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Article

Improved Fly Ash Based Structural Foam Concrete with Polypropylene Fiber

by
Alexey N. Beskopylny
1,*,
Evgenii M. Shcherban’
2,
Sergey A. Stel’makh
3,
Levon R. Mailyan
4,
Besarion Meskhi
5,
Valery Varavka
6,
Andrei Chernil’nik
3 and
Anastasia Pogrebnyak
7
1
Department of Transport Systems, Faculty of Roads and Transport Systems, Don State Technical University, 344003 Rostov-on-Don, Russia
2
Department of Engineering Geology, Bases, and Foundations, Don State Technical University, 344003 Rostov-on-Don, Russia
3
Department of Unique Buildings and Constructions Engineering, Don State Technical University, Gagarin Sq. 1, 344003 Rostov-on-Don, Russia
4
Department of Roads, Don State Technical University, 344003 Rostov-on-Don, Russia
5
Department of Life Safety and Environmental Protection, Faculty of Life Safety and Environmental Engineering, Don State Technical University, 344003 Rostov-on-Don, Russia
6
Research and Education Center “Materials”, Don State Technical University, 344003 Rostov-on-Don, Russia
7
Department of Metal, Wood, and Plastic Structures, Don State Technical University, 344003 Rostov-on-Don, Russia
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2023, 7(2), 76; https://doi.org/10.3390/jcs7020076
Submission received: 25 December 2022 / Revised: 9 January 2023 / Accepted: 10 February 2023 / Published: 12 February 2023
(This article belongs to the Special Issue Polymer Composites and Fibers)
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Abstract

:
The direction of construction science that is associated with the development of the theory and practice of creating a new generation of foam concrete is particularly interesting and relevant. The development of improved structural foam concrete using polypropylene fiber and industrial waste, namely fly ash (FA), is prompted by the existing environmental threat posed by FA; this threat is a result of the operation of the fuel energy industry, as well as the possibility of using foam concrete not only as thermal insulation, but as the main material for load-bearing structures that have a certain level of responsibility. The aim of this work was to create and optimize the recipe technological parameters to produce non-autoclaved fiber foam concrete (FFC) using FA as a component. The study used standardized methods for assessing the properties of FFC, and the method of optical microscopy to analyze the structural characteristics of the material. It has been revealed that the replacement of cement with FA in an amount of 10% to 40% helps to reduce the dry density (DD) of FFC. The lowest DD was recorded for samples with 40% FA. The best results for the compressive strength (CS) and flexural strength (FS) were recorded for FFC samples with 10% FA instead of cement. The increase in CS was 12%, and the increase in FS was 23%. The best thermal insulation properties of FFC, and in terms of resistance to freezing and thawing, were recorded in samples with a 10% replacement of cement with FA. The maximum decrease in thermal conductivity was 14%.

