Enhancing the Properties of Cement Composites Using Granulated Hemp Shive Aggregates

Abstract

In recent years, civil engineers have been exploring innovative methods of constructing buildings using environmentally friendly materials. The beneficial properties of hemp harl, an agricultural waste that is gaining popularity in construction, prompted the idea of strengthening its properties through the granulation process and using it as an aggregate in cement composites. This work aimed to investigate whether the use of hemp husk in the form of granules would have a positive effect on the properties of cement composites compared to their traditional form (stems). Potato starch was introduced as an additional factor in the granulation process to improve the material. Experimental tests were carried out on organic fillers, fresh mixtures, and hardened composites. Physical, mechanical, and structural tests (SEM imaging) were carried out. The highest strength was demonstrated by samples containing hemp shive aggregate (1.186 MPa), while the use of hemp shives in the form of granules had a positive effect on the consistency and density, and it also reduced water absorption by 30% during the production of the composite. The apparent density of composites with hemp shives in the form of hemp pellets was higher (1042 ÷ 1506 kg/m³) than in the case of composites with shives in the form of harl (727 ÷ 1160 kg/m³). Nevertheless, hemp shive in both forms can be used as a natural aggregate in cement composites.

1. Introduction

The world’s population and standard of living are both steadily expanding, resulting in increased global energy consumption, carbon emissions, and waste generation. The construction industry has a significant role in this process in developed countries, accounting for approximately 40% of energy use and CO₂ emissions. When embodied carbon (EC) and embodied energy (EE) are included, these percentages rise to roughly 50%. Consequently, it is essential to find ways to reduce the high energy and carbon demands of the building sector to achieve greater sustainability and a lower environmental impact [1].

Concrete is the most extensively utilized material in the construction industry, with an annual production of 10 billion tons used across various applications. Additionally, cement has a significant embodied carbon (EC) footprint, as the production of each ton of cement releases approximately one ton of CO₂ into the atmosphere [2,3].

Therefore, it is imperative to develop sustainable construction materials to lessen the environmental impact of the building sector. One innovative concept that is gaining momentum is Hempcrete, also referred to as Lime–Hemp Concrete (LHC). LHC is a novel sustainable construction material composed of hemp shives used as bio-aggregates and lime serving as the binder [4].

The physical and mechanical characteristics of lightweight cement composites are predominantly influenced by the type of aggregate employed and its porosity. Typically, these composites exhibit a low compressive and bending strength, typically ranging between 1 and 3 MPa. These properties dictate their applications, often found in prefabricated elements or utilized within wooden frame structures to fill walls, floors, and ceilings.

Organic composites exhibit an open structure due to the reduced fine aggregate content, aimed at occupying free spaces within the material. These products boast a favorable heat transfer coefficient, albeit subject to various factors such as mixture density, composition, and moisture content [5]. Organic-filled composites achieve heat transfer coefficients between 0.08 and 0.16 W/(m²·K), with variations stemming from filler fraction and binder percentage. Notably, larger filler fractions and higher binder concentrations diminish the product’s insulating properties [6].

Studies reveal that cement biocomposites can achieve a superior performance, potentially obviating the need for additional insulation, while maintaining reasonable wall thicknesses. Hemp shives, with a thermal conductivity (λ) of 0.04–0.06 W/(m·K) [7], emerge as a viable alternative to conventional insulation methods.

Organic-filled composites also excel in sound absorption and frost resistance due to the air voids within their structure. Mineralizers can further mitigate mold and fungi issues, while the carbonized coating formed at high temperatures prevents combustion and internal temperature escalation. Interestingly, the combustible nature of organic aggregates compromises these protective features.

