Structure and Properties Improvement by Recipe Factors of Geopolymer Basalt Fiber Reinforced Concrete for Building Enclosing Structures

Abstract

The application of geopolymer concrete in buildings and structures is becoming widespread because of its low cost and high strength characteristics. At the same time, the capabilities of geopolymer concrete are not fully used, especially to strengthen flexural properties. The article examines the problems of developing an effective composition of geopolymer concrete based on ground granulated blast furnace slag (GGBS) by selecting the optimal composition of the alkaline activator and the amount of basalt fiber (BF). To determine the degree of effectiveness of the proposed formulation solutions, the characteristics of geopolymer fiber-reinforced concrete (FRC) were determined. It has been investigated the most effective composition of an alkaline activator is an activator containing a NaOH solution with a molarity of 12 M. The most optimal dosage of BF is 1.5% by weight of GGBS. The increase in compressive and flexural strength for the most effective composition of geopolymer FRC 12 M/BF1.5, which combines the most effective parameters of formulation solutions, compared to the least effective composition 8 M/BF0 was 40.54% and 93.75%, respectively, and the decrease of water absorption was 45.75%. The obtained scientific result represents a significant empirical basis for future research in the field of geopolymer FRC. The developed effective composition of geopolymer FRC is ready for use in practical construction.

1. Introduction

Construction and building technologies are the most studied and dynamically developing areas throughout the world. At the same time, such an applied engineering knowledge as construction science is impossible without conducting preliminary fundamental research aimed at finding the most useful and promising properties of various raw materials and sources that can become for buildings and structures a reliable bastion of their durability in the future [1]. In this regard, the main task of materials scientists involved in the building construction is a new recipe of building materials design, the study and development of the properties of existing materials, as well as proposals for the rationalization of currently known materials [2,3]. The reliability and quality of building materials is not the only area that should be developed. An essential aspect is the environmental friendliness of the building material and its belonging to green technologies. One of the most striking examples of recent decades in the field of green technologies is geopolymer concrete. The relevance of geopolymer concrete is increasing every year. All over the world, scientific research is being conducted to study the beneficial properties, as well as fundamental dependencies in the structure formation of geopolymer concretes and materials [4,5].

The high demand for geopolymer concretes, as well as confirms the relevance of research in this direction, their poor knowledge regarding the fundamental nature of the formation of their properties and structure depending on the composition. From a practical point of view, there is a serious problem with some limitations that geopolymer concrete has, in particular in terms of mechanical and physical properties, and this results in a poor standardization of the material and the need for additional research [6,7]. The geopolymer itself is a construction conglomerate, which comprises various constituent components, such as an aluminosilicate component, fine and coarse aggregate and an alkaline activator. For the production of geopolymer concrete, a fairly large range of different types of aluminosilicate components are used, which are mainly technogenic waste [8,9,10]. For example, in works [11,12,13,14], the authors developed various compositions of geopolymer concretes, where fly ash was used as an aluminosilicate component, and the strengths of the composites varied from 15 MPa to 70 MPa. In studies [15,16,17], various types of slag were used as a binder, and the strength of geopolymer composites varied from 40 MPa to 70 MPa. Also, in addition to the above types of aluminosilicate components, metakaolin, microsilica and ground waste from concrete and brick waste are used for the production of geopolymer composites [18,19,20]. The combination of these types of aluminosilicate components with each other in optimal quantities makes it possible to obtain durable composites with high mechanical characteristics [21,22,23,24].

Besides the recipe and technological factors in geopolymer concrete, the most important role is played by the solution of already known problems that are inherent in the studied recipe and technological parameters, which make it possible to create geopolymers with known characteristics. The need for additional research to study the improvement of existing compositions of geopolymer concrete is also confirmed by the wide variety of properties that can be imparted to geopolymer concrete by changing certain parameters [25,26]. One of the most promising types of research for opening new directions and obtaining a base of new knowledge on the compositions, structure formation and properties of geopolymer concretes is the development of new compositions of alkaline activators [27]. Geopolymer concrete heavily relies on the alkaline activator, which greatly influences the characteristics and behavior of the material. In a particular study [28], the authors examined how the molarity of the alkaline activator impacts the durability and strength of geopolymer concrete made from fly ash. It has been established that the most effective type of alkaline activator is a solution with a molarity of 16 M. Similarly, in [29,30], the impact of various concentrations of alkaline activators on the geopolymer characteristics was studied and the best compositions of hardening activators were selected for specific types of aluminosilicates.

