3.1.1. Acoustic Properties
The values for the sound absorption coefficient are presented in
Figure 2 and
Figure 3. The first figure presents the values obtained for each Category within the frequency range of 0–6400 Hz, while the second figure presents the data for the sound absorption coefficient across the standardized frequency bands of 250 Hz, 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz.
The results presented in
Figure 2 indicated that the highest peak for Category I (bottom of the figure) was approximately 0.3, which corresponded to an absorption rate of approximately 30% at a frequency of 800 Hz. This value was associated with the composition in which clay was present. In comparison to the other binders, the peak was observed to follow a descending order, with hydrated lime, then cement and plaster.
The highest values of sound absorption for Category I were observed within the frequency range of 500–2400 Hz. In the case of Category II (middle figure), the highest peak was approximately 0.25, which was defined by the same composition as clay. However, the tendency for this composition was to exhibit a similar absorption rate across the entire frequency range. The decreasing performance of the binders were similar in Category I: clay, hydrated lime, cement and plaster.
In Category III (top of the figure), an higher peak was observed for the composition with cement at a frequency of 1000 Hz, which then rapidly decreased until reaching a frequency of 2400 Hz. Thereafter, an increase was noted, starting around 4000 Hz. For this Category, the highest value was around 0.4, and it is important to note that for the other binders, the values of the sound absorption peak were mostly identical, around 0.28.
The data provided in
Figure 3 on the standard frequency bands indicated an increasing trend in Category I for the sound absorption coefficient values at 250 Hz, 500 Hz and 4000 Hz, beginning with the composition of cement, then plaster, hydrated lime and clay. However, a variation was observed at the 1000 Hz and 2000 Hz bands, where cement had the lowest value.
At the ratio of 2:1 hemp to binder for Category II of the composite materials, all of the binders showed similar values of sound absorption; however, just for the bands at 500 Hz and 4000 Hz, the clay composition values were more notable than the other materials.
For Category III, the most optimal behavior was defined by the cement–hemp composition, having the highest value at 1000 Hz, while the other standardized frequency bands showed the highest values for the clay–hemp composition.
It would be necessary to inquire why a hemp–cement composition indicated an improvement of acoustic characteristics in Category III, given that the ratio of hemp to cement was 3:1. A possible reason for this situation is the increased porous structure of the composition, which leads to an internal structure in which sound waves are scattered and lose energy, thus increasing the absorption.
The understanding of the sound absorption coefficient values from the current study required a comparison with those found in the scientific literature [
30,
31,
32,
33,
34] and are presented in
Table 8. This evaluation should consider the impact of different binder compositions and densities on the acoustic performance of the materials at varying frequencies. The objective is to facilitate the determination of the performance of each material in absorbing sound and to support the selection of appropriate binders for specific acoustic applications.
From the scientific literature, the composites based on hemp–cement had high absorption coefficient values in the 1000 Hz segment according to the study [
34], particularly between 0.65–0.95, and with increasing frequency; towards high frequencies, the absorption coefficient decreased and fell between 0.35–0.5. In cement and lime-based composites [
33], a maximum alpha at 700 HZ was obtained with an absorption value of 0.9. Compared to the present study, the aforementioned values were better performing; in the current study, the maximum absorption capacity of the materials was not higher than 40% or more than 0.4.
The study [
30] indicated that the acoustic absorption properties of hemp–clay and hemp–lime were dependent on density, with the following reported ranges: 180–195 kg/m
3, 339–340 kg/m
3 and 461–470 kg/m
3. An increase in density affected both materials in a similar manner; the absorption peak shifted to lower frequencies with a lower amplitude. The acoustic behavior of hemp–clay and hemp–lime mixtures was comparable. Both the hemp–clay and hemp–lime samples were tested in three ways, defined as light, medium and heavy compaction. The samples with clay showed the maximum absorption: α
maxcl = 1 at 1000 Hz for light, α
maxcm = 0.75 at 600 Hz for medium and α
maxch = 0.4 at 2000 Hz. Based on the current study, the values of the hemp–clay composite arrived at a maximum of 0.3 on low frequency (700–800 Hz) in all three cases.
In the case of hemp–lime composites, the scientific studies presented a variety of potential matrix options for the binder, with a recommended absorption range of 0.2–0.6 depending on the frequency [
28]. These values were comparable to those of the current study, where the absorption range was between 0.2 and 0.3, and also to the results of [
30], where the maximum for heavy compaction was approximately 0.2 and the absorption profile was similar to that observed in the current study. When the compaction was medium or light, the absorption increased to approximately 0.82 at 600 Hz and 0.98 at 1000 Hz.
A review of the scientific literature revealed no studies investigating the use of hemp–gypsum composites. However, the current study presented values for the absorption range between 0.1 and 0.2, with optimal performance observed at lower frequencies.
The hemp–clay composites exhibited higher absorption coefficients across all plots in the current study at mid-to-high frequencies, indicating that this configuration is more effective in absorbing sound within those ranges.
