Study: “Fire behavior of hemp blocks: A biomass-based construction material”

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^aDepartment of Civil Engineering, Stellenbosch University, Stellenbosch, 7600, South Africa

^bFaculty of Civil and Environmental Engineering, Jimma Institute of Technology, Jimma University, Jimma, Ethiopia

^cIgnis Testing, Cape Town, South Africa

Received 26 August 2023, Revised 30 October 2023, Accepted 12 November 2023, Available online 14 November 2023, Version of Record 17 November 2023.

https://doi.org/10.1016/j.jobe.2023.108147 Get rights and content

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open access

Highlights

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Hemp blocks were tested for fire behaviour and resistance.
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Fire resistance, heat release rates, calorific value, and smouldering were studied.
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Discoloration and charring were observed and quantified.
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Hemp blocks have a low propensity to contribute to fire severity and spread.
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2-h fire resistance rating achieved.

Abstract

Hemp blocks, also known as hempcrete, are eco-friendly and sustainable construction materials composed of hemp, lime, and water. In this study, the fire behaviour and structural performance of hemp-based materials were experimentally investigated. Fire exposure scenarios using raw hemp shives, hemp blocks, and non-load-bearing hemp block walls were examined. Tests conducted include cone calorimeter, bomb calorimeter, standard furnace, heat-transfer rating inducing system (H-TRIS), and small-scale elevated temperature material tests. Hemp shives exhibit ignition with sustained flaming, a relatively high heat release rate (HRR), and a relatively low critical heat flux (CHF). However, the hemp blocks exhibited no flaming ignition, only smouldering combustion, and an HRR an order of magnitude lower. Hemp blocks and hemp shives produced minimal smoke. Hemp blocks charred, and associated discoloration zones have been documented. Tests indicate that limited structural capacity is lost up until 200 °C, whereas at 300 °C, the residual material strength is almost negligible. The hemp block walls maintained their structural stability and integrity for 2 h of standard fire testing. The ambient-temperature compressive strength of the hemp blocks was determined to be 1.0 MPa. This work is the first comprehensive study on the fire behaviour of hemp blocks and highlights their good performance, whereby they are likely to have a limited impact on fire risk in buildings. Plastered walls will have a fire performance exceeding those reported here.

Keywords

Hemp blocks

Biomass

Construction material

Fire behavior

Fire testing

Fire safety

1. Introduction

In recent years, the development of sustainable and eco-friendly construction materials has gained attention, and there has been a growing need for building materials, especially biomass, that meet high energy efficiency standards whilst having a smaller environmental footprint than the traditional options [1]. Hemp-based materials, such as hempcrete, are examples of biomass materials. Hemp is a high-growing cultivar of the Cannabis sativa plant that can be harvested in as little as 60 days and is gaining popularity as a versatile crop with a wide range of uses [2]. This variety of hemp does not have medicinal or narcotic properties.

Hemp concrete, commonly known as hempcrete or hemp blocks, is manufactured by combining a mixture of hemp shiv (or its internal stem), water, and a lime-based binder to make the construction material [[2], [3], [4]]. Hempcrete is a “carbon-negative” or “better-than-zero-carbon” substance, as it stores more carbon over the lifetime of buildings than is emitted during construction [[2], [3], [4], [5]] and can serve as a self-supporting thermal insulator with low thermal conductivity [1,6]. Hempcrete can be used as an alternative to concrete and standard insulation in buildings, and can potentially be recycled at the end of the lifespan of buildings [[2], [3], [4]]. It is non-toxic, energy-efficient, and resistant to moulds and insects. It can be used for wall insulation, flooring, roofing, and other applications [4]. However, the porous composition of hemp blocks reduces their mechanical strength and increases their water retention ability, which can limit their potential applications [5]. Several researchers have reported that hempcrete has a low compressive strength of less than 0.5 MPa. However, results have varied depending on the percentage of hemp shives (20–40 %). In Ref. [7] the compressive strength ranged between 0.044 MPa and 0.462 MPa, while in Ref. [8] ranged between 0.02 MPa and 0.39 MPa. Hence, they are typically unsuitable for load-bearing walls [1,5].

Hempcrete exhibits good thermal characteristics. The thermal conductivity of hempcrete can vary depending on its moisture content (influenced by relative humidity) and densities (258–460 kg·m⁻³). In Ref. [9], the measured conductivity varied between 0.061 W/(m⋅K) and 0.166 W/(m⋅K), whereas in Ref. [10] it ranged between 0.091 W/(m⋅K) and 0.12 W/(m⋅K). In comparison, concrete has a thermal conductivity in the range of around 1 W/(m⋅K). According to Ref. [10], the specific heat capacity of hempcrete is within the range of 0.28 kJ/(kg⋅K) to 0.5 kJ/(kg⋅K). It has been observed that if hempcrete is used above ground level, it is waterproof and resistant to rotting [4].