In terms of the closest analogues of geopolymer concrete, namely cement concrete, the technology of fiber reinforcement is being widely developed and improved. Dispersed fiber, called fiber, produced from various materials, be it steel, basalt, polypropylene or other materials, is a very promising modifying additive that can significantly improve the mechanics and physics of the properties of cement concrete [31,32,33,34,35]. Geopolymer concrete, as a construction conglomerate, is no exception in terms of the prospects of reinforcing it with fiber. For example, in a study [36], the introduction of 0.3% steel fiber into the composition of geopolymer concrete made it possible to achieve the best compressive strength values. In studies [37,38,39,40,41], reinforcing geopolymer composites with glass, polypropylene, basalt, and steel fibers in optimal amounts improved their physical and mechanical properties. However, the issues of geopolymer concrete, not to mention geopolymer FRC, are poorly covered in regulatory and technical documents and currently do not allow mass production flow construction. Moreover, from a research point of view, joint recipe and technological solutions for varying the alkaline activator and different kinds of fibers to improve the geopolymer concrete characteristics are even less studied [42,43,44,45,46,47,48]. This reveals a notable deficit in scientific and practical understanding, specifically regarding the dependencies between the composition, their structural features, and geopolymer properties. The deficit is related to the insufficient understanding of how alkaline activator type and basalt fiber quantity affect things. Our goal is to find a solution to this problem through our research.

The scientific novelty of the research lies in the identification of patterns of interdependence of the compositions, structure and properties of geopolymer concrete based on GGBS manufactured using alkaline activators of various compositions and basalt fiber. Based on the identified scientific data, fundamental knowledge was obtained and several practical results were identified on this basis, which were used in the applied direction and recommendations for the construction industry. These recommendations are expressed in specific rational formulations, dosages and other formulation parameters for obtaining the highest quality geopolymer FRC using various types of alkaline activators.

The tasks of the research were to study the dependence of the compositions, structure and properties of geopolymer FRC on the composition of the alkaline activator and the amount of basalt fiber and, on the basis of this, put forward new recommendations for the real practical construction sector.

2. Materials and Methods

2.1. Materials

For the design of geopolymer FRC, a number of the following raw materials were used:

-

ground granulated blast furnace slag (GGBS) (Severstal, Cherepovets, Russia);

-

quartz sand (QS) (Arkhipovskoye v., Russia);

-

crushed sandstone (CrS) (Solntsedar-Don, Rostov-on-Don, Russia);

-

basalt fiber (BF) (Pascal, Dzerzhinsk, Russia);

-

sodium hydroxide (NaOH) (Khimprom, Novocheboksarsk, Russia);

-

sodium liquid glass (Na₂O(SiO₂)n) (Kerami-NSK, Novosibirsk, Russia);

-

plasticizer MasterGlenium 115 (P) (BASF Construction Systems, Moscow, Russia).

The main characteristics of the raw materials are presented in

Table 1. Attributes of raw materials.

Name of Raw Material Component	Indicator	Actual Value (Provided by the Manufacturer)
GGBS	Loss on ignition, 1000 °C (%)	11.38
	Silicon oxide SiO2 (%)	32.15
	Aluminum oxide Al2O3 (%)	5.92
	Iron oxide Fe2O3 (%)	0.73
	Calcium oxide CaO (%)	40.56
	Magnesium oxide MgO (%)	5.83
	Titanium oxide TiO2 (%)	0.28
	Phosphorus oxide P2O5 (%)	0.04
	Sulfur oxide total. SO3 (%)	3.11
	Density (kg/m3)	911
QS	Bulk density (kg/m3)	1415
	Apparent density (kg/m3)	2579
	The content of dust and clay particles (%)	0.05
	Content of clay in lumps (%)	0.08
	Organic and contaminant content (%)	No
	Fineness modulus	2.07
CrS	Bulk density (kg/m3)	1438
	Apparent density (kg/m3)	2665
	Resistance to fragmentation (wt %)	11.1
	The content of lamellar and acicular grains (wt %)	8.0
BF	Fiber length (mm)	From 12 to 15
	Density(g/cm3)	2.6
	Modulus of elasticity (GPa)	76
NaOH	Mass fraction of sodium hydroxide (%)	99.6
	Density (kg/m3)	2130
(Na2O(SiO2)n)	Mass fraction of SiO2 (%)	33.7
	Mass fraction of Na2O(%)	14.8
	Mass fraction of H2O (%)	51.5
	Density (kg/m3)	1270
P	Density (kg/m3)	1064
	pH	5.04

.

The actual values given in the Table 1 were provided by the manufacturers of these materials.

Figure 1 presents the characteristics of raw materials, namely the particle size distribution (Figure 1a–c) and the GGBS diffraction pattern (Figure 1d).

To construct size distribution curves for sand (Figure 1a) and crushed stone (Figure 1b) particles, sieves with cell sizes of 0.16, 0.315, 0.63, 1.25, 2.5, 5, 12.5, 20 and 25 (mm) were used. GGBS was additionally ground in a planetary ball mill “Activator-4M” for 24 h at 800 rpm. After additional processing, its granulometric composition was determined using a Microsizer model 201C (VA Insalt, Saint Petersburg, Russia) (0.2–300 μm) device and its phase composition, which was determined using an ARLX’TRA diffractometer. The results of granulometric and X-ray phase analysis of GGBS particles are presented in Figure 1c and 1d, respectively. According to the particle size distribution presented in Figure 1c, it can be seen that particle sizes vary from 0.1 µm to 93.3 µm. The peak particle distribution of 12.5% falls on particles with a size of 17.8 μm. The largest proportion of particles, up to 96.0%, are in the range from 1 µm to 40.75 µm. X-ray phase analysis of GGBS (Figure 1d) revealed its strong amorphization. Calcite (olivine-Ca*) reflections are visible and an albite phase is present.