Regarding thermal mass, hempcrete’s ability to absorb and gradually release heat proves advantageous, with a specific heat of 1300 J/(kg·K). This favorable outcome is attributed to the lime and cellulose present in shives, enhancing comfort despite external temperature fluctuations [8]. Typically, organic fillers predominantly originate from two industries: wood and agriculture. Wood-derived fillers, such as sawdust, shavings, and chips generated in sawmills, paper, and furniture industries, are sourced from both coniferous and deciduous trees. Among the most suitable tree species are spruce, pine, birch, and poplar [9].

Agricultural waste, including hemp harl and shive, flax stalks, common reed, or cereal straw, arises after these plants are utilized in industries such as textiles, paper, and pharmaceuticals. Hemp (Cannabis sativa) thrives without the need for high-quality soil, though it flourishes best in neutral-to-alkaline soils. Its extensive root system effectively aerates and improves soil conditions, absorbing harmful minerals and incorporating them into its structure. Rapid growth suppresses weed development by outcompeting space and sunlight, eliminating the need for pesticides. The plant can grow up to 30 cm per week and reach heights of up to 4 m during its 4-month growth cycle. Over this period, it can absorb approximately 1.8 tons of CO₂ from the atmosphere, with the potential for double absorption, as it can be grown twice annually. With an estimated yield of around 10 t/ha, the decortication process can yield up to 7 tons of shives [10]. Hemp bast fibers, known for their length and strength, are obtained by processing the plant’s woody cores [7]. Despite the increasing demand for hemp-derived products across industries, national and EU (European Union) regulations currently dissuade potential growers due to trace amounts of psychoactive substances. Favorable growing conditions may lead to THC (tetrahydrocannabinol) levels exceeding the permitted 0.2%, posing a risk of crop destruction and legal repercussions, including imprisonment for up to 3 years. Stringent control measures are enforced, requiring meticulous record-keeping of invoices and seed packages. However, discrepancies in seed content detected during inspections can present challenges [11].

Hempcrete is created by blending hemp shives and harl with a lime-based binder [12,13]. Due to its low compressive strength in this form, hempcrete is not suitable for load-bearing purposes in structures. However, it serves well as a filling material, typically poured into temporary or lost formwork during the construction of walls and insulation for roofs and ceilings. Its vapor permeability makes it particularly useful in renovating old buildings, where traditional concrete might harm wooden structures by tightly insulating them [7].

An intriguing alternative involves producing prefabricated hemp products like ready-made blocks or panels. This method streamlines the process, eliminating the challenges of mixing and lengthy drying times. Additionally, these blocks can be easily cut and adjusted to fit a building’s wooden frame [6].

The production of one ton of cement results in one ton of carbon dioxide emissions. As concrete production continues to rise, there is a growing need to reduce cement usage in line with sustainable development principles. Lime production emits approximately 740 kg of CO₂ per ton, but this process can be reversed when carbon dioxide is absorbed by walls containing lime binders [14]. Additionally, incorporating natural fillers such as hemp waste into cement composites increases the amount of CO₂ captured in building materials [15]. As Boltryk et al. [15] show, the natural filler could absorb CO₂ from the atmosphere via the carbonation of nonmineralized aggregate and an aggregate mineralized with aluminum sulfate solution and calcium hydroxide, as well as the cement composites with an organic aggregate. Moreover, the mechanism by which hemp waste contributes to CO₂ capture is primarily through the natural growth process of the hemp plant. Hemp absorbs significant amounts of CO₂ from the atmosphere during its rapid growth cycle. When hemp waste, such as shives, is incorporated into cement composites, the CO₂ that was sequestered by the plant during its growth is effectively stored within the composite material. Additionally, the lime-based binder used in the cement composite can further absorb CO₂ from the atmosphere over time through a process called carbonation, where CO₂ reacts with calcium hydroxide to form calcium carbonate [16,17,18]. However, it is crucial to maintain appropriate physical and strength properties. Therefore, this study aimed to assess the impact of the use of organic fillers, such as hemp granules, on selected physical and mechanical properties of cement composites.