3.1.2. Thermal Properties
For the current research, the thermal properties of a composite material were defined in terms of density and thermal conductivity. In accordance with
Figure 4, it was observed that no values were available for the clay composition, as the determination was not possible at 90 days, when the samples exhibited weak, fragile behavior.
The results showed a decrease in density in all three categories, namely cement, plaster and hydrated lime. The lowest density values were observed in the hydrated lime compositions, which may be attributed to the reduced quantity of powder incorporated into the matrix of the novel composite materials. The density of the cement-based composition was approximately 1200–1600 kg/m3, whereas the values for the hydrated lime-based composition were approximately half those of the cement-based composition.
In contrast with the density, the thermal conductivity presented close values for the plaster and hydrated lime in all the three categories.
It is important to note the situation of the hemp–cement composition, which decreased the density values by almost 50% when increasing the hemp ration compared to the binder. In this situation, where the density remained relatively constant but thermal conductivity decreased significantly, further investigation was interrogated. An explanation of the raw materials and how they were expressed in the matrix composition of the new composite materials can be provided. The amounts expressed in kilograms did not show noticeable variation in the mixture of composite materials when expressed in volumes. However, an improvement in thermal conductivity was observed for the cement samples. By increasing the hemp volume, the composite materials increased their thermal conductivity performance, becoming better than gypsum in the third part, where the hemp–cement sample had a 3:1 ratio. This trend could be explained in the case of cement, by the fact that increasing the hemp volume may contribute to reducing the pore size between the hemp shivs and subsequently, the sample became more compact.
The density of hemp-based materials can influence their thermal and mechanical properties. Accordingly, the values for hemp-based composite materials and different binders have been collected from previous studies published in the scientific literature and are summarized in
Table 9 [
34,
35,
36,
37,
38,
39,
40,
41,
42].
In the scientific literature, hemp and lime-based composites showed a range of densities and thermal conductivity values that varied based on the specific recipes used. For most studies, the density of these composites generally fell between 374 and 1750 kg/m3, with thermal conductivity values ranging from 0.08 to 0.135 W/mK.
When focusing on lime and hemp composites specifically, Williams et al. obtained densities between 374 and 432 kg/m
3, with corresponding thermal conductivities between 0.08 and 0.13 W/mK [
41]. Similarly, Šadzevičius et al. recorded a density of 482 kg/m
3 and a thermal conductivity of 0.114 W/mK [
39], while Horszczaruk et al. reported a density of 655 kg/m
3 with a conductivity of 0.12 W/mK [
40]. Subanesh et al. found slightly higher values, with a density of 780 kg/m
3 and a conductivity of 0.135 W/mK [
42]. In the current study, the composite reached a density of approximately 700 kg/m
3, with thermal conductivities ranging from 0.1605 to 0.1699 W/mK. Although these values were slightly higher than previous findings, they still fall within a comparable range, supporting the suitability of hemp–lime composites in applications requiring moderate thermal insulation.
For hemp–clay composites, the current study was unable to derive any meaningful data as these composites were too fragile to be tested. In addition, there was a lack of research in the literature that provided information on this type of material.
In contrast, for hemp–gypsum composites with densities between 900–1050 kg/m
3 and thermal conductivities between 0.2003 and 0.2656 W/mK, this study agreed with the results of study [
36], which reported densities around 500–1000 kg/m
3 and thermal conductivities between 0.14 and 0.3 W/mK. However, study [
35] showed even higher performance with densities of 366–392 kg/m
3 and thermal conductivities of 0.098–0.102 W/mK.
For hemp–cement-based materials, the present study recorded density values at least 10% higher than any previously documented, in the range of 1300–1500 kg/m3. Thermal conductivity values were comparable to previous studies, particularly those approaching 0.2 W/mK.
3.1.3. Mechanical Properties
The mechanical characteristics, both flexural (fctm) and compressive (fcm) strengthswere plotted not on the basis of the three categories of materials reported for the binder group (as initially was presented for the acoustic and thermal properties), but by each binder individually for the three situations of 1:1, 1:2 and 1:3. The explanation for this lies in the values obtained, and the different intervals between them, making graphical representation difficult.
Compressive Strength
Figure 5 illustrates the values obtained at 3, 7, 14 and 28 days for all the binders, with the addition of 90-day data for clay and lime.
As shown by the data, the best performing composition of hemp–cement was Ce + H1, which approached 12.5 MPa, while those of gypsum plaster Pl + H2 achieved 4 MPa, with both values presented at 28 days. The best value for the lime compositions was due to Hl + H2, which presented a value of 1.2 MPa, and for clay Cl + H3 at around 0.9 MPa at 90 days.
In the case of the first binder, cement (
Figure 5-bottom), the transition from one category to the other was characterized by significant variation. A reduction in the volume of hemp resulted in a notable improvement in the compression performance. In contrast, the categories for plaster exhibited a relatively consistent pattern across all testing days. However, the behaviors of lime and clay were markedly different. In these cases, the composition with the highest volume of hemp demonstrated the most favorable performance across a range of testing days.
