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Article

Optimizing Masonry Mortar: Experimental Insights into Physico-Mechanical Properties Using Recycled Aggregates and Natural Fibers

by
Daniel Ferrández
1,*,†,
Alicia Zaragoza-Benzal
1,†,
Rocío Pastor Lamberto
2,
Paulo Santos
3 and
Jacek Michalak
4
1
Departamento de Tecnología de la Edificación, Escuela Técnica Superior de Edificación, Universidad Politécnica de Madrid, Avda. Juan de Herrera, 6, 28040 Madrid, Spain
2
Departamento de Arquitectura y Tecnologías de la Edificación, Escuela Técnica Superior de Arquitectura y Edificación, Universidad Politécnica de Cartagena, C/Real, 3, 30201 Cartagena, Spain
3
University of Coimbra, ISISE, ARISE, Department of Civil Engineering, 3030-788 Coimbra, Portugal
4
Research and Development Center, Atlas sp. z o.o., 2, Kilinskiego St., 91-421 Lodz, Poland
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Appl. Sci. 2024, 14(14), 6226; https://doi.org/10.3390/app14146226
Submission received: 2 July 2024 / Revised: 15 July 2024 / Accepted: 16 July 2024 / Published: 17 July 2024
(This article belongs to the Special Issue Geomaterials: Latest Advances in Materials for Construction and Engineering Applications (2nd Edition))
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Abstract

:
The European Green Deal establishes the efficient management of construction resources as one of its main lines of action. In this sense, the recovery of construction and demolition waste for its reincorporation into the manufacturing process of new sustainable materials has become necessary for the industry. This work deals with the physical and mechanical characterization of cement mortars made with recycled concrete aggregates and reinforced with natural fibers. The reinforcement fibers used (abaca, coconut, and toquilla) are more environmentally friendly compared to traditional synthetic reinforcements. The aim of this research is to analyze the main physico-mechanical properties of these sustainable cement mortars. The results show that mortars made with recycled sand have a lower density and better thermal performance than traditional mortars. In addition, with the incorporation of these natural fibers, the flexural strength of the mortars with recycled aggregate increased by up to 37.6%. Another advantage obtained from the incorporation of these natural fibers is the reduction in shrinkage in the masonry mortars during the drying process, giving them greater dimensional stability and making their behavior similar to that of traditional mortars. Thus, this work shows the potential application of masonry mortars produced under circular economy criteria and their application in the building sector.

1. Introduction

According to the latest estimations, the building materials market will reach USD 1.5 trillion by 2027, a consequence, among other factors, of the accelerated growth of infrastructure in emerging economies [1]. Thus, this increase in market share linked to construction’s intensive nature will be related to an increase in the consumption of natural resources and energy, leading to a strong environmental impact in terms of greenhouse gas emissions and waste generation [2]. Construction and demolition waste (CDW) is produced in the process of construction, maintenance, demolition and deconstruction of infrastructure and buildings [3]. This waste, mainly solid, has a very heterogeneous and generally inert nature [4]. According to the latest UN figures, each citizen in an economically developed region contributes approximately two tons of CDW per year, which implies more than 500 million tons of CDW per year [5].
Unfortunately, the vast majority of these CDW are disposed of in landfills in large cities, posing a risk to health, the environment and wasting resources [6]. The European Union, aware of this global problem, has promoted the efficient use of resources in construction for responsible economic growth through the European Green Deal [7]. For this reason, in recent decades, several lines of research have emerged that are committed to the incorporation of circular economy criteria and the redesign of traditional construction products. In this work, the study of the physical and mechanical properties of masonry mortars made with recycled concrete aggregate and natural fibers is addressed, thus seeking to promote the application of eco-friendly materials in the construction sector.
Concrete is the most consumed building material on the planet, generating a strong environmental impact due to the high energy consumption required for its production [8], while at the same time it is responsible for most CDW (between 100 · 109 and 340 · 109 kg in the EU alone [9]). The use of recycled concrete aggregates (RCA) is essential for CDW recovery and revalorization, as it allows the production of sustainable mortars and concretes with a lower demand for original raw materials [10]. In general terms, the physical and mechanical characteristics of RCA, such as their high content of surface impurities as a result of the cementitious layer adhered to the aggregate during the crushing and grinding of these wastes [11], make it difficult to use them in structural elements. This mortar layer that covers the RCA decreases its mechanical strength the lower the quality of the old mortar adhered and can also compromise the durability of the composites made with these secondary raw materials as a result of their higher porosity and water absorption capacity compared to natural aggregate [12].
On the other hand, these RCA have a higher content of fine particles, which means that it is necessary to use a higher content of mixing water or to use chemical superplasticizing agents in the dosing of mortars [13]. This aggregate particle size can be corrected to some extent by introducing washing and sieving stages prior to the use of the aggregate, although it is true that qualitatively its physical properties do not usually improve with respect to natural aggregate [14]. Another consequence of the use of these aggregates in the manufacture of masonry mortars is the increase in shrinkage during the setting process, which causes surface cracks that can compromise their durability [15].
For this reason, reinforcement fibers have become a technically and economically viable alternative to improve the properties of mortars and concrete made with these recycled aggregates.