2.2. Methods

The methodology employed to examine the connection between compositions, structures, and properties of new geopolymer FRC strictly adhered to regulatory and technical guidelines. Using techniques for geopolymer concrete reinforced with basalt fiber, consistent with the research techniques for traditional concrete with a cement binder, was important in order to maintain the purity of the experiment. Test conditions, selected compositions and recipe-technological parameters for manufacturing geopolymer FRC were used with strict adherence to the principle of repeatability, with verification and mandatory verification of outlier results from the general series, as well as with the linking of each specific factor and its analysis after receiving the results.

In this study, 20 mixtures were developed to develop the optimal composition of geopolymer FRC. In the compositions of concrete geopolymer mixtures, the molarity of the NaOH solution in the alkaline activator and the amount of basalt fiber were varied. The experimental research program and testing scheme are presented in Figure 2, and the recipes for concrete mixtures are given in

Table 2. Compositions of geopolymer FRC mixtures.

Mixture Type	GGBS (kg/m3)	QS (kg/m3)	CrS (kg/m3)	NaOH (kg/m)3/Molar Concentration	Na2O(SiO2)n, (kg/m3)	BF (kg/m3)	P (kg/m3)	Water (L/m3)
8 M/BF0	408	554	1094	40 (8 M)	110	0	24.5	55
8 M/BF0.5	408	554	1094	40 (8 M)	110	2.0	24.5	55
8 M/BF1.0	408	554	1094	40 (8 M)	110	4.1	24.5	55
8 M/BF1.5	408	554	1094	40 (8 M)	110	6.1	24.5	55
8 M/BF2.0	408	554	1094	40 (8 M)	110	8.2	24.5	55
10 M/BF0	408	554	1094	40 (10 M)	110	0	24.5	55
10 M/BF0.5	408	554	1094	40 (10 M)	110	2.0	24.5	55
10 M/BF1.0	408	554	1094	40 (10 M)	110	4.1	24.5	55
10 M/BF1.5	408	554	1094	40 (10 M)	110	6.1	24.5	55
10 M/BF2.0	408	554	1094	40 (10 M)	110	8.2	24.5	55
12 M/BF0	408	554	1094	40 (12 M)	110	0	24.5	55
12 M/BF0.5	408	554	1094	40 (12 M)	110	2.0	24.5	55
12 M/BF1.0	408	554	1094	40 (12 M)	110	4.1	24.5	55
12 M/BF1.5	408	554	1094	40 (12 M)	110	6.1	24.5	55
12 M/BF2.0	408	554	1094	40 (12 M)	110	8.2	24.5	55
14 M/BF0	408	554	1094	40 (14 M)	110	0	24.5	55
14 M/BF0.5	408	554	1094	40 (14 M)	110	2.0	24.5	55
14 M/BF1.0	408	554	1094	40 (14 M)	110	4.1	24.5	55
14 M/BF1.5	408	554	1094	40 (14 M)	110	6.1	24.5	55
14 M/BF2.0	408	554	1094	40 (14 M)	110	8.2	24.5	55

.

The production of geopolymer FRC samples took place in several stages. First, 24 h before use, a solution of a hardening activator based on liquid glass and sodium hydroxide was prepared. Liquid glass and sodium hydroxide with different molar concentrations from 8 to 14 mol/L were used. Then the dosage of raw materials was carried out in accordance with the presented recipe. To prepare these compositions in a laboratory mixer BL-10, GGBS, sand and crushed stone were first mixed dry for 60 s. Then a solution of a hardening activator and water with a plasticizing additive dissolved in it were added to the mixture of dry components. The entire mixture was mixed for 60 s, and then fibers were added as a percentage of the weight of GGBS. Next, entire mixture was remixed again until it become smooth. The finished composition was loaded into several molds and compacted on a special platform with vibrating mode SMZh-539-220A (IMash, Armavir, Russia). The compacting time was 60 s. The geopolymer samples were stored in the lab for 24 h before being transferred to a regular hardening chamber for 27 days.

Geopolymer concrete mixtures’ workability was assessed following methodology requirements [49]. Mold (cone) for preparing a test sample, made of metal resistant to cement paste, 1.5 mm thick. The mold is shaped like a hollow cone with specific internal measurements: base diameter of 200 mm, top diameter of 100 mm, and height of 300 mm. The mold has two handles at the top and clamps at the base for stability. A bayonet with a round cross-section, straight, with rounded ends, made of steel, 16 mm in diameter and 600 mm in length was used to distribute and compact the mixture in a cone. The process of filling the cone involved three distinct stages, wherein each stage contributed roughly one-third of its height after compaction. A total of 25 bayonet blows were used to compact each layer. The effects were uniformly distributed throughout the cross section of every layer. The surface of the concrete mixture had excess removed after compacting the top layer. Then, the cone was cautiously lifted vertically to remove it. From the moment we started filling to when we removed the cone, it took around 150 s without any breaks. As soon as the cone was removed, the settlement was measured accurately to within 10 mm. This measurement was defined as the difference between the mold’s height and the highest point of the settled test sample.

