Reinforcement of cementitious mortars with hemp ﬁ bers and shives

: Polypropylene, steel, and carbon ﬁ bers are used to increase the ductility and toughness of concrete materials. In recent years, studies on the use of natural ﬁ bers have increasingly continued. Among natural ﬁ bers, hemp ﬁ ber has the highest tensile strength value. The aim of this study was to discuss and present the results of the experimental study on the use of cement - based hemp shives and ﬁ bers to increase the ductility of mor - tars. Another aim is to increase the use of hemp in the construction ﬁ eld by using more economical and natural ﬁ bers. Various binders and di ﬀ erent amounts of ﬁ bers and shives were added and their e ﬀ ects on ﬂ exural strength were investigated. Water absorption and ﬂ exural tests and scanning electron microscope and X - ray di ﬀ rac - tion analysis were performed on the produced samples. Particle and hole size distributions and stereo microscope images of the samples are given. The use of hemp ﬁ bers was observed to improve ﬂ exural strength, while shives were not signi ﬁ cantly improved ﬂ exural strength.


Introduction
Hemp concrete can be recycled at the end of the building's lifespan. In addition, hemp can be harvested in as little as 60 days. Hemp concrete is a better-than-zerocarbon material because it absorbs carbon from the atmosphere while the hemp plant is grown. It is one of the few materials that has the ability to absorb carbon even after it has been used in construction. In addition, the cannabis plant removes heavy metals from water and soil [1]. Hemp and lime is a material that consumes CO 2 through carbonation inside the concrete matrix [2]. In a study, they found the total carbon sequestration amount of hemp concrete as 307.26 kg CO 2 per m 3 (Figure 1) [2,3]. Hemp concrete is considered a green building material not only because of its low carbon content but also because of its ability to regulate heat, humidity, and relative humidity [4]. Hemp concrete is also known to exhibit acoustic insulation [5]. Response to fire is an important factor to consider as it relates to building safety. However, interestingly, all of the research has agreed that hemp concrete is suitable for fire resistance [6].
Hempcrete, also known as lime-hemp concrete, is a breakthrough concept that has attracted attention [7]. Lime is a binder that is 500°C lower than the temperature used to make cement (1,450°C) and is produced in a limestone kiln at 950°C. In addition, during the carbonation process that occurs during its hardening, most of the CO 2 released as a result of the chemical reaction that creates it is absorbed back into the system. Therefore, the carbon emission is significantly smaller or even negative than that of cement [8].
Hemp shives are a byproduct of the hemp fiber industry. They constitute 65-70% of the total production (by mass) of the hemp plant. The usage areas of the hemp plant are given in Figure 2. In a study that has been done, the 2-year evolution of the functional properties of two hemp concretes formulated with two different binders according to different environmental conditions was investigated. These binders are a natural cement (NC) and a lime-based binder (FL). Thermal conductivity increases during aging and this increase is greater for FL than for NC [9]. In this case, although lime is a sustainable building material, it could not provide durability such as cement mixtures in aging tests.
In the last few decades, several studies have been conducted on the use of natural fibers in cementitious concrete to improve properties [10]. Figure 3 shows the optimum replacement percentage of various natural fibers in cement concrete [11].
Different binders have been reported to produce hemp concrete, such as ordinary Portland cement, lime, and lime combination [12][13][14]. It is confirmed by a large number of studies and has mechanical properties that make natural fibers suitable for use in building materials [15][16][17][18]. It is also important to state that building materials made with natural fibers have lower production costs than those made with synthetic fibers [19]. Hemp concrete is a viable solution to increase the energy efficiency of buildings [20].
As it is known, hemp fibers are one of the highest tensile strengths among natural fibers [10,21]. It has been shown that long sisal fibers are more effective than short sisal fibers in increasing the bending capacity of the sandwich panel [22]. Table 1 includes the mechanical properties of natural and synthetic fibers. It is seen that the tensile strength values of hemp are greater than other natural fibers but less than carbon and glass fibers. It can be used to increase toughness and ductility instead of using it as reinforcement in concrete. In addition, due to its good insulation properties, its usage areas can be increased as an insulation material. In this study, hemp was used as shive and fiber to increase the toughness and   ductility of concrete. The effects of these two forms on concrete were investigated by mechanical experiments, X-ray diffraction (XRD) experiments, stereo microscope, scanning electron microscope (SEM) analysis, and gap and particle size distribution.

Materials
Hemp shives and some fibers were soaked in water for 24 h before use due to their hydrophilic properties. The other shives were used in dry form to compare. Wet preservation of hemp is equivalent to other pre-treatment of hemp fibers [19]. The particle size distribution for a sample of hemp shives is listed in Table 2. The chemical compositions and physical properties of hemp fibers and shives are given in Table 3. Figure 4 shows the chemical forms found in hemp. Four different sizes of shives and fibers used in the samples are given in Figure 5. In Figure 6, samples containing shives and fibers are given.