Literature Review on Mortars with Natural Fibers: Coconut, Abaca and Toquilla

For centuries, natural fibers have been present in the execution of construction works, extending their application as reinforcement in the elaboration of materials [16]. So much so that, thanks to their low cost and local availability, they have been frequently used in housing and rural structures, contributing to improve the thermal resistance of the base material and being a light and easy to repair addition [17]. Research has also shown how replacing synthetic fiber reinforcement with natural fibers can help mitigate adverse air pollution effects [18]. The high carbon footprint and a large amount of energy required to produce commercial synthetic fibers are some of the reasons that have driven research to look for alternative and eco-friendly solutions for the reinforcement of composite materials [19]. The use of natural fibers still needs to be improved, but thanks to continuous research, the development of new methods, and the growing awareness that they are a sustainable alternative to conventional materials, their use is increasing in various industries [20].
The origin of these natural fibers is very diverse, e.g., hemp, bamboo, sisal, wheat straw, date palm, rice husk, pineapple, jute or banana, and many research have been carried out in this area [21]. In this research, three natural fibers have been used as reinforcement material for mortars: coconut, abaca and toquilla straw.
First, a review of the most recent studies on coconut fiber reinforced cement mortars is presented in Table 1.
Firstly, there is a discrepancy in the ideal percentage of fiber to add, as well as in the optimum fiber length. While it is true that additions in mortar volume between 1 and 2% and lengths between 15 and 20 mm provided in all cases improvements in mechanical flexural strengths [28,30]. In addition, it has been observed that the addition of these fibers improved the toughness and prevented mortars cracking [26], while increasing their thermal resistance [31] and the durability of these cement composites [23]. Negative effects include the possible degradation of the fibers when subjected to moisture–dryness cycles [29].
Similarly, a brief review of the most recent research findings on abaca fiber reinforced mortars is shown in Table 2.
In view of the obtained results by other researchers shown in Table 2, it can be seen that abaca fibers also allow to improve the mechanical behavior to bending in masonry mortars [37]. Additionally, the good hygroscopic properties of these fibers and their potential application to reduce shrinkage are highlighted [35,36]. Finally, the importance of pre-treatment of natural fibers is emphasized, with the aim of increasing their durability and favoring the integration of the fiber into the mortar matrix [34].
No research has been found on the use of toquilla straw fibers in the manufacture of cement mortars, however, the potential applications of toquilla straw as a thermoplastic reinforcement for composite materials were reported [38]. Toquilla straw or Carludovica palmata has been commonly used in hot and humid regions of South America for the manufacture of hats and handicrafts. Although its application in construction has not yet been widely exploited, other similar natural fibers such as wheat straw [39], kenaf straw [40], rice straw [41] or cotton straw [42] have been widely used. In general terms, these fibers have been used as structural reinforcement material in mortar and concrete, with the aim of improving the mechanical strength, toughness and deformation capacity of cement-based composite materials, as well as obtaining improved thermal properties [43].
Moreover, there are constantly new reports about the possibility of using new natural fibers, such as guaruma (Ischinosiphon koern) of Amazonian origin—natural fibers still unexplored with great potential for use in cementitious systems [44]. Many review articles, often critical ones, have examined the potential use of natural fibers to reinforce cementitious systems [45,46,47]. In recent years, the need to reconcile technological development in the cement industry with the preservation of the environment has led to the use of silica-rich ash residues, from the calcination of plant fibers such as corn straw, sugar cane straw or rice straw [48]. This novel application has become a promising alternative for the partial substitution of cement in the development of new eco-friendly mortars, which in turn have physical-mechanical properties that make them competitive in the market [49]. However, although the use of natural fiber has gained prominence in the area of materials engineering due to the versatility of the use of natural fibers in cementitious composites. There are also some disadvantages of using these fibers in the matrix of mortars. As demonstrated in some research [44], the incorporation of these reinforcing materials can result in a loss of workability of the mix or wettability, as well as the difficulty to distribute the fiber homogeneously or the need for prior treatments that improve the behavior and durability of the fiber for application in matrices.
In short, based on the current state of knowledge, the objective of this research is to analyze the physical and mechanical properties of masonry mortars made with recycled concrete sand and reinforced with natural fibers. For this purpose, a characterization of the developed cement materials, to which coconut, abaca, and toquilla straw fibers have been added, will be carried out. To date, no research work has been found that comparatively analyzes the effect of these three types of reinforcement on cement mortars made with RCA. This may be of particular interest for the development of sustainable mortars in South American countries where these three types of natural fibers are highly abundant. The aim is to contribute to the development of eco-friendly mortars and to promote their application in the building sector. The research thus falls within the framework of the Sustainable Development Goals (SDGs) included in the 2030 Agenda, and more specifically, SDG 9, “Industry, Innovation and Infrastructure” [50]. Moreover, this work is also essential in terms of the contemporary perception of the environmental impact of construction products, namely that using natural materials of plant origin is crucial [51], Any positive example of using recycled materials in construction is additionally significant because demolition/recycled materials are perceived as less valuable in construction for fear that they will not meet specific requirements in terms of mechanical strength [52]. Moreover, the innovative simultaneous use of RCA and natural fibers in masonry mortars is crucial for this research.

2. Materials and Methods

This section describes the raw materials used to make the cement mortars studied in this research, as well as their dosage and manufacturing process. Subsequently, the experimental campaign conducted is described, indicating the equipment and methods applied.

2.1. Materials

The materials used to make the masonry mortars were cement, water, natural aggregate, recycled concrete aggregate, and natural coconut, abaca, and toquilla straw fibers.

2.1.1. Cement

The binder used for the mortars’ preparation was CEM II/B-L 32.5 N cement. This material commonly used in building construction, is made up of Portland clinker, limestone, and up to 5% additions [53]. Table 3 shows the elemental composition of this raw material obtained by X-ray fluorescence.
As shown in Table 3, different oxides constitute the cement used in this research, with CaO, SiO2, SO3, Al2O3, and Fe2O3 being the majority. These oxides are essential for the formation of the potential compounds of cementitious materials: tricalcium silicate (SC3), dicalcium silicate (SC2), tricalcium aluminate (AC3), and tetracyclic aluminoferrite (AFC4) [54]. Finally, it should be noted that the appearance of sulfur oxide in the cement composition is linked to the use of gypsum compounds during the manufacturing process, which are added to mitigate setting time [55].

2.1.2. Water

In the development of the different samples elaborated in this work, drinking water from the Canal de Isabel II of the Community of Madrid has been used. This tap water has been successfully used in previous research [56], according to Council Directive 98/83/EC [57]. Its main characteristics are its combined Cl content of 1.13 mg/L, pH 7.8, and conductivity of 116.7 μS/cm [58].