Hemp shiv has maximum water absorption capacity and it can absorb up to four times its original weight, primarily because of its porous structure and its hydrophilic properties [11]. It possesses remarkable hygric characteristics, exhibiting a moisture buffer value within the range of 2.07–2.36 g/(m² % relative humidity (RH)) [12]. Likewise, hempcrete exhibits a remarkable hygric characteristics, including a notable moisture transfer capability with a water vapor permeability of roughly 2.5 × 10⁻¹¹ kg/(m s Pa), substantial moisture storage capacity, and a significant moisture buffering capacity of 2.0–2.15 g/(m² %RH) [[13], [14], [15], [16]]. Furthermore, the use of hemp concrete for the building envelope greatly enhances thermal comfort and it naturally regulates humidity levels in the surrounding air, leading to improved ambiance and stabilised relative humidity levels [17].

One example of the recent application of hemp blocks is the infill walls of a 12-storey building, with 50 apartments, at 84 Harrington Street, Cape Town, South Africa, which has been named the tallest building made of hempcrete in the world [18]. Hempcrete construction is currently underway in various countries, and similar methods are employed. However, there are limited guidelines or standards governing the use of hempcrete in building construction. Furthermore, there is a substantial lack of information on its fire-resistance properties. Previous studies have primarily focused on fire properties, such as thermal conductivity [9], specific heat capacity [10], HRR and mass loss rate (MLR) of hemp shiv only (i.e. not hempcrete) [19], and cone calorimeter properties of hemp-gypsum-based moulds [14], whereas there are negligible data in the literature on heat release, charring, discoloration, smouldering combustion, structural stability, integrity, and insulation properties under exposure to various heat and temperature scenarios of hemp blocks. This lack of data poses a significant challenge for ensuring the safety of buildings constructed with hempcrete.

The present study aims to experimentally characterise the fire behaviour of industrially produced hemp shives, hemp blocks and non-load-bearing hemp block walls. The findings of this study can serve as the basis for the development of guidelines and standards. As a summary of the experimental tests, the cone calorimeter developed by Ref. [10] with specifications in ASTM E1354 [11] and ISO 5660 [12], was used to quantify the HRR, MLR, ignitability, and smoke release [10]. H-TRIS testing involved the application of high intensity radiation to study localised behaviour, based on the work by Ref. [13]. Standard fire resistance furnace testing for 2 h was performed to evaluate the non-load-bearing capacity, structural integrity, resistance to fire penetration, heat transfer, and insulation properties [14,15]. A bomb calorimeter was used to measure the gross heat of combustion. Elevated temperature material tests were used to quantify the residual compressive strength of the hemp blocks.

2. Materials and methods

This section describes the various tests and materials used in each test conducted in this study. This research studied two materials: hemp shives and hemp blocks. Hemp shiv “comes from the woody part of the hemp stem, and it is increasingly commercialised as a fibre by-product” [21]. Hemp shives were employed in the production of hemp blocks, which were then assembled to construct a hemp block non-load bearing wall. A series of fire and thermo-mechanical property tests were conducted. Fig. 1 provides a summary of the materials used and the tests conducted, along with the relevant codes applicable to the study.

2.1. Hemp-based materials description

The materials used in this study were obtained from Afrimat Hemp, South Africa [22]. Fig. 1 shows the hemp shiv and hemp blocks. The hemp shives utilized consist of a range of sizes, from fine-cut shives of approximately 3 mm to coarse-cut shives averaging around 18 mm in length. The hemp block dimensions were as follows: length, 390 ± 1 mm; depth, 110 ± 1 mm; and height, of 180 ± 1 mm. According to the supplier, the hemp blocks were manufactured using a volume-based formulation approach, consisting of 40 % formulated lime and 60 % hemp shiv in the presence of water, and had an average density of 650 kg·m⁻³. The collected blocks were stored in a clean area and kept above the ground until use.

2.2. Compressive strength and moisture content of hemp blocks

Compressive strength tests were performed on the three hemp blocks to evaluate their strength. A displacement-controlled method was used during the compression tests at a loading rate of 1 mm/min. The figures and results of this test can be found in Section 3.3. They were placed there to compare their strengths after 2 h of heating. The moisture content of the hemp blocks was determined using the loss on drying moisture content determination test (the samples were heated in an oven at 105 ± 2 °C for 24 h, and the mass was measured before and after heating).

2.3. Cone calorimeter test

Cone calorimeter tests form the basis of the reaction-to-fire assessment in this study. The cone tests are based on standards such as ASTM E1354 [23], ISO 5660-1 [24], and NFPA 271 [25]. In this study the oxygen consumption cone calorimeter test was performed according to ISO 5660-1:2022 [24] at Ignis Testing, Cape Town. The HRR, peak heat release rate (pHRR), total heat release (THR), MLR, time to ignition (TTI), CHF, effective heat of combustion (EHC), and smoke release were measured and recorded during the test. Cone calorimetry has been extensively used for the evaluation of biomass materials [20,[26], [27], [28]] with an emphasis on combustible products.