1. Introduction

Currently, there is a scientific problem in building practice and, accordingly, in building material science; this lies in the insufficiency of recipe technological methodologies aimed at finding new directions to ensure the proper quality of heat-insulating building materials [1,2,3]. The requirements for energy efficiency and energy saving are growing, and the operating conditions of various buildings and structures are being tightened.
This leads to the fact that it is necessary to create new heat-insulating materials, which, in addition to their good heat-insulating characteristics, will have other characteristics necessary for their completely safe and reliable operation [4,5,6,7,8]. In this regard, developments aimed at finding the most optimal solutions in the field of the formulation and technology of heat-insulating building materials, one of which is cellular concrete [9,10,11], are currently underway around the world. The cellular concretes are subdivided into a number of different, but somewhat similar concretes. The main types are foam concrete (FC) and aerated concrete. As is known, they are different in the technology of their manufacture. Currently, aerated concrete is historically and practically considered to be a more used and reliable material, due to the high costs of its manufacture and the significantly greater complexity of its production. Thus, it turns out that the cost of producing a cubic meter of aerated concrete significantly exceeds the cost of producing the same volume, for example, of FC [12,13,14,15,16,17,18]. Therefore, the direction of construction science associated with the development of the theory and practice of obtaining new generation FC seems to be especially interesting and relevant [19,20,21,22,23,24,25].
At the same time, it is impossible not to note the existing environmental problem inherent in all regions of the world. The building material industry is one of the most suitable industries for the competent, optimal disposal of industrial waste by processing it into building materials. Industrial, agricultural, household and other wastes are used as components of building materials, or as building materials in general [26,27]. Such a recipe technological solution did not bypass the production of FC [28,29,30,31]. There are a number of developments aimed at improving the properties of FC and other cellular concretes at the expense of industrial waste. The literature [32,33,34,35,36,37,38,39,40,41] presents methods for modifying and improving the mechanical and physical characteristics, as well as the durability characteristics of FC using various formulation and technological solutions.
According to [32], with an increase in the dosage of hydrogen peroxide, the compressive strength (CS) and thermal conductivity (TC) decrease, so the recommended dosage of hydrogen peroxide is 5%. The best values of CS and TC were recorded with the introduction of microsilica in the amount of 5%. As for the effect of polypropylene fiber on the same characteristics, its dosage in the amount of 0.1% is the most optimal, and the introduction of polypropylene fiber also improves frost resistance.
The authors of one study [33] found that the use of cement together with a synthetic foaming agent is more effective than with a protein-based foaming agent. In addition, when replacing part of the cement with metakaolin, siliceous fly ash (FA) and calcareous FA, an improvement in CS and TC was recorded only when metakaolin was added. In [34], the authors determined that the HT-type foaming agent was more efficient than sodium dodecyl sulfate and that it produces more stable foams. Further, the best ratios of the mortar part of the FC mixture and the foam itself, at which the CS of FC is higher than 2 MPa, are 2:1 and 3:1. As for the types of binder, this paper considers the partial replacement of Portland cement with FA, and the best percentage of replacement is an ash content of 40–55%. In [35], the authors studied the replacement of part of the binder with FA and spent lime mud (0%, 50%, 60%, 70%, 80%) in FC.
The mechanical properties of FC increase with an increase in density or a decrease in the dosage of FA and lime mud. In general, the CS and flexural strength (FS) of all experimental samples exceeded the minimum requirements of the standard. Indeed, “the use of FA in [36] makes it possible to reduce the shrinkage of FC during drying, however, the low rate of FA hydration adversely affects the CS in the initial period of hardening. In turn, the introduction of light aggregates has a positive effect on CS” [36]. In [37], the authors studied the possibility of manufacturing FC on a geopolymer basis. In [37], geopolymer concretes show the best CS of 18.2 MPa at a hardening temperature of 60 °C. In comparison, the porosity and water absorption of samples hardened at room temperature were 15.29% and 2.35%, respectively, and for samples hardened at a temperature of 60 °C, they were 6.78% and 1.22%, respectively. “The addition of polypropylene fibers makes it possible to improve the mechanical characteristics of the foamed geopolymer” [38]. Thus, the CS and FS increased as the dosage of fibers increased from 0.25% to 0.5%.
The addition of polypropylene fibers contributes to the filling of voids and an increase in the bond strength at the phase boundaries between the fibers and the geopolymer matrix. In another study [39], the authors developed cellular geopolymer foam concretes, where aluminum powder was used as a foaming agent. These concretes have higher mechanical characteristics than traditional cement-based concretes. “In mixtures with a high foam content, the introduction of microsilica provides better CS and higher CS/TC ratios than FA” [40]. In [41], microsilica has the best effect on the value of mechanical characteristics, as in [40]. The optimal foam content is its dosage in the amount of 28%, and the optimal water-binding ratio is at the level of 0.60.
Let us focus on creating an improved structural FC using polypropylene fiber and industrial waste, namely FA. The choice of the topic for research is due to the existing environmental threat created by FA as a result of the operation of the fuel and energy complex, as well as structural FC; this is used not only as thermal insulation, but also as the main material for the load-bearing structures of buildings and structures of a certain level of responsibility [42,43,44,45,46,47,48]. Thus, this study intends to eliminate the scientific deficit, which consists of a lack of proper theory in regard to structure formation and the formation of properties at the micro- and macrolevels of foam concrete on fly ash. The second mission of the article is the creation of practical and recipe technological recommendations for real production and their subsequent implementation at enterprise—industrial partners.
Thus, the purpose of this study is to create and optimize recipe technological parameters to produce non-autoclaved fiber foam concrete (FFC) using FA as a component. The objectives of the study will be the selection, justification, and implementation of recipe technological proposals for the creation of improved structural FFC based on FA; in addition, this study will carry out large-scale experimental and microstructural studies with the identification of fundamental and applied dependencies between recipe technological factors and output parameters, which will be the improved properties and improved micro and macrostructure of new foam concrete.
The scientific novelty of the ongoing research is the use of a recipe technological method of using FA as a component of structural FC with polypropylene fiber. That is, for the first time, a denser microstructure of FFC with a dense packing of particles, obtained by the least expensive non-autoclave method and via simple foaming, was studied; this ultimately flows from scientific novelty to practical significance.

2. Materials and Methods

2.1. Materials

As initial raw materials for the manufacture of FC with polypropylene fiber based on FA, materials were used to ensure the accuracy of the results obtained and to adequately assess the impact of the recipe decisions made. The physical, mechanical and chemical properties of the materials used in the work are presented in Table 1, Table 2, Table 3, Table 4 and Table 5.
Before the addition of FA into the concrete mixture, it was subjected to additional mechanical processing in a planetary ball mill for 5 h at 600 rpm. Figure 1 shows the graphical dependence of the content on the particle size of FA before and after its additional grinding. The size distribution of FA particles was estimated using a MicroSizer-201C (“VA Install”, St. Petersburg, Russia).
Figure 1 shows that the majority of FA particles (90%) before mechanical activation are between 6 µm to 106 µm in size. After FA grinding, the size range is reduced and amounted to 6 to 70 microns, and the total number of particles in this range is 91%. Thus, mechanical activation contributes to the grinding of particles in the range of 70 to 280 microns to a more finely dispersed state.