2.1.3. Aggregates

Two different types of aggregates have been used for the development of this research: natural sand (NA) standardized for reference tests distributed by the Eduardo Torroja Institute of Construction Sciences (Madrid, Spain), and recycled sand from concrete waste (RCA) supplied by the TEC-REC plant (Madrid, Spain). First, the physical properties of these raw materials were analyzed in Table 4, following the recommendations of the reference standards.
As can be seen in Table 4, the fine particle content of the recycled aggregates was higher than that obtained for the standard sand. This is due to the process of crushing and obtaining these secondary raw materials, and if their particle size is not controlled by sieving before use, they can cause a decrease in the mechanical strength of masonry mortars [64]. Furthermore, these recycled sands have a higher friability coefficient and a lower density, which implies a decrease in the mechanical properties of the mortars, as has been demonstrated in another research [65]. It should also be noted that these RCA had a higher water absorption coefficient, which results in a higher water demand during mortar mixing [66].
Figure 1 shows the granulometric curve obtained for applying these aggregates in the manufacture of cement mortar. The aim was to obtain a continuous particle size that favors mixture workability, using a back-and-forth sieve shaker and a series of standardized sieves according to UNE-EN 933-2 [67].
Finally, an elemental analysis was carried out to determine the chemical composition of the recycled concrete aggregates. The results obtained using the X-ray fluorescence technique are shown in Table 5.
As can be seen in Table 5, RCA used have a majority composition of SiO2 (57.92%) as a consequence of the natural aggregate used in the manufacture of concrete. In addition, CaO and SO3 impurities are observed, generally coming from gypsum waste mixed in the building demolition process [69]. High content of sulfur impurities can be detrimental to the durability of reinforcement or other steel elements that come into direct contact with these mortars. Therefore, it is important to insist on the need for selective demolition processes at the end of the life of the building [70]. Finally, ignition losses (11.68%) are a consequence of the organic matter content of these secondary raw materials.

2.1.4. Natural Fibers

Three natural fibers have been used as reinforcement material: coconut, abaca and toquilla straw. These fibers are shown in Figure 2.
All fibers used and shown in Figure 2 were manually cut to a length of 20 ± 1 mm. In addition, these fibers have the properties shown in Table 6.
Regarding the natural fiber treatment, the vast majority of studies have opted for using NaOH solutions in the range of 1–15% [71]. According to a review of the current literature [72,73], this type of pre-treatment improves durability, tensile strength, and Young’s modulus compared to untreated natural fibers. Thus, in this research, the three types of fibers used were pre-treated by soaking the fibers in a 5% NaOH solution by mass for 2 days, washing them with water and manually separating the fibers, drying the fibers for 24 h at a temperature of 45 ± 5 °C, and finally, they were stored in airtight containers until their use.

2.2. Production Process

For the preparation of the different types of mortar used in this research, the recommendations of the UNE-EN 196-1 standard [74] were followed. The same techniques and methods outlined in Figure 3 have always been used, using an IBERTEST planetary mixer model IB32-040V01 (Madrid, Spain).
As can be seen in Figure 3, all the samples were cured under the same conditions prior to the tests (temperature 22 ± 1 °C and ambient humidity 95 ± 2%) and for a period of 28 days.
Regarding dosages used for the preparation of the samples, Table 7 shows the mass proportions used for each type of mortar.
As seen in Table 7, two proportions of cement/aggregate by mass (1:3 and 1:4) commonly used for manufacturing of mortars have been used. A percentage of natural fiber addition corresponding to 1.5% by weight of cement has been selected, based on the results obtained by other researchers and analyzed in the literature review.
Additionally, with this added fiber proportion it was possible to obtain a plastic and workable consistency corresponding to a mortar diameter of 175 ± 10 mm in the shaking table test, following the UNE-EN 1015-3 standard [75]. It should be noted that in no case superplasticizers were used, to eliminate environmentally harmful chemical compounds and taking into consideration that the main application of these mortars would be as a non-structural lining or bonding material [14]. Consequently, the water/cement ratio had to be increased compared to mortars made with natural aggregate, since otherwise, as highlighted in the research by Marvila et al. there is a significant difficulty to obtain an adequate workability of the mixture [76]. Finally, it should be noted that two reference mixes were prepared with standardized sand for both cement/aggregate ratios used in this research.

2.3. Experimental Programme

A schematic representation of the experimental programme developed is shown in Figure 4.
In this way, in the first phase, a study of the masonry mortar’s physical properties has been conducted through the following tests:
  • Bulk density of mortars according to UNE-EN 1015-10 [77]. To determine this value, the average of three measurements taken on samples of dimension 4 × 4 × 16 cm3 is used. The bulk density is obtained by dividing the mass of the specimen previously dried in an oven for 24 h (65 ± 5 °C) by the apparent volume of the hardened mortar sample.
  • Mortar thermal conductivity was determined according to the UNE-EN ISO 8990:1997 standard [78]. For this purpose, a mini thermal hot-box equipped with thermocouples and a datalogger for accumulating temperature data was used. A total of three samples were tested for each type of mortar, with dimensions of 24 × 24 × 3 cm3. The measurements were taken 24 h after the start of the test to obtain a stationary heat flux.
  • Water absorption coefficient by capillarity according to UNE-EN 1015-18 [79]. In this way, six half-samples are obtained from 4 × 4 × 16 cm3 samples so that the fractured face with the open matrix is the one in contact with the water. Thus, these half-samples, previously dried for 24 h in an oven (65 ± 5 °C), are immersed vertically in water to a depth of 10 ± 1 mm. The capillary absorption coefficient is determined with the help of Equation (1):
C = 0.1 · M 90 − M 10  
where C is the capillary absorption coefficient, and M 10 and M 90 correspond to the mass of the sample at 10 and 90 min of testing.
  • Open porosity determined according to UNE-EN 1396 [80]. It is defined as the ratio between the accessible pore volume and mortar apparent volume. To determine this index, a total of three samples of each type were used and Equation (2):
O p e n P o r = W s a t − W 0 W s a t − W i m m × 100
where O p e n P o r shows the open porosity index in percentage, W s a t is mass of water saturated sample, W i m m immersed weight of the water-saturated sample determined by hydrostatic balance, and W 0 is the weight of the dried sample.
  • Mortar shrinkage over time according to the recommendations of the UNE 80-112-89 standard [81]. For this test, three samples of each type of mortar of dimension 2.5 × 2.5 × 28.7 cm3 were used. This test allows the determination of the dimensional stability of the mortars over time and is expressed as the longitudinal variation in mm/m experienced by the samples over 120 days.
On the other hand, the mechanical properties of these eco-friendly cement mortars have been studied. The tests carried out in this second phase include:
  • Mortar bonding strength according to UNE-EN 1015-12 [82]. For this purpose, a layer of each mortar type of approximately one centimeter thickness is applied to a previously moistened ceramic block. The ceramic piece is moistened immersed in water for 24 h, in such a way that the suction of the water from the mortar mixing by the ceramic piece is avoided. Subsequently, with the help of adhesion equipment, metal discs with a diameter of 50 mm are glued with epoxy resin on the hardened sample. The tensile test determines the adhesion strength between the mortar and the application surface. A total of five samples were tested for each mortar type.
  • Mortar surface hardness, determined by using a Shore D durometer (Smooth-On, Inc., Macungie, PA, USA), following the recommendations of UNE-EN-ISO 868 standard [83]. A total of three 4 × 4 × 16 cm3 samples were analyzed for each dosage, taking five measurements on the two plane-parallel faces that were in contact with the mold and separating each measurement by a minimum distance of 2 cm from each other.
  • Mechanical resistance to bending and compression in standardized samples of 4 × 4 × 16 cm3 according to UNE-EN 196-1 [74]. The flexural strength test consists of a three-point breakage test, where the vertical load is applied perpendicular to the longitudinal axis of the sample and at the center of the span. Subsequently, each of the two pieces generated in this bending test is subjected to a compression breaking test using a uniform load on a surface of 4 × 4 cm2. For this test, an IBERTEST hydraulic press model AUTETEST 200-10SW has been used (Madrid, Spain).
  • Interior microstructure analysis of the different mortars developed in the research using scanning electron microscopy (SEM). A Jeol JSM-820 (Mitaka, Tokyo) operating at 20 kV and equipped with Oxford EDX analysis was used to carry out this analysis. The test samples were extracted from the inner matrix of the 4 × 4 × 16 cm3 mortar samples. Additionally, to ensure a good surface conductivity of electrons, the test samples were coated with a thin gold foil using a Cressington 108 model metallizer (Watford, UK).
Finally, at the end of the presentation and discussion of the results, a last subsection has been included where a critical review of the results obtained for the different properties analyzed is addressed. In this way, the application possibilities of the masonry mortars produced under the circular economy criteria studied in this research are explored.