The experimental design consisted of two distinct test material configurations, assessment of (a) hemp shives and (b) hemp block samples. Owing to the dispersed nature of the hemp shives, the preparation of hemp shives involved a volumetric approach. Prior to the test, the samples were oven-dried at a temperature of 105 ± 2 °C for 24 h to minimise the impact of moisture on the fire properties, enabling accurate estimation of intrinsic factors. The mass of all samples with a precision of 0.01 g was obtained. The hemp blocks were cut to measure 100 mm × 100 mm with a thickness of 50 mm. Fig. 2a and b depicts the samples. Except for the top surface, the sides of the specimens were covered with a layer of aluminum foil with a thickness ranging from 0.025 mm to 0.04 mm. The specimens were then placed in a standard specimen holder and subjected to irradiances of 35 kW m⁻² and 50 kW m⁻². A typical pilot spark igniter was used in this study. Data collection continued for at least 30 min subsequent to the point of sustained flaming or in the event that the specimen remained unignited for 30 min.

Various cone calorimeter parameters were measured and analysed. HRR was established using measurement of the oxygen consumption and the measured concentration of gases (O₂, CO, CO₂, and moisture) as well as the flow rate in the exhaust from the combustion stream. The TTI was derived through computerised recordings captured at 1-s intervals. The EHC was determined by dividing the measured HRR from ignition by the total mass loss at the end of the test [29]. EHC denotes the combustion efficiency of materials, which is generally lower than the heat of combustion measured in a bomb calorimeter [30]. THR was calculated as the integral of the HRR curve [31].

MLR is the rate at which the mass of a sample changes during burning. A lower mass loss rate of combustible materials suggests a lower propensity for flame spread [32] and burning [33]. The Savitzky-Golay (SG) smoothing filter (polynomial of degree 2) was employed for the measured cone calorimeter mass loss result to obtain relatively noise-free estimates of the mass loss rate [[34], [35], [36]]. The fire growth rate index (FIGRA) was calculated from the cone calorimeter test results.

FIGRA is the ratio of the pHRR to the time to reach the pHRR [37] and is used to predict the potential fire growth and fire spread across various products and scenarios [38]. A higher FIGRA indicates greater potential for fire growth and spread, which translates to a higher fire hazard.

CHF testing was performed for the hemp shiv to determine the lowest irradiance necessary for ignition. A range of irradiance levels in 0.5 kW m⁻² increments was employed until ignition did not occur in the 30 min duration of the tests. CHF testing was exclusively conducted on hemp shives because the hemp block samples did not ignite.

Following the determination of CHF, the ignition temperature (T_ig) was calculated. The ignition temperature refers to the surface temperature of the specimen just prior to the point of ignition [39]. Although it is challenging to directly measure T_ig in experiments, it can be estimated using predictive models [40]. The cone calorimeter results can be used to estimate this ignition parameter [41]. To calculate T_ig for hemp shiv, which was found to be thermally thick based on the Biot number $(B i = h L / k = 15 > 0.1)$ , where h is the convective heat transfer coefficient taken as 0.013 kW m⁻² K⁻¹ for the cone calorimeter apparatus [39], L is the thickness of the specimen (0.05 m), and k is the thermal conductivity of the hemp shiv taken as 0.05 W m⁻¹·K⁻¹ [42], Equation (1) discussed in Ref. [39] was employed.(1) ${\dot{q}}_{c r}^{″} = h_{c} (T_{i g} - T_{o}) + σ (T_{i g}^{4} - T_{o}^{4})$ where ${\dot{q}}_{c r}^{″}$ is the critical heat flux obtained from tests (Fig. 9), $h_{c}$ is the heat transfer coefficient given above, $T_{i g}$ is the ignition temperature, $T_{o}$ is the ambient temperature (20 °C) and $σ$ is the Stefan-Boltzmann constant (5.67 W⋅m⁻²⋅K⁻⁴).

2.4. Bomb calorimeter

A CAL3K–S bomb calorimeter was used to measure the amount of combustion heat and determine the calorific values [43,44] of the hemp shives and hemp blocks. The test used a sample of hemp shives and hemp blocks weighing 0.25 g.

2.5. H-TRIS test

An H-TRIS test was conducted to examine the initial flammability, charring, thermal properties, and compressive strength of the hemp blocks after 2 h of radiation exposure. Three hemp block samples were tested and each block was placed in front of the H-TRIS radiant heat panels at a distance of 10 mm. To maintain the heating of the boundary condition, the boundary of the sample was enveloped with a ceramic fibre blanket, while the outer frame was constructed using calcium silicate boards. Fig. 3 displays the experimental setup. Temperature measurements were obtained using 1.5 mm diameter K-type thermocouples. These thermocouples were placed at five positions on both the exposed and unexposed faces of the block, as shown in Fig. 3a and b. In addition, measurements were taken at the midsection of the blocks at distance of 27.5 mm and 55 mm from the exposed face.

The applied time history of the incident radiant heat flux using the H-TRIS for a heating period of 2 h is shown in Fig. 3c. The temperature of the radiant heating panels stabilised at around 947 °C and the heat flux applied was estimated at 82 kW m⁻². The H-TRIS, as shown in Fig. 3d, was built using nine symmetrically arranged radiant heating panels. The effects of H-TRIS heating on the mass and residual compressive strength were examined. The mass of each sample was measured before and after testing and the resultant total mass loss was calculated. Post-exposure compression tests were performed on the samples. The aim of this test was to evaluate the residual strength and charring behaviour in an environment in which block-level phenomena can be readily studied.

