Bio-based materials are of great interest owing to their abundance and the immense potential they display as an ideal alternative to widely used industrial construction materials (that directly and indirectly harm the environment). In this scope, an in-depth experimental study is presented here on clay-based materials aimed to enhance their properties through the addition of other bio-based components such as fibers, in the present case alfa fiber. The thermal conductivity and mechanical properties (compressive and flexural tensile strengths) of the composite clay-alfa material are analyzed with the percentage of alfa fiber in the matrix ranging from 0% to 4%. It is concluded that a percentage of alfa fibers between 1% and 2% is the most effective to get the required optimization of thermal and mechanical behavior of this bio-based material.

The construction industries are one of the largest energy-consuming sectors and it contributes significantly to environmental stress [

One of the studied bio-based materials in recent years is clay. The growing interest in studying clay as a building material is due to its good physical properties, availability, and low environmental impact. However, the earth (clay) alone suffers from cracking, shrinkage, and low durability [

An extensive review of different types of natural fibers (such as leaf fibers, seed or fruit fibers, and wood fibers) with bio-composites and their different properties is conducted by Ramesh et al. [

In this perspective, our work consists of studying the thermal and mechanical properties of Alfa fiber which is a natural fiber. This type of fiber is used with clay to form a biomaterial based on clay and alfa plant.

For the thermo-mechanical characterization, the thermal conductivity is determined using the asymmetrical hot plate method on the composite clay-based material with alfa fiber percentages ranging from 0% to 4%. In addition to it, the compressive and flexural tensile strengths are obtained using a mechanical press. Then three dimensionless parameters are introduced in order to assess the best composition for a composite with good thermal and mechanical properties.

The materials used in this study are earth (clay) and Alfa fibers.

The soil used comes from Zahjouka in the region of Ksar El Kebir in the north of Morocco. The chemical analysis of this type of clay used is given in the table below (

Chemical component | Percentage of the chemical element (%) |
---|---|

SiO_{2} |
45, 79 |

Al_{2}O_{3} |
15, 68 |

Fe_{2}O_{3} |
12, 83 |

MnO | 0, 17 |

MgO | 6, 26 |

CaO | 9, 6 |

Na_{2}O |
3, 54 |

K_{2}O |
1, 39 |

TiO_{2} |
2, 84 |

P_{2}O_{5} |
0, 6 |

P.F | 0, 31 |

The Alfa’s scientific name is Stipa tenacissima L. The alfa fiber used in this study for its availability and low cost. Alfa fiber bundles are characterized by an average diameter of 113 μm (90–120 μm) and a density of 0.89 g/m^{3}. The fibers are washed beforehand with water and air-dried for 72 h, then cut into 4 cm lengths.

The water used is the drinking water of the city of Salé. The amount of water needed to obtain a paste with normal consistency is:

The unfired clay is mixed with water and several mass concentrations of the Alfa fiber (0.5%; 1%; 2%; 3%; 4%). After drying the soil in an oven for 24 h at 60°C, we measure the mass of the soil each time until it becomes constant. Then, it is mixed with water until the mixture becomes more manageable. Finally, the fibers are gradually added.

The mixture of clay and alfa fibers is transferred into molds that have the following shapes:

Parallelepipeds: Dimensions of 10 cm × 10 cm × 2 cm to carry out the thermal tests (

Cylindrical: Of dimensions 11 cm × 22 cm to carry out the compression tests (

Prismatic: Of dimensions 4 cm × 4 cm × 16 cm to carry out the tests of flexion (

The samples are then dried in a 60°C oven until they reach a constant mass.

Samples | Alfa fiber percentage (%) | Number of samples |
---|---|---|

100% clay | 0 | 3 |

Clay + 0.5% Alfa | 0.5 | 3 |

Clay + 1% Alfa | 1 | 3 |

Clay + 2% Alfa | 2 | 3 |

Clay + 3% Alfa | 3 | 3 |

Clay + 4% Alfa | 4 | 3 |

The apparent densities of the samples may be calculated using the dry mass and dimensions of the samples (

Samples | Alfa fiber percentage (%) | Average density (Kg/m^{3}) |
---|---|---|

100% clay | 0 | 2213.19 |

Clay + 0.5% Alfa | 0.5 | 2075.603 |

Clay + 1% Alfa | 1 | 1916.70 |

Clay + 2% Alfa | 2 | 1806.66 |

Clay + 3% Alfa | 3 | 1642.97 |

Clay + 4% Alfa | 4 | 1583.25 |

We observe that increasing the percentage of Alfa fibers decreases the density of the samples, allowing us to gain in terms of lightness.

The density of the sample containing 0% Alfa fibers is 2213.19 kg/m^{3}, while the density of the sample containing 4% Alfa fibers is 1583.25 kg/m^{3}. We have a gain in lightness of 28.46% between these two samples.

The thermal conductivity is determined using the asymmetric hot plate method in a steady-state regime [

We place the sample of dimensions 100 × 100 × 20 mm^{3} on a heating element. Below this heating element, an insulating foam of dimensions 10 × 100 × 100 mm^{3} and thermal conductivity λ_{2 }= 0.04 W/m.K is placed. The system (heating element, sample, and insulating foam) is then placed between two 50 × 100 × 100 mm^{3} aluminum blocks to allow the system to reach thermal equilibrium as quickly as possible.

In order to measure the thermal conductivity of the sample, we need to measure using a thermocouple the temperature T_{0} at the center of the heated face of the sample, the temperature T_{1} at the center of the unheated face of the sample, and the temperature T_{2} at the center of the unheated face of the insulating foam. The following figure illustrates the method used (

We can write the following relations:

ϕ_{1} represents the heat flux passing through the sample, ϕ_{2} is the heat flux passing through the insulating foam, and ϕ is the total heat flux emitted by the heating element. λ_{1} and e_{1} are respectively the thermal conductivity and thickness of the sample to be characterized. λ_{2} and e_{2} are respectively the thermal conductivity and thickness of the insulating foam (λ_{2 }= 0.04 W/m.K; e_{2 }= 10 mm).