This study is pioneering in its exploration of using granulated hemp shives as an aggregate in cement composites. While existing research has extensively studied hempcrete and the use of hemp fibers, the application of hemp shives in a granulated form for cement composites has not been reported in the literature before. The primary advantages of this novel approach include the use of granulated hemp shives, thus simplifying the mixing and forming processes in cement-composite production. Moreover, incorporating hemp shives into cement composites reduces the need for cement, thereby lowering the CO₂ emissions associated with cement production. Additionally, the hemp shives enhance the composite’s CO₂ sequestration capabilities. Therefore, this research broadens the scope of hemp utilization in construction by demonstrating the feasibility and benefits of using granulated hemp shives, which could lead to more sustainable building practices.

2. Materials and Methods

2.1. Materials

2.1.1. Organic Filler

The granulate was prepared using hemp shive with a size fraction of 1–5 mm, as depicted in Figure 1. Hemp shive, derived from hemp straw, is commonly referred to as industrial hemp.

2.1.2. Cement

Portland cement CEM I 42.5 R was used as the cement binder. The fundamental characteristics of the CEM I 42.5 R cement employed are outlined in

Table 1. Properties of CEM I 42.5R cement [19].

Property	Test Standard	Standard Requirements	Test Results
Initial setting time, min	EN 196-3 [20]	≥60.0	187
Final setting time, min	EN 196-4 [21]	-	248
Specific surface area, cm2/g	EN 196-6 [22]	-	3746
Soundness of cement, mm	EN 196-3 [20]	≤10.0	1.6
Compressive strength, MPa	EN 196-1 [23]
-after 2 days		≥20.0	25.6
-after 28 days		≥42.5 ≤ 62.5	54.3
SO3 content, %	EN 196-2 [24]	≤4.0	2.98
Cl content, %	EN 196-21 [25]	≤0.1	0.079
Insoluble residue, %	EN 196-2 [24]	≤5.0	0.79
Loss on ignition, %	EN 196-2 [24]	≤5.0	3.24
Alkali content as Na2O eq, %	EN 196-21 [25]	-	0.60
Specific density (g/cm3)	EN 196-6 [22]	-	3.10

.

2.1.3. Bituminous Emulsion

Weber.tec 901 Eurolan 3k bituminous emulsion was employed in this research, known for its plasticizing effect that positively influences the water-to-cement ratio. It serves to create a moisture-proof and protective coating. The fine particles of the emulsion penetrate the pores of mortars, enhancing waterproofing and resistance to aggressive substances. Furthermore, the emulsion is solvent-free, rendering it environmentally friendly, with a high solids content.

Table 2. Technical data of the Weber.tec 901 Eurolan 3k bitumen emulsion [26].

Property	Value
Solvent	lack
Consistency	liquid
Color	black
Density	ca. 1000 kg/m3
Dry residue	60%wt.

outlines the basic technical specifications of the emulsion.

2.1.4. Plasticizers and Mineralizers

In the mixture, the CHRYSO Alpha 304 plasticizer was added, providing plasticizing and water-reducing properties. It was incorporated at a ratio of 1% of the cement mass.

Table 3. Technical data of the CHRYSO Alpha 304 plasticizer [27].

Property	Value
Consistency	Liquid
Color	Colorless
Density	1020 ± 2 kg/m3
pH	4 ± 1
Cl− content	≤0.1%
Alkali content as Na2O eq.	≤1.0%

shows the basic technical parameters of the plasticizer.

To counteract the effects of the harmful compounds present in organic fillers and mitigate their water absorption, mineralizers were introduced. Aluminum sulfate (Al₂(SO₄)) was utilized at a proportion of 9% of the filler weight, while hydrated lime (Ca(OH)₂) was employed at 18% of the filler weight. When dissolved in water, aluminum sulfate typically generates an acidic pH range from 3 to 5. However, the addition of hydrated lime to the mixture neutralizes this acidic reaction and enhances the workability of the composite blend.