3. Results

This section presents the results obtained after carrying out the planned experimental campaign, as well as a discussion of these results.

3.1. Physical Properties

This section presents the physical properties analyzed for the different mortar types. Firstly, Figure 5 shows the results obtained for bulk density and thermal conductivity.
As can be seen in Figure 5, mortars made with recycled concrete aggregate had a lower bulk density compared to traditional mortars made with natural sand. Additionally, incorporating natural fibers in these recycled mortars causes a slight reduction in density. This trend is observed for both cement/aggregate ratios, where it should be noted that mortars with a 1:4 ratio have a lower bulk density. This slight decrease in density has already been observed in other studies [84]. However, in the case of this research, it was not very significant since the natural fibers are incorporated as an addition and not as a replacement for the aggregate.
On the other hand, it is observed that the bulk density decreases experienced by mortars made with RCA translates into a decrease in their thermal conductivity. This improvement in thermal resistance has become crucial to the application of these products made with secondary raw materials in the construction sector [85]. Furthermore, no significant differences in terms of thermal conductivity were observed between samples made with cement/aggregate ratios of 1:3 and 1:4 by weight, except for the reference sample of conventional mortar. Benmansour et al. also corroborate how the addition of natural fibers reduces the thermal conductivity of masonry mortars [86]. In this sense, the addition of toquilla straw fiber produced the most significant improvements, decreasing the thermal conductivity of the recycled mortars by 24.2% and 29.5% for dosages with cement/aggregate ratios of 1:3 and 1:4, respectively. This decrease in thermal conductivity was expected because the fibers used have a lower thermal conductivity compared to the mortar matrix. In addition, the incorporation of fibers in the matrix tends to generate porosity and air in the matrix, improving the thermal resistance of the composite.
On the other hand, the behavior of the different mortar types produced against the action of water has been analyzed. Table 8 shows the results of the capillary water absorption and open porosity tests.
As seen in Table 8, the capillary water absorption coefficient of mortars made with recycled sand is much higher than that of traditional mortars made with natural sand. This phenomenon, attributed to the type of aggregate, has already been observed in previous research, where it was related to the higher water absorption capacity of recycled aggregates [87]. In addition, capillary absorption is inversely proportional to pore size, so mortars made with a cement/aggregate mass ratio of 1:4 have a lower absorption, although they would have a higher water vapor permeability [88]. Finally, as Santo de Lima et al. point out, capillarity is linked to the internal communication between the pores, so the incorporation of natural fibers causes a reduction in the capillarity coefficient that can be attributed to the clogging of the pores by these reinforcing materials [44]. For the three types of natural fibers studied, the addition of abaca fiber was the one that achieved the greatest reduction in capillarity in mortars with RCA.
On the other hand, the open porosity of the mortars with a cement/aggregate ratio of 1:4 by mass was higher than that obtained for the samples made with a 1:3 ratio. It can also be seen that this volume of accessible pores is higher in the mortars made with RCA and increases with the incorporation of natural fibers.
Figure 6 shows shrinkage evolution during the hardening process of the different mortars designed for this research.
The cement paste shrinks over time and causes surface cracks that contribute to a decrease in the mechanical strength and durability of the mortars [89]. Mainly, shrinkage in masonry mortars made with RCA occurs as a consequence of the higher porosity and water absorption capacity of these sands [90]. Thus, in Figure 6a,b, it can be seen how traditional mortars made with natural sand have a lower shrinkage over time. However, in agreement with previous research [65], the beneficial effect of incorporating fibers into the mortar matrix to improve its dimensional stability and reduce shrinkage is confirmed. In this sense, it was the abaca fibers that reduced the shrinkage of the mortars made with RCA to the greatest extent, approaching the values experienced by the reference mortar with natural aggregate. It can also be seen that shrinkage is higher in mortars with a 1:3 cement/aggregate ratio than in those with a 1:4 cement/aggregate mass ratio. Thus, according to Miah et al., a good distribution of natural fibers during the manufacture of cement mortars would reduce shrinkage during drying and increase the application potential of these sustainable construction materials [91].