2.2. Methods

The process of manufacturing and hardening samples is illustrated by the block diagram shown in Figure 2.
The accepted proportions of the concrete mixture in accordance with [24]: the water/binder + sand ratio was 0.47; the sand/binder ratio was 0.3; and the foaming agent content was 4.2 kg/m3. The binder consisted of either only cement without fly ash, which was the control composition (CC), or a mixture of cement and ash, in which ash was used as a replacement for cement: FA × 100/(cement + FA). The quantity of sand for 1 m3 of the mixture was 140 kg/m3. Dispersed reinforcement with polypropylene fiber for all samples, including control ones, amounted to 1% by weight of cement.
The average density of the FFC samples was determined on samples previously dried to constant weight according to the method of GOST 12730.1-2020 “Concretes. Methods of determination of density” [49]. The temperature of drying the samples to constant weight was (105 ± 5) °C. The mass was considered constant if the results of two successive weight measurements differed by no more than 0.1%.
To determine the TC, samples were made in the form of a rectangular parallelepiped, the largest faces of which were in the form of a square with a side of 100 × 100 (mm). The thickness of the samples was 20 ± 2 mm. The determination of TC was carried out in accordance with the requirements of GOST 7076-99 “Building materials and products. Method of determination of steady-state thermal conductivity and thermal resistance” [50].
The CS and FS (3 point bending test) was determined in accordance with GOST 10180-2012 “Concretes. Methods for strength determination using reference specimens” [51] on samples dried to constant weight.
Frost resistance was assessed according to the method of GOST 25485-2019 “Cellular concretes. General specifications” [52]. In the process of testing to assess frost resistance, the main and control samples were made. The main and control samples of concrete before testing for frost resistance were saturated with water at a temperature of (18 ± 2) °C. The duration of one freezing cycle at a steady temperature in the chamber of −(18 ± 2) °C was at least 4 h. After 15 cycles of freezing and thawing (CFT), the main samples were tested: their weight, moisture content, and CS were determined. The control samples were tested for CS after they were kept in the thawing chamber for the entire time corresponding to the number of CFT.
The necessary calculation formulas for calculating the physical (PC) and mechanical characteristics (MC) of FFC are presented in Table 6.
Table 7 presents the technological and testing equipment, as well as the measuring instruments used during the study.
The program of test studies is shown in Figure 3. In total, 7 series of samples were made based on the results of the test studies.
Photos of the sample testing process are shown in Figure 4.
The nature of the collapse of the sample, revealed during the tests, differs significantly from the nature of the collapse of concrete of normal density and cellular concrete without dispersed reinforcement. Due to the presence of fibers, the fracture pattern becomes more viscous in contrast to concretes without fibers with increased strength.

