Nanomodified Basalt Fiber Cement Composite with Bottom Ash

Abstract

Directed control of the process of structure formation of a cement composite from modern positions must be carried out taking into account the synergistic effect of its components. In particular, the cement composite, when applied with pozzolanic additives and fiber reinforcement, is transformed into a more complex material with excellent performance. The aim of the article is to study the combined action of nanomodified basalt fiber (NBF) and bottom ash (BA) as structural elements of concrete. To achieve this aim, a number of tasks were performed, including the development of nanomodified-basalt-fiber–bottom-ash–cement concretes, as well as the study of their fresh, physical and mechanical properties (flowability, average density, compressive and flexural strength, elastic modulus and crack resistance) and durability characteristics (water, frost and abrasion resistance). A series of nanomodified basalt-fiber-reinforced concretes (from 0 to 7 wt.% NBF) were developed, in which the economical Portland cement CEM I 32.5 N was replaced by up to 45 wt.% mechanically activated bottom ash residue (400 m²/kg). An economical superplasticizer with a high water-reducing capacity (35%) made it possible to achieve uniform flowability of the mixes (slump 20–22 cm and slump flow 45–52 cm). The combined effect of BA and NBF provides control over the structure formation of cement materials, which ensures the redistribution of internal stresses from shrinkage deformations throughout the entire volume of the composite; under loading, the process of crack formation slows down, the stress concentration near structural defects decreases, and stresses are redistributed in the microstructure of the cement composite between its components. Perfect values of mechanical properties (compressive strength up to 59.2 MPa, flexural strength up to 17.8 MPa, elastic modulus up to 52.6 GPa, critical stress intensity factor 0.507 MPa m^0.5) are explained by the complex action of the ash residue and nanomodified basalt fibers. A mix with 30 wt.% BA and 5 wt.% NBF is characterized by water resistance grade W18, frost resistance class F400 and abrasion resistance 0.59 g/cm², which confirms the high wear resistance of the developed materials.

1. Introduction

By the beginning of the 21st century, traditional reinforced concrete had evolved into a cementitious composite, which, with the addition of pozzolanic additives and reinforcing fibers, was transformed into a more complex material with superior performance [1]. These benefits are achieved by using various silica substituents of the cement, chemical modifiers and fibers [2].

Some pozzolanic additives with silica or aluminosilicate composition are used as substitutes for part of the cement. A fairly well-studied additive in binders is fly ash, both acidic and basic, with an effective replacement for Portland cement up to half by weight [3,4,5]. In addition, various materials studied in sufficient detail include metakaolin [6], red mud [7], ground blast furnace slag [8], rice husk ash [9] and coal ash [10]. Promising material such bottom ash (BA) was studied in [11,12,13], where the high potential of its use as a structure-forming component of the concrete mix was proved. Chindasiraphan et al. [14] indicated that concrete with 50% bottom ash cement replacement could be classified as high-strength concrete at 7 days, achieving a compressive strength of approximately 80.9 MPa at 28 days. Summarizing and analyzing recent papers [15,16,17] revealed a maximum increase in mechanical properties by 20–30% (compressive and flexural strength, modulus of elasticity) as well as durability performances by 25–40% (water absorption, water resistance, frost resistance, abrasion resistance) due to the inclusion of BA. At the same time, the optimal dosage at a level of 15–20%, as well as the required degree of enrichment of the bottom ash, is important. In the case of using the original (unenriched) BA, the concrete will weaken at any dosage. Despite this, the existing work on bottom ash is clearly not enough; moreover, there are no studies on the combined effect of BA and new material such as nanomodified basalt fiber on new-generation cement composites.