3.2. Mechanical Properties

In this section, the mechanical properties obtained during the characterization of the developed mortars are presented. Firstly, Table 9 shows the results for surface hardness and bond strength.
As can be seen in Table 9, there are no significant differences between the surface hardness of mortars made with natural sand and mortars made with RCA. Likewise, the cement/aggregate ratio used is not a discriminating factor for this property. However, a slight decrease in Shore D hardness is observed in those mortars that incorporate natural fibers in their composition (approximately 6–7% on average). A similar behavior was experienced by Piña et al., in their research with mortar made with the addition of recycled mineral wool fibers [57].
On the other hand, in terms of bond strength, greater differences were observed between the reference mortar and the mortar made with RCA. For both cement/aggregate ratios, the adhesion of the traditional mortars was superior to that of the RCA samples, with better adhesion for the samples with higher cement content (1:3 cement/aggregate ratio). This effect was already observed by García López de la Osa in his doctoral thesis, which showed a non-linear relationship between the increase in adhesion and the cement content of the mortars [92]. On the other hand, a slight decrease in the adhesion of mortar on ceramic walls is observed when natural reinforcement fibers are incorporated. This effect may be due to the water retention capacity of these natural fibers, as well as to the weakening of the mortar’s internal microstructure at the cement-aggregate interface. Of all the mortar types analyzed, those with the incorporation of toquilla straw fiber were the least resistant to adhesion. In any case, it is worth noting that, during the test, the breakage of the samples with fibers occurred in all cases at the mortar-ceramic piece interface.
Figure 7 and Figure 8 show the results obtained for the flexural and compressive strength tests of the mortars.
Figure 7 shows that mortars with a higher cement content (1:3 cement/aggregate mass ratio) have a higher flexural strength. Also, in accordance with the results obtained in other investigations, the total substitution of the natural aggregate by RCA results in a decrease in the mechanical flexural strength [93]. For this reason, the incorporation of reinforcement fibers has become a recurrent solution to improve the mechanical strength of these construction materials produced under circular economy criteria. Thus, it is observed that in all cases, the addition of natural fibers led to an improvement with respect to the flexural strength values obtained for recycled mortars without fibers. In addition, it can be seen that coconut and abaca fibers showed a similar behavior, being the ones that increased the flexural strength to the greatest extent. These results are in agreement with others analyzed in the literature review, where natural fibers increased the deformation capacity and reduced cracking in cement mortars [27,33].
Regarding to mortar compressive strength, shown in Figure 8, it is also observed that the traditional mortars surpassed the mortars made with recycled aggregate. In this sense, the dosages with a 1:4 cement/aggregate mass ratio show a lower strength compared to their counterparts with a higher amount of cement. This effect on the compressive strength of recycled aggregates has already been observed by other researchers [94], who highlight this characteristic as one of the main limitations to their widespread use in the construction sector. In addition, it can be observed that for this property, the incorporation of reinforcement fibers does not result in an improvement in strength. In other studies, improvements were only obtained for flexural strength, even reducing the compressive strength as a consequence of the addition of fibers [95].
In view of the results obtained, a potential application of these cement mortars would be in masonry works where the mechanical requirements need not be very high [96]. However, it is important to take into consideration the compatibility between the mortar and the ceramic pieces in order to avoid excessively stiffening the wall [97]. In this sense, other potential applications for this type of mortar according to the literature are the production of prefabricated elements, floor slabs or urban furniture, among others [98,99]. Finally, to complete the discussion of the mechanical results, SEM images have been included to show the microstructure of the mortars produced in this research. The results are presented in Figure 9. It should be noted that these images were obtained with the aim of capturing as much information as possible and thus providing a complementary discussion of the mechanical properties analyzed. For this reason, the magnifications were selected for each mortar according to the diameter of the natural fiber added and the characteristics of the mortars studied.
Regarding mortars made with coconut fiber, a slight cracking of the matrix around the fibers can be observed in Figure 9a. This weakening may be due to the absorption of hydration water from the cement by the fiber during the hardening process. Also, although fiber dispersion was good, there are preferential points of fiber accumulation. Figure 9b shows in detail the interface between coconut fiber and the cementitious matrix, where the good adhesion between the two and the formation of compounds derived from the hydration of the cement on its surface can be seen. It is well known that the arrangement of strong, stiff, and brittle fibers, as well as a higher strength of the fiber–matrix bond, results in better mechanical properties and reduced ductility [25].
Figure 9c shows the integration of abaca fiber into the cement mortar matrix. As highlighted by some authors [100], the bonding between added fibers and mortar matrix is directly linked to the lignin and cellulose content of these natural reinforcement materials. These biopolymers are shown in Figure 9d, showing one of these fibers in detail. In general, natural fibers exhibit a bridging effect similar to that of existing synthetic fibers, which can reduce autogenous shrinkage by connecting the micro-cracks in the matrix and inhibiting the propagation of cracks at an early stage [36].
Finally, Figure 9e shows in detail the interior of the toquilla straw fiber where its composition can be seen. These larger diameter fibers were the ones that, to a lesser extent increased the flexural strength of the mortars made with RCA. Although it is true that, as seen in Figure 9f, there is a good integration between the reinforcement and the matrix. However, it can be seen how toquilla straw fiber broke after the flexural test, as illustrated in the SEM image obtained.

3.3. Critical Discussion of the Potential Application of the Developed Mortars

In this section, a critical discussion of the results obtained is carried out in order to explore the application potential of the developed cementitious materials. Thus, Figure 10 and Figure 11 show a classification of the mortars developed based on the different properties studied. The values presented in Figure 10 and Figure 11 are expressed in terms of the increase or reduction experienced concerning traditional mortar in each of the properties determined.
Comparatively, Figure 10 and Figure 11 show how the decrease in bulk density and thermal conductivity are one of the main advantages of mortars made with recycled aggregates and natural fibers. Thus, a potential application of these composite materials would be the development of prefabricated elements for modular construction with improved thermal performance. These lighter and more thermally resistant parts would reduce material execution times, transport costs, and improve the energy rating of the building [101]. Another key factor to consider is the shrinkage experienced during setting, which, although it is true that it increases when using recycled sands, the incorporation of natural fibers becomes an economically viable alternative that would improve the dimensional stability of the mortars. In this sense, it would be interesting to analyze the durability of these cementitious materials, since, due to their greater porosity and water absorption by capillarity, they could be difficult to apply in climates where they could be exposed to freeze–thaw cycles.
Regarding mechanical properties, a decrease in flexural and compressive strength is observed with respect to traditional mortar. However, the incorporation of natural fibers significantly improves the flexural strength and avoids brittle fracture of the mortar pieces. In any case, these properties may not be discriminating in terms of the final application of the mortars, since they would be suitable for use in non-structural walls. Finally, the decrease in adhesion on ceramic walls suggests the application of these mortars as reinforcement materials. In this regard, recent research has suggested the use of these materials as cladding materials in traditional adobe houses to improve their performance in earthquakes [17].
Figure 12 shows the process for possibly manufacturing mortar blocks made with the dosages proposed in this research.
First, it should be noted that the mortars produced in this research did not use a superplasticizer. This chemical agent, which improves the workability of the mixture without impairing its mechanical properties, becomes one of the environmentally harmful agents during the production of mortars with recycled aggregates [14]. In this sense, it is crucial to approach the final application of these materials with the aim of reducing the consumption of these chemical compounds, as they can sometimes be dispensable for non-structural applications.
From the point of view of the process presented in Figure 12, the transport of the raw materials and the final distribution of the prefabricated parts to the point of service are understood as critical phases. This effect has been observed previously by other researchers [102], where it was noted that the most significant environmental impact was derived from the distribution of the product, sometimes offsetting the beneficial effect obtained by using secondary raw materials. Likewise, the design and packaging of the products also become a key factor in optimizing their distribution, a key factor here being the decrease in density experienced by mortars made with recycled aggregate [103]. Thus, a greater lightening of the precast products thanks to the use of these RCA would lead to a reduction in logistics costs and a lower environmental impact associated with this distribution. In addition, this lower density, and consequently better thermal behavior, would have a positive impact on the improvement of energy efficiency during the use phase. Finally, it is worth highlighting the need to address a waste management plan and carry out careful deconstruction processes, as this would allow CDW to be classified at source and enhance its subsequent recovery, recycling, and revaluation [104].