3. Results

3.1. PC and MC of Samples of FC with Polypropylene Fiber Based on FA

The test results of the influence of the percentage of the replacement of cement with FA on the PC and MC of FFC are presented in Figure 5, Figure 6, Figure 7 and Figure 8.
Figure 5 shows the dependence of the density of FFC samples in a dry state (dry density—ρd) at various percentages of FA.
Figure 5 shows that the minimum value of the DD of FFC was recorded with FA in the amount of 40% and that it was 773 kg/m3. In general, when replacing cement with fly ash from 0 to 40%, a decrease in density was observed, and its peak was recorded at 40%. However, with a further increase in cement replacement from 50 to 70%, an increase in density was observed. Therefore, when FA was in an amount of 10%, the DD value was 843 kg/m3, 849 kg/m3, 874 kg/m3. The decrease in the DD of FFC when replacing cement with FA up to 40% is primarily due to the fact that the density of FA is significantly lower than the density of cement. In addition, the emerging area of density reduction at the optimal dosage of FA is apparently explained by the following phenomenon. In view of the fact that the nature of porosity improves, interpore partitions are strengthened, and macroporosity increases by reducing the microporosity of interpore partitions. All this leads to the fact that the degree of shrinkage of cellular concrete decreases, the volume of concrete remains at the level of the poured concrete mixture, and while maintaining the mass, such a stable structure leads to a regular decrease in density, which is an effective optimum between the strength and density characteristics of the resulting composite. However, when replacing more than 50%, the opposite trend is observed in the form of an increase in DD. This is due to the fact that a very high content of FA affects the foaming process, and the stability of the formed bubbles decreases and more trapped gas escapes, thereby increasing the density of the FFC [15,16]. In percentage terms, the change in the density of FFC modified with the addition of FA, in comparison with the CC, is presented in Table 8.
Figure 6 shows the dependence of the CS of FFC samples on the amount of FA added instead of cement. R2 (Figure 6, Figure 7 and Figure 8) is the coefficient of determination during the approximation of experimental data by the polynomial of 4-th degree.
The effect of FA on CS is shown in Figure 6. The best CS is for FFC samples containing FA in an amount of 10%. A further increase in the percentage of FA leads to a decrease in strength. This is due to the fact that the fluidity of the foam concrete solution increases as the number of bubbles per unit volume increases; however, the wall thickness decreases. Reducing the thickness of interpore partitions leads to a decrease in strength. In addition, with an increase in FA in the FFC system, the content of cement decreases; as a result, the formation of calcium hydrosilicates (CSH) decreases. However, the decrease in strength with an FA content of 20–30% is not so significant, which can be explained by the pozzolanic effect. The silica components in FA (Al2O3 and SiO2) and cement are mixed, and a gel of CSH is formed, as a result of which the strength development is slower [15,16,53]. For FA dosages of 10%, 20%, 30%, 40%, 50%, 60% and 70%, the CS values were 3.83 MPa, 3.32 MPa, 3.12 MPa, 2.85 MPa, 2.72 MPa, 2.32 MPa, and 2.01 MPa, respectively. In percentage terms, the change in CS of FFC modified with the addition of FA, in comparison with the CC, is presented in Table 9.
Figure 7 shows the dependence of the FS of FFC samples on the amount of FA introduced instead of cement.
The nature of the curve in Figure 7 is similar to Figure 6. The peak FS is observed when the FA is 10%. Therefore, for dosages of 10%, 20%, 30%, 40%, 50%, 60% and 70%, the values of FS were 1.13 MPa, 0.78 MPa, 0.66 MPa, 0.51 MPa, 0.45 MPa, 0.43 MPa, and 0.40 MPa, respectively. In percentage terms, the change in FS of FFC modified with the addition of FA, in comparison with the CC, is presented in Table 10.
Figure 8 shows the dependence of the TC of FFC samples on the amount of FA introduced instead of cement.
As a rule, the TC of FC is more dependent on the gaseous phase of foam concrete than on the solid phase. The fewer defects the porous structure has, the better the heat-insulating effect of the porous material will be. Figure 8 shows that when replacing part of the cement with FA from 10% to 30%, a decrease in the TC of FFC samples is observed; the lowest value of TC was recorded with a FA amount of 10%. This can be explained by the fact that the interpore walls are additionally compacted by the gel of hydrated calcium silicate, which is formed as a result of the secondary reaction of FA hydration. However, at 40% fly ash or more, the opposite effect is observed, and TC increases. This is primarily due to the excess content of FA and the fact that the ash particles themselves have a sufficiently large specific surface area, which, to a certain extent, hinders heat transfer and causes a significant decrease in TC. In addition, there is an increase in the fluidity of the suspension, which leads to a decrease in the number of formed pores and, as a result, an increase in the DD of the material and TC. Thus, with the content of fly ash in the amount of 10%, 20%, 30%, 40%, 50%, 60% and 70%, the TC values were 0.157 W/(m×°C), 0.168 W/(m×°C), 0.172 W/(m×°C), 0.189 W/(m×°C), 0.195 W/(m×°C), 0.201 W/(m×°C), and 0.213 W/(m×°C), respectively. In percentage terms, the change in the TC of FFC modified with the addition of FA, in comparison with the CC, is presented in Table 11.

3.2. Durability Characteristics of Foam Concrete with Fly Ash Based Polypropylene Fiber

Based on the results of assessing the PC and MC of FFC with different percentages of cement replaced by FA, it was found that their best values are observed at a 10% replacement. In this regard, it was decided one of the most important durability parameters—frost resistance—would be evaluated. The assessment of the effect of replacing cement with FA on the frost resistance of FFC samples was carried out on two compositions—a control and a composition with 10% FA. Since this test is quite laborious and requires a large number of samples, it was not reasonable to evaluate this indicator for other compositions with the replacement of part of the cement over 10%, which showed lower strength characteristics (SC). The results of testing samples of FFC in terms of “frost resistance” are presented in Table 12, Table 13 and Table 14.
According to the results of the research, it was found that the frost resistance of experimental samples with 10% FA instead of cement is better than the frost resistance of the FFC samples of the CC. This is expressed in the loss of SC and weight loss after 15 cycles of freezing and thawing. These experimental results can be explained by the fact that the samples modified with the addition of FA have an initially higher SC, and the introduction of FA in this established optimal amount contributes to the additional formation of calcium hydrosilicates, which leads to the formation of stronger and denser interpore walls with fewer microcracks.