The density of the mixture was calculated in accordance with the requirements of [50]. A water-resistant metal container, capable of withstanding brief contact with cement paste, was used to compact the concrete mixture, which had a smooth inner surface. There were 5 L of space in the container. After weighing it, the container was filled with the mixture in two layers. The concrete mixture was compacted on a vibrating platform until there was no excessive delamination and release of laitance. The container with its contents was weighed and its mass was determined. The mass of the mixture was determined as the difference in the masses of the container with the mixture and the container without the mixture. The density was calculated by dividing the total mass by the container volume.

The method [51] was employed to determine the density of hardened geopolymer concrete.

The methods [52,53,54,55,56,57] were used to determine the compressive and flexural strength. Water absorption of geopolymer FRC samples was recorded in accordance with the requirements [58,59].

The micro-structure of the geopolymer composite was studied using ZEISS CrossBeam 340 microscope (Carl Zeiss AG, Jena, Germany) with 500× magnification.

3. Results and Discussion

The results of determining the characteristics of fresh geopolymer concrete are presented in Figure 3 and Figure 4. Figure 3 shows a graphical interpretation of the dependence of the density of fresh geopolymer concrete on the composition of the alkaline activator and the amount of basalt fiber.

As can be seen from Figure 3, the considered recipe solutions, namely changes in the composition of the hardening activator and the amount of basalt fiber, do not have a natural effect on the density of fresh geopolymer concrete. When the molarity of the alkaline activator changes, its amount in the geopolymer mixture remains unchanged, and when the amount of fiber increases from 0% to 2%, the mass of the mixture increases slightly. However, fibers cannot significantly affect the density of fresh geopolymer concrete due to small dosages. Figure 4 shows the change in workability of fresh geopolymer concrete, characterized by mixture slump, depending on the composition of the alkali activator and the amount of BF.

The workability of fresh geopolymer concrete is negatively impacted by increasing the concentration of NaOH solution in the alkaline activator composition, as shown in Figure 4. The reduction in cone settlement for compositions of type 10 M/BF0, 12 M/BF0, 14 M/BF0 compared to composition of type 8 M/BF0 was 2.27%, 4.55% and 9.09%. The introduction of fiber into the composition of geopolymer concrete also negatively affects its workability parameters. As the amount of fiber increases, this negative effect also increases. For geopolymer concrete compositions in which a NaOH solution with a molar concentration of 8 M was used as an alkaline activator, with the introduction of BF in amounts of 0.5%, 1.0%, 1.5% and 2.0%, the cone slump decreased by 5.68%, 9.09%, 14.77%, and 20.45%, respectively. For compositions where a NaOH solution with a molar concentration of 10 M was used, with similar BF dosages, the reduction in cone settlement was 5.81%, 10.47%, 13.95% and 19.77%, respectively. For geopolymer concrete compositions with a molar concentration of NaOH solution of 12 M, the reduction in cone settlement was 5.95%, 10.71%, 15.48% and 21.43%, respectively. Finally, for concrete compositions with a molar concentration of NaOH solution of 14 M, the reduction in cone slump was 6.25%, 10.0%, 15.0% and 21.25%, respectively.

It can be concluded that changing the molarity of NaOH solution from 8 M to 14 M in the alkaline activator does not significantly affect the density of geopolymer mixtures, but does affect their workability, reducing cone settlement. This is due to the fact that higher concentrations of sodium hydroxide solution accelerate the setting process of fresh geopolymer concrete and increase its viscosity, while reducing workability parameters [60]. For example, in [61], self-compacting concrete mixtures with a higher content of sodium hydroxide (12 M) showed lower fluidity than mixtures where an activator with a lower content of sodium hydroxide (8 M) was used. Similarly, in studies [62,63], the use of a more concentrated NaOH solution in the composition of alkaline activators impairs the workability of fresh geopolymer concrete, reducing cone settlement. Dispersed reinforcement with basalt fiber also does not significantly affect the density of fresh geopolymer concrete, but reduces its workability. As the amount of BF increases, the mixture slump decreases more and more. The decrease in workability of fresh geopolymer concrete with the introduction of fiber is explained by the fact that the fibers create additional friction forces between the mortar part and the coarse aggregate. In addition, part of the solution is spent on lubricating the BF, so the greater the amount of fiber, the greater the amount of the solution part is spent on lubricating the fibers and the more the cone settlement decreases [64,65]. In a study [66], increasing the weight content of BF from 1% to 2% led to a decrease in the slump of self-compacting geopolymer mixtures. In studies [67,68], the same picture was observed, namely, with an increase in the BF content, the workability of the mixture decreased.

The characteristics of hardened geopolymer concrete have been determined and the results are as follows. The density of geopolymer concrete in Figure 5 varies with the molarity of the activator and the amount of BF.

