Mechanical Properties of Composite Materials Obtained with Clay Matrices and Plant Waste Inserts

Abstract

In a global context where sustainability is becoming a priority in construction, this paper analyzes the use of composite materials based on clay and plant waste, offering an ecological alternative to conventional materials. This article analyzes the mechanical properties of composite materials made from clay with walnut shell inserts, shredded biomass from corn stalks, wheat straw, and wool waste. These materials are developed for sustainable rural construction. The study evaluates flexural and compressive strength based on measurements at varying insert concentrations. The results indicate that mechanical strength decreases as insert concentration increases. The materials are suitable for partitions and insulating walls, and in lightweight buildings without floors, they can be used as load-bearing walls if reinforced with wood or metal. Future research directions include improving the estimation of mechanical behavior, studying rheological characteristics under environmental conditions, and expanding the application of clay and plant waste composites.

1. Introduction

Composite materials based on clay matrices and plant waste inserts are inspired by traditional construction methods such as adobe, which has been used for thousands of years. On the other hand, the last century of construction materials history has seen the rise of composite materials [1,2]. This work presents research on the properties of construction materials obtained from clay as a matrix and inserts from plant waste. Finally, a significant benefit of producing such materials is the reduction in vegetable waste accumulation and the reduction in carbon dioxide emissions produced by burning them when they are not used as inserts.

Composite materials have been used throughout history, both naturally and through human intervention [3,4]. Examples found in nature include wood, soil, and the bones of living organisms [5]. The first man-made composite materials were bricks made from a mixture of straw and mud, used in construction since ancient times [6]. Among the most traditional building materials is adobe [7], a compound made from clay, straw, and dung, dried in the sun. Houses built with this material, often supported by wooden structures, have been found across the globe throughout history.

Composite materials with a clay matrix and plant waste inserts are inspired by traditional construction methods, such as adobe, but benefit from modern manufacturing techniques [8,9,10].

The production of these materials involves mixing clay with four types of plant waste (walnut shells, corn stalks, wheat straw, and wool), followed by mechanical pressing and controlled drying for two weeks [1,2]. This method ensures better material cohesion and allows for testing of the mechanical properties, such as compressive and flexural strength [11,12,13,14,15,16,17,18,19,20].

A comparative analysis with other construction materials shows that this class of composites is suitable for lightweight structures and insulation, but not for high-load-bearing applications [21,22]. This study examines the mechanical performance of these composites, identifying their limitations and advantages in sustainable construction [23,24,25,26,27,28]. Other solutions include coconut fibers [29], straw [30], sisal [31,32], bioenzymes [33], and recycled optical fibers, all having an impact on mechanical strength and sustainability [34,35,36,37,38,39,40,41].

Unlike conventional materials, these composites contribute to sustainability by reducing agricultural waste and lowering CO₂ emissions associated with their incineration [42,43,44,45]. Composite materials are being researched for specific applications, highlighting their versatility and benefits in various industries [46,47,48,49], utilizing different manufacturing technologies and developing new solutions for recycling agricultural and industrial waste [50,51,52,53,54,55,56].

This paper presents experimental results that give the mechanical characteristics of composite materials with clay matrix and plant waste inserts. The dependencies of the mechanical characteristics of the materials depending on the concentration of the insert are also given.

This study hypothesizes that the mechanical strength of clay-based composites decreases with increasing plant waste content due to weaker cohesion between the fibers and the matrix. However, certain types of inserts, such as walnut shells and corn stalks, may provide better structural integrity compared to others, such as wool and wheat straw.

It is expected that composite materials with lower insert concentrations (below 15%) will exhibit sufficient mechanical properties for lightweight construction applications, whereas higher insert concentrations will lead to a significant reduction in mechanical strength.

Composites with walnut shells and corn stalks will demonstrate superior mechanical properties compared to those containing wool and wheat straw, which may fail under higher concentrations due to poor cohesion.

Adjustments in processing parameters (such as pressure, drying time, and temperature) could enhance the mechanical performance of these materials, making them more viable for construction applications.

2. Materials and Methods

The base material consists of a clay matrix and various plant waste inserts. The composite materials were mechanically tested to analyze their flexural and compressive strength. The samples were subjected to compression and forced drying, which improved the hardness of the material. The tests were carried out using high-precision mechanical testing equipment.