3.3. Analysis of the Structure of FFC Based on FA

Figure 9 shows photographs of the pore structure of FFC samples of the CC and composition with 10% FA.
From Figure 9, it is visually observed that the structure of the FFC of the CC (Figure 9a) has fewer pores than the structure of the FFC modified with FA (Figure 9b). In addition, there is a visual difference between the pore sizes of the sample of the control composition (Figure 9a) and the sample with 10% FA (Figure 9b. As can be seen, the sample of the control composition is characterized by a large number of pores of smaller diameter. These differences in the structure can be explained by the fact that the introduction of FA in an amount of 10% increases the fluidity of the FC solution, and the number of bubbles increases accordingly. In addition, the pozzolanic effect at a given dosage of FA has a positive effect, and it contributes to the additional formation of CSH, which strengthens the structure of interpore walls; as a result, the SC increases, which is in good agreement with [15,16,53].

4. Discussion

As is known, the actual properties of foam concrete are largely determined by its pore structure; to a large extent, the properties are influenced by the size of the pores, the thickness and density of the interpore partitions, as well as the number of microcracks. The main prescription factors that affect the SC and PC of FFC are presented in the form of an Ishikawa cause-and-effect diagram (Figure 10).
Undoubtedly, an important factor that affects the quality of fiber foam concrete is the parameter of fiber reinforcement. In this study, the optimal parameters for the reinforcement of foam concrete with polypropylene fibers, already carefully selected by the authors, were adopted, considering the increase in the density of FFC [24]. During the study, these parameters remained unchanged. Therefore, in the cause-and-effect diagram presented in Figure 10, an important factor that affects the quality of FFC is deliberately absent. Each component that is part of the fiber foam concrete plays an important role in its quality. Particular attention should be paid to the foaming agent and binder, namely their origin, composition, and dosages. The parameters of the pore structure depend on this, and, consequently, all of the PC and MC of the composite. Moreover, FC is more sensitive to any minor changes in the recipe than ordinary concrete.
In [54], the authors studied various characteristics of FC with fly ash. It was revealed that with an increase in the content of FA up to 60%, the SC of FC decrease. The density, on the contrary, first decreases when replacing concrete with no more than 40%, and then increases. The same can be said about thermal conductivity. As in [54], in the presented study, a similar trend in the change in compressive strength, and especially in dry density, is observed depending on the dosage of fly ash instead of cement. The change in thermal conductivity in the presented study is somewhat different than in [54]; at the same time, the fact that the thermal conductivity is greater than that of the control composition, with a fly ash content of 40% or more, is similar.
In [55], the authors developed high-strength alkali-activated FC based on fly ash and slag, and studied their properties. The strength of the developed alkali-activated FC in compression ranged from 0.50 MPa to 44.98 MPa, and in bending, ranged from 0.22 MPa to 13.86 MPa, respectively, at different densities. In general, the mechanical strength of alkali-activated fly ash FC is higher than that of conventional Portland cement-based FC of the same density. In general, the recipe solutions of the authors in the study [55] are quite effective and reflect the positive effect of the use of industrial waste in FC of various densities.
The study [40] investigated the effect of foam content, as well as the addition of FA and microsilica on some of the PC and MC of FC. According to the results of the study, the values of density, water absorption, CS and TC of FC turned out to be within 873–1998 kg/m3, 3.5–35.9%, 1.5–88.1 MPa, and 0.239–0.942 W/(m×°C), respectively. In addition, the balanced content of fly ash and microsilica in the foam concrete mixture provides adequate strength and physical characteristics. However, mixtures with a high content of foam and microsilica showed higher CS values than similar mixtures with fly ash. The study [40] is one of the many confirmations that the use of industrial waste in the form of fly ash, slag, microsilica, separately or together in rationally selected optimal dosages, improves the PC and MC of FC in comparison with conventional FC based on Portland cement.
In [56], the authors studied the mechanism of strengthening lightweight cellular concrete filled with FA. After 28 days, the FA pozzolanic reaction and the cement hydration reaction mutually contribute to the compaction of the skeletal structure. During the SEM analysis, the pore structure and skeletal hydration products were examined, and as a result of this analysis, the recommended fly ash content was determined, which is approximately 25%. In [56], the features of FC with FA, regarding the compaction of the structure and porosity parameters, have similarities and are in good agreement with the observations given in the current study. However, the optimal dosage of fly ash is different.
The use of industrial waste in the form of fly ash in a carefully selected dosage in fiber foam concrete improves its durability and physical and mechanical characteristics, in comparison to conventional foam and fiber foam concrete in Portland cement; this is confirmed by laboratory studies on the properties and structure of the composite material.
In addition, the special role of the formation of high strength properties and a developed pore structure in structural FC should be noted. In comparison to heat-insulating FC, in which the main task is to increase transportability, prevent premature cracking and destruction, the task of structural FC is to ensure, to a certain extent, the reliability of buildings and structures erected from it. In view of the rather complicated technology, simpler in comparison with aerated concrete, but more complex in comparison with heavy concrete, structural FC occupies an intermediate position; therefore, its use in certain types of buildings and structures is not always effective. As a result of our study, an effective method for the formulation and technological regulation of the structure of FC, which has a dispersed fiber reinforcement and additional centers of crystallization, arising from the rational introduction of the optimal dosage of the modifier in the form of FA, has been determined; this, in addition to improving the structure, also allows one to control its uniformity and homogeneity. Thus, the structure becomes maximally developed, and the dense packing of particles in the interpore partitions is achieved. In addition, all this leads to an improvement in the performance properties of structural FC in comparison with analogues. A fairly close analogue in terms of properties is autoclaved aerated concrete, which is much more difficult to manufacture. The cost and expenditure of energy, materials and labor in the manufacture of autoclaved aerated concrete is much higher than for structural FFC on FA. Thus, this development is important for the applied industry in order to replace expensive autoclaved analogues with cheaper and simple non-autoclaved types that also have an improved FC with fiber and FA. The strength obtained as a result of the modification of structural FC with FA with the addition of dispersed fibers makes it possible to achieve values, albeit lower than those of conventional concrete, but that take into account a significantly lower density and a strength relative to density that exceeds those of other competitive materials. Thus, it is advisable to evaluate the SC of the obtained structural FC, improved with FA and dispersed reinforcement, in terms of the “strength relative to density”, which certainly makes it competitive and efficient in comparison to similar building materials that have more weight.

