This article is focused on the investigation of the mechanical and thermal properties of composite material that could be used for the production of plaster or plasterboards. This composite material is made of gypsum and reinforcing natural fibers. The article verifies whether this natural reinforcement can improve the investigated properties compared to conventional plasters and gypsum plasterboards made of pure gypsum. From this composite material, high-strength plasterboards could then be produced, which meet the higher demands of users than conventional gypsum plasterboards. For their production, natural waste materials would be used efficiently. As part of the development of new building materials, it is necessary to specify essential characteristics for their later use in civil engineering. Crushed wheat straw and three gypsum classes with strengths G2 (2 MPa)—gypsum Class I., G5 (5 MPa)—gypsum Class II. and G16 (16 MPa)—gypsum Class III. were used to create the test samples. Samples were made with different ratios of the two ingredients, with the percentages of straw being 0%, 2.5%, and 5% for each gypsum grade. The first part of the article describes how the increasing proportion of straw affects the composite’s mechanical properties (flexural strength and compressive strength). The second part of the article focuses on the change of thermal properties (thermal conductivity and specific heat capacity). The last part of the article mentions the verification of the fire properties (single-flame source fire test and gross heat of combustion) of this composite material. The research has shown that the increasing proportion of straw reinforcement caused a deterioration in the flexural strength (up to 56.49% in the 3. series of gypsum Class II.) and compressive strength (up to 80.27% in the 3. series of gypsum Class III.) and an improvement in the specific heat capacity and thermal conductivity (up to 31.40% in the 3. series). This composite material is thus not suitable for the production of high-strength plasterboards, but its reduced mechanical properties do not prevent its use for interior plasters. Based on the performed fire tests, it can be said that this composite material can be classified as a non-flammable material of reaction to fire Classes A1 or A2. From an ecological point of view, it is advantageous to use a composite material with a higher straw content.
Gypsum and products made from gypsum are standard building materials and there are many ways of using them in civil engineering [
Natural crushed wheat straw, without chemical additives, was chosen for this research. One of the advantages of using straw as a building material is the reduction of energy consumption from the environmental point of view. These fibers are usually agricultural waste, so they consume less energy in production. They also offer good thermal properties due to their porous nature, which leads to reduced energy consumption in the operation of buildings [
The question is how adding natural fibers to the composite will affect its mechanical properties. Yang et al. [
The disadvantage of straw reinforcement may be its smooth surface, which reduces the cohesion with the binder. This was the subject of research in which straw fibers were modified by treatment with acrylic acid coating and the mechanical properties of straw fiber-reinforced gypsum composite were investigated. The results of this research showed that by the treatment with acrylic acid coating, the roughness of the straw fibers increased markedly, and the dry flexural strength and the dry compressive strength of the gypsum composite increased respectively by 71.3% and 52.4% [
The aim of this research is to verify whether these natural fibers can increase the strength of the plasterboards as well as synthetic fibers. These would be used indoors just like common gypsum plasterboards, but would also meet higher mechanical user requirements. Due to the reinforcing fibers, they could be stronger and have a higher load-bearing capacity. These composite plasterboards differ from commonly available high-strength plasterboards precisely in that they are not reinforced with synthetic fibers but effectively use natural fibrous materials. The straw was mixed with gypsum in various proportions. The percentage amount of straw and the class of gypsum that influence the mechanical and thermal properties investigated can also affect the ignitability of composite materials. This research focuses on investigating the basic properties of a new composite material made of gypsum and straw, which can be further used for the production of interior plasters and gypsum plasterboards.