A modern material such as fiber-reinforced concrete is a kind of cement composite, a feature of which is the use of dispersed fibers in the concrete mix [18]. Dispersed fibers can be either natural (sisal [8], jute [19], pineapple [20], etc.) or synthetic (steel [21], glass [22], polypropylene [23], etc.). The fiber-reinforced concrete is far superior to its predecessors [24,25]. It has number of positive qualities: the material is resistant to abrasion and chemical attack [26]; it has high strength and may exhibit high stiffness [27]; with such a frame, sometimes there is no need for additional reinforcement [28]; it perfectly perceives flexural and tensile forces [29] and perfectly counteracts the formation of chips and cracks [30]; it can be used in conditions of frost and heat and is moisture resistant [31]. Due to small reinforcing elements, the material can be given any shape [32]. The durability of buildings erected from fiber-reinforced concrete exceeds the usual indicators by several times [33]. The advantage of basalt fiber over metal ones is that, firstly, basalt fiber does not have a negative cathodic effect in products, and is also not subject to any corrosion [34]. In terms of volume, one metal fiber with a diameter of 1 mm corresponds to more than 600 basalt fibers, while the surface area of the basalt fiber is 25 times greater [35]. Specific gravity of steel fiber is 7.8, and that of basalt is 2.8 [36]. This allows the use of 2.7 times less basalt fiber by weight, so such building products are lighter [37]. Basalt-fiber concretes are radio transparent and do not have the effect of a conductor [38]. Steel fibers are produced in different configurations including wavy, with flattened and bent ends to increase anchoring, due to the weak adhesion of the metal and the cement matrix [39]. High adhesion of basalt fiber with cement paste allows the use of fibers with no additional configuration changes [40]. Superior modification of basalt fiber is the application of fulleroid-type nanoadditives to its surface [41]. During a literature search [42,43], very few studies were found on the use of nanomodified basalt fibers; however, high potential for the whole complex of physical and mechanical properties and durability performance was noted. Depending on the content of nanomodified basalt fibers used in the dispersion method, compressive strength increases from 15.7% to 23.3%, and flexural strength increases from 28.5% to 31.1%. Pozzolanic additives and basalt fiber have a high water demand and affect concrete workability, so it is necessary to use various chemical modifiers in them [44,45]. The optimal dosage of nanomodified basalt microfiber is 1.0–1.5% by weight of the binder [46,47,48,49,50].

In the literature analysis, a lack of research on the integrated use of bottom ash and nanomodified basalt fiber in composites based on low-grade Portland cement (CEM I 32.5 N) was revealed to improve the complex of fresh, physical and mechanical properties and durability characteristics. Therefore, the aim of this article is to study the combined effect of NBF and BA as structural elements of concrete. To achieve this aim, a number of tasks were performed, such as the development of nanomodified-basalt-fiber–bottom-ash–cement concretes, as well as the study of their fresh, physical and mechanical properties (flowability, average density, compressive and flexural strength, elastic modulus and crack resistance) and durability characteristics (water, frost and abrasion resistance).

2. Materials and Methods

2.1. Raw Material Characterization

The basis of the binder was Portland cement CEM I 32.5 N (Spasskcement, Russia). Portland cement t with the chemical composition shown in

Table 1. Chemical and mineral composition of the Portland cement used, wt.%.

Chemical Composition					Mineral Composition
CaO	SiO2	Al2O3	Fe2O3	MgO	C3S	C2S	C3A	C4AF
64.7–65.2	21.1–21.5	5.2–5.4	3.7–4.2	2.3–3.0	62–65	11–14	6–7	11–14

and properties shown in

Table 2. Physical and mechanical properties of CEM I 32.5 N.

Compressive Strength, MPa		Setting Time, min		Passing through the Sieve 008, %	Specific Surface Area, m2/kg	Standard Consistency, %
2 Days	28 Days	Start	End
14.0–18.8	38.0–47.0	130–240	225–360	88–90	290	22.25–26.25

is the most common in wide construction, and the results obtained are of great practical importance.

Photographs of the Portland cement, bottom ash, superplasticizer, sand and nanomodified basalt fiber are shown in Figure 1.

BA from the Primorskaya thermal power plant (Luchegorsk, Russia) was used. An SEM image and the EDS pattern of the BA are shown in Figure 2. EDS was taken at 16 points of the sample, while the results shown in Figure 2 are representative of any part of the ash.