Bending and compression tests are essential for understanding how these materials will react under real-world usage conditions. For example, composite samples containing shredded biomass from corn stalks and walnut shells showed a satisfactory mechanical strength for light structural applications.

The technology used to produce these composite materials was based on mixing clay with four types of plant waste materials of different concentrations (from 0 to 50%), pressing them and drying them in a controlled environment for two weeks [1], after which conclusions were drawn regarding the cohesion of the insert material with the clay matrix, as well as the optimal insert concentration.

In this paper, mechanical testing methods applied to composite materials with a clay matrix and various inserts at different concentrations are described.

2.1. Methodology of Experiments for Bending Stress

The bending test of clay and plant waste samples involves testing their ability to withstand forces that cause flexural deformation. This type of test is essential for evaluating the structural behavior of bricks under specific loading conditions and can provide valuable information regarding their use in construction.

The samples for the bending test of construction materials (clay with shredded biomass from corn stalks, clay with walnut shells, clay with wool, and clay with wheat straw) were made using a mold with dimensions of 40 × 40 × 160 mm. After two weeks of drying, the composite material samples (Figure 1) were subjected to mechanical bending tests on a 50,000 N ZDM testing machine (Figure 2).

The samples made of composite materials are positioned between the supports of the machine at a length of 100 mm. For the bending stress on the sample, a force was applied (which varied from the minimum value of 30 N to the maximum value of 420 N), obtained by operating the lever at a constant speed until it yielded, and the value of the displayed force was recorded (Figure 3).

Next, we calculate the bending strength of the analyzed composite materials, based on relations (1) and (2) [57].

$$R_{inc}={\displaystyle \frac{1.5·F·l}{b·h^2}}$$

(1)

and the average bending strength is:

$$R_{inc med}={\displaystyle \frac{R_{inc1}+R_{inc2}}{2}}$$

(2)

where:

• F—pressing force (N);
• b—length of the sample (mm);
• h—height of the sample (mm);
• l—length between supports, (mm);
• $R_{inc}—$ bending resistance (N/mm²);
• $R_{inc med}$—average bending strength (N/mm²).

2.2. Methodology of Compression Experiments

The compression test provides information on how the samples respond to vertical pressures, such as those generated by the weight/mass of the supporting structure or other loads [3,4]. The composite material samples are positioned on the lower platen of the testing machine (Figure 4a); the zero point is adjusted on the dial (Figure 4a); the upper platen is lowered to approximately the height of the sample (Figure 4b) by operating the lever. A uniform and constant vertical force is then applied to the sample (Figure 4c) until it fails (Figure 4d), at which point the force value displayed on the dial is recorded.

For the calculation of the compressive strength, relations (3)–(5) were used [57].

$$A=b·h$$

(3)

$$R_{comp}={\displaystyle \frac{F}{A}}$$

(4)

$$R_{comp med}={\displaystyle \frac{R_{comp1}+R_{comp2}+R_{comp3}}{3}}$$

(5)

where:

• A—area of the composite material, (mm²);
• $R_{comp}$—compressive strength, (N/mm²);
• $R_{comp med}$—average compressive strength, (N/mm²).

3. Results

The numerical data collected during the bending and compression experiments were processed statistically. All of the physical quantities depend on the parameters of the composite material produced in the process. The main parameter for analysis is the concentration of the insert in the composite, a parameter that can be controlled.

Apart from the concentration of the insert, there are several parameters that influence the process: density, initial humidity, viscosity, cohesion and adhesion of the composite components, the temperature at which the process is carried out, and the compression speed. The dependence of the technological process parameters solely on the concentration of the insert in the clay composite and the insert materials was studied. Naturally, the final qualitative parameters will also depend on the initial ones.

3.1. Results Regarding the Mechanical Resistance of Bricks Made of Composite Material

3.1.1. Bend Request

The results of the bending test of the samples built from the four composite materials formed with the clay matrix and insert from walnut shells, shredded biomass from corn stalks, wool and straw, are summarized in Figure 5 and

Table 1. Statistical data of the bending test.

The Concentration of the Insert	Walnut Shell Insert	Insert Shredded Biomass from Corn Stalks	Wool Insert	Wheat Straw Insert
0	0.3083	0.3083	0.3083	0.3083
5	0.205	0.2221	0.3042	0.1668
10	0.0352	0.0477	0.1768	0.1428

.