4. Conclusions

This research has addressed the development and physico-mechanical characterization of cement mortars made with RCA and reinforced with three types of natural fibers. Thus, this research contributes to progress towards the circularity of construction products and the characterization of new materials produced under circular economy criteria. The main conclusions of this study are:
  • Mortars made with RCA have a lower bulk density than those made with natural aggregate. This decrease in density of close to 10% is even greater for those samples made with natural fibers. Thus, this lower density and the incorporation of fibers of vegetable origin have a positive effect on the thermal conductivity of the mortars. For the best case, with the incorporation of toquilla straw fibers, the RCA–1:3–1.5%(T) dosage obtained a thermal conductivity 37% lower than that of the reference mortar (NA–1:3).
  • Capillary water absorption was much higher in those mortars made with RCA due to the nature of these sands. However, the incorporation of natural fibers managed to reduce the capillary water absorption, although without reaching the values obtained for the traditional mortar made with natural sand.
  • The incorporation of natural fibers reduces shrinkage during the drying of mortars made with RCA, thus approaching the behavior experienced by traditional mortars. It has been observed that the mortars made with abaca fiber showed greater dimensional stability.
  • Surface hardness was not affected by the type of aggregate used in the manufacture of the mortars, although it was slightly reduced by the incorporation of natural fibers.
  • The production of mortars with RCA has a negative impact on bond strength. Thus, the adhesion of traditional mortar is superior to mortar with RCA and, in turn, the incorporation of natural fibers has an impact on the decrease in this mechanical property. Mortars with higher cement content showed higher bond strength.
  • Flexural strength of mortars made with recycled aggregate is lower than that of traditional mortar and is reduced as the cement content in the mix decreases. Despite this, the incorporation of natural fibers has a positive impact on the improvement of the flexural strength of mortars made with RCA, with coconut and abaca fibers showing the best performance.
  • The incorporation of fibers did not significantly improve the compressive strength of the mortars, the type of aggregate being the determining factor for this property. Thus, the traditional mortars showed a higher compressive strength than their counterparts made with RCA.
This study’s limitations and future lines of research include the need to carry out accelerated ageing tests, which would make it possible to estimate the durability of these mortars. In the same way, for its use as a facing material, it would be convenient to analyze the optimum fiber length that allows an improvement in the mechanical properties, without excessively reducing the workability of the mortar. Finally, its application in the development of blocks or prefabricated pieces would be interesting for the future industrialization of these products.
Moreover, taking into account the availability of natural fibers whose impact was examined in this study, i.e., abaca, coconut, and toquilla straw, and that the most beneficial use of these natural raw materials in terms of environmental impact will be in their place of origin, the results of this study are also an indication for professionals involved in construction in areas where these natural fibers occur. The research described in this article fits well with the expectation that recycling and reuse of demolition materials and using renewable raw materials are/will be a traditional and common practice in the construction sector, bringing us closer to sustainable development.

Author Contributions

Conceptualization, D.F. and A.Z.-B.; methodology, A.Z.-B. and D.F.; software, A.Z.-B. and R.P.L.; validation, D.F., P.S. and J.M.; formal analysis, D.F. and R.P.L.; investigation, A.Z.-B., D.F. and R.P.L.; resources, D.F. and P.S.; data curation, D.F. and A.Z.-B.; writing—original draft preparation, D.F. and A.Z.-B.; writing—review and editing, A.Z.-B., P.S., R.P.L. and J.M.; visualization, P.S. and J.M.; supervision, P.S. and J.M.; project administration, D.F. 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

All data have been included in the research results.

Acknowledgments

The authors would like to thank the kindness and availability shown by the ATLAS® company, as well as the staff of the laboratories involved in carrying out the tests presented in this work.