This paper first describes the production of composite material and methods to test the individual properties of this composite. Specifically, this research is focused on the determination of the following characteristics: flexural strength
For the production of the test samples, gypsum was selected as the binder. It is a powder mixture of hemihydrate of calcium sulfate (CaSO4·1/2 H2O). For research, three types of gypsum with strengths G2 (2 MPa)—gypsum Class I., G5 (5 MPa)—gypsum Class II. and G16 (16 MPa)—gypsum Class III. were tested [
The variant of crushed wheat straw as reinforcing fiber was chosen due to its relatively high tensile strength [
For experimental testing, three series of samples were prepared. The series and numbers of test samples produced are described in
The samples were created in a technical laboratory adapted to this. The ingredients of each series were weighed in proportion and amount and the test samples were made in the number according to
Gypsum class | Series | Percentage proportions of gypsum (%) | Weight of gypsum (kg) | Percentage proportions of crushed straw (%) | Weight of crushed straw (kg) | Testing samples (pcs) |
---|---|---|---|---|---|---|
I. | 1. | 100.0 | 1.00 | 0.0 | 0.000 | 4 |
2. | 97.5 | 0.975 | 2.5 | 0.025 | 3 | |
3. | 95.0 | 0.950 | 5.0 | 0.050 | 3 | |
II. | 1. | 100.0 | 1.00 | 0.0 | 0.000 | 4 |
2. | 97.5 | 0.975 | 2.5 | 0.025 | 6 | |
3. | 95.0 | 0.950 | 5.0 | 0.050 | 6 | |
III. | 1. | 100.0 | 1.00 | 0.0 | 0.000 | 4 |
2. | 97.5 | 0.975 | 2.5 | 0.025 | 6 | |
3. | 95.0 | 0.950 | 5.0 | 0.050 | 6 |
After weighing and mixing the dry ingredients, a powdery mixture was formed and mixed with water for 1 min [
The resulting mixture filled the prepared steel moulds, which were vibrated on the vibrating table for one min. to achieve complete filling of the mould. After 24 h, the samples were removed from the moulds, dried at 40°C to steady weight, and then stored for seven days in the test environment (temperature (23 ± 2)°C and relative air humidity (50 ± 5)%) [
Samples twenty-eight days old were weighed and the dimensions of each sample were measured. The bulk density of the measured composite material was determined as the arithmetic mean of the weight of the test samples of each series divided by the volume of the test samples. Samples of the I. gypsum class were tested in thermal conductivity and specific heat capacity tests. After these tests, the flexural strength measurement was performed on each sample. The compressive strength values were then measured in the individual halves of the samples. The indoor air temperature and relative humidity during the sample testing were 21.9°C and 55.0%. Ignitability and gross heat of combustion were determined in the samples for fire properties tests.
The main purpose of this test was to measure the force needed to break the test samples.
The samples were placed in the FormTest press and oriented so that their horizontal axis was perpendicular to the supports of the test press machine. Therefore, the load was perpendicular to the direction of filling the test moulds. The load roller transferred the load perpendicular to the opposite surface of the test sample. The load velocity was set to 10 N/s for all samples. The load was evenly increased until the sample broke [
The measured values of the maximum applied load
Calculation of flexural strength [
where:
Half of the samples were created by the flexural strength test of each original sample and were marked by the proportion of straw content. Immediately after this test, these new samples were tested for compression strength. The samples were placed in the FormTest press so that the load was perpendicular to the direction of filling; the samples were centered in relation to the load boards. The area of the load boards is 40 mm × 40 mm, so the entire area of the load board was in contact with the sample. The load velocity was set to 50 N/s for all samples. The load was evenly increased until the damage of the sample [
The measured values of the maximum applied load
Calculation of compressive strength [
where:
1600 40 mm × 40 mm is the area of load boards (mm2).
The measurement was performed on only one set of samples. Samples containing gypsum class G2 (I.) were selected to measure thermal properties using the ISOMET 2114 device. This device can determine the value of thermal conductivity
where:
Another quantity that the ISOMET device can measure is the volumetric heat capacity
where:
For measurements, the test samples were placed in a test chamber where the temperature (23°C) and air humidity (50%) were constant. The results of each measurement were recorded.