2.6. Full-scale fire resistance test furnace

The fire resistance behaviour of a non-load-bearing wall made of hempcrete blocks was investigated using a fire-resistance furnace test apparatus. This test was conducted to examine the structural stability, integrity, and insulation properties of the wall. The test was performed by Ignis Testing according to SANS 10177:2005 [45], which follows the ISO 834 curve. A wall specimen of 2200 mm × 2400 mm was constructed, installed, and tested 31 days after installation. Fig. 4a shows the non-load-bearing hemp block wall layout and thermocouple arrangements, where R1 to R5 refer to the thermocouples located on the right and L1 to L5 are those situated on the left.

The temperature in the furnace was controlled by the average of ten plate K-type thermocouples. High-temperature 10 × 1200 mm plate-type stainless-steel K-type thermocouples were used. The furnace temperature was measured at 5-s intervals. Prior to the commencement of the test, all the thermocouples were calibrated at ambient temperature. The thermocouples were found to be within a tolerance of ±2 °C and cross-referenced to the calibrated test device, which had an accuracy of ±4 °C. Surface temperatures were measured using two groups of five thermocouples. Additional thermocouple measurements were taken at distances of 27.5 mm, 55 mm, and 82.5 mm from the unexposed face of the blocks to determine the temperature gradient, as shown in Fig. 4b.

10 mm mortar joints made of lime binder and water (1 water to 3 lime binder ratio) was utilized for the construction of the wall. These mortar joints provide structural stability and cohesion between hemp blocks. The mortar joint composition was different from that of the hemp blocks.

The test samples were firmly fixed to a furnace frame on all four sides. The joints were sealed according to a typical installation. The deflection measurements were limited to the central portion of the sample.

Fig. 4c and d show the experimental test setup of an unexposed and exposed surface of the wall, respectively, on the test day after construction was completed. The maximum deflection measured was 15 mm from the outside, and it was observed that deflection occurred gradually. During the test, observations such as deformation and time of failure; formation of cracks, holes, or openings through which flames or gases can escape; flame ignition; smoke emissions; and discolouring were made. Table 1 summarises the furnace tests.

Table 1. Summary of the furnace test.

Item	Description
Type	Walling System (Non-load bearing)
Hemp block wall nominal size	2200 mm x 2400 mm
Hemp block wall thickness	110 mm
Heat direction	Symmetrical
Furnace type Plaster configuration	Vertical Not plastered

Fig. 5 shows a comparison of the obtained furnace temperatures and required limits on furnace temperatures (upper and lower limits) recommended by SANS 10177-2 [45], along with the standard fire, ISO 834, curve [46].

2.7. Elevated temperature material tests

This test was designed to measure the residual compressive strength of hemp blocks that were exposed to the target temperature. The hemp block samples were cut into four pieces along their longer dimensions (measuring 97.5 × 180 × 110 mm) to fit inside a small furnace, as shown in Fig. 6a. The samples were heated for 60 min at 100, 200, and 300 °C. The samples were then allowed to cool to their ambient temperature before the compression test (see Fig. 6b) was conducted. Study [47] described this method as the easiest approach, most found in literature, and accounts best for the cooling effect. The compression tests were performed using a Zwick Z250 compression machine. A displacement-controlled method was used at a rate of 1 mm/min, and the test was stopped when the load decreased by 50 % of the ultimate load or when the deformation reached 15 mm.

3. Results and discussion

3.1. Cone calorimeter results

This section presents an analysis of the results, the fire behaviour and thermal hazards exhibited by hemp shives and hemp block samples based on cone calorimeter testing. The analysis aimed to provide insight into the combustion behaviour of these materials and their potential fire hazards. In addition, the study examined the influence of irradiance variations on these parameters and explored the differences between the hemp shiv and hemp block samples.

3.1.1. Visual observations

Fig. 7 depicts the hemp shiv behaviour observed during the cone calorimeter test. The pre-ignition stage is illustrated in Fig. 7a, with pyrolysis gases being produced. The samples experienced no discernible mass loss and the HRR remained at a minimal level. Fig. 7b depicts the ignition stage. As the combustion reaction progressed, the HRR eventually reached pHRR. This occurs during the third stage, as shown in Fig. 7c. As shown in Fig. 7d, during the sustained flaming period, the sample continued to burn with low flame intensity for the remainder of the testing period. The combustion behaviour is similar at applied irradiance levels of 35 and 50 kW m⁻².

The results of a cone calorimeter test, depicted in Fig. 8, demonstrate the different combustion behaviour between the hemp shives (8a) and hemp blocks (8b). Upon exposure to heat, the hemp shives entirely burned with little residual matter. On the other hand, the hemp blocks exhibited a slow charring effect, with a measured char depth of only 15 mm after 30 min. This enhanced behaviour can be attributed to the presence of the lime binder within the blocks, which resulted in a non-combustible matrix encapsulating the shiv particles.

The hemp blocks exhibited a very low level of smoke production. The limited amount of smoke generated, and the nature of the samples, suggest that hemp block emissions are relatively non-toxic; however further research is required to quantify the smoke emission characteristics.