5. Conclusions

Based on the results of the study, the following conclusions can be drawn.
(1) Replacing cement with FA in an amount of 10% to 40% allows a reduction in the DD of FFC. The lowest DD was recorded for samples with 40% fly ash;
(2) The best results in CS and FS are recorded in fiber-reinforced concrete samples with 10% fly ash instead of cement. The increase in CS was 12%, and the increase in FS was 23%.
(3) The best thermal insulation properties of FFC have been recorded in samples with a 10% replacement of cement with fly ash. The maximum decrease in thermal conductivity was 14%.
(4) The freeze and thaw resistance of samples with 10% fly ash instead of cement is higher than of the CC.
(5) FA in an amount of 10% is optimal and provides an improvement in physical and mechanical characteristics; replacement in an amount of 20% to 30% is acceptable and does not lead to significant changes in material properties compared to the control composition.
(6) The practical significance of the study was the development of technological maps for the finding of the composition of improved structural fiber foam concrete containing fly ash with a strength that exceeds analogues by 20%.
(7) Prospects for the development of the study are defined as a projection of the developed approach in relation to foam concrete reinforced with other types of fibers, particularly those obtained from vegetable waste.

Author Contributions

Conceptualization, S.A.S., E.M.S., V.V. and A.C.; methodology, S.A.S., E.M.S. and A.P.; software, S.A.S., E.M.S., A.N.B. and A.C.; validation, A.C., S.A.S., E.M.S. and A.N.B.; formal analysis, A.C., S.A.S. and E.M.S.; investigation, A.P., L.R.M., S.A.S., E.M.S., A.N.B. and B.M.; resources, B.M.; data curation, S.A.S., E.M.S. and A.C.; writing—original draft preparation, S.A.S., E.M.S. and A.N.B.; writing—review and editing, S.A.S., E.M.S. and A.N.B.; visualization, S.A.S., E.M.S., A.N.B. and V.V.; supervision, L.R.M. and B.M.; project administration, L.R.M. and B.M.; funding acquisition, A.N.B. and B.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The study did not report any data.

Acknowledgments

The authors would like to acknowledge the administration of Don State Technical University for their resources and financial support.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

CFTCycles of Freezing and Thawing
CCControl Composition
CSCompressive Strength
DDDry Density
FAFly Ash
FCFoam Concrete
FFCFiber Foam Concrete
FSFlexural Strength
MCMechanical Characteristics
PCPhysical Characteristics
SCStrength Characteristics
TCThermal conductivity