In this research framework, the ignitability and gross heat of combustion were measured. The single-flame source fire test of composite material made of gypsum and reinforced with natural fibers has been performed [
The single-flame source fire test was carried out on three test samples to test fire properties. On the exposed surface of the samples, two horizontal axes were marked. The first was 40 mm above the bottom edge of the testing sample, and the second was 150 mm above the first axis, as shown in
The test to determine the gross heat of combustion (calorific value)
Gypsum class | Series | Bulk density (kg·m−3) | Average value of flexural strength |
Standard deviation of flexural strength |
Average value of compressive strength |
Standard deviation of compressive strength |
---|---|---|---|---|---|---|
I. | 1. | 1434 | 3.55 | 0.39 | 9.23 | 1.78 |
2. | 1142 | 2.61 | 0.20 | 5.87 | 0.32 | |
3. | 1111 | 2.09 | 0.10 | 3.09 | 0.30 | |
II. | 1. | 1314 | 5.74 | 0.93 | 22.52 | 1.70 |
2. | 1165 | 4.15 | 0.34 | 9.25 | 1.06 | |
3. | 971 | 2.50 | 0.23 | 4.77 | 0.24 | |
III. | 1. | 1674 | 7.38 | 0.27 | 39.73 | 1.18 |
2. | 1388 | 5.51 | 0.24 | 14.93 | 0.87 | |
3. | 1235 | 3.93 | 0.20 | 7.84 | 0.81 |
The smooth surface of the individual stalks of crushed straw can be the reason for the decrease in flexural and compressive strength. According to Bouasker et al. [
On the contrary, Yang et al. [
Gypsum class | Series | Average value of thermal conductivity |
Standard deviation of thermal conductivity |
Average value of specific heat capacity |
Standard deviation of specific heat capacity |
---|---|---|---|---|---|
I. | 1. | 0.484 | 0.012 | 1092.559 | 0.664 |
2. | 0.406 | 0.055 | 1300.932 | 15.196 | |
3. | 0.332 | 0.015 | 1330.786 | 22.819 |
It is clear from
It is worth mentioning that the thermal conductivity
The next step in the research of fire properties was to verify whether the composite material can be classified as a non-flammable material. Thus, into the reaction to fire A1 or A2 class. Whether this composite material meets the requirements for classification as a non-flammable material was verified by the gross heat of combustion (calorific value) fire test [
Gypsum class | Series | Gross heat of combustion |
---|---|---|
I. | 1. | 0 |
2. | 0.418 | |
3. | 0.836 |
Based on the fire tests performed, it can be said that the composite material is non-flammable (Classes A1, A2 according to its reaction to fire). The single-flame source test excluded Classes E and F for the composite material. Depending on the behavior of the material during this fire test, it can be assumed that the SBI (Single Burning Item) fire test would also exclude Classes B, C, and D. The SBI fire test is used in practice to classify building materials into Classes A2 to D [
Moreover, Yue et al. [
The research contains basic information on the mechanical, thermal, and fire-resistant properties of the composite material made of gypsum and natural fibers. It is evident from the measurements made that with an increasing proportion of straw reinforcement, the flexural strength, the compressive strength, and the thermal conductivity decrease. On the contrary, the specific heat capacity increases with the increasing proportion of straw. The fire resistance of the composite material does not deteriorate.
Thus, this research has shown that adding reinforcing straw fibers to the gypsum binder does not improve its mechanical properties and therefore this composite material cannot be used as high-strength plasterboards. The largest decrease in flexural strength (by 56.49%) occurred in the sample with 5% crushed straw (3. series) of gypsum Class II., and the largest decrease in compressive strength (by 80.27%) occurred in the sample with 5% crushed straw (3. series) of gypsum Class III. The reason for the decrease in strength can be that with an increasing proportion of straw, the intergranular porosity increases, and the amount of binder decreases. Another reason may be the smooth surface of the individual stalks of crushed straw. This composite material should be used primarily for the production of cladding plasterboards or plasters, because research has shown that the added straw fibers improve its thermal properties and do not worsen fire resistance. Their thermal insulation properties were better, but not as much as insulation boards that would replace other insulations. Surprisingly, even with 5% crushed straw, the composite material is still non-flammable (reaction to fire Classes A1, A2).
In any case, we consider the use of straw stalks, rice, corn or other natural fibers with various binders to be beneficial in areas where they are readily accessible because it could allow a reduction in the manufacturing costs of conventional products by using inexpensive and renewable natural fillers. This is advantageous in terms of economic, environmental and life-cycle analysis using different impact assessment indicators.
The authors would like to thank the staff of the laboratories of the Department of Building Materials and Diagnostics of Structures, Faculty of Civil Engineering, VSB–Technical University of Ostrava.