3.1. Chemical Composition and Microstructure
The structure formation of modified foam concrete using mineral fiber and complex modifiers was investigated to determine its structural and phase composition. In the process of studying the quality of foam concrete using mineral fiber and complex modifiers, the results of an energy dispersive X-ray spectroscope were obtained. These studies are necessary to ensure the stability and quality of foam concrete, taking into account the structural features of foam concrete and its changes during structure formation, which eliminate all possible risks in the technological process of manufacturing the material.
The results of the scanning electron microscope allowed us to determine the structure of the cell skeleton and its integrity and quality.
To study the structure of foam concrete, a sample with a density of 400 kg/m
3 was taken. In
Figure 6, we see an uneven pore structure of foam concrete, as well as defects and cracks in larger cells that violate the integrity of the pores.
These results of the scanning microscope show that, during the preparation of the foam concrete solution:
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firstly, the foam is unstable and its delamination leads to the formation of uneven pores, and an increase in pores leads to an increase in the surface area of the pore walls, and as a result, the foam is unable to maintain its structure since the solution has a high load on the foam due to the large area;
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secondly, the internal stress of concrete is created after the beginning of setting and hardening, and the reason is large pores, a large area, and a thin structure of the pore wall contributing to the formation of microcracks.
Thus, in order to obtain low-density thermal insulation foam concrete, it is necessary to use polymer components to reduce internal stress. The polymer component creates a film on the crack areas and thereby ensures the integrity of the pore structure. One of the important factors is the stability of the foam, and it is difficult to achieve its stability because, as the foam bubble increases, its area increases and the solution affects it, thereby forming defects in the pore or destroying the pores. To reduce the pore structure and density of foam concrete, a reinforcing component is needed, since the created frame will reduce the load on the foam despite its multiplicity, thereby allowing the solution to take shape and harden.
The obtained results of modified foam concrete showed that the presence of a polymer reduces the internal stress of concrete due to an additional polymer shell on the walls and in the open spaces of the pore structure. However, the use of a polymer provides only a reduction in high internal stress of foam concrete.
Figure 7 shows a Type 4 sample using mineral fiber and a complex modifier. According to the results of the scanning electron microscope, a cell skeleton was obtained, which had a structure without microcracks and chips, and there are no large fibers in the image that violate the integrity of the cell. This study confirms that, during the mixing process, the mineral fiber is destroyed but into smaller fibers that allow the cell walls to be structured, reinforcing only them and not creating their own frame, thereby allowing the surface of the finished product to be polished after laying, eliminating the release of fibers on the surface that violate the aesthetics of the surface. Given the thickness and length of the fibers, it can be concluded that the fibers are destroyed by mixing the mixture, but during the formation of the structure, the destroyed fibers, although small, reinforce the cell structure.
Figure 8 shows mineral fibers and their dimensions. The maximum fiber size is 9.38 µm per minute.
Thus, the results obtained confirm that mineral fiber can significantly strengthen the walls of cells, creating the effect of their microreinforcement, and when using a complex modifier based on polymer components, a protective film is formed that ensures the formation of cracks and chips on the surface of the cell, thereby allowing high-quality material to be obtained.
Having studied the changes in the chemical processes of hydration, we see that there are no significant changes in the quantitative parameters of the active oxides involved in the hydration process. These studies show reinforcement of the cellular concrete wall structure with mineral fiber, as well as a complex modifier consisting of polymer components.
The presented studies show an increase in the activity of portlandite, as well as an increase in silicon in the composition of foam concrete with mineral fiber, as well as with a complex modifier.
Figure 9 shows a control sample where we observe a large amount of portlandite. According to chemical analysis, the composition of calcium oxide was 93.64%, which indicates a high activity of the cement binder. However, there is also a negative side to the high activity of cement, which is the risk of high alkalinity and, as a result of corrosion of the material (peeling and cracks and chips), peeling occurs due to the migration of free lime, which promotes the movement of salts through the structure of the material, creating locations for its maximum accumulation, thereby causing corrosion processes.
The chemical and mineralogical composition of the control foam concrete specimen was determined through EDS analysis, and the results are presented in
Table 6.
To further analyze the elemental composition of the foam concrete samples containing mineral fiber and a complex modifier, an EDS spectrum was obtained, as shown in
Figure 10.
The elemental and oxide composition of the foam concrete containing mineral fiber and a complex modifier was determined using EDS analysis. The corresponding results are presented in
Table 7.
Mineral fibers from basalt insulation waste consist of silicon oxide minerals, which means that, during the hydration process, the entire mixture, from cement to inert materials, is coated with polymer, and the interaction during the hydration of cement and mineral powder is identical to that of inert materials such as sand, with the difference being the thread-like shape. These components structure the form of foam concrete according to chemical analysis, increasing the silicon content compared to the control sample, which indicates that the mineralogy of the cell structure is changing and it can be concluded that the mineral fiber subsequently reacts with the cement binder, not enveloping it but forming a single mineral. The polymer component creates a shell in the cell wall structures, eliminating its micropores and thus ensuring the monolithicity of the pore walls.
The conducted studies showed the effectiveness of the applied additives, taking into account the microstructure images, and the chemical and mineralogical composition can be concluded based on the improvement of the structure of foam concrete. The research of physical–mechanical and construction–technical properties of foam concrete with the use of mineral fiber and complex modifiers in comparison with the control sample showed a significant difference in the improvement of foam concrete with additives and mineral fiber.