Preparation of mortars
In this study, 32 series of concrete mixes were produced, in which the amount of materials used changed at certain rates. Due to the hydrophilic properties of cannabis, it was washed by soaking in water for 24 h before use. Two different types of hemp, shive and fiber, were used. In shivecontaining samples; the effects of hydrated lime and brick dust in the K samples and gypsum in the I samples on the flexural strength and water absorption values were    investigated. While some of the shives were used wet and some dry, all of the fibers were used wet. While hemp fibers were placed in longitudinal form in one group of samples, they were placed in mixed form in the other group of samples. Figure 7 illustrates the classification of the materials used in the design of the mortar.  Each experiment was performed three times. After 24 h, the poured samples were removed from their molds and cured under laboratory conditions for 28 days. The ratio of shives was fixed for comparison. The materials used in the samples with the shive content giving optimum values were then used in the fiber mixtures. The mixtures were prepared per the mixing procedure in the TS EN 196-1 standard. The content ratios of the materials used in the mortar mixture are given in Tables 4 and 5.

Experimental methods
To assess water absorption, the samples were immersed in a water bath at laboratory conditions (20°C) for 48 h. The percentage of water content was determined using the following equation: where W wc is the weight of the water-saturated sample and W dc is the weight of the dry sample.

Characterization
SEM analysis was performed to examine the physicochemical and structural changes of the mortar samples and their surface properties were determined. A ZEISS Supra 40VP model was used for SEM imaging and energy-dispersive spectrometer (EDX) analysis. We conducted an X-ray diffraction (XRD) analysis using a Panalytical Empyrean Xray diffractometer. In addition, the LEICA S6 D model was used for stereo microscope images.

Water absorption
It was not expected that water absorption values of lime were higher than gypsum [29,30]. However, in the samples used in the shives, the gypsum-containing ones absorbed less water than those containing lime (Figure 8). Since the water absorption rate is higher in limes, only gypsum was used in the fibers. It was observed that the water absorption rate of the samples increased as the size of the shive used increased. Soaked shives exhibited better water absorption performance than dry shives. The best water absorption values; among the samples containing wetted shive, the I3 sample has a water absorption value of 7.14%. Among the fibrous samples, the D3 sample has the lowest fiber content and its water absorption value is 8.11%. The highest water absorption values; samples containing unwetted shive and samples with the highest fiber content.
It is observed that the water absorption values of the samples using cement adhesive are higher than that of cement. The water absorption values of the samples placed by mixing the fibers with mortar into the mold gave better results than the fibers placed straightly.

Flexural strength
As shown in Figure 9, the flexural strength values of the fibrous samples are higher than the samples with the shive content. The flexural strength values of the samples placed by mixing the fibers with mortar into the mold gave better results than the fibers placed straightly. The highest flexural strength value among the shives was the I5 sample, which was 1.03 MPa. Among the fibers, the KL1 sample contains mixed fiber and cement and is 9.28 MPa. As the fiber ratios increased, the flexural strength values increased [31]. Cementitious ingredients gave better results than cement adhesive ingredients. In a study, cementitious composites containing hemp showed the best performance [32].

SEM analysis
SEM images of fracture surfaces of cured cement paste are shown in Figure 10. Block-shaped portlandite  (calcium hydroxide) crystals exhibiting a typical cubic behavior are seen in sample D1. The tubular structure of the hemp was seen in sample I1 [33]. The images in the I3 sample show that the bonding of the cellulose microfibers with the mortar is good due to the surface textures and non-deformable hemp fibers.   Figure 11 shows the results of EDX analyses confirmed by XRD analysis. The predominance of calcium, silicon, aluminum, and oxygen was determined. Figure 12 shows the holes in the shives samples. Large pore sizes have developed due to the presence of fibers. Despite these flaws, it increased the bending strength [34]. Figure 13 shows the hole size distribution of the K7 sample and the hole size distribution found in the cells of the hemp shive in the I1 sample. It is seen in both the SEM image and size distribution analysis that the gap size distributions of hemp cells are very different from each other.

Stereo microscope analysis
In Figure 14, the fibers used in the stereo microscope images are seen in the D1 and K1 samples and the shives in the I1 and I3 samples. Analysis of the fragment distribution of holes and shives is given in Figure 15. It is seen that the hole size distribution of the K7 sample obtained from SEM analysis in Figure 13 and the hole size distribution of the I3 sample obtained from the stereo microscope in Figure 15 are close to each other. In this case, it is seen that the stereo microscope and SEM images support each    other. The presence of cavities significantly affects the mechanical behavior of concrete. As the cavity diameter increased from 0 to 20 mm, the compressive strength values of the samples containing the inner cavity decreased nonlinearly [35].

Conclusıon
As expected, the flexural strength values of the fibrous samples in the produced samples gave nine times better results than the shive samples. The fibers increased the toughness of the samples compared to the shives samples. In fact, as the fibers' ratios increased, the flexural strengths increased even more. The use of hemp fibers can be a good alternative to steel and synthetic fibers. It was observed that the shives formed voids in the samples, lightening the sample, but decreasing the strength values. The water absorption and porosity of the shives were higher than the fibrous samples. In this case, research can be made for their use in thermal insulation materials to shed light on future studies.
Acknowledgments: The author would like to thank Bilecik Şeyh Edebali University Scientific Research Projects Coordinatorship for their support to the project numbered 2022-01.BŞEÜ.03-08 at Bilecik Şeyh Edebali University Central Campus.
Funding information: There is no funding for this article.
Author contributions: The author confirms sole responsibility for the following: study conception and design, data collection, analysis and interpretation of results, and article preparation.

Conflict of interest:
The author declares no conflict of interest.
Ethical approval: The conducted research is not related to either human or animal use.
Data availability statement: Derived data supporting the findings of this study are available from the corresponding author on request.