2.1.5. Water

Tap water, meeting all standard requirements for mixing water in concrete, was utilized in preparing composites containing organic fillers. This water served multiple purposes, including creating aqueous solutions of aluminum sulfate and hydrated lime, as well as acting as mixing water for the cement hydration process.

2.2. Methods

2.2.1. Properties of Organic Fillers

Water Adsorption

The absorbability test for organic filler was conducted following the EN 1097-6:2013 standard, which outlines the assessment of mechanical and physical properties of aggregates, specifically focusing on determining grain density and water absorption [28]. To begin, the organic filler, in the form of shives and granules, underwent a drying process until reaching a constant weight at 40 °C. Subsequently, the samples were carefully weighed and placed within a fine mesh to prevent dispersion. Once submerged in water, measures were taken to ensure the fillers remained submerged and did not float out. Upon removal from the water, any excess moisture was drained, and the samples were re-weighed. The water absorption capacity was then calculated as an average value based on three independent samples.

Bulk Density in the Loose and Compacted State

The test procedure adhered to the specifications outlined in the EN 1097-3:1998 standard, which governs the evaluation of mechanical and physical properties of aggregates, specifically focusing on the determination of grain density and water absorption [29]. Before testing, the samples underwent a drying process until reaching a consistent weight at 40 °C. This temperature was set to protect the structure of the organic filler. Due to the presence of a plant filler, the drying temperature should not be too high so as not to damage its structure. Bulk density measurements were then conducted, with the average value derived from three independent measurements.

2.2.2. Composite Mixture Testing

Composite Mixture Consistency

The consistency of the composite mixture was assessed utilizing the slump test method, under the EN 12350-2:2019 standard, which outlines procedures for testing concrete mixtures, specifically employing the cone slump [30]. Each series of tests involved the evaluation of two independent samples.

Density of the Composite Mixture

The examination followed the protocols specified in the EN 12350-6:2019 standard, which governs tests conducted on concrete mixtures, particularly focusing on density assessments [31]. Density measurements were obtained as the mean value derived from three measurements within each series.

2.2.3. Cement-Composite Testing

Compressive Strength of Cement Composite

The compressive strength assessment of the cement composite followed the procedures outlined in EN 12390-3:2019 [32]. Testing occurred after a 28-day maturation period under air-dry laboratory conditions. Each series comprised six samples, each with dimensions of 10 × 10 × 10 cm.

Apparent Density of Cement Composite

The density assessment of the cement composite adhered to the procedures specified in the EN 12390-7:2019 standard, which governs the testing of hardened concrete, specifically focusing on density measurements [33]. Following a 28-day maturation period, three samples, each measuring 10 × 10 × 10 cm, were subjected to the test. Before testing, the samples were dried to a constant weight at 60 °C.

Water Absorption of Cement Composite

The water absorption assessment of the cement composite was carried out on cubic specimens measuring 10 × 10 × 10 cm, following the guidelines outlined in the standard EN 206:2013 + A2:2021 [34]. Each specimen underwent full saturation with water before being dried to a constant mass at 60 °C.

Capillary Water Absorption

The capillary water-absorption test was conducted following the guidelines outlined in the Polish standard PN-85/B04500 for building mortars, focusing on the evaluation of physical and strength characteristics [35]. The experiment involved monitoring the mass increase in samples over time. Each series comprised three samples, each with dimensions of 10 × 10 × 10 cm, which were initially dried to a constant weight at a temperature of 60 °C. To ensure controlled water penetration, the side walls of the samples were sealed with paraffin, allowing water to enter only through the base of the sample. Subsequently, the samples were placed in a container, and water was added until it reached a height of 1 cm from the base of the sample. Weight measurements were taken at intervals of 1, 24, 48, and 168 h. Throughout the test duration, the water level was maintained by replenishing it as necessary to keep it at a consistent height. The weight gain was calculated using the following formula:

$$m={\displaystyle \frac{m_c-m_s}{m_s}}·100\%$$

(1)

where m is the sample weight increase, %; m_c is the mass of the sample moistened by capillary action, kg; and m_s is the mass of the sample in a dried state (coated with paraffin), kg.

