1. Introduction
The increased demand in agriculture and industrialization and accelerated growth of population has brought about large amounts of waste and pollution. In the field of construction, this trend is heavily manifested as environmentally harmful methods of manufacturing have caused the deterioration of the environment. Therefore, the motivation for the development of an innovative material in construction has become of interest to researchers and practitioners. Versatility, in terms of low energy consumption, sustainable and highly functional, is the new norm for these new materials. In order to integrate these alternative materials to modern society, it is vital for technology to progress, adapt, and increase the acceptance of such products. Hence, searching for alternative approaches that are environmentally friendly materials could lead to a more sustainable future.
One of the emerging topics in sustainable construction building materials is the use of bio-composite material, such as mycelium. Mycelium is comprised of hyphae and root-like plant structures with the ability to allow the fungus to consume the nutrients from its waste substrate. In the study of Abhijith et al., mycelium-based materials have the potential to be a material of choice for various applications [
1]. Mycelium would bind material together by growing into its substrate. The material could have the capacity to grow on lignocellulosic substrates. This could also be vital to the strength and its capability to activate its physiological properties [
2]. The world’s second most abundant biopolymer is Chitin which is a chemical molecule that comprises mycelium. Chitin is a high tensile strength, poly-crystalline polymeric acetylglucosamine, which is responsible for the tensile strength of the mycelium, thus, providing reinforcing capacity strength to the fibrous network [
3]. There are studies on enhancing properties through natural additives which cause stronger bond at the interfaces [
4].
Several studies related to building materials, for both structural or non-structural applications, were done from literatures. Use of mycelium to construction, design, and educational sectors is available [
5]. In the construction sector, development of load-bearing mycelium bricks and concrete with an average compressive strength of 5.7 MPa and 22.5 MPa, respectively, was achieved [
6]. Biopolymers for architectural cladding has also been developed towards sustainable construction and building materials [
7]. The growth of mycelium can be studied and use it for intervention in existing architecture and solid building materials grown from fragmented waste materials [
8]. Since bio-composites may react with moisture, materials with mycelium can be maximized once water vapor is removed [
9].
It is recommended to explore the development of new stabilization agents. This was done through a review on life cycle assessment comparing traditional bricks and alternative bricks with organic and inorganic materials [
10]. In order to achieve sustainable construction alternatives for bricks used in interior walls were no substantial wetting and drying occur, this study focused on the utilization of waste products such as: Rice bran (RB), sawdust (SD), and coconut husk (CH) with mycelium. The RB/CH and SD are byproducts of rice milling/coconut processes and woodworking operations, respectively. The binder that holds the substrate together is with the use of a bio-composite called mycelium (M). With the utilization of agricultural and industrial waste materials as substrate and reduction on the use of cement binders, the researchers aim to formulate and produce mycelium bricks that can achieve an environmentally friendly product that is of par with standard construction building materials. The contribution of this paper in the field of sustainable construction material are the design mix, methodology, and the results of compressive and flexural strength.
2. Materials and Method
The binder in the produced bricks is from mycelium. The materials used for the development of mycelium are rice and sugarcane molasses (SCM). The matrix that the mycelium binds are the substrates RB, SD, and CH. The RB, SD, and CH contains nutrients that achieve mycelium growth in the matrix. As a control specimen, clay (C) brick was also produced, which are common building materials. Seen in
Table 1 are the types of specimens and the number of specimens for the compressive and flexural test. The RB design mix contains a ratio of 5:2:1 by mass for C:RB:CH with 350 mL water added to the mix. The SD mix contains a ratio of 5:3:1 by mass for C:SD:CH with 450 mL of water added to this mix. The C mix was designed using conventional brick production with varying water in attaining a workable mix.
SCM ingredients were commercially available, while the RB and CH were acquired from local farms and factories and SD was obtained from lumber wastes. In addition, paper, two (2) plastic containers, and airtight wrappers were also gathered.
Figure 1a shows the items mentioned. Seen in
Figure 1b is the steel brick mold used which was measured to have the dimensions, 200 mm length × 90 mm width × 60 mm height. The mold comprises of 4 bricks in one production. After creating the bricks with 33 days of incubation, a laboratory oven (kept at 110° to 115 °C for not less than 24 h) was used for drying. Drying of the bricks is required to prohibit any further growth of the mycelium.