Figure 5 demonstrates that changes in alkaline activator composition and BF dosage within the tested ranges have minimal impact on the density of hardened geopolymer concrete. This influence is difficult to describe as a natural dependence. The density values of geopolymer concrete samples vary from 2268 kg/m³ to 2294 kg/m³. Figure 6 shows a graphical dependence of the compressive strength of geopolymer concrete on the molarity of the activator and the amount of BF.

Change in compressive strength f_cm of geopolymer concrete depending on the composition of the alkaline activator and the amount of BF (x in equations) was approximated by a polynomial of 3rd degree with the coefficient of determination R²

$$f_{cm}^{8M}=14.82+0.5023x+1.857x^2-0.866x^3,\hspace{1em}{R}^2=0.990$$

(1)

$$f_{cm}^{10M}=16.52+0.5643x+2.142x^2-1.0x^3,\hspace{1em}{R}^2=0.988$$

(2)

$$f_{cm}^{12M}=18.12+0.6262x+2.428x^2-1.133x^3,\hspace{1em}{R}^2=0.979$$

(3)

$$f_{cm}^{14M}=17.32+0.6333x+2.0x^2-0.933x^3,\hspace{1em}{R}^2=0.981$$

(4)

Figure 6 demonstrates that among the compositions of geopolymer concrete, unreinforced with BF, the most effective is the composition of the 12 M/BF0 type with a compressive strength of 18.1 MPa, where a NaOH solution with a molarity of 12 M was used in the preparation of the alkaline activator solution. The least effective was the 8 M/BF0 type composition with a compressive strength of 14.8 MPa, where an NaOH solution with a molarity of 8 M was used as an alkaline activator. The increases in compressive strength of compositions such as 10 M/BF0, 12 M/BF0, 14 M/BF0 compared to composition 8 M/BF0 were 11.5%, 22.3% and 16.9%, respectively. Dispersed BF reinforcement of all four types of compositions, where NaOH solution with different molarity was used, with an amount of fibers from 0% to 2.0%, has a positive effect on the compressive strength of geopolymer concrete. The values of increases in the strength of geopolymer concrete depending on the percentage of dispersed reinforcement BF are presented in

Table 3. Values of incremental compressive strength (∆f_cm) of geopolymer concrete.

BF (%)	∆fcm (%)
	Molarity of Solution NaOH
	8 M	10 M	12 M	14 M
0	0.0	0.0	0.0	0.0
0.5	4.7	4.8	5.0	4.6
1.0	9.5	9.7	9.9	9.2
1.5	14.2	14.5	14.9	13.9
2.0	10.1	10.3	10.5	10.4

. Compositions 8 M/BF0, 10 M/BF0, 12 M/BF0 and 14 M/BF0 were taken as control values for assessing the effectiveness of the influence of fiber on the compressive strength of geopolymer concrete.

Based on the increases in compressive strength presented in Table 3, it was found that the introduction of BF in an amount from 0% to 2% has a positive effect on this indicator for all compositions. Note that for all types of compositions, the peak value of the strength increase was observed at a BF amount of 1.5%, and at a BF of 2.0%, the efficiency of dispersed reinforcement decreased. Accordingly, the use of BF for these compositions of geopolymer concrete based on GGBS in an amount of more than 2% is impractical [68]. Next, Figure 7 demonstrates the dependence of the flexural strength of geopolymer concrete on the same two recipe indicators.

Change in flexural strength R_tb of geopolymer concrete depending on the composition of the alkaline activator and the amount of BF (x in equations) was approximated by a polynomial of 3rd degree with the coefficient of determination R²

$$R_{\mathrm{tb}8M}=1.629+0.0211x+0.7885x^2-0.3533x^3,\hspace{1em}{R}^2=0.974$$

(5)

$$R_{\mathrm{tb}10M}=1.839+0.0545x+0.9086x^2-0.4066x^3,\hspace{1em}{R}^2=0.983$$

(6)

$$R_{\mathrm{tb}12M}=2.124+0.0731x+1.0942x^2-0.4866x^3,\hspace{1em}{R}^2=0.975$$

(7)

$$R_{\mathrm{tb}14M}=2.0-0.1290x+1.202x^2-0.5066x^3,\hspace{1em}{R}^2=0.986$$

(8)

The flexural strength of geopolymer concrete (Figure 7) shows a similar trend to the compressive strength. The highest flexural strength among non-BF-reinforced geopolymer composites was possessed by the 12 M/BF0 type composition, where a NaOH solution with a molarity of 12 M was used as an alkaline activator, and the 8 M/BF0 type composition had the lowest strength. The increases in compressive strength of compositions 10 M/BF0, 12 M/BF0 and 14 M/BF0 compared to composition 8 M/BF0 were 13.0%, 30.3% and 22.8%, respectively. The flexural strength of all four compositions was also separately evaluated to determine the impact of dispersed BF reinforcement, using NaOH solutions of varying molarities.