Scanning Electron Microscopy

To perform SEM and take photos of the microstructure of composites was used a high-resolution microscope (Quanta 250 FEG, FEI, ThermoFisher Scientific, Waltham, MA, USA). Magnitudes of up to 10,000 were used to investigate the structural properties of composites.

2.3. Preparation of Samples

2.3.1. Pelletization

The granulate was manufactured from hemp shives, supplemented with potato starch as a binding agent, employing a P-300 granulator (Protechnika, Trzebieszów, Poland) with a power rating of 7.5 kW (Figure 2). The operational mechanism of the granulator comprised two stationary compaction rollers and a rotating matrix featuring holes measuring 6 mm in diameter and 40 mm in length; therefore, the resulting pellets had a diameter of 6 ± 0.2 mm and a length of 40 ± 5 mm. The setup was described in detail elsewhere [36]. The rotational speed of the matrix averaged approximately 270 rpm. Material introduction into the operational system was facilitated by a vibrating feeder, with a mass flow rate set at 40 kg/h.

Two types of granulates were produced (Figure 3). The first type was made from hemp shives, which were moistened to a moisture content of approximately 22% 24 h before pelletization. The second type was prepared by mixing hemp shives with potato starch, with the starch constituting 10% of the mixture by weight. This mixture was also moistened to a moisture content of approximately 22% 24 h before pelletization. The starch acted as a binder, enhancing the structure of the granules and reducing the energy consumption during the pelletization process [36].

2.3.2. Mineralization of Granules and Hemp Shives

In the initial stage of mineralization, aluminum sulfate was dissolved in one portion of water, which was then added to the granules/shives. The mixture was stirred in a laboratory mixer for 3 min. Concurrently, a solution of hydrated lime was prepared in the remaining water to neutralize the acidic effect. After introducing the Ca(OH)₂ solution to the mixture, the components were mixed again for an additional 3 min to ensure even distribution over the granules. The mineralization process of organic fillers, developed at the Białystok University of Technology, is further detailed in References [15,37].

2.3.3. Shaping and Care of Samples

Sand, cement, and either asphalt emulsion or plasticizer mixed with the remaining mixing water were added to mineralized granules and hemp shives. The mixing process continued until all components were uniformly combined. The samples were then formed in two layers, with each layer compacted using 25 blows from a tamper. After demolding, the samples were stored in air-dry conditions (temperature of 20 ± 5 °C and humidity of ca. 40%) for an additional 27 days before testing (Figure 4).

2.3.4. Development of the Experiment Plan

During the experiment, the ingredients and their proportions in the composite mixture were selected based on the authors’ expertise. A consistent quantity of cement and water was maintained, with a constant water-to-cement ratio (w/c) of 1.44 across all series. The quantities of aluminum sulfate and calcium hydroxide were set at 9 wt.% and 18 wt.% of the mass of the organic aggregate, respectively. A constant amount of sand was assumed (182 kg/m³), and a constant volume of organic aggregate was assumed, which was 452 dm³. For the research, the variables considered for the granules included the presence or absence of potato starch during production and the impact of using either asphalt emulsion or plasticizer in the mixture. Notably, hemp shive granulates had not been previously utilized as an organic aggregate.

Table 4. Composition of cement composite with organic filler per 1 m³.