3.1.2. Compression Request

The results of the compression tests of the samples from the four composite materials formed with a clay matrix and insert from walnut shells, shredded biomass from corn stalks, wool and straw are summarized in Figure 6 and

Table 2. Statistical data of the compression test.

The Concentration of the Insert	Walnut Shell Insert	Insert Shredded Biomass from Corn Stalks	Wool Insert	Wheat Straw Insert
0	3.06	3.06	3.06	3.06
5	1.5	1.49	1.85	1.41
10	0.89	1.34	1.88	0.9
15	0.89	0.95		0.64
20	0.49	1.6
25	0.72	1.16
30	0.9	1.04
35	0.56	0.8
40	0.7	1.03
45	0.55	0.36
50	0.32	0.41

.

As the concentration of plant waste in the composite material increases, its resistance to bending and compression decreases. This reduction is mainly due to the poor cohesion between the insert fibers and the clay matrix, which diminishes the ability of the composite to withstand mechanical loads.

Composite materials with clay matrix and inserts of shredded biomass from corn stalks and walnut shells demonstrated a better mechanical performance compared to similar materials with wool or straw inserts, which failed to withstand tests at concentrations higher than 15% due to the lack of cohesion.

4. Discussion

The analysis of the research results is conducted according to the insertion concentration, because it represents the basis of all other developments.

The obtained results suggest that adjusting some parameters (forming pressure, working temperature and humidity) could significantly improve the mechanical properties of these composites.

Mechanical Resistance Properties

In general, composite materials with a clay matrix and plant waste inserts are not used for high bending loads. In fact, they are not even produced in the sizes required for insert concentrations greater than 10% (Figure 5). The critical bending strength of the investigated materials ranges between 0.05 and 0.3 MPa. Wood has a flexural strength of 10 MPa [58], and unfired clay is measured at 4.8 MPa [59].

The bending test of bricks made of composite materials with a clay matrix was possible only for low insert concentrations (below 10%).

The compression test experiments were conducted on composite materials with a clay matrix and inserts of shredded biomass from corn stalks and walnut shells. Composite materials of the same type with wheat straw inserts could only be tested at insert concentrations below 15%, while those with wool waste inserts were tested only at concentrations below 10%.

The bending and compressive strengths of all of the tested materials decrease as the insert concentration increases.

The variation in the mechanical properties of composite materials due to the insert concentration is related to the microstructure of these materials. Based on both intuition and a review of the literature on the variation of the modulus of elasticity in composite materials, one can hypothesize that adhesion between the fiber and the matrix of the material is among the properties involved in this phenomenon. To validate this, it would be necessary to specify an average value of the adhesion between the matrix and the insert. However, determining this value is complex, as it likely depends on multiple parameters of the composite formation process, such as molding pressure, material grain size, working temperature, initial and final humidity, moisture content at the time of testing, matrix material density, and insertion density.

The effect of insert concentration on the mechanical properties of composite materials is a subject for future research, particularly for the materials studied in this work. There are no similar studies in the specialized literature. Some preliminary studies in this direction have already been conducted [59], but further progress requires extensive and highly detailed experiments, especially considering that the mechanical strength of composite bricks changes over time, depending on the storage or usage conditions.

The composite materials obtained in this research have compressive strength values comparable to those of weaker construction materials (e.g., porous limestone: 2.7 MPa, light concrete: 8.1 MPa, clay brick: 9.8 MPa, Heluz P15 25 brick: 15 MPa). However, their strength is sufficient for use in light construction, filling, or insulating walls, but not for tall buildings or heavy load-bearing structures. It is observed that increasing the insert concentration reduces the compression resistance.

Using the linear regression program and the Mathcad professional program [60,61], linear or non-linear regressions are obtained for the compressive strength of composite materials with a clay matrix. For the composite material with an insert of shredded biomass from corn stalks, the linear regression of two variables is obtained

$$R_c=1.318081-0.058803c+0.710093{\tau }_p,$$

(6)

where $R_c$ is the compression resistance; c is the insertion concentration of the raw material; ${\tau }_p$ is the loading or pressing time for brick formation. Regression (6) achieves the coefficient of determination R² = 0.84, which means that the two parameters, together, explain 84% of the variation in compressive strength.

For the composite material with a clay matrix and walnut shell insert, the calculated linear regression [60] is:

$$R_c=0.555358-0.087545c+0.0154119F_c,$$

(7)

where $F_c$ is the pressing force of the material in the forming mold. The level of significance reached is 0.05, and the coefficient of determination is 0.79; the two significant variables for compressive strength together explain 79% of the resistance variation.