Conflicts of Interest

Author Jacek Michalak was employed by the company ATLAS. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Aggregate particle size curves compared with limits set out in NBE FL-90 standard [67] and adapted to sieves series indicated in UNE-EN 933-2 [68].
Figure 1. Aggregate particle size curves compared with limits set out in NBE FL-90 standard [67] and adapted to sieves series indicated in UNE-EN 933-2 [68].
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Figure 2. Natural fibers used: (a) abaca fiber; (b) coconut fiber; (c) toquilla straw fiber.
Figure 2. Natural fibers used: (a) abaca fiber; (b) coconut fiber; (c) toquilla straw fiber.
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Figure 3. Manufacturing process diagram of the mortar samples used in this research.
Figure 3. Manufacturing process diagram of the mortar samples used in this research.
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Figure 4. Experimental programme scheme.
Figure 4. Experimental programme scheme.
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Figure 5. Thermal conductivity (bars) and bulk density (points) of the analyzed mortars.
Figure 5. Thermal conductivity (bars) and bulk density (points) of the analyzed mortars.
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Figure 6. Shrinkage experienced by mortars over time. (a) Samples with cement/aggregate mass ratio 1:3. (b) Samples with cement/aggregate mass ratio 1:4.
Figure 6. Shrinkage experienced by mortars over time. (a) Samples with cement/aggregate mass ratio 1:3. (b) Samples with cement/aggregate mass ratio 1:4.
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Figure 7. Flexural strength test results.
Figure 7. Flexural strength test results.
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Figure 8. Compressive strength results.
Figure 8. Compressive strength results.
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Figure 9. SEM analysis: (a,b) RCA–1:3–1.5%(C); (c,d) RCA–1:3–1.5%(A); (e,f) RCA–1:3–1.5%(T).
Figure 9. SEM analysis: (a,b) RCA–1:3–1.5%(C); (c,d) RCA–1:3–1.5%(A); (e,f) RCA–1:3–1.5%(T).
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Figure 10. A comparison between the different mortar typologies studied according to the cement/aggregate ratio used, 1:3 ratio by mass.
Figure 10. A comparison between the different mortar typologies studied according to the cement/aggregate ratio used, 1:3 ratio by mass.
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Figure 11. A comparison between the different mortar typologies studied according to the cement/aggregate ratio used, 1:4 ratio by mass.
Figure 11. A comparison between the different mortar typologies studied according to the cement/aggregate ratio used, 1:4 ratio by mass.
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Figure 12. Product system for the manufacture of mortar blocks.
Figure 12. Product system for the manufacture of mortar blocks.
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Table 1. Coconut fiber-reinforced cement mortar studies.
Table 1. Coconut fiber-reinforced cement mortar studies.
Ref.Fiber Addition Fiber Length [mm]Main Results
[22]0, 4, 6, and 8% by weight of cement16.7In all cases the flexural strength was increased, however, for the compressive strength a decrease was observed from 4% added fiber. Capillary water absorption in the mortars is increased. Energy consumption and CO2 emissions during production decreased by more than 11% compared to the control mortar without coconut fiber.
[23]0, 0.2, 0.4, and 0.6% mass fractions of cement and soil mix24.0The coconut fiber reinforcement increased mortar blocks’ residual strength under compression, as well as their ductility and energy absorption. In addition, a significant improvement in mortar durability against freeze–thaw and moisture–dryness cycles was obtained.
[24]0.5, 1.0, 1.5, and 2.0% relative to the weight of cement30.0Properties of cement mortars with coconut fiber in masonry walls were evaluated. The best results were obtained for addition of 0.5% fiber, significantly improving flexural strength and slightly increasing diagonal compression in walls.
[25]0, 0.25, 0.5, 0.75, 1.0, 1.25 and 1.5% by volumeLess than 3.0As the interface between added fibers and mortar matrix weakened, there was an improvement in the mortars’ cushioning properties. However, there was a compressive strength loss and an increase in porosity.
[26]0.125, 0.25, 0.5 and 0.75 by weight of mortar mix24.0While there was no significant improvement in flexural and compressive mechanical strengths, post-cracking properties such as ductility, residual strength and toughness increased with a higher fraction of coconut fiber in the mortar matrix.
[27]0, 1, 2.5, and 4% by volume17.0Coconut fiber addition positively influenced the first crack deflection, improving the toughness indices, plastic cracking and impact strength of the produced cementitious mortars.
[28]0, 1, 2 and 3% by volume of mortar10.0–20.0 The best results were obtained with additions of coconut fiber at 2% by volume of mortar, significantly increasing flexural strength compared to the reference, improving toughness, and preventing cracking.
[29]0, 1, 2 and 3% by volume of mortar16.0–18.0The mortars were subjected to moisture–dryness cycles and an increase in wetting was observed in coconut fiber samples. Natural degradation of the fibers in the mortar matrix occurred, which decreased the mechanical strengths.
[30]1.5% by volume of mortar15.0Higher flexural strengths, increased fracture energy and increased capillary water absorption were obtained compared to traditional mortar.
[31]0, 5, 10, 15, and 20% by weight of cement60.0The potential use of coconut fiber reinforced mortars as a façade cladding material was analyzed. It was found that these mortars with plant fibers could reduce dwelling interior temperatures by 0.5–1.5 °C.
[32]0.25 and 0.5% by weight of cementMore than 10The coconut fiber reinforced mortars significantly improved flexural and compressive mechanical strengths, satisfying the criteria for application as mortar in masonry walls according to Colombian standards.