No.	Cement CEM I 42.5R	Water	Plasticizer	Asphalt Emulsion	Sand	Fillers			Mineralizers
						Hemp Harl Pellets		Loose Hemp Harl	Aluminum Sulfate Al2(SO4)3	Hydrated Lime Ca(OH)2
						Starch	-
	kg	dm3	kg	Kg	kg	kg	kg	kg	kg	kg
1	360	519	-	3.6	182	337	-	-	30	60
2	360	519	3.6	-	182	337	-	-	30	60
3	360	519	-	3.6	182	-	337	-	30	60
4	360	519	3.6	-	182	-	337	-	30	60
5	360	519	-	3.6	187	-	-	224	20	40
6	360	519	3.6	-	187	-	-	224	20	40

shows the composition of the cement composite with the used granules or shives per 1 m³.

3. Results

3.1. Properties of Organic Fillers

Table 5. Density and water absorption by the organic filler.

Property	Hemp Harl Pellets	Hemp Harl + Starch Pellets	Loose Hemp Harl
Density in loose state, kg/m3	725 ± 2	726 ± 2	409 ± 1
Density in a compacted state, kg/m3	745 ± 1	746 ± 1	495 ± 1
Water absorption, %wt	287 ± 1.5	285 ± 0.6	343 ± 1

displays the mean values of test results for the bulk density in both the loose and compacted states, as well as the absorbability of organic fillers, including hemp shives and granules, both with and without potato starch.

Upon analyzing the study results, it was noted that the bulk density of granules is roughly six times higher compared to hemp shives. For granules, the density in both loose and compacted states with and without starch is nearly identical. Specifically, the average density of granules without starch was 725 kg/m³ in the loose state and 745 kg/m³ in the compacted state, whereas with starch, it was 726 kg/m³ and 746 kg/m³, respectively. In contrast, hemp shives exhibited an average bulk density of 409 kg/m³ in the loose state and 495 kg/m³ in the compacted state. The marginal differences observed among granule types may be attributed to variations in granule length, which significantly influences volumetric outcomes. The notably lower density of shives compared to granules stems from the substantial voids between the woody fragments of shives and their porous structure, which is compressed in granules. Furthermore, the average absorbability of granules with and without starch is 285% and 287%, respectively, while hemp shives demonstrate a higher water absorption by weight of approximately 343%.

3.2. Consistency of the Composite Mixture

The average results of testing the consistency of the composite mixture, conducted on two independent samples in each series, are depicted in

Table 6. Consistency of the composite mixture.

No.	Mixture Composition	Slump, cm	Consistency Class
1	Hemp harl–starch pellets + asphalt emulsion	5.8	S2
2	Hemp harl–starch pellets + plasticizer	8.0	S2
3	Hemp harl pellets + asphalt emulsion	8.7	S2
4	Hemp harl pellets + plasticizer	13.1	S3
5	Loose hemp harl + asphalt emulsion	0.4	S1
6	Loose hemp harl + plasticizer	3.9	S1

.

Based on the consistency test results, it can be inferred that the addition of a plasticizer has a more beneficial impact on the workability of the mixture compared to the bituminous emulsion utilized. Ensuring a consistent test duration was crucial during the testing phase, as the presence of hydrated lime accelerated the drying of the mixture before formation. Among the series tested, the one containing hemp shive exhibited the poorest workability (S1) due to its high water-absorption properties.

One of the primary factors contributing to the reduced consistency of the concrete mixture in series 5 and 6 was the high water absorption of hemp (Figure 5). This issue was compounded by hemp’s large surface area, which increased its water demand. In contrast, using pellets with the same water content in the mix greatly improved workability. This improvement was due to the pellets’ significantly lower water absorption and smaller total surface area compared to loose hemp, resulting in a reduced water demand.

3.3. Bulk Density of the Composite Mixture

Figure 6 presents the average results of density testing for the composite mixture, conducted on six independent samples in each series.

It was observed that the influence of the plasticizer on the density of the fresh mixture containing hemp granules is minimal compared to that of the bituminous emulsion. Both additives enhance the workability of the composite mixture and affect particle packing within the mixture. No significant disparity was noted in the density between mixtures containing granules with and without starch. However, in the series involving hemp shives, the presence of the plasticizer led to an approximate 14% increase in mixture density compared to the series employing bituminous emulsion. Conversely, substituting hemp shives for granules resulted in a density reduction of approximately 26% in the mixture, primarily attributable to the significantly lower bulk density of this filler.