The analysis of the particle size distribution of the binder obtained by joint grinding of Portland cement and bottom ash in a ball mill for 30 min is shown in Figure 3, from which it can be concluded that the average particle size is 12.17 µm.

River sand was used as an aggregate, having an average fineness modulus of 2.36 (Razdolnoye deposit, Russia). Specific gravity of sand was 2.6, bulk density was 1400 kg/m³, maximum particle size was 2.5 mm, and water absorption by weight was 0.5%. The composition of the sand includes a large number of minerals, mainly quartz, feldspars, calcite and mica. The chemical composition of the sand includes 98% SiO₂.

The mixing water used did not contain dissolved acids or alkalis that prevent the normal setting or hardening of binders; harmful impurities; or decomposing plant substances that can have a harmful effect on concrete hardening.

The superplasticizer C-3 based on lignosulfonate in liquid form (Vladimirsky ZhBK, Russia) was used to reduce the water–cement ratio.

Nanomodified basalt fiber (Alliance, Russia) was used as a dispersed reinforcement (

Table 3. Characteristics of NBF.

Characteristics	Value
Tensile strength	3.5 GPa
Elastic modulus	75 GPa
Fiber diameter	13 µm
Fiber length	6 ± 1.5 mm
Elongation coefficient	3.2%
Melting temperature	1450 °C
Density	2600 kg/m3
Alkali resistance	high
Corrosion resistance	high

).

2.2. Mix Design

The mix design was developed to evaluate the mutual influence of bottom ash and nanomodified basalt fiber on various concrete characteristics (

Table 4. Mix proportions, kg.

Mix ID	CEM	BA	NBF	Water	SP	Sand
0-0	500	0	0	175	5.0	1500
0-3	500	0	15	175	5.3	1500
0-5	500	0	25	175	5.6	1500
0-7	500	0	35	175	5.9	1500
15-0	425	75	0	175	6.0	1500
15-3	425	75	15	175	6.3	1500
15-5	425	75	25	175	6.6	1500
15-7	425	75	35	175	6.9	1500
30-0	350	30	0	175	7.0	1500
30-3	350	30	15	175	7.3	1500
30-5	350	30	25	175	7.6	1500
30-7	350	30	35	175	7.9	1500
45-0	275	45	0	175	8.0	1500
45-3	275	45	15	175	8.3	1500
45-5	275	45	25	175	8.6	1500
45-7	275	45	35	175	8.9	1500

). The dosage of the superplasticizer was adopted to ensure that almost equal slump and slump flows are achieved.

Milling and mixing of the binder components (Portland cement and bottom ash) were carried out in a ball mill for 30 min until a specific surface area of 400 m²/kg was reached (for an adequate comparison, the mixes without BA were also subjected to similar grinding). After that, the components were mixed for 10 min in a concrete mixer, where the fiber was introduced in doses and in stages to avoid clumping.

After the concrete mix was brought to the desired consistency, molds measuring 70 × 40 × 160 and 40 × 40 × 160 mm³, as well as cylindrical molds with a diameter of 150 mm and a height of 30 mm, were filled. Initially, 1/3 of a mold was filled and consolidated, then 2/3 of the mold was filled and consolidated again, after which the mold was filled “with a slide” and placed on a vibrating table (Figure 4). Compaction of the concrete mix was carried out in accordance with the requirements of the Russian Standard GOST 7473-2010 [51].

Cutouts in the manufacture of cubic samples with an edge of 70 mm (for crack resistance testing) were formed by inserting wooden rulers 2 × 25 mm in size into the mold form. Superglue (cyanoacrylate) was used to bind the rulers in the shape of the letter “H” so that the two rulers were in the same plane with edges to the opposite walls of the mold from the inside and reached the bottom of the mold. A third ruler was attached on top of the walls of the mold and connected them, in order to avoid displacement during vibration. In addition, the rulers were glued to the form itself to ensure the purity of the experiment. The possibility of removing the rulers without harm to the samples was provided by the fact that the ends inside the sample were wrapped with cling film.