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Jcs 07 00076 g001a 550Jcs 07 00076 g001b 550
Figure 1. Graphical distribution of fly ash (FA) particles: (a) particle distribution, (b) cumulative curve, 1—before grinding; 2—after grinding.
Figure 1. Graphical distribution of fly ash (FA) particles: (a) particle distribution, (b) cumulative curve, 1—before grinding; 2—after grinding.
Jcs 07 00076 g001aJcs 07 00076 g001b
Jcs 07 00076 g002 550
Figure 2. The process of manufacturing experimental samples.
Figure 2. The process of manufacturing experimental samples.
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Figure 3. Test research program.
Figure 3. Test research program.
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Figure 4. CS testing of FFC specimens: (a) during loading; (b) after collapse.
Figure 4. CS testing of FFC specimens: (a) during loading; (b) after collapse.
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Figure 5. Dependence of the DD of FFC samples in a dry state on various percentages of replacing cement with fly ash.
Figure 5. Dependence of the DD of FFC samples in a dry state on various percentages of replacing cement with fly ash.
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Figure 6. Dependence of the CS of FFC samples on the amount of fly ash introduced instead of cement.
Figure 6. Dependence of the CS of FFC samples on the amount of fly ash introduced instead of cement.
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Figure 7. Dependence of the FS of FFC samples on the amount of FA introduced instead of cement.
Figure 7. Dependence of the FS of FFC samples on the amount of FA introduced instead of cement.
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Figure 8. Dependence of the TC of FFC samples on the amount of FA.
Figure 8. Dependence of the TC of FFC samples on the amount of FA.
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Figure 9. Structure of FFC: (a) control composition; (b) composition with 10% fly ash instead of cement (6 times magnification).
Figure 9. Structure of FFC: (a) control composition; (b) composition with 10% fly ash instead of cement (6 times magnification).
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Jcs 07 00076 g010 550
Figure 10. Causal diagram of factors affecting the quality of fiber foam concrete.
Figure 10. Causal diagram of factors affecting the quality of fiber foam concrete.
Jcs 07 00076 g010
Table
Table 1. Properties of cement.
Table 1. Properties of cement.
Material TitleSpecific Surface, m2/kgNormal Density of Cement Paste, %Grinding Fineness, Sieve Pass No. 008, %Flexural Strength (FS), MPaCompressive Strength (CS), MPa
Portland cement CEM I 42.5N (“Novoroscement”, Novorossiysk, Russia)3402598.14.2 (2 days)
7.7 (28 days)		26.2 (2 days)
45.3 (28 days)
Table
Table 2. Physical characteristics of sand.
Table 2. Physical characteristics of sand.
Material TitleSize ModulusBulk Density, kg/m3True Density, kg/m3The Content of Dust and Clay Particles, %Clay Content in Lumps, %
Quartz sand (“Arkhipovsky quarry”, Arkhipovskoe, Russia)0.81145627690.20.1
Table
Table 3. Characteristics of polypropylene fiber.
Table 3. Characteristics of polypropylene fiber.
Material TitleFiber Length, mmDiameter, µmDensity, kg/m3Oil
Polypropylene fiber (“Fibropolymer”, Moscow, Russia)620–25910Silastol
CUT 70
Table
Table 4. Characteristics of the foam generator.
Table 4. Characteristics of the foam generator.
Material TitleDensity, kg/m3Hydrogen Index (pH) of the Foam Generator AgentFoam Stability, c
Foam generator PB-2000 (Ivkhimprom, Ivanovo, Russia)12008360
Table
Table 5. Chemical composition of fly ash.
Table 5. Chemical composition of fly ash.
Material TitleSiO2, %SO3, %TiO2, %CaO, %K2O, %MgO, %Al2O3, %Fe2O3, %
Fly ash (Novocherkasskaya GRES, Novocherkassk, Russia)29.784.131.0911.561.433.4232.9815.61
Table
Table 6. Formulas for calculating the PC and MC of FFC.
Table 6. Formulas for calculating the PC and MC of FFC.
Defined CharacteristicFormulaDefinition
Dry density (DD) ρ d = m V · 1000 ρ d is DD (kg/m3);
m is the mass of the sample (g);
V is the sample volume (cm3).
CS R = ∝ F A K w R is CS (MPa);
F is breaking force (N);
A is the area of the loaded section of the sample (mm2);
∝, δ are scale factors (∝ = 0.95, δ = 0.92 for specimen cross-sectional side 100 mm);
Rbt is FS (MPa);
a, b, l are width, height of the cross-section of the prism and the distance between the supports, respectively, when testing specimens for tensile bending (mm);
K w is coefficient depending on the moisture content of the samples during the test (Kw = 1.0 for 10% moisture content).
FS R b t = δ F l a b 2 K w
Frost resistance R r e l = ( 1 − R m t n R m t k ) · 100 R r e l is relative decrease in the strength of the main samples (%);
R m t n is the average value of the strength of the main samples after a given number of test cycles (MPa);