Table 4. Values of increment in flexural strength (∆R_tb) of geopolymer concrete.

BF (%)	∆Rtb (%)
	Molarity of Solution NaOH
	8 M	10 M	12 M	14 M
0	0.0	0.0	0.0	0.0
0.5	13.0	13.7	15.2	10.0
1.0	25.3	27.9	28.9	25.0
1.5	40.7	43.7	47.4	45.0
2.0	22.8	26.8	29.9	25.1

presents the values of flexural strength increases based on the introduced fiber amount.

The best enhancement in flexural strength was observed at 1.5% BF, similar to compressive strength. Importantly, the improvement in flexural strength exceeds the improvement in compressive strength, implying superior fiber performance in the composite under bending loads [69].

Based on the results of determining the compressive strength and flexural strength of geopolymer composites, it was found that the use of an alkaline activator made with the addition of NaOH solution (12 M) is most suitable for activating GGBS particles with a given granulometry and allows obtaining the highest strength characteristics. In studies [18,70], the use of a 12 M NaOH solution in the composition of alkaline activators used for mixing geopolymer concrete with GGBS showed the best efficiency. The best values of strength characteristics were recorded when 1.5% BF was added to the geopolymer concrete mixture. The optimal amount of fibers in the geopolymer matrix makes it possible to obtain a composite with higher compressive and flexural strength. However, an excessive amount of BF in this case, 2% or more, leads to poor compaction and increased porosity, which will reduce the strength properties [60]. The positive effect of basalt fiber at optimal dosages on the strength characteristics of geopolymer composites is also confirmed by a number of the following studies [57,58,64,65]. In general, the mechanism of action of BF at an amount of 1.5% can be explained as follows. Fibers, both at the macro and micro levels, act as a kind of bridge and are connected to the geopolymer matrix by adhesion forces. Accordingly, when exposed to destructive loads, cracks form in the body of the geopolymer composite and, when approaching the fiber, additional energy is required to detach the fiber from the matrix. Thus, the presence of fiber in the geopolymer takes on part of the destructive load, and the composite itself does not fail brittlely, but with many small cracks branched over the entire surface [71]. The process of destruction of geopolymer FRC with the formation of many small cracks on its surface is presented in Figure 8.

The mechanics of geopolymer concrete’s destruction are altered by the addition of BF, as shown in Figure 8. When a load is applied, the destruction of the sample occurs as follows. First, deformation of the sample is observed, and then, at a critical load value, destruction occurs with the formation of a network of cracks on all its faces. Analysis of the nature of the destruction of the sample after failure made it possible to identify the peculiarities of the influence of the addition of BF on this process. The addition of BF made it possible to give the sample a more viscous fracture pattern. The fracture mechanics of the sample with fiber has changed compared to the twin sample without BF. This fracture toughness leads to increased deformability, which in some cases will be a significant advantage. For example, in case of application in non-traditional operating conditions, for example, in seismic areas. The nature of the destruction of the sample after the introduction of fiber shows that for some time after failure the sample continues to deform due to the fibers introduced into it, and the mechanics of the process makes it possible to move from a brittle nature of destruction to a more ductile one.

Next, we will consider the influence of the applied formulation solutions on the change in water absorption of geopolymer concrete (Figure 9).

Change in water absorption W (%) of geopolymer concrete depending on the composition of the alkaline activator and the amount of BF (x in equations) was approximated by a polynomial of 3rd degree with the coefficient of determination R²

$$W_{8M}=6.485-0.1043x-1.183x^2+0.520x^3,\hspace{1em}{R}^2=0.987$$

(9)

$$W_{10M}=5.525+0.01111x-1.231x^2+0.526x^3,\hspace{1em}{R}^2=0.976$$

(10)

$$W_{12M}=4.309-0.0397x-0.914x^2+0.393x^3,\hspace{1em}{R}^2=0.982$$

(11)

$$W_{14M}=4.879-0.1156x-0.8585x^2+0.3793x^3,\hspace{1em}{R}^2=0.981$$

(12)

Figure 9 shows that among compositions of the 8 M/BF0, 10 M/BF0, 12 M/BF0 and 14 M/BF0 types, geopolymer concrete with the alkaline activator 12 M NaOH has the lowest water absorption. Compared to the 8 M/BF0 composition, the water absorption of the 10 M/BF0, 12 M/BF0 and 14 M/BF0 compositions decreased by 14.37%, 33.23% and 24.42%, respectively. Dispersed BF reinforcement of a geopolymer composite up to 1.5% inclusive helps reduce water absorption, however, with a fiber content of 2% its effectiveness decreases. The effect of dispersed BF reinforcement on water absorption, as in the case of strength characteristics, was assessed separately for all four types of compositions, where NaOH solutions with different molarities were used. The differences in water absorption of geopolymer concrete compositions depending on the amount of BF are presented in

Table 5. Values for reducing water absorption of geopolymer concrete compositions.