As a result, the cubic samples acquired notches in the middle of two opposite sides with a depth of 20 mm. After 24 h, the samples were removed from the molds and left to harden for up to 28 days under standard temperature (18–22 °C) and humidity (relative humidity 100%) conditions (Figure 5).

Each test was investigated on six samples. The error of all obtained experimental results was less than 5%.

2.3. Equipment and Methods

2.3.1. Material Morphology

To study the Portland cement mineral composition, a D8 Advance AXS X-ray powder diffractometer (Bruker, Billerica, MA, USA) was used (wavelength λ = 1.5418 Å) using Rietveld refinement. The percentage of oxides and minerals in the Portland cement was determined by the standard method of X-ray fluorescence analysis.

Microstructure of the bottom ash and concrete was studied using a MIRA3 scanning electron microscope (SEM) (Tescan, Brno, Czech Republic), which makes it possible to carry out energy dispersive spectroscopy. The samples used in the SEM analysis of binders were internal fragments of the sample after fracture. When preparing the surface of the SEM samples, polishing was not performed.

2.3.2. Granulometry

The specific surface of bulk raw materials was studied using a PSH-11 device (Khodakov Devices, Moscow, Russia). The granulometry of the particles of the raw materials was evaluated using a laser analyzer Analysette 22 (Fritsch, Idar-Oberstein, Germany).

According to the Russian standard GOST 8736-2014 [52], the sand fineness module is determined as follows. From 2 kg of sample, particles larger than 5 mm are first separated using sieves, and 1 kg is taken from the remaining mass and sieved sequentially through sieve cells ranging in size from 2.5 mm to 0.16 mm (5 sieves in total). The amount of sand, as a percentage of 1000 g, not sieved through each sieve, is recorded. Screening during operations is stopped when the sand stops passing through the cells. Sand size modulus is calculated by the formula: Mk = (A2.5 + A1.25 + A0.63 + A0.315 + A0.16)/100, where A is the total residue on five sieves as a percentage of the total weight.

2.3.3. Fresh Properties

The setting time of the Portland cement was determined by the loss of flowability. The beginning of the loss of flowability (or plasticity) of the cement paste is taken as the starting point, and some of its hardening is taken as the end point.

The essence of the method for testing the standard consistency of the cement paste is to determine, by successive attempts, the amount of water required for the cement to form a paste of standard consistency (in %). During testing, the pestle in a Vicat device should not reach the bottom (plate) by 5–7 mm.

The slump of the concrete mix is determined by laying a metal ruler with an edge on top of the cone and measuring the distance from the lower edge of the ruler to the top of the concrete mixture with an error of up to 0.5 cm. The slump flow of the concrete mix is determined by measuring the diameter of the spread paste with a metal ruler in two mutually perpendicular directions with an error of not more than 0.5 cm.

2.3.4. Physical and Mechanical Properties

The value of the average density of the samples was calculated by dividing the mass by the volume. The compressive strength was determined according to the standard method of the Russian standard GOST 310.4-81 [53] on cubes with an edge of 70 mm. The flexural strength was determined by the three-point method on specimens of prismatic shape 40 × 40 × 160 mm.

To determine the modulus of elasticity of concrete by the magnitude of elastic-instantaneous deformations, tests are carried out on specimen prisms with a size of 40 × 40 × 160 mm according to the standard method of the Russian standard GOST 310.4-81 [53]. Measurement of prism deformations is carried out with an accuracy of at least 1 × 10⁻⁵ relative units using strain gauges installed on each face of the sample. Prior to the implementation of the stepwise loading mode of the prism, the test loads are centered along the physical axis of the sample. As a first approximation, the prism is mounted on the base plate of the press centered along the geometric axis. Then, at a speed of 0.2–0.3 MPa/s, a force is transferred to the sample up to the size of the first loading stage, the value of which is assumed to be 10% of the expected destructive force (P_u). In this case, the largest difference in the increment of deformations on opposite faces should be no more than 20% of the average deformation of the sample; otherwise, the centering of the load is refined by moving the prism towards the faces that deform to a greater extent. When the load is removed during the alignment process, the initial readings of the strain gauges are refined by introducing corrections to compensate for the “zero drift” of the instruments. Further loading of the sample is carried out in steps of 0.1 × P_u. At each stage of the load, a constant force is maintained for 5 min, during which readings are taken on the instruments at the time of application of the next stage of the load (at the beginning of the stage) and after exposure (at the end of the stage). Based on these measurements, the fraction of the elastic component is separated from the total deformations of concrete, and the elastic modulus is calculated from the value of this component by Equation (1):