3.4. Compressive Strength of the Cement Composite

Figure 7 displays the average results of compressive strength testing for the cement composite after 27 days, conducted on six independent samples with dimensions of 10 ×10 × 10 cm in each series.

Based on the test results, it can be deduced that samples containing granulate exhibit notably low compressive strength after 28 days, not surpassing 0.35 MPa. Throughout testing, these samples were prone to easy cracking, with frequent chipping of the sample walls—particularly the molded ones—leading to complete destruction. Conversely, samples containing shives were initially crushed and only subsequently delaminated, resulting in markedly superior outcomes compared to granule-containing samples.

Samples incorporating hemp shives demonstrated an approximately 2–3 times higher compressive strength than those containing granules. Additionally, a notably favorable effect of the plasticizer was observed, with its presence increasing compressive strength by approximately 40% compared to samples containing bituminous emulsion.

Pantawee et al. [38] analyzed the impact of the two-stage mineralization of hemp aggregate with aluminum sulfate and lime on the properties of composites. Using both mineralizers and aluminum sulfate alone, they achieved compressive strength in the range of 15–17 MPa after 28 days of maturation. In the absence of mineralization, the compressive strength was much lower, at the level of 3–4 MPa, which confirms the effectiveness of mineralization.

3.5. Water Absorption of the Cement Composite

The average results of the cement composite absorbability test are presented in Figure 8. The tests were conducted on three samples with dimensions of 10 × 10 × 10 cm in each series.

Composite samples containing granules exhibited high water absorption of approximately 40%, which remained consistent across all series, regardless of the admixture used (plasticizer or bituminous emulsion). However, samples with hemp shive showed a significant increase in water absorption, reaching 61% with the emulsion and 51% with the plasticizer. The higher absorbability in the series with hemp shives is attributed to the greater absorbability of this filler compared to granules.

Additionally, it was observed that samples with granules swelled significantly when soaked in water, leading to cracking and even splintering upon removal from the water, indicating partial destruction of the granulate structure during soaking. In contrast, no such detrimental changes were observed in the samples with shives after soaking.

3.6. Apparent Density of the Cement Composite

Figure 9 shows the average values from the results of testing the apparent density of the cement composite, conducted on six samples with dimensions of 10 × 10 × 10 cm in each series.

Based on the results, it was observed that the average apparent density of all batches with granules exceeds 1000 kg/m³. However, there was a significant increase of approximately 50% in the density of the composite containing the plasticizer compared to the series with bituminous emulsion. However, no effect of the presence of starch on this tested feature was observed. The lowest composite density of approximately 730 kg/m³ was obtained in the series with hemp shives and asphalt emulsion and was approximately 30% lower than the density of samples with granules. In the presence of hemp shives and plasticizer, the density increased by approximately 60% compared to the bituminous emulsion. To sum up, the tested composites with granules and with hemp shives and plasticizer can be classified according to the density classes EN 12390-7:2019 [33] as class D1.2 (>1000 and ≤1200 kg/m³), while the series with hemp shives and bituminous emulsion is not included in the classification because its average apparent density is below 800 kg/m³. This low apparent density of composites improves their thermal insulation properties, which is confirmed by other authors [39,40].

Figure 10 shows the relationship between the compressive strength of the composite and its density.

Typically, in tests, an increase in the density of a composite correlates with an increase in its compressive strength. However, in this case, the highest strength was observed in the test series with the lowest density, specifically those containing loose hemp shives. This material exhibited the lowest bulk density and the highest water absorption, affecting the test results. When hemp shives pellets and hemp shives + starch pellets were used, the composite’s density increased, but there was a notable decrease in compressive strength. This reduction in strength resulted from the distinct nature of sample destruction due to granule damage, as detailed in Section 3.4.