BF (%)	Water Absorption Reduction (∆W), %
	Molarity of Solution NaOH
	8 M	10 M	12 M	14 M
0	0.0	0.0	0.0	0.0
0.5	5.26	5.60	5.79	5.73
1.0	10.97	11.19	11.81	11.04
1.5	17.31	18.95	19.44	18.0
2.0	12.06	12.45	13.66	12.90

.

Thus, experimental studies show that the lowest values of water absorption of polymer concrete are observed in compositions reinforced with BF in an amount of 1.5%; in general, a decrease in water absorption caused by a decrease in the introduction of BF in the optimal amount is associated with an improvement in strength characteristics and the determination geo-content distribution of particles in the stage of polymer concrete. composite [18,19].

Assessing the composition of the alkaline activator and dispersed reinforcement with basalt fiber in the mechanical and physical characteristics of geopolymer composites, the following conclusions can be drawn.

The most effective composition of the alkaline activator contains a NaOH solution with a molarity of 12 M. The higher the molarity of the NaOH solution, the greater its conservative activity. A high open capacity ensures the activation of a larger number of GGBS particles, which ultimately leads to a stronger and denser composite structure [60]. The aluminosilicate component used in this case in the form of GGBS with a certain granulometry has the best compatibility when included with an alkaline activator, where a NaOH solution with a molarity of 12 M is used. The deterioration of the properties of the composite when using an alkaline activator, which includes a NaOH solution with a molarity of 14 M, is due to the fact that with these components the viscosity of the NaOH solution of the polymer geomixture, and the geopolymer composite itself hardens faster. All this can lead to the formation of shrinkage cavities and microcracks in the body of the composite, which subsequently reduces the strength and maintains the geoconcrete polymer [63].

The most effective dosage of basalt fiber is 1.5% by weight of GGBS. Dispersed reinforcement of BF geopolymer concrete in this document is the most justified and allows increasing tensile and flexural strength, as well as reducing water absorption [40]. The increase in tensile and flexural strength with dispersed reinforcement is due to the fact that when stresses arise in the body of the geopolymer composite, it transfers the main share of stressed basalt fibers due to tangential forces, which are retained at the phase interface. Basalt fibers have a fairly high modulation of elasticity and absorb the greatest part of the stresses, thereby maximizing the strength of the composite [72].

When analyzing the results of experimental studies, attention should be paid to the microstructure of geopolymer concrete (Figure 10).

The SEM analysis of the microscopic structure of the forming geopolymer composites revealed the following aspects. Firstly, it should be noted that microscopic analysis was carried out on specially prepared samples aimed at studying the influence of the molarity of the alkaline activator used in geopolymer concretes. The special samples studied showed significant differences between the control composition sample, manufactured using the least rational alkaline activator, and the best sample, which was manufactured with the most rational dosage of the alkaline activator. These differences are expressed in the following. Firstly, the packing of particles in the best geopolymer concrete is significantly superior in terms of the density of its arrangement to its counterpart in the form of a control composition. Secondly, the micrograins of the structure look more structured and closer to each other, that is, the contact surface of the grains is in a more perfect form, which makes it possible to form a structure not only based on the particle packing density, but also based on the location of these grains in space in the concrete body. The grains of the forming geopolymer concrete have a conventional oblong shape, which can be located in the form of “plane-to-plane” contacts, or in the form of “point-to-point” contacts. In this case, we observe that in the most rational geopolymer composition, using a rational dosage of an alkaline activator, the grains are located with the best combination of conventional planes and sharp edges. Under such circumstances, at the level of SEM analysis when studying the microstructure, the effectiveness of the applied composition with an alkaline activator molarity of 12 M is confirmed.

The experiments carried out and the results obtained made it possible to draw a parallel with already known information regarding geopolymer concretes [73,74]. The discussion of the results obtained will be divided into two main branches. This is the research scientific novelty of the data obtained and practical applied novelty, defined as the usefulness of new knowledge. In terms of the research scientific novelty of the study, it is worth reflecting for the first time the patterns of the complex influence of two recipe and technological factors. Firstly, this is dispersed reinforcement of geopolymer concrete with fiber. In itself, this method is already quite effective, a proven option for changing the characteristics and structure of geopolymer concrete, giving it a different nature of destruction, more viscous, as well as the entire mechanics of the structure made from such concrete. This is in good agreement with the works of other authors who used various types of fiber in geopolymer concrete technology [36,37,38,39,40,41].

In terms of additional comprehensive research, it should be noted that the obtained effect from the influence of a rational choice of the composition of the alkaline activator in combination with the most effective fiber is important. Basalt fiber has proven its superiority and such a complex recipe-technological method as the use of the most optimal composition of an alkaline activator and the most optimal fiber, made it possible to obtain an increase in compressive and flexural strength of 14.92% and 47.39%, respectively, in the composition of type 12 M/BF1.5, and also a reduction in water absorption by 18.75% compared to composition type 12 M/BF0. In comparison with the 8 M/BF0 type composition, which has the worst characteristics, the increases in compressive and flexural strength were 40.54% and 91.98%, and water absorption decreased by 46.21%.