$${\mathrm{E}}_{\sigma }=\frac{{\sigma }_1}{{\epsilon }_{1\mathrm{y}}}$$

(1)

where σ₁ is the strain increment up to 30% of the breaking load; ε_1y is the increase in specimen deformation corresponding to the load level P₁ = 0.3P_b, where P_b is the breaking load, and P₁ is the corresponding increase in external load.

Cubic specimens with cutouts were used to study the crack resistance of concrete composites. Figure 6 shows the specimen form and scheme of its loading. A SHIMADZU AGS-10kN testing machine was used for eccentric compression tests. Loading rate of the specimens was set according to the speed of movement of the loading plate of the press of 0.1 mm/s; in this case, the testing time for each specimen was 2–3 min. The critical value of the stress intensity factor was determined by Equation (2) according to the Russian standard GOST 29167-2021 [54]:

$$K_{1c}=\frac{F}{b{L}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}(1.83{\lambda }^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}-430{\lambda }^{\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}+3445{\lambda }^{\raisebox{1ex}{$5$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}-11076{\lambda }^{\raisebox{1ex}{$7$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}+12967{\lambda }^{\raisebox{1ex}{$9$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$$

(2)

where λ = (a + a₀)/L; F is the load at which failure occurs, MN; b is sample width, m; L is sample height, m; and a₀ is the depth of cutouts, m.

The loading rate of the specimens was uniform throughout the entire test time and was 7 kN/min (compressive strength and modulus of elasticity) and 3 kN/min (flexural strength).

2.3.5. Water Resistance, Freeze–Thaw Resistance and Abrasion Resistance

Cylindrical specimens with a diameter of 150 mm and a height of 30 mm were used for the tests to study the water resistance according to the wet spot method with a gradual application of pressure. Freeze–thaw resistance of concrete was studied, guided by the Russian standard GOST 10060-2012 [55], on specimens with an edge of 10 cm, saturated with mineralized water. Specimen abrasion resistance was studied according to the method of the Russian standard GOST 13087-2018 [56] using an LKI-3 abrasion wheel (Moscow, Russia) on cubic samples with an edge of 70 mm. The calculation method was used to estimate the difference in the mass of the sample between the initial state and after passing 600 m of the abrasive wheel.

3. Results and Discussion

3.1. Slump and Slump Flow

Uniform flowability of the fresh mixes (slump 20–22 cm) was achieved by varying the dosage of an economical superplasticizer with a high water-reducing ability (35%) (

Table 5. Fresh properties of the concrete mixes.

Mix ID	Slump, cm	Slump Flow, cm
0-0	22 ± 0.5	52 ± 1
0-3	22 ± 0.5	52 ± 1
0-5	22 ± 0.5	51 ± 1
0-7	22 ± 0.5	51 ± 1
15-0	22 ± 0.5	50 ± 1
15-3	22 ± 0.5	50 ± 1
15-5	22 ± 0.5	49 ± 1
15-7	22 ± 0.5	49 ± 1
30-0	22 ± 0.5	49 ± 1
30-3	22 ± 0.5	48 ± 1
30-5	22 ± 0.5	48 ± 1
30-7	22 ± 0.5	47 ± 1
45-0	20 ± 0.5	46 ± 1
45-3	20 ± 0.5	46 ± 1
45-5	20 ± 0.5	45 ± 1
45-7	20 ± 0.5	45 ± 1

). A slight decrease in the slump flow (up to 45 cm) was observed at a dosage of bottom ash in the amount of 45 wt.% for unmodified mixes (47–52 cm depending on the superplasticizer content).