3.1.2. Fire behavior and thermal hazards of hemp shives and hemp blocks

In this section, the results of TTI, CHF, HRR, THR, MLR, and FIGRA are presented along with a detailed discussion. A summary of the results is presented in Table 2. The results typically indicate that the blocks are very stable under fire conditions and provide minimal contribution to fire behaviour, whereas the hemp shiv is readily combustible.

Table 2. Summary of cone calorimeter test results.

Fire properties	Hemp shiv		Hemp block
Empty Cell	35 kW m⁻²	50 kW m⁻²	35 kW m⁻²	50 kW m⁻²
TTI (s)	11.2	8.4	^a	^a
pHRR (kW m⁻²)	189.3	225.1	22.9	22.3
TpHRR (s)	55	55	135	135
EHC (MJ kg⁻¹)	11.7	14.9	5.7	5.9
MLR_10-90 (g m⁻² s⁻¹)^b	3.4	3.6	1.9	2.2
THR (MJ m⁻²)	71.6	98.5	21.6	22.1
FIGRA (kW m⁻² s⁻¹)	4.1	3.4	0.17	0.17
CHF (kW m⁻²)	5 ± 0.5		^a

a: Hemp blocks did not ignite.
b: MLR_10-90 is the average mass loss rate per unit area between 10 % and 90 % of mass loss.

Hemp shives exhibited low TTI values under both irradiances, with higher irradiances leading to a lower TTI. Moreover, the susceptibility of hemp shives to ignition is evident when they are exposed to open flames or high temperatures. Fig. 9 shows the measured time to ignition of the hemp shiv for the respective irradiance applied. The CHF of hemp shiv was determined to be 5 ± 0.5 kW m⁻², which is significantly lower than other that of construction materials such as wood, with some studies indicating values such as 12 ± 2 kW m⁻² [48]. These results are likely influenced by the fibrous geometry of the sample.

The ignition temperature of hemp shiv calculated using Eq. (1) was 206 °C. This value is slightly lower than the ignition temperatures typically observed for wood and other cellulose-based materials, which range from 220 to 250 °C, as discussed in Ref. [39]. This indicates that hemp shiv has a lower ignition temperature compared to wood, making it more susceptible to ignition. The lower ignition temperature of hemp shives can be attributed to their geometry and potentially the pyrolysis rate [49],

Hemp blocks were subjected to the same conditions; however, flaming ignition did not occur during the test period. Fig. 10 illustrates the HRR and THR results of the cone calorimeter test conducted on the hemp shiv and hemp blocks under two different irradiances, 35 kW m⁻² and 50 kW m⁻². Note that the y-axis values are different for visual clarity, with the pHRR of the shives being an order of magnitude higher than the blocks, i.e., 22 vs. 225 kW m⁻² at higher irradiance. For the blocks, the initial increase in the HRR occurred until a char layer was formed, after which the HRR began to decrease as the char layer thickened. The lower HRR of the blocks is attributed to the lime binder, which absorbs energy, reduces the heat transfer to the shiv, and restricts the production of pyrolysis gases. The results obtained from the hemp block samples support the study conducted by Ref. [20], who reported that the HRR curve was lower than 20 kW m⁻² for a sample made from a mixture of gypsum and hemp shiv. However, the study [20] showed no pHRR but rather a trend that stabilised to a constant HRR value. Since the cone measures oxygen depletion, leading to the calculation of the plotted HRR, it means that a certain degree of combustion is occurring, even if flaming is not observed for the blocks. As discussed in Section 3.3, smouldering combustion within the hemp blocks has been identified, and appears to be the phenomena here leading to an HRR.

According to the test results given in Table 2 and Fig. 10, the time required for both samples to reach their pHRR was not significantly affected by whether the irradiance is 35 kW m⁻² or 50 kW m⁻². It was found that hemp shives reached pHRR faster than the hemp block samples.

The results in Table 2 indicate that hemp shives have a higher EHC than hemp block samples, as expected, as lime has a negligible calorific value. Although the EHC values should have remained constant, a slight variation was observed because of the applied irradiances. These variations could be attributed to factors such as the fire environmental conditions of heat transfer as well as the physical condition of the samples, as noted by Ref. [50].

Fig. 11 shows the MLR of the hemp shives and hemp blocks. From the data obtained, it was observed that the MLR increased sharply and reached its peak before decay for both samples. This decay was attributed to the exhaustion of the reactants. However, the MLR curve of hemp shives eventually stabilised at a near-zero rate after the consumption of the materials. Furthermore, an increase in irradiance was found to result in an increase in MLR, with hemp blocks exhibiting a significantly greater MLR compared to hemp shives. This appears to be primarily due to the presence of moisture in the blocks. It is important to note that the mass of the hemp block samples (285 ± 5 g) used in the test was greater than that of the hemp shiv (60 g). In addition, hemp blocks have a higher thermal conductivity than raw hemp shives. The latter has a thermal conductivity of ∼0.05 W/(m·K), as reported by Ref. [42]. The increased thermal conductivity of the hemp blocks facilitated the heat transfer within the material. This leads to additional smouldering combustion process and a higher MLR.

