors of [40]. Figure 11 shows the process of strength reduction in RHC compositions with different content of MCU-95 (No. 2-6) and the reference composition (No. 1) without MCU-95 at the age of 28 and 180 days during the test for frost resistance in 10% NaCl.  Figure 11 shows a general trend towards improved frost resistance index in the RHC compositions modified with MCU-95 (with higher MCU-95 content leading to higher indicators) compared to the reference composition of conventional cement RHC. The Initial weight of samples, g: Figure 11. Decrease in the strength of RHC compositions after frost resistance tests. * Reference composition.
Based on the data collected on the strength of RHC samples during the early hardening period at 2 and 7 days, it can be noted that almost all compositions, except those using pure cement, achieved the target of reaching a compressive strength of 70% or more of concrete grade C 25/30 at 2 days. Interestingly, increasing the amount of cement beyond the reference composition did not lead to higher early strength, which was an unexpected result. However, this finding was partially explained by previous studies [6].
The results of the compressive strength tests conducted at 2 and 7 days of hardening for the remaining three types of samples were as expected, with all of them exhibiting a compressive strength of at least 70%. The presence of active SiO 2 in compositions with the replacement of some of the cement with microsilica facilitated the formation of an early structure with the densest packing of crystals, as evidenced by [38], which consisted primarily of low-base hydroxide calcium silicate (Ca 5 (OH) 2 Si 6 O 16 *4H 2 O ⇒ (CaO/SiO 2 < 1.5)).
The use of polycarboxylate plasticizer in compositions with an increase in the amount of cement up to 15% and hyperplasticizer AR Premium up to 20% resulted in good early strength gain dynamics of RHC up to 110% at the age of 2 days, accompanied by a decrease in the B/C ratio. The addition of CaCl 2 in the concrete mixture led to an increase in early strength gain due to early bonds between portlandite Ca(OH) 2 and calcium chloride CaCl 2 , resulting in the formation of gel-like calcium oxychloride CaCl 2 *3Ca(OH) 2 12H 2 O, up to a certain amount equal to 2% of the weight of cement [30,31]. Further increase in the amount of CaCl 2 had no additional effect on the strength gain at the ages of 2 and 7 days. The frost resistance tests on the samples of RHC showed that the best indicators of frost resistance were observed in the samples where the cement part was replaced by microsilica in the quantity of up to 50 kg per 1 m 3 . The performance was better with a higher content of microsilica, indicating the higher operational reliability of products made of concrete with active microsilica. The results confirmed the theory of the creation of additional centers linking hydrolyzed lime into an additional amount of strong, cementitious calcium hydrosilicates and showed the compliance of the obtained results with the theoretical assumptions about the creation of additional centers of crystallization and the reduction of pore space in the concrete body when using reactive pozzolanic additives.

Conclusions
The results of the tests confirmed the purpose of the research, which was to select RHC compositions that achieved a strength gain of 70% or more in the first 2 days of hardening. Out of the four series of samples tested, three samples with different modifier and binder contents were able to achieve the required early strength indicators.
Based on the calculations and laboratory tests, it can be concluded that the use of modified RHC is justified both from technical and scientific points of view. Compared to ordinary cement concrete compositions, RHC with modifiers can accelerate the construction process. RHC with MCU-95 microsilica accelerated the strength gain rate and exhibited the best frost and strength indicators at 90 and 180 days after placement, indicating the absence of destructive processes in concrete due to the use of microsilica and good performance characteristics.
The test results demonstrated the consistency of the obtained data in RHC with the addition of microsilica with the theoretical assumptions regarding the emergence of additional centers of crystallization and the reduction of pore space in the concrete body during the use of reactive pozzolanic additives (active SiO 2 ). The process of binding Ca(OH) 2 by the active mineral additive SiO 2 into the low-solubility compound-calcium hydrosilicate-occurred according to the equation: Ca(OH) 2 + SiO 2 + mH 2 O = CaO*SiO 2 *nH 2 O. Experiments with accelerated curing using a specific amount of CaCl 2 as well as compositions with simultaneously increased amounts of cement and hyperplasticizer based on the polycarboxylate AR Premium demonstrated an improvement in the early strength gain of RHC. However, microscopic examination of the concrete samples revealed a heterogeneous and weakly crystallized structure, indicating the poor quality and durability of the resulting RHC, especially since chlorides should be used with caution in reinforced structures.
The compositions of RHC (No. 4-6) with MCU-95 from Table 13 that showed high strength gain at 2 and 7 days can be recommended for application in production. Furthermore, these compositions maintained their mass and compressive strength after frost resistance tests at 90 and 180 days, which is an important indicator of durability. Summarizing the foregoing, it can be said that the research team fulfilled the objectives of this study and obtained satisfactory results.