3.7. Capillary Rise

The capillary-rise test was conducted on three samples from each of the six series. The dried and paraffin-coated samples were weighed and placed in a container on a plastic grid, allowing free access for water to penetrate from the bottom. During the test, the immersion level of the samples was kept constant at 3 cm. The samples were weighed after 1, 24, 48, and 168 h.

Unfortunately, it was observed that samples with hemp granules swelled when the base was immersed in water, creating cracks in the walls that allowed water to penetrate through the side walls. This phenomenon was not observed in the series with hemp shives. Figure 11 and Figure 12 show the changes in weight of the individual series of samples with bituminous emulsion and plasticizer, respectively.

The process of capillary rise in samples over time was similar across all tested series, with an intense rise observed up to 48 h, after which the process slowed down. No significant effect on the kinetics of capillary rise was observed between samples containing emulsion and those with plasticizers. Both admixtures produced similar final values, with a slight advantage in favor of the plasticizer. However, it was noted that samples containing hemp shives absorbed more water than those with granules, attributable to their lower apparent density.

3.8. Structure Imagining

Figure 13, Figure 14 and Figure 15 show SEM photos of the microstructure of composites with visible hemp fibers. Figure 1 shows the shives’ hemp cells, which are characteristic of organic fibers. Figure 13 and Figure 15 show hemp fibers and granules embedded in the cement matrix and covered with cement paste. The sample in Figure 15 presents that the bonding of the cellulose microfibers with the mortar is good due to the surface textures and non-deformable hemp fibers. Large pore sizes developed due to the presence of fibers and could increase the water absorbability of hemp composites.

4. Summary

Over the years, construction engineers have sought innovative methods to construct buildings using ecological materials. The advantageous properties of hemp shives inspired the authors to explore their potential as an aggregate for composites. To this end, granulate was produced and used as an aggregate for cement composites to assess its beneficial effects on their properties. Based on the research conducted, the following conclusions were drawn:

-

The use of potato starch in the production of granules did not significantly affect their properties. The anticipated reduction in water absorption could potentially be achieved by altering the binding substance used in granule production. The bulk density of granules with starch is very similar to that of granules without starch.

-

In the compressive strength test, composites with granules exhibited a low compressive strength of 0.3 MPa. Samples with granules were completely destroyed, unlike those with only shives, which were crushed and delaminated.

-

The hemp granulation process positively impacted the consistency and density of the mixture and reduced the absorbability of cement composites by approximately 10–20%. However, there was a marked decrease in the compressive strength of composites with granules compared to those made with shives.

The apparent density of composites using hemp granulate is higher than that of cement composites with shive, a consequence of the granulation process. Nonetheless, both materials could serve effectively as insulators.

-

Capillary action tests indicated that composites with granules had a lower weight gain of 10–20% compared to composites with shives. However, after a few days, cracks appeared on the walls of the granule samples, indicating that prolonged contact with water causes irreversible structural changes in the composite.

Hemp granulation is a time-consuming and challenging process. An effective method to improve the parameters of cement composites could involve using a bituminous emulsion after the mineralization process until the initial setting or impregnating the granules with an emulsion to create a waterproof barrier.

-

A viable solution could be to use a different type of binder in granule production.

In conclusion, this study demonstrates that the use of granular hemp shives in cement composites positively impacts the material’s consistency and density while reducing water absorption. The mechanical properties, such as compressive strength, are directly related to the enhanced microstructure observed through SEM imaging. The granulated form of hemp shives results in a denser and more uniform distribution within the composite, reducing porosity and water absorption. Future research should further explore these relationships to optimize the performance characteristics of hemp-based cement composites. Moreover, future experiments should also focus on modifying the composition of mixtures and mineralizers used, or adding other additives to reduce the absorbability of composites or enhance their strength.