In terms of THR, hemp shives released a significant amount of heat compared to hemp block samples at 225 and 22 MJ m⁻², respectively, at higher irradiance. A higher THR value of hemp shives indicates that the material has the potential to release more heat and contribute to a more severe fire hazard, although combining hemp shives with a lime binder lowers the THR for hemp blocks. As presented in Table 2, both samples exhibit a relatively small FIGRA, at 3.4 and 0.17 kW m⁻² s⁻¹ respectively at the higher irradiance. Furthermore, the FIGRA of the hemp blocks was determined to be lower than that of the hemp shives, as expected based on the above results.

3.2. Bomb calorimeter result

The results of the bomb calorimeter test showed that the gross heat of combustion of the hemp shiv was 23.2 MJ kg ⁻¹ (Table 3). This result is somewhat comparable to a reported gross calorific value of hemp (in pellet form for biomass fuel) of 18.5 MJ kg⁻¹ [51]. Similar material such as rice husks have reported value of 13.8 MJ kg⁻¹ [52], wheat straw has 17.7 MJ kg⁻¹ [53] and pine wood has 21.3 MJ kg ⁻¹ [54]. Compared with these results, the gross heat of combustion for hemp shiv was a bit higher. In contrast, the calorific value of the hemp blocks was 7.4 MJ kg⁻¹. The higher heat of combustion in hemp shiv compared to that hemp blocks may be attributed to its elevated cellulose and lignin contents, which possess higher calorific values, whereas the lower caloric values of hemp blocks relative to shiv can be attributed to the inclusion of a lime-based binder with a negligible energy content. In addition, the heat of combustion of both samples from the bomb calorimeter test was higher than that from the cone calorimeter tests. This variation is due to the influence of moisture content, as the cone measures a net heat of combustion.

Table 3. Comparison of the heat of combustion results from bomb and cone calorimeters.

Sample	Gross heat of combustion (MJ kg ⁻¹)^a		Effective heat of combustion (MJ kg ⁻¹)^b
	Hemp shiv	Hemp block	Hemp shiv	Hemp block
1	23.1	7.8	14.0	6.1
2	22.9	7.2	14.8	4.3
3	23.6	7.3	15.8	7.4
Average	23.2	7.4	14.9	5.9

a: Bomb calorimeter results.
b: Cone calorimeter results at 50 kW m⁻²

3.3. H-TRIS testing results

The temperatures measured by various thermocouples placed on the exposed and unexposed faces, as well as in the mid-section, at distances of 27.5 mm and 55 mm from the exposed face, are shown in Fig. 12.

After 2 h of heating using the H-TRIS, the temperature on the exposed side stabilised at approximately 560 °C. The temperature on the unexposed side remained stable at around 45 °C from around 70 min onwards. The maximum temperature recorded on the unexposed face during this period was 45 °C, which suggests that the hemp blocks provided effective thermal insulation.

Temperatures in the middle region did not remain constant but displayed a trend of steady increase, reaching a value of approximately 723 °C after a period of 2 h. The temperature was the same at the exposed face and mid-section (27.5 mm away from the exposed face) at 62.5 min, with both measuring 560 °C. As the exposed face of the hemp blocks was consumed and turned into ash, no material remained to fuel the combustion process and generate additional heat. This may have resulted in a stabilisation of the temperature at the exposed face. However, the midsection of the material continued to experience smouldering combustion due to the presence of the remaining unburnt material, leading to a gradual increase in temperature over time. However, due to reduced convective cooling and increased insulation relative to the exposed face, the internal region became hotter due to smouldering combustion. Surface oxidation occurs at the sample surface when the surface temperature of the sample is higher than the inner surface temperature [55].

During the test, it was evident through visual observation that the inner section of the blocks exhibited a glowing appearance, whereas the outer surface underwent ash formation, as shown in Fig. 13 [39]. Study [40] noted that this glowing ignition is a result of smouldering combustion, which is an oxidising process that leads to a significant increase in temperature. It is noteworthy that the surface undergoing this process can also exhibit glowing and charring effects [40], however, this was not observed in the test conducted.

Fig. 14 shows the temperature gradient across the sections of the hemp block sample at 30 min time intervals.

Fig. 15 shows the hemp blocks after exposure to 2 h of H-TRIS heating. Different discoloration regions were observed, and their respective measured thicknesses are presented. These layers can be described as white (23 % of the total thickness), grey (25 %), brown (26 %), and cream (26 %). The results showed that 74 % of the hemp block was affected by the applied heat.

White discoloration (layer 1) indicates that the hemp block underwent thermal degradation and turned into ash. The material is completely consumed and no longer exhibits structural integrity. Grey discoloration (layer 2) indicates that the hemp block has undergone charring, the surface of the material is partially burned, but the underlying material is still intact. This can provide a degree of fire resistance because the charred layer can act as an insulating layer that slows down the rate of heat transfer to the interior of the material. However, if charring is extensive, it can weaken the structural integrity of the material and increase the risk of collapse. Brown discoloration (layer 3) indicates incomplete combustion, which means that the material is partially burned. The cream colour indicates the unaffected portion of the hemp block. This indicates that the outer section of samples was not significantly affected by heat and can potentially provide a barrier against the spread of fire and smoke.