R m t k is the average value of the strength of control samples (MPa).
Δ m = m n ( 1 − w n ) − m n ¯ ( 1 − w n ¯ ) m n ( 1 − w m ) Δ m is mass loss of the main samples (%);
m n is the average value of the mass of the main samples after water saturation (g);
w n is the average moisture content of control samples in parts of a unit after water saturation;
m n ¯ is the average value of the mass of the main samples after passing through the established or intermediate number of freeze and thaw cycles (CFT) (g);
w n ¯ is the average moisture content of the main samples in parts of a unit after passing through a specified or intermediate number of CFT.
Table
Table 7. Technical support of the pilot study process.
Table 7. Technical support of the pilot study process.
Technological laboratory equipmentBall planetary mill “Activator-4M” (“Plant of chemical engineering”, Novosibirsk, Russia)
Laboratory foam concrete mixer SA 400/500 (DSTU, Rostov-on-Don, Russia)
Test laboratory equipmentPress PM-20MG4 (RNPO RusPribor, St. Petersburg, Russia)
Thermal conductivity meter ITP-MG4
(SKB “Stroypribor”, Chelyabinsk, Russia)
Low-Temperature Climatic Chamber NTKK-1.8/4.2 (Research Institute of Building Physics, Moscow, Russia)
Measuring instrumentsMicroscope MBS-10 (Measuring equipment, Moscow, Russia)
Table
Table 8. Change in DD of FFC samples depending on the percentage of cement replacement with FA.
Table 8. Change in DD of FFC samples depending on the percentage of cement replacement with FA.
IndicatorChange in % DD of FFC Samples with FA Content Instead of Part of Cement (%)
10203040506070
Dry density−2−6−8−10−5−12
Table
Table 9. Change in CS of FFC samples depending on the percentage of FA.
Table 9. Change in CS of FFC samples depending on the percentage of FA.
Indicator% Change in CS of FFC Specimens Containing FA Instead of Part of the Cement (%)
10203040506070
Compressive strength 12−3−9−16−20−32−41
Table
Table 10. Change in FS of FFC samples depending on the percentage of FA.
Table 10. Change in FS of FFC samples depending on the percentage of FA.
IndicatorChange in % of FS of FFC Samples with FA Content (%)
10203040506070
Tensile strength in bending 23−15−28−45−51−53−57
Table
Table 11. Change in the TC of FFC samples depending on the percentage of FA.
Table 11. Change in the TC of FFC samples depending on the percentage of FA.
IndicatorChange in % of TC of FFC Samples with the Content of FA (%)
10203040506070
Thermal conductivity −14−8−5471017
Table
Table 12. Results of determining the CS of control samples.
Table 12. Results of determining the CS of control samples.
Sample NumberCS of FFC Samples of the Control Composition, MPaCS of FFC with 10% Replacement of Cement for FA, MPa
13.403.81
23.423.87
33.383.82
43.463.75
53.423.76
63.463.78
Rmtk1 = 3.42Rmtn2 = 3.80
Table
Table 13. Results of determining the CS of the main samples.
Table 13. Results of determining the CS of the main samples.
Sample NumberCS of FFC Samples of the CC, MPaCS of FFC Samples with 10% Replacement of Cement for FA, MPa
13.053.55
23.083.52
32.983.5
43.013.51
52.953.49
62.983.48
72.953.57
82.923.59
93.093.58
102.943.54
112.903.5
123.043.52
Rmtn1 = 2.99Rmtn2 = 3.58
Table
Table 14. Results of frost resistance assessment.
Table 14. Results of frost resistance assessment.
Composition TitleRelative Strength Reduction in the Main Specimens, %Mass Loss of the Main Samples, %
Fiber foam concrete of control composition124.6
Fiber foam concrete with 10% fly ash instead of cement63.4
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Beskopylny, A.N.; Shcherban’, E.M.; Stel’makh, S.A.; Mailyan, L.R.; Meskhi, B.; Varavka, V.; Chernil’nik, A.; Pogrebnyak, A. Improved Fly Ash Based Structural Foam Concrete with Polypropylene Fiber. J. Compos. Sci. 2023, 7, 76. https://doi.org/10.3390/jcs7020076

AMA Style

Beskopylny AN, Shcherban’ EM, Stel’makh SA, Mailyan LR, Meskhi B, Varavka V, Chernil’nik A, Pogrebnyak A. Improved Fly Ash Based Structural Foam Concrete with Polypropylene Fiber. Journal of Composites Science. 2023; 7(2):76. https://doi.org/10.3390/jcs7020076

Chicago/Turabian Style

Beskopylny, Alexey N., Evgenii M. Shcherban’, Sergey A. Stel’makh, Levon R. Mailyan, Besarion Meskhi, Valery Varavka, Andrei Chernil’nik, and Anastasia Pogrebnyak. 2023. "Improved Fly Ash Based Structural Foam Concrete with Polypropylene Fiber" Journal of Composites Science 7, no. 2: 76. https://doi.org/10.3390/jcs7020076

APA Style

Beskopylny, A. N., Shcherban’, E. M., Stel’makh, S. A., Mailyan, L. R., Meskhi, B., Varavka, V., Chernil’nik, A., & Pogrebnyak, A. (2023). Improved Fly Ash Based Structural Foam Concrete with Polypropylene Fiber. Journal of Composites Science, 7(2), 76. https://doi.org/10.3390/jcs7020076

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