All mixes have almost the same slump (20–22 cm), but slightly different slump flow (46–52 cm). These are related to the methodology of these tests and are supported by references to relevant previous literature [57,58].

Experimental results show a decrease in the slump flow with an increase in the content of both BA and NBF (Figure 7). At the same time, with a content of up to 15% of BA, the dosage of bottom ash does not have any effect on the tendency to reduce the slump flow. When the content of BA increased from 15 to 45 wt.%, a mutual influence of bottom ash and nanomodified basalt fiber on fresh properties is noted.

3.2. Density, Air Content, Strength and Elastic Modulus

Density, air content, strength and elastic modulus of the developed concretes are listed in

Table 6. Physical and mechanical properties of the composite mixes.

Mix ID	Density, kg/m3	Air Content, %	Compressive Strength, MPa	Flexural Strength, MPa	Flexural Strength/Compressive Strength	Elastic Modulus, GPa
0-0	2090 ± 1	2.0 ± 0.05	35.6 ± 0.3	3.9 ± 0.1	0.11	28.7 ± 0.6
0-3	2098 ± 3	1.9 ± 0.04	36.9 ± 0.7	5.9 ± 0.2	0.16	28.9 ± 0.3
0-5	2104 ± 2	1.8 ± 0.02	38.2 ± 1.3	9.6 ± 0.2	0.25	29.3 ± 0.9
0-7	2109 ± 1	1.9 ± 0.05	37.4 ± 0.3	6.7 ± 0.2	0.18	29.0 ± 0.6
15-0	2099 ± 2	1.7 ± 0.02	44.6 ± 0.6	6.2 ± 0.2	0.14	40.1 ± 0.9
15-3	2009 ± 3	1.6 ± 0.03	47.9 ± 1.3	9.1 ± 0.2	0.19	40.6 ± 0.3
15-5	2113 ± 1	1.5 ± 0.05	48.2 ± 0.4	13.5 ± 0.3	0.28	40.8 ± 0.9
15-7	2120 ± 3	1.6 ± 0.04	47.4 ± 0.9	10.0 ± 0.3	0.21	40.7 ± 0.6
30-0	2111 ± 2	1.3 ± 0.05	56.6 ± 1.2	9.1 ± 0.2	0.16	51.6 ± 1.1
30-3	2117 ± 1	1.1 ± 0.05	56.9 ± 1.4	12.4 ± 0.3	0.21	51.7 ± 0.3
30-5	2125 ± 3	1.1 ± 0.03	59.2 ± 1.3	17.8 ± 0.3	0.30	52.6 ± 1.1
30-7	2128 ± 2	1.3 ± 0.05	58.4 ± 0.5	13.4 ± 0.3	0.23	52.3 ± 0.6
45-0	2109 ± 1	1.6 ± 0.05	43.6 ± 1.3	3.9 ± 0.1	0.09	40.2 ± 1.1
45-3	2119 ± 2	1.5 ± 0.04	48.9 ± 0.5	6.8 ± 0.2	0.14	40.5 ± 0.3
45-5	2123 ± 3	1.4 ± 0.05	49.2 ± 0.7	11.3 ± 0.3	0.23	40.9 ± 0.9
45-7	2120 ± 1	1.5 ± 0.05	47.5 ± 0.3	7.6 ± 0.2	0.16	40.6 ± 0.6

.

The calculated air content (from the composition and the density) is in the normal range, but each 1% of air is equivalent to 10 L/m³ of water for the W/C ratio.

The increase in the ratio of flexural to compressive strength by a factor of 3 (from 0.11 to 0.30) can be caused by a longer plastic zone before failure of the developed cement composite.