Table 4 summarises the results of the masses of the hemp blocks at ambient conditions and after 2 h of H-TRIS heating. The samples lost approximately 826 g, or 13.5 % of their original mass. This result is less than the 36.9 % mass loss observed in hardwood timber as documented in Ref. [56]. As the samples were heated, they underwent a range of physical and chemical transformations, such as evaporation, decomposition, and oxidation, which resulted in a reduction in their overall mass. However, the samples could withstand the heat without ignition.

Table 4. Measured masses of hemp blocks at ambient and after 2 h H-TRIS heating.

Sample	Mass at ambient (g)	Mass at 2 h heating (g)
1	6384	5690
2	6260	5421
3	5638	4692
Average	6094	5268

Based on the observations in Fig. 16a, it is evident that the block being tested split/delaminate at the interface between the charred and unaffected original layers. Heating had a significant impact on the compressive strength of the hemp blocks. Fig. 16b shows the strength at ambient temperature and after 2 h of heating (average and upper and lower limits). The decrease in compressive strength can be attributed to various factors, such as the loss of moisture and breakdown of the structural integrity of the material (both shiv and lime). The factors that compromise the structural integrity and overall performance of the hemp blocks are the decomposition of organic hemp shive fibers, calcination of the lime binder in which the lime binder loses its water content and becomes less effective as a binding agent, thermal expansion induced stress in the blocks, and reduced thermal insulating properties of the hemp blocks.

The mechanical property test showed hemp blocks (390 mm × 110 mm) have a compressive strength of approximately 1 MPa (can resist up to 43 kN compressive load, as shown in Figs. 16) and 3.73 % moisture content. The hemp blocks exhibit low compressive strength so are not suitable for load-bearing application, as discussed above.

3.4. Furnace testing analysis

Fig. 17 shows the unexposed surface temperature and internal temperature of the blocks measured at 27.5 mm, 55 mm, and 82.5 mm from the exposed surface obtained from the furnace test. The maximum average unexposed surface temperature was 75 °C, indicating low conductivity and good thermal stability of the blocks. The recorded temperature on the unexposed surface remained below the prescribed limit of 140 °C set by standards such as SANS 10177-2 [45] and EN 1363-1 [57].

Fig. 18 illustrates the physical changes observed in the wall during the test. Discoloration of the blocks was observed on the exposed surface after 3 min of heating (Fig. 18a), and limited cracks were formed in the mortar between the blocks after 50 min (Fig. 18b). The limited cracks observed in the mortar were attributed to the compositional difference between the mortar and hemp blocks, as mentioned in Section 2.5. After 2 h exposure, the blocks remained intact and maintained their structure. No significant holes or damage were observed, and the high exposure temperature of 1050 °C resulted in the block glowing by the end of the test, as shown in Fig. 18c. After being allowed to cool for 24 h, the blocks experienced thermal decomposition, forming a few small holes and gaps in the mortar, and visible discoloration (Fig. 18d).

An observation of the unexposed surface revealed the discoloration of blocks, with more permeable blocks having a higher relative temperature at 25 min (Fig. 18e), and after 98 min (Fig. 18f), there were noticeable differences in block composition and discoloration. Based on visual observation, where it seems blocks with higher density (well-compacted) retained their white colour and structural integrity, while blocks with lower density (less compacted) exhibited a brown discoloration and increased permeability. These differences in appearance and characteristics can be attributed to the variations in the density and compaction levels of the blocks.

After 24 h, a portion of the front of the blocks crumbled, resulting in a powdery texture. This is evidenced by the knife being able to easily penetrate the soft surface of the blocks, as depicted in Fig. 19 (a). Similar discoloration zones are observed in Fig. 19b, as per Fig. 15. The maximum depth of the burnt-through material was determined to be 65 mm, giving a resultant charring rate of around 0.54 mm/min. The charring rate of hemp blocks closely aligns with the findings from studies on various softwood glulam species, which range between 0.53 and 0.91 mm/min [58], as well as tropical hardwood species, which range from 0.36 to 0.71 mm/min [59].

Based on the furnace test results, the hemp block wall system exhibited good performance in terms of fire resistance characteristics, such as stability, integrity, and insulation, and demonstrated its ability to withstand high temperatures for up to 2 h. In addition, after 24 h of testing, a thorough inspection of the wall system indicated no adverse impact on its stability, and the deflection was minimal.

3.5. Residual strength of hemp blocks

The results of the small furnace tests and compressive strength tests carried out on various samples at temperatures of 100 °C, 200 °C, and 300 °C are shown in Fig. 20. These samples exhibited unexpected inconsistencies. These inconsistencies are likely due to the variations in the densities of the blocks. In the case of ambient temperature, as well as 100 °C and 200 °C, the compressive strength exhibited substantial variations among the different samples (five samples tested). Therefore, instead of presenting an average value, the individual results for each sample are presented (Fig. 20a–c). On the other hand, when the samples were subjected to a temperature of 300 °C, the compressive strength remained stable and exhibited minimal variation in terms of values and a significant reduction in strength was observed. It was observed that at lower temperatures (100 °C and 200 °C), there were insufficient changes in the blocks. However, at 300 °C, chemical changes stabilised, moisture had been effectively driven off, and the blocks attained a more stable and uniformly distributed temperature effect. These factors resulted in more consistent strength at 300 °C. This consistency enabled the calculation of an average value, which is depicted in Fig. 20d based on a minimum of three test results.