The results of this study confirm that the mechanical strength of clay-based composites decreases as the concentration of plant waste inserts increases. This trend aligns with previous research on fiber-reinforced composites [62], which analyzed fiber-reinforced mud bricks and reported similar reductions in strength at higher fiber contents. Similarly, [63] demonstrated that adding recycled asphalt to mud bricks influenced mechanical properties by altering the cohesion level within the composite.

Comparing our results with those obtained by other authors [64], who studied fiber-reinforced adobe bricks, we observe that our materials exhibit a lower bending and compressive strength than traditional unfired clay bricks, but retain suitable properties for lightweight, non-load-bearing structures. Additionally, research conducted by [30] on coconut fiber-reinforced cement bricks highlights that certain plant-based materials improve insulation properties, albeit at the expense of the mechanical performance, a trade-off also observed in our study.

Regarding sustainability, studies conducted by [21,65] suggest that integrating plant waste into composite construction materials can significantly reduce environmental impact while maintaining sufficient durability for practical applications. Our conclusions align with these studies, supporting the viability of clay-based composites with plant waste inserts as sustainable construction solutions.

Future research should focus on optimizing processing techniques, such as increasing compaction pressure and refining drying conditions, to mitigate the reduction in mechanical strength observed at higher insert concentrations. Additional studies integrating microstructural analyses (e.g., SEM, FTIR) would provide deeper insights into the interactions between fibers and the matrix, as presented in the study on biologically structured smart materials [66,67].

In conclusion, this study contributes to the research on sustainable construction materials and highlights the potential of clay-based composites with plant waste inserts for environmentally friendly applications.

5. Conclusions

Composite materials consisting of a clay matrix and inserts of agricultural waste, such as shredded walnut shells, shredded biomass from corn stalks, wool, and straw, showed variable mechanical strengths depending on the type and concentration of the insert. Materials with shredded biomass from corn stalks and walnut shells demonstrated a better mechanical performance compared to those made with wool and straw.

Increasing the insertion concentration of agricultural waste in the composite had a negative effect on the mechanical strength. This decreases both the flexural and compressive strength, as the cohesion between the clay matrix and the insert fibers becomes insufficient to support high mechanical loads. The mechanical resistance to bending and compression, for all the tested materials, falls within the resistance ranges of the construction materials of which they are a part, more precisely in the minimum area of these ranges.

Complete testing over the entire range of insert concentrations of 0–50% was only possible for materials with an insert of shredded biomass from corn stalks and shredded walnut shells, the other two materials with cohesion of only up to 10–15% insertion concentration. For these reasons, the following statements will refer only to the composites with biomass from corn stalks and walnut inserts.

A characteristic valid for the results of all of the experiments and characteristics measured and processed statistically is that the relatively large spread of the values and their variation as a sign in relation to the average shows that the manufacturing process of the materials and the properties of the materials created have a pronounced random character. For these reasons, the study was stopped when checking the global trends of increase or decrease in the characteristics measured in relation to the concentration of insertion. The respective variations (bending and compressive strength) have minimums and maximums for which the causes and their stability cannot be specified, because the granulation of the insert material was not considered.

Although the tested composite materials are not suitable for applications requiring high flexural or compressive strength, they may be useful for lightweight construction such as infill or insulation walls, particularly where structural loads are not high.

The study highlights the importance of continued research to better understand how the adhesion between the clay matrix and the insert fibers influences the mechanical properties of composites. This research may lead to the development of more efficient building materials.

To improve the mechanical strength of the samples, future research will explore the following directions:

Optimization of processing parameters: Increasing the compaction pressure and adjusting the initial moisture content could improve the cohesion between the fibers and the matrix.

Addition of binding agents: Investigating the use of ecological binders, such as modified starch or lignin-based substances, to improve the interaction between clay and plant fibers.

Advanced microstructural analyses: Applying SEM (Scanning Electron Microscopy) and FTIR (Fourier Transform Infrared Spectroscopy) techniques to better understand the molecular level interaction mechanisms between the matrix and the inserts.

Long-term durability testing: Conducting studies on the behavior of the material under variable humidity conditions and freeze–thaw cycles to evaluate its stability in real-world use environments.

These directions will contribute to the development of composite materials with an improved performance, while maintaining the ecological advantages of using plant waste.