Experimental results show an increase in flexural strength with increasing content, as the BA increases up to 30%, after which there is a decrease (Figure 8). A similar trend is observed for all dosages of NBF. The limit of fiber saturation is 5%, which allows the creation of an optimal structure for work, both in compression and in flexion. The curves do not intersect, so there is no mutual influence of bottom ash and nanomodified basalt fiber.

The significant increase in the entire complex of physical and mechanical properties is explained by the comprehensive effect of bottom ash and nanomodified basalt microfiber as a result of an increase in the volume of new hydrate products due to the acceleration of the hydration kinetics of clinker minerals and the formation of the second-generation hydration new growth.

3.3. Critical Stress Intensity Factor

Table 7. Crack resistance of the basalt fiber concrete at 28 days.

Mix ID	F, N	K1c, MPa·m0,5
0-0	2000	0.351
0-3	2200	0.358
0-5	2680	0.383
0-7	2450	0.370
15-0	2350	0.366
15-3	2550	0.373
15-5	3410	0.367
15-7	2900	0.400
30-0	2810	0.393
30-3	3300	0.418
30-5	4000	0.507
30-7	3750	0.434
45-0	2000	0.351
45-3	2500	0.368
45-5	3680	0.383
45-7	2850	0.397

presents the applied forces and the obtained results of the critical stress intensity factor.

As can be seen from Table 7, the developed mixes show a significantly longer plastic zone before the onset of irreversible brittle fracture, which confirms the higher values of the critical stress intensity factor.

Experimental results show the absence of interaction between NBF and BA at all dosages of bottom ash and 0, 3 and 7% of nanomodified basalt fiber (Figure 9). On the other hand, at 5% fiber and 30% BA, the critical stress intensity factor K_1c increases significantly (by 25–40%), which is explained by the combined structure-forming action of NBF and BA.

3.4. Water Resistance, Freeze–Thaw Resistance and Abrasion Resistance

Experimental results show an increase in water resistance with an increase in the content of BA up to 30%, after which there is a decrease (Figure 10). At the same time, a similar trend is observed for all dosages of NBF. Limit of the fiber saturation is 5%, which allows the creation of an optimal structure for work in wet conditions. Curves practically do not intersect (except for 3 and 7% fiber), so there is mutual influence of bottom ash and nanomodified basalt fiber, but it does not have a decisive effect on the pore microstructure. Mixes with 30% BA and 5% NBF achieved water resistance grade W18, i.e., the ability to withstand water pressure of 1.8 MPa for a long time.

Showing a similar trend with the results of water resistance, with an increase in the content of BA up to 30%, an increase in freeze–thaw resistance is noted, after which there is a decrease (Figure 11). A similar trend is observed for all dosages of the nanomodified basalt fiber. Limit of the fiber saturation is 5%, which allows the creation of an optimal structure for work in wet conditions. Curves practically do not intersect (except for 3 and 7% fiber), so there is mutual influence of bottom ash and nanomodified basalt fiber, but it does not have a decisive effect on the pore microstructure. The mixes with 30% BA and 5% NBF reached freeze–thaw resistance grade F400, which means the ability to withstand 400 cycles of alternate freezing and thawing. These results correlate with the water resistance values. All of these results confirm the durability of structures built from the developed material.

Unlike the other results, for abrasion, a more complex interaction between the content of BA and NBF is observed, especially in the range between 15 and 30 wt.% of the content of bottom ash, and it is possible to predict the expansion of the boundaries of this range from 10 to 35 wt.% (Figure 12). The rational fiber content (7%) shows a good and uniform tendency to create low-abrasion cementitious composites. The largest decrease in abrasion is predicted in the presence of 18–22 wt.% bottom ash and 3 or 7 wt.% NBF, respectively.

3.5. Microtructure Formation

The increase in the whole complex of operational characteristics is explained by the combined action of bottom ash and nanomodified basalt fiber. The coefficients of linear thermal expansion of the basalt fiber and cement paste are quite close, 8 × 10⁻⁶ and 10 × 10⁻⁶ °C⁻¹, respectively, which ensures good adhesion of these components. Moreover, basalt fiber is an amorphous material, having about 50% SiO₂ in its chemical composition. These prerequisites allow the fiber to work not only as a dispersed reinforcement, but also as an active structure-forming component.