3.6. Summary of performance and analysis of results

The findings of this study revealed numerous important observations regarding hemp blocks in fire. The blocks did not ignite when subjected to heating at fluxes up to 50 kW m⁻². This indicates that the blocks display significant resistance to flaming combustion, although smouldering combustion was observed. The HRR and THR of the hemp blocks were also measured. Hemp blocks have low HRR and THR values of approximately 23 kW m⁻² and 22 MJ m⁻², respectively. This implies that hemp blocks have a reduced propensity to contribute to the severity and spread of fires. This would be further reduced if a plaster layer was applied in front of blocks.

After heating, the blocks exhibited different discolorations, including white, grey, brown, and cream layers after exposure for 2 h. These layers accounted for varying proportions of the total thickness of the blocks, with approximately 74 % of the blocks affected by the applied heat. The study also investigated the impact of heating on the mass and compressive strength of hemp blocks. The blocks experienced a mass loss of ∼13.5 % after 2 h of heating. This loss can be attributed to various physical and chemical transformations such as evaporation, decomposition, or oxidation. However, despite the mass loss, the blocks could withstand heat without igniting.

Regarding compressive strength, the study revealed a significant reduction in the strength of the hemp blocks when subjected to 300 °C heating, at around 0.48 MPa (i.e., 53 % reduction). Nevertheless, the samples exposed to temperatures of 100 °C and 200 °C showed greater variations, making it challenging to draw definitive conclusions regarding the effects of heating at these specific temperature values. Observations indicate that under load, the charred layer and unaffected original layer debond. Factors contributing to the reduction in strength include moisture loss, release of trapped air/steam, and breakdown of structural integrity due to thermal expansion and contraction. In terms of fire resistance characteristics, the hemp block wall system demonstrated good performance in terms of stability, integrity, and insulation during the furnace tests. The blocks were able to withstand 2 h of standard fire exposure without adverse effects on the stability or significant deflection.

4. Conclusion

This study investigated the fire properties of hemp shives and hemp blocks, as well as the fire resistance of non-load-bearing hemp block walls. Hemp blocks, made from a mixture of hemp shives, lime binder, and water, can provide a viable and sustainable alternative to traditional construction materials, with the added advantage of being eco-friendly. Although hemp blocks have several advantages for sustainable construction, their mechanical properties must be considered when utilizing them in building projects because of their low compressive strength.

The cone calorimeter test provides an understanding of the flammability, combustion behavior, and level of fire severity of these materials. The hemp blocks exhibited low HRR and MLR, an order of magnitude lower than shiv or typical timber, indicating a reduced risk of fire spread. The H-TRIS test results indicated that the hemp blocks underwent discoloration and charring under heat, experienced a decrease in compressive strength, and exhibited mass loss without igniting, although smouldering combustion does occur.

The results of the standard furnace test on the hempcrete wall system indicates that the system could withstand exposure for the duration of the 2h furnace test. The maximum recorded exposed surface temperature is 75 °C. The results indicate that the system exhibited good fire resistance characteristics, including stability, integrity, and insulation. After 24 h, the wall system showed minimal deflection and negligible impact on stability, suggesting its suitability as a partitioning component for non-load-bearing walls. This suggests that the hempcrete wall system has potential as a building material because it can provide fire resistance in addition to its other desirable properties, such as thermal insulation and sustainability.

The hemp blocks possibly exhibited cases of scatter in results attributed to the inherent variations in density and mixing due to the nature of the hemp shiv. Nevertheless, this concern can be addressed by employing appropriate mixing techniques to ensure consistent results, but also highlights the need for a large number of tests to be conducted such that statistical distributions can be generated. Further research is needed to investigate the long-term durability of hemp as a construction material and to assess its suitability for various construction applications. Moreover, plastering hemp blocks can improve fire resistance properties further. This additional plaster layer offers further protection against ignition and reduces the risk of fire spreading. Consequently, the fire-resistance performance of hemp blocks is likely to be more reliable and robust in construction applications than indicated here when plastered.

CRediT authorship contribution statement

Yohannes Werkina Shewalul: Conceptualization, Methodology, Investigation, Writing – Original Draft, Writing – Review & Editing, Visualization. Natalia Flores Quiroz: Writing – Review & Editing. Dirk Streicher: Investigation, Writing – Review & Editing. Richard Walls: Resources, Validation, Writing – Review & Editing, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors gratefully acknowledge the SFPE Foundation for providing funding for this research and Afrimat Hemp for providing the materials required for this research. The tests in this research were conducted with the assistance of the Ignis Testing.

Data availability

Data will be made available on request.

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Tags: hempblocks, chalk, hazard, fireproof

Studies

Study: “Fire behavior of hemp blocks: A biomass-based construction material”