3.2. Compressive Strength and Density
To analyze the combined effect of polymer components and mineral fibers, three grades of foam concrete were investigated: D 400 (thermal insulating), D 800 (structural thermal insulating), and D 1100 (structural).
Based on the results of qualitative indicators of strength we can conclude that the greatest strength effect is achieved when using M3. This is due to the fact that the composition of M3 contains both polymeric component and mineral fiber strengthening the structure of foam concrete, which provides a framework at the time of setting and hardening of foam concrete, and the polymeric component having in its composition surfactant provides effective wetting and plasticity of the mixture and, as a result, quality cell walls of foam concrete. As a result of the obtained data we see an increase in the strength of foam concrete from the control sample of M0 by 82%. M2 showed an increase in strength by 64% while M1 showed an increase in strength by 19%. The results are explained by the fact that because of the instability of the frame during setting and curing caused by the presence of surfactants M1 has not gained high strength and no stable density, presented in
Figure 11. High density indicates the uneven distribution of density in the structure of foam concrete as there was shrinkage of the material. We seea similar result in the M0 control sample.
Studies of physical–mechanical and construction–technical properties of heat insulating constructive foam concrete D 800 with the use of mineral fiber and complex modifiers in comparison with the control sample showed the following results: M3 showed the highest quality indicators;
Table 8 shows an increase in the strength of M3 by 85%, while M2 showed 74%, and M1 showed an increase in strength by 42%. The obtained results of thermal insulating and structural foam concrete in terms of percentage ratio are higher than the results of thermal insulating foam concrete. This fact is explained by the fact that the density is higher and in the process of setting and curing, by reducing the size of the cells, the internal stress on the walls of the cells is reduced, thereby allowing effective hydration.
To analyze the effect of the modifier and mineral fiber on the density of foam concrete, the density values for all mixtures were determined. The results are shown in
Figure 12.
The study of structural foam concrete D 1100 as well as heat insulating and heat insulating constructive showed similar results of physical and mechanical properties. Maximum strength was achieved in the sample M3 compared to M0 (control) (64%), and strength of the sample M2 was 43% and that of M1 was 36% but not significantly different compared to the control sample. This fact is because high-density cells in foam concrete are much smaller and their walls are much thicker in contrast to the foam concrete with low density.
3.4. Water Absorption
Low water absorption of foam concrete was also shown by the sample of M3, 28% lower than that of the control sample. This effect is explained by the fact that the integrity of the closed cell walls and the polymeric component contained in the additive provides a protective framework of the cell. The results are shown in
Table 10.
Water absorption of insulating and structural foam concrete showed the following results: M1 showed a 29% reduction in water absorption, sample M2 showed a result of 14%, and M3 showed a 28% reduction in water absorption.
Water absorption of foam concrete with density 1100 (M1) with the use of complex modifiers significantly decreased, because the composition contains a polymer component, and amounted to 23.5%. M3 showed the lowest water absorption of 27.6% as the composition contained not only complex modifiers but also mineral fiber, which improved the structure of foam concrete. M2 showed a decrease in water absorption of only 8.2%, which is explained by the lack of complex modifiers. The results are presented graphically in the
Figure 14.
An analysis of existing technologies for producing high-quality foam concrete materials has been shown [
16,
36], and the use of polymer fiber improves the structure of foam concrete [
28]. However, given that the fiber is a polymer and has low adhesion due to low internal stress, it subsequently loses strength and resistance to aggressive environments [
37,
38].
Based on compressive strength results, the mixes were classified according to standard thresholds for lightweight and cellular concretes. The D 400 series (1.1–2.0 MPa; ≈400–500 kg/m3) corresponds to non-structural, thermal insulating concretes used for void filling and roof insulation. The D 800 series (4.3–7.9 MPa; ≈800–900 kg/m3) represents semi-structural concretes suitable for self-supporting or enclosure elements. The D 1100 series (10.0–15.4 MPa; ≈1100 kg/m3) approaches the lower limit of load-bearing lightweight concrete. Although below the ≈17 MPa structural threshold defined in ACI 213R, these results indicate potential for limited structural applications in low-rise construction.
Regarding to the results of the study, it can be concluded that the use of a complex of modified additives based on a polymer component and mineral fiber allows the highest quality to be achieved: The polymer strengthens the skeleton structure by polymerizing all components of the cement binder, aggregate, and mineral fiber, and the mineral fiber reinforces the cell walls, removing the load from the upper part of the mixture and ensuring uniform density in the early stages of foam concrete structure formation.
During the research, the composition of modified foam concrete using mineral fiber and complex modifiers was optimized, and the work carried out made it possible to establish the effective ratios of all components of modified foam concrete.
The work carried out to study the structure formation of modified foam concrete using mineral fiber and complex modifiers showed that mineral fibers come into contact during the hydration process and not only improve the structure of foam concrete in the early stages of hardening but also form a single stone in the pore walls, increasing the physical–mechanical properties of foam concrete, while the presence of a modifier enveloping all mineral components creates both an integral structure and the hydrophobicity of the material. This process is confirmed by a twofold increase in silicon in the foam concrete composition compared to the control sample. This is because mineral fibers consist of more than 50% silicon.
Tests conducted on the physical, mechanical, and structural properties of foam concrete of various densities using mineral fiber and complex modifiers confirmed the effectiveness of mineral fiber and complex modifiers for all types of foam concrete, from thermal insulation to structural. The results obtained for the optimal compositions showed stability in production and high-quality indicators, as reflected in the article.