Basalt fiber application leads to a significant increase in the entire complex of physical and mechanical properties and performances. The results obtained are explained by good adhesion between the cement paste and the microfiber, selected according to the law of similarity of the components of the cement composite. Consequently, the shear stress between the microfiber and the cement paste is greatly increased (Figure 13).

The “BA + NBF” specimen structure includes a larger amount of low-basic calcium silicate hydrates CSH(I) (Figure 13a), while in the cement matrix without additives, there are more highly basic calcium silicate hydrates CSH(II), and hexagonal hydration products plates are present [59] (Figure 13b).

The primary and secondary microstructures of new growths of the composite system are formed in the process of hydration of the developed binder. In this case, amorphous hydration products in the interpore space form the primary structure.

The joint effect of the bottom ash and nanomodified basalt fiber provides control over the structure formation of cement materials, which ensures the redistribution of internal stresses from shrinkage deformations throughout the entire volume of the composite. When loading, the process of crack formation slows down, the stress concentration near structural defects decreases, and stresses are redistributed in the microstructure of the cement composite between its components.

4. Conclusions

The novelty of this work lies in the fact that the joint effect of nanomodified basalt fiber and bottom ash on fresh, physical and mechanical properties and durability of concrete was studied. The following main conclusions were obtained:

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Nanomodified-basalt-fiber-reinforced concretes (from 0 to 7 wt.% fiber) were developed, in which the economical Portland cement CEM I 32.5 N was replaced by up to 45 wt.% mechanically activated bottom ash (400 m²/kg).

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Equal flowability of the compositions (slump 20–22 cm, slump flow 45–52 cm) was achieved by varying the dosage of an economical superplasticizer with a high water-reducing ability (35%); when the content of BA increased from 15 to 45 wt.%, the mutual influence of bottom ash and nanomodified basalt fiber on fresh properties was noted.

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High values of mechanical properties (compressive strength up to 59.2 MPa, flexural strength up to 17.8 MPa, elastic modulus up to 52.6 GPa) were explained by the complex effect of bottom ash and nanomodified basalt fiber as a result of an increase in the volume of new growths due to acceleration kinetics of hydration of clinker minerals and formation of the second-generation new growths.

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A longer plastic zone was established before the onset of irreversible brittle fracture for the developed compositions, which confirms the higher values of the critical stress intensity factor K_1c; at 5% fiber and 30 wt.% BA, K_1c increased significantly (by 25–40 wt.%), which is explained by the combined structure-forming action of NBF and BA.

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For the composition with 30% BA and 5% NBF, water-resistance grade of W18 was achieved, which means the ability to withstand water pressure of 1.8 MPa for a long time.

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Similar to the results of water resistance, there was an increase in frost resistance with an increase in the content of BA up to 30%, after which there was a decrease; this, in turn, proves the durability of structures built from the developed material.

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In contrast to other results, for abrasion, a more complex mutual influence of BA and NBF content was observed, especially in the range between 15 and 30 wt.% of the content of bottom ash, and it is possible to predict the expansion of the boundaries of this range from 10 to 35%; the maximum reduction in abrasion is predicted in the presence of 18–22 wt.% BA and 3 or 7 wt.% fiber, respectively.

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Joint effect of BA and NBF provides control of the structure formation of cement materials, which ensures the redistribution of internal stresses from shrinkage deformations throughout the entire volume of the composite; during loading, the process of crack formation is retarded, stress concentration near structural defects is reduced, and stresses are redistributed in the microstructure of the cement composite between its components.

The limitations of this study are in compliance with the identified proportions of the components, in particular 30 wt.% bottom ash and 5 wt.% nanomodified basalt fiber, as well as the need for mechanically activating BA up to a specific surface area of 400 m²/kg.