[33]0, 3, 6, 9, 12, and 15% by weight of cement10.0–30.0Composite with a combination of 10% silica fume, 10% metakaolin and 6% coconut fibers showed superior mechanical and physical properties to the reference mortar, with a compact microstructure, making it a sustainable and environmentally friendly alternative.
Note: Search in Web of Science with the keywords: ((“Coco” OR “Coconut”) AND (“Fiber*” OR “Fiber*”) AND (“Mortar*”)).
Table 2. Studies on abaca fiber-reinforced cement mortar.
Table 2. Studies on abaca fiber-reinforced cement mortar.
Ref.Fiber AdditionFiber Length [mm]Main Results
[34]0.2, 0.3 and 0.4% over the total solid weight20.0, 25.0, 30.0 and 35.0The best results were obtained in mortars made with treated fibers with a 3% NaOH solution, with a length of 30 mm and incorporated at 0.2% of the mortar weight. With this composition, it was possible to improve the mechanical resistance to bending and compression, with a good fiber integration in the matrix.
[35]0.2% over the total solid weight30.0Flexural strength was improved by 20%, compressive strength by 28% and tensile strength by 26% compared to the reference mortars. In addition, the abaca fiber mortars improved their dimensional stability by reducing shrinkage.
[36]0, 0.25, 0.5, 1 and 2% of the weight of cement4.5The good hygroscopic properties of abaca fibers reduce the autogenous shrinkage of mortars, improving their mechanical performance and mitigating cracking risk. In addition, the presence of these fibers delays mortar setting time.
[37]1% over the total solid weight10.0, 30.0, 80.0, 100.0 and 300.0Abaca fibers showed great potential for application in rehabilitation mortars for masonry houses. The optimum fiber length for improving flexural strength and wall ductility was found to be 80 mm.
Note: Search in Web of Science with the keywords: ((“Abacá” OR “Abaca”) AND (“Fiber*” OR “Fiber*”) AND (“Mortar*”)).
Table 3. Elemental composition of cement type CEM II/B-L 32.5 N. Results in percentage by mass.
Table 3. Elemental composition of cement type CEM II/B-L 32.5 N. Results in percentage by mass.
CaOSiO2SO3Al2O3Fe2O3K2OTiO2MgOSrOBaOMnOP2O5I. Loss
67.1116.984.564.383.560.710.230.250.090.070.090.041.93
Table 4. Aggregates’ physical characterization.
Table 4. Aggregates’ physical characterization.
PropertiesNARCANormative
Fine Content (%)1.23 ± 0.063.87 ± 0.10UNE–EN 933-1 [59]
Particle FormNot relevantNot relevantUNE–EN 13139 [60]
Fineness Modulus (%)2.56 ± 0.144.11 ± 0.08UNE–EN 13139 [60]
Friability Coefficient (%)14.11 ± 0.2524.15 ± 0.23UNE–EN 146404 [61]
Bulk Density (kg/m3)1560 ± 171387 ± 14UNE–EN 1097-3 [62]
Dry Density (kg/m3)2569 ± 32398 ± 16UNE–EN 1097-6 [63]
Water Absorption (%)0.54 ± 0.045.87 ± 0.09UNE–EN 1097-6 [63]
Table 5. Elemental composition of recycled concrete aggregates. Results in percentage by mass.
Table 5. Elemental composition of recycled concrete aggregates. Results in percentage by mass.
AggregateAl2O3CaOFe2O3K2OMgOSiO2MnOTiO2SO3P2O5NaO2I. Loss
RCA10.8112.031.562.410.9857.920.430.651.030.090.4111.68
Table 6. Properties of the natural fibers used in this research.
Table 6. Properties of the natural fibers used in this research.
FiberBulk Density (kg/m3)Young’s Modulus (GPa)Diameter (mm)
Abaca1230 ± 822.7 ± 1.30.1–0.4
Coconut1340 ± 1516.4 ± 0.90.1–1.2
Toquilla straw670 ± 1211.5 ± 0.70.7–1.1
Table 7. Mass dosages used for the manufacture of mortars.
Table 7. Mass dosages used for the manufacture of mortars.
TypeCement [g]Sand [g]Water [g]Water/Cement RatioNatural Fiber Reinforcement [g]
AbacaCoconutToquilla Straw
NA–1:3450.01350234.00.52———
RCA–1:3450.01350328.50.73———
RCA–1:3–1.5%(A)450.01350328.50.736.75——
RCA–1:3–1.5%(C)450.01350328.50.73—6.75—
RCA–1:3–1.5%(T)450.01350328.50.73——6.75
NA–1:4337.51350183.00.54———
RCA–1:4337.51350283.50.84———
RCA–1:4–1.5%(A)337.51350283.50.846.75——
RCA–1:4–1.5%(C)337.51350283.50.84—6.75—
RCA–1:4–1.5%(T)337.51350283.50.84——6.75
Table 8. Capillary water absorption and open porosity test results.
Table 8. Capillary water absorption and open porosity test results.
SampleCapillarity Water Absorption (kg/m2·min0.5)Open Porosity (%)SampleCapillarity Water Absorption (kg/m2·min0.5)Open Porosity (%)
NA–1:30.41 ± 0.0367.3 ± 0.9 NA–1:40.39 ± 0.0170.1 ± 0.3
RCA–1:30.63 ± 0.0182.4 ± 1.2 RCA–1:40.57 ± 0.0383.3 ± 0.3
RCA–1:3–1.5%(A)0.50 ± 0.0376.5 ± 0.7RCA–1:4–1.5%(A)0.44 ± 0.0176.4 ± 1.3
RCA–1:3–1.5%(C)0.55 ± 0.0277.1 ± 0.9RCA–1:4–1.5%(C)0.46 ± 0.0378.8 ± 0.5
RCA–1:3–1.5%(T)0.57 ± 0.0276.8 ± 0.4RCA–1:4–1.5%(T)0.50 ± 0.0278.5 ± 0.5
Table 9. Results obtained for surface hardness and adhesion strength tests.
Table 9. Results obtained for surface hardness and adhesion strength tests.
SampleSuperficial Hardness (Shore D Units)Bonding Strength (MPa)SampleSuperficial Hardness (Shore D Units)Bonding Strength (MPa)
NA–1:384.1 ± 0.70.53 ± 0.05NA–1:483.7 ± 0.60.42 ± 0.02
RCA–1:383.8 ± 0.40.41 ± 0.02RCA–1:483.9 ± 0.50.35 ± 0.01
RCA–1:3–1.5%(A)78.9 ± 0.40.35 ± 0.04RCA–1:4–1.5%(A)79.2 ± 0.80.29 ± 0.03
RCA–1:3–1.5%(C)78.1 ± 1.10.38 ± 0.01RCA–1:4–1.5%(C)78.5 ± 0.70.30 ± 0.03
RCA–1:3–1.5%(T)79.3 ± 0.60.30 ± 0.02RCA–1:4–1.5%(T)78.1 ± 0.40.26 ± 0.02
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Ferrández, D.; Zaragoza-Benzal, A.; Pastor Lamberto, R.; Santos, P.; Michalak, J. Optimizing Masonry Mortar: Experimental Insights into Physico-Mechanical Properties Using Recycled Aggregates and Natural Fibers. Appl. Sci. 2024, 14, 6226. https://doi.org/10.3390/app14146226

AMA Style

Ferrández D, Zaragoza-Benzal A, Pastor Lamberto R, Santos P, Michalak J. Optimizing Masonry Mortar: Experimental Insights into Physico-Mechanical Properties Using Recycled Aggregates and Natural Fibers. Applied Sciences. 2024; 14(14):6226. https://doi.org/10.3390/app14146226

Chicago/Turabian Style

Ferrández, Daniel, Alicia Zaragoza-Benzal, Rocío Pastor Lamberto, Paulo Santos, and Jacek Michalak. 2024. "Optimizing Masonry Mortar: Experimental Insights into Physico-Mechanical Properties Using Recycled Aggregates and Natural Fibers" Applied Sciences 14, no. 14: 6226. https://doi.org/10.3390/app14146226

APA Style

Ferrández, D., Zaragoza-Benzal, A., Pastor Lamberto, R., Santos, P., & Michalak, J. (2024). Optimizing Masonry Mortar: Experimental Insights into Physico-Mechanical Properties Using Recycled Aggregates and Natural Fibers. Applied Sciences, 14(14), 6226. https://doi.org/10.3390/app14146226

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