Figure 2 shows the raw ingredients used. For the design mixes, Clay (C) and Coco Husk (CH) remained as the constant ingredient for the substrate in the four design mixes with agricultural wastes. Sugar Cane Molasses (SCM) was utilized to act as the mycelium in serum form for the researchers to securely handle and later add the mycelium to the substrate mixes. An immensely common agricultural food waste in the Philippines, Rice Bran (RB) can be easily obtained from local farms and gardens in the country. RB was employed as a substrate material with the intention of obtaining a faster rate of growth in the material and acted both as an additional filler material and stabilizer for the substrate. Hardwood Sawdust (SD) is a commercially available waste material generated from wood workshops and construction sites which were sieved to smaller particles. Coco Husk (CH) was collected from a coconut farm as it is a highly common agricultural waste in the Philippines that is usually disposed of after use. In the mix design, CH was used as an additional substrate for mycelium nutrients for growth. CH was also sieved into fine particles and was utilized for all the design mixes.
Seen in
Figure 3 is the process made in the production bio-composite brick. The methodology begins with the acquisition of the mycelium fungi from underneath a bamboo shoot. One kilogram of rice in a plastic container, as shown in
Figure 3a, was the initial main substrate used and buried below the soil near the bamboo shoot. The container was left underground for a total of five (5) days. This was vital to allow the substrate to hold the fungal tissue. After the said period, the mycelium was segregated by the researchers from the rice substrate. This can be observed in
Figure 3b. After the segregation, the mycelium was mixed with 1 L of sugarcane molasses (SCM) which was stored in a 1.5 L PET bottle. There is then an inoculation period of five (5) days in constant ambient temperature, before it was used in the brick production as a mycelium serum.
The researchers arranged substrate design mix based on material availability and from the waste substrates utilized by Zurbano et al. (2017) for mycelium growth [
11]. Mass was utilized as the basis for the design mix ratio. Sieve no. 150 μm was used to achieve the particle size of SD and CH. Water used in all substrate mixes is distilled water. As seen in
Figure 3c, the agricultural wastes were mixed together manually to form the substrate. Afterwards, the mycelium serum was added to the main substrate design mix, as observed in
Figure 3d. One tablespoon of mycelium serum for one kilogram of ingredients was used for the mycelium serum-substrate ratio. After mixing, the mycelium bricks placed in mold are stored inside a room with no sunlight at a constant temperature for a period of twenty-five (25) days incubation as seen in
Figure 3e. During this incubation period, the weight of the bricks specimens was recorded daily until all the mechanical tests were performed. After the incubation, the mycelium bricks were oven-dried for 1 day in order to prohibit the growth of the mycelium in the bricks. The range of temperature used in the oven drying the RB, RBM, SD and SDM bricks was at 110 to 115 °C, while oven-fired bricks for C and CM observed a minimum temperature of 900 °C and a maximum temperature of 1100 °C. A further four days was allowed for, so the bricks could harden after the oven process, as shown in
Figure 3f. Another four days was consumed for the curing period of the bricks. The total number of days from the start of production until testing is 34 days.
Compressive and flexural tests were performed after the production of the bricks after 34 days. The universal testing machine (UTM) was used to administer mechanical tests. The compression tests were implemented in accordance with ASTM C67. The compressed bricks, after testing, are seen in
Figure 4a. In accordance with ASTM E518, the flexural tests were performed. The set-up for the latter test was a three-point loading which obtained the flexural capacity of the brick. This flexural test set-up can be seen in
Figure 4b, which shows the specimen with equally spaced supports on both ends of the brick. The midpoint displacement, due to the force applied, was recorded.
4. Conclusions
The production of bio-composite mycelium bricks as sustainable construction building materials using agricultural waste was achieved. There were six design mixes consisting of rice bran (with and without mycelium), sawdust (with and without mycelium), and pure clay (with or without mycelium). Based on the Indian Standard IS1077, all the brick design mixes with mycelium reached an adequate average compressive strength result and were well above the minimum requirement of 3.5 MPa. Additionally, RBM and SDM obtained a higher compressive strength as compared to their non-mycelium counterparts with an impressive increase of 38.5% and 31.0%, respectively. The addition of mycelium in the flexural test improves the ductility of the brick specimens by producing fewer cracks. The mycelium content at 34 days of age increases from the design mixes CM, RBM, and SDM. It showed that when the mycelium content increases, the linear dimensional change increases. There is a presence of fiber seen in the stereoscopic microscope, which proved the natural fibers from mycelium acted as a binder to the building material. Future recommendations include the use of material characterization of ingredients, more tests on dimensional stability, more mechanical tests, acoustic tests, water absorption tests, and non-destructive tests with different design mixtures from available agricultural wastes.