Hemp-based materials utilized as plaster finishes are classified as non-load-bearing elements and are typically characterized by the lowest values in compressive strength. In fact, even hempcrete, which is defined as hemp concrete, is utilized as insulation materials without exhibiting any significant load-bearing characteristics.
A comparative analysis of the data from the current study with the scientific literature [
11,
35,
36,
38,
39,
40,
41,
42,
43,
44] is presented in
Table 10, which outlines the compressive strength of various hemp composite materials.
The majority of studies identified in the literature focused on the hemp–lime composite, which is typically defined as hempcrete. The data for compressive strength in this Category exhibited a range of 0.15 to 3.85 MPa, varying according to the percentage of lime in combination with other binders. Looking only at studies where the materials were 100% lime, the values of compressive strength reported included 0.693 MPa [
39], 2.64 MPa [
42], 0.15–0.45 MPa [
41], and 2.56 MPa [
40]. When these results were viewed in terms of material density, there was a clear trend that higher density correlated with higher compressive strength. In comparison, data from recent research on lime-based materials with densities between 600–800 kg/m
3 showed compressive strengths at 90 days ranging from 0.9 to 1.2 MPa—approximately 50% lower than those reported in existing studies. A possible reason for this discrepancy may be the storage conditions: unlike other studies, these samples were stored in a dry laboratory environment with no additional humidity control, which may have influenced the results.
A review of the literature revealed a lack of uniformity and correlation in the reported values for hemp and cement-based composite materials. This makes it challenging to establish a relationship between the reported values and the density of the final products. For example, in the study conducted by Sahin [
11], the values of compression strength ranged from 0.28 to 1.24 MPa for densities between 312 and 928 kg/m
3. In the study conducted by Horszczaruk et al., the values for a cement-based recipe with a density of 988 kg/m
3 reached a strength value of 9.56 MPa. Furthermore, the study conducted by Subanesh et al. reported relatively low strength values of 2.97 MPa observed at a density of 1228 kg/m
3. In the case of the current study, the cement-based material exhibited densities between 1200 and 1500 kg/m
3, with compressive strength values ranging from 7 to 12 MPa at 28 days. Therefore, these values were approximately three to four times higher than those reported in previous studies for composites with similar densities.
In compositions using clay earth as a binder with hemp, compression strengths have been documented as between 0.39 and 0.48 MPa, with corresponding densities ranging from 370 to 510 kg/m
3, as reported by Clay Brahim Mazhoud et al. [
44]. These findings aligned with the data obtained for the three categories of samples in the current research, where compressive strengths ranged between 0.1 and 0.9 MPa.
In the category of samples composed of hemp and gypsum plaster, previous studies reported compressive strengths ranging from 0.28 to 0.55 MPa, with an associated density of approximately 380 kg/m3. In the present study, the densities were approximately 50% higher than those reported in previous studies, and the compressive strengths were significantly greater, ranging from 3.5 to 4.5 MPa—approximately eight to nine times higher than previously documented values.
Flexural Strength
Figure 6 presents the data collected at the same time interval as that used in the comparative strength diagrams.
The values for cement exhibited a range of 2–4 MPa. The composition Ce + H2 demonstrated optimal performance at 28 days. The results for the hemp–plaster materials ranged from 1.2 to 2.4 MPa. The composition Pl + H1 showed the optimum performance compared to the three compositions during all the test periods. With regards to the values of lime and clay composite materials, where the values ranged between 0.1 to 0.5 MPa, it was observed that the best performing materials were the ones with the lowest volume of hemp (Hl + H1 and Cl + H1).
Flexural strength is a fundamental property of materials utilized in applications where bending or tensile forces may be encountered, such as in the fabrication of panels and boards. The data obtained from the scientific literature on hemp-based materials are presented in
Table 11 [
41,
42,
43,
44,
45]. The composite materials, which consisted of hemp wood fiber (hemp shiv) and various types of binders at varying percentages, demonstrated flexural strength values ranging from 0.02 to 1.5 MPa.
The present research findings suggested that hemp–cement composite materials exhibited higher values, approximately 50% higher than those observed in previous studies [
42,
45].
The hemp–lime materials from the ongoing study demonstrated a range of values between 0.05 and 0.5 MPa, which were comparable to the findings of Williams et al. [
38], where the values ranged from 0.1 to 0.3 MPa. However, flexural strength exhibited the lowest values in comparison to the study conducted by Subanesh et al. [
42].
The investigation carried out by Mazhoud et al. regarding hemp–clay composites revealed values between 0.02 and 0.026 MPa, which were the lowest recorded compared to the present study where the highest values reached 0.5 MPa [
44].
Despite an exhaustive search, no data regarding hemp–gypsum composites were found. However, based on the findings of the current study, the range values of flexural strength were determined to be between 1.2 and 2.2 MPa.