It’s important to analyze the underlying research interpretation of the result. Geopolymer concrete is a conglomerate that undergoes complex physical and chemical transformation s to develop a stone-like structure with a particle arrangement determined by its physical properties. By interfering in the process of structure formation, directing and regulating this process with recipe and technological factors, it is possible to achieve a redistribution of the structure in terms of particle packing, making it denser. This is in good agreement with the results of [42,60,63].

As for compressive strength, we are talking about a complex effect that arises not only from the addition of fiber itself, but also from a change in the molarity of the activator used. It should be noted that in its original form, as a control composition, a geopolymer concrete composition with the lowest molarity and also without basalt fiber was used. However, in the best composition, in addition to the rational dosage of basalt fiber, an activator with the most rational molarity was also used. In total, these two recipe and technological aspects made it possible to obtain a peak increase in the “compressive strength” indicator. That is, this increase occurred not only as a result of one fiber, but as a result of the complex effect of “fiber + activator molarity”. Therefore, this is precisely what explains such a fairly significant increase in this indicator.

Controlling the structure of geopolymer concretes makes it possible to change the mechanics of their work in structures from an applied point of view. The practical novelty of the research is as follows. It turned out that the introduction of basalt fiber in combination with the best type of alkaline activator affects the levels of micro- and macrostructure formation of the new geopolymer FRC, changing the nature of internal stresses in concrete when creating a load. This was well confirmed by the results of strength tests, which showed a completely different level, and the quantitative excess of the compressive and flexural strength of geopolymer FRC of composition 12 M/BF1.5 with the best type of alkaline activator turned out to be 14.92% and 47.39% better than that of geopolymer composition 12 M/BF0 without fiber. Thus, having discussed the results obtained and analyzed them from fundamental and applied points of view, we can note the clear promise of continuing these studies in the future. We believe that the prospects for the development of this research lie in the study of other types of fibers in geopolymer concretes, in the search for the best combination of different types of fibers with different types of alkaline activators and fillers of geopolymer concretes. At the same time, it is important to understand that in each specific case there will be a specific relationship between the compositions, structure and properties of the geopolymer FRC being created. However, our research provides an excellent basis for future research to build on. Recommendations for the applied construction industry consist in the proposed specific recipe and technological recommendations for the creation of building structures, both compressed and bendable, from new geopolymer FRC based on basalt fiber. Geopolymer concrete, as a compressible material, will be even more effective from the introduction of fiber, but the effect will be even more pronounced if the new geopolymer FRC is used in bendable structures. The fiber will increase the tensile strength during bending of the concrete itself, and will also create micro-reinforcement due to dispersed fiber, and all this can give bendable fiber geopolymer concrete structures better load-bearing capacity when used at construction sites.

4. Conclusions

Research has been conducted on different formulation solutions to enhance geopolymer concrete properties by incorporating ground granulated blast furnace slag. The fresh properties and physical and mechanical characteristics of geopolymer concrete were evaluated based on the composition of the alkaline activator and dispersed reinforcement with basalt fiber.

(1)

The introduction of basalt fiber into the composition of geoplastic concrete in an amount from 0% to 2%, as well as the use of activators of various compositions, does not have a significant effect on the change in the density of both fresh and hardened concrete. At the same time, the workability of the geopolymer mixture changes. A decrease in cone settlement is observed with increasing concentration of the alkaline activator and the amount of fiber.

(2)

Increasing the molarity of NaOH solution from 8 M to 14 M has a positive effect on the strength characteristics and water absorption of geopolymer concrete. As is known, as the concentration of NaOH solution increases, the reactivity also increases. The higher the reactivity, the more active the geopolymerization reaction occurs and the more GGBS particles are activated. The most effective alkaline activator composition for GGBS particles of this granulometry is an activator with a 12 M NaOH solution. The strength of geopolymer concrete increased by 22% in compression and by 30% in bending, and water absorption decreased by 33%.

(3)

The introduction of basalt fiber into the composition of geopolymer concrete has a positive effect on its properties. For all applied compositions, the best characteristics were recorded when fibers were introduced in an amount of 1.5%. The increase in strength was: in compression-up to 15%, in bending-up to 48%, water absorption decreased to 20%.

(4)

In the course of experimental studies, an effective composition of geopolymer FRC based on ground granulated blast furnace slag with the following recipe was developed: GGBS—408 kg/m³; QS—554 kg/m³; CrS—1094 kg/m³; NaOH (12 M)—40 kg/m³; Na₂O(SiO₂)n—110 kg/m³; BF—6.1 kg/m³; P—24.5 kg/m³; water—55 kg/m³).

Thus, based on the results of the study, an effective geopolymer FRC with a compressive strength of 20.8 MPa, a flexural strength of 3.11 MPa and a water absorption of 3.48% was developed. A composite with these physical and mechanical characteristics can be used both for the manufacture of various construction products and for the manufacture of monolithic structures on a construction site. Continuation of the research is planned in the study of other types of fibers in geopolymer concrete, in search of the best combination of different types of fibers with different types of alkaline activators and geopolymer concrete fillers.