The heating element is an electrical resistance R dissipating a flux by Joule effect when the current (I) flows through under the effect of an electrical voltage (U), hence:

When combining the above equations, we find:

We use a hydraulic press in order to determine the compressive strength. The cylindrical samples (110 mm × 220 mm) are centered between two mechanical plates (norm NF P 18 406).

The tensile strength by bending is determined using the prismatic samples of 40 mm × 40 mm × 160 mm (norm NF P 18 407).

The following table shows the values of thermal conductivity determined using the asymmetric hot plate method in a steady state regime (

Samples | Alfa fiber percentage (%) | Thermal conductivity (W/m.K) |
---|---|---|

100% clay | 0 | 0.938 |

Clay + 0.5% Alfa | 0.5 | 0.683 |

Clay + 1% Alfa | 1 | 0.640 |

Clay + 2% Alfa | 2 | 0.509 |

Clay + 3% Alfa | 3 | 0.419 |

Clay + 4% Alfa | 4 | 0.372 |

The thermal conductivity of the composite decreases as the Alfa fiber content in the composite increases, as seen in the table. The thermal conductivity decreases from 0.938 W/m.K for the earth alone to 0.372 W/m.K for a percentage of 4% of Alfa fibers. There is a gain in terms of insulation.

The results of thermal conductivity is interpreted in

As demonstrated in

The composite studied (clay-Alfa) is generally used for its thermal advantages. But it is required that the composite exhibits a minimum of mechanical properties as well because the material is subjected to the forces of its own weight and the loads it supports when it’s going to be used in the envelope of a building.

We will evaluate the compressive strength of the composite in addition to its flexural tensile strength.

Samples | Alfa fiber percentage (%) | Compressive strength (MPa) |
---|---|---|

100% clay | 0 | 4.63 |

Clay + 0.5% Alfa | 0.5 | 4.5 |

Clay + 1% Alfa | 1 | 4.42 |

Clay + 2% Alfa | 2 | 4.05 |

Clay + 3% Alfa | 3 | 3.5 |

Clay + 4% Alfa | 4 | 2.9 |

We notice from

For the second mechanical property, we have the flexural tensile strength. The results are displayed in

Samples | Alfa fiber percentage (%) | Flexural tensile strength (MPa) |
---|---|---|

100% clay | 0 | 1.15 |

Clay + 0.5% Alfa | 0.5 | 1.38 |

Clay + 1% Alfa | 1 | 1.58 |

Clay + 2% Alfa | 2 | 2.01 |

Clay + 3% Alfa | 3 | 1.75 |

Clay + 4% Alfa | 4 | 1.59 |

The results shown in

We can see from

The flexural strength of clay alone is 1.15 MPa, while the composite clay with 4% Alfa has a flexural strength of 1.59 MPa, which means a 38% increase in flexural strength between the two samples.

Galán-Marín et al. [

Fiber content, thermal conductivity and, mechanical resistance are crucial parameters for choosing the right insulating material. The goal is to find an optimal point on which we will have a better ratio: mechanical resistance/thermal resistance.

For this purpose, we introduced three normalized dimensionless parameters:

K_{therm}, K_{com} and K_{flex} are respectively normalized dimensionless parameters of thermal resistance, compressive strength and flexural strength.

R_{measured} is the thermal resistance in m².K/W. R_{com,measured} and R_{flex,measured} are respectively the compressive strength and the flexural strength in MPa.

_{therm} curve of thermal resistance and the K_{com} curve of compressive strength and K_{flex} flexural strength is located between 1% and 2% of Alfa fibers.

Beyond the 2% of Alfa fibers, the homogeneity assumption is no longer valid. This explains the decrease in resistance.

It can be concluded that for the samples tested, between 1% and 2% of the alfa fiber is the recommended percentage for optimal use of the alfa fiber consolidated with the raw earth material.

Several studies have been conducted on the use of fibers as natural insulators that contribute to the improvement of thermal and mechanical parameters of a composite material. Nitcheu et al. [

In the current paper, an experimental study is carried out on the thermal and mechanical properties of a bio-based composite material made of clay and alfa fibers. The experimental results show that the increase of alfa fibers in the matrix of clay results to a decrease in thermal conductivity thus a gain in lightness and in insulation.

For the mechanical properties, it is shown in this study that the more alfa fiber percentage added, the less the compressive strength the composite material has. However, for the flexural tensile strength, it increases and reaches a peak in 2% of fiber content.

In order to provide an adequate ecological material for construction purposes, it is essential to find an optimal point between the thermal and mechanical characteristics. Using normalized dimensionless parameters taking into consideration the thermal and mechanical aspects, a percentage of alfa fibers between 1% and 2% is recommended for optimal use of this bio-based material in construction.

Alfa fibers do have properties that can justify their use in construction. The developed material (clay-alfa) can be considered as the optimum balance between three requirements: a low cost, a satisfactory mechanical resistance, and good thermal properties. Furthermore, the content of this paper can be used as the main basis to conduct even more in-depth studies on bio-based materials of clay and alfa fibers for the building and construction sectors.

Thermal conductivity (W/m.K)

Heat flux (W)

_{therm}

Thermal dimensionless normalized parameter

_{com}

Normalized dimensionless parameter of compressive strength

_{flex}

Normalized dimensionless parameter of flexural strength

_{measured}

Thermal resistance (m².K/W)

_{com,measured}

Compressive strength (MPa)

_{flex,measured}

Flexural strength (MPa)