Introduction

The growing awareness of environmental concerns and the imperative to adopt sustainable practices within the construction industry has spurred an exploration and implementation of a variety of bio-based materials. Among these materials, cork has emerged as a compelling and innovative contender, due to its unique and versatile properties.

Cork, is sourced from the bark of the cork oak tree (Quercus suber L.), has a rich history that dates back thousands of years, with its origins traceable to ancient civilizations like Egypt. These cork oak forests predominantly thrive in semi-arid regions around the western Mediterranean, encompassing countries such as Portugal (which holds approximately one-third of the world’s cork tree area and is the source of about 50 percent of global cork production), southern France, Spain, specific parts of Italy, North America, and even China. In India, the cork oak tree predominantly grows in the central part of the country [1].

These woodland areas play a crucial role in combating desertification, enhancing soil permeability and hydrological control, encouraging soil preservation, and offering various environmental advantages such as carbon dioxide capture and regulation of the water cycle. Moreover, they provide a favourable habitat for a wide range of plant and animal species, fostering biodiversity [2].

This particular tree thrives in conditions that demand abundant sunlight and an uncommon blend of minimal rainfall coupled with moderately elevated humidity. The thickness of the tree’s bark fluctuates depending on the unique growth environment it experiences.

Methodology

This research relies on a literature-based and case study approach to explore the use of cork material in the construction industry. Initially, an extensive review of existing scholarly works on the topic is conducted to gather insights, theories, and findings. This literature review encompasses research from 1998 until today, reflecting over two decades of exploration into the qualities and environmental benefits of cork material in the construction field. It is important to note that major research have been carried out in Portugal, the largest manufacturer of cork, which plays a pivotal role in advancing our understanding of cork’s applications in construction. This extensive review serves as the foundation for identifying gaps and shaping the research questions. Subsequently, multiple case studies of construction projects employing cork materials will be undertaken to gain practical insights into the benefits, challenges, and outcomes of cork applications in diverse settings. The case studies will involve in-depth examinations of the selected projects, including data collection through interviews, site visits, and document analysis.

Additionally, this research acknowledges the valuable contributions of scholars like Luis Gil, who have conducted extensive research on cork material, particularly in Portugal, and their work provides essential context and insights. The combined analysis of the literature, case study findings, and the insights from scholars like Luis Gil will provide a holistic understanding of the role of cork in construction. Furthermore, the paper will delve into the exploration of various cork agglomerates, considering their unique properties and applications in construction. The research will also address the environmental implications of cork utilization, discussing its eco-friendly attributes and contribution to sustainable construction practices. Ethical considerations will be paramount in the research process, ensuring that all data is collected and used in accordance with best practices and academic standards.

Properties and benefits of cork

Cork is composed of numerous tiny cells, exceeding 40 million per cubic centimetre. These cells are structured with five walls made up of cellulose, lignin, tannins, suberin, and waxes [2]. The cell membranes posses a degree of impermeability and are filled with a gas resembling texture of air. When compressed, cork cells flex and fold without much lateral expansion, and they subsequently regain their original shape. Cork also disperses deformation energy, providing a comfortable tactile sensation, effective energy absorption, and anti-slip characteristics [3].

Cork is a lightweight and flexible substance that is nearly impervious to both liquids and gases. Its physical and mechanical characteristics make it an excellent choice for thermal insulation, especially in scenarios where it’s subjected to compressive forces, such as in cold storage facilities. It also excels in absorbing sound, such as in recording studios, and provides effective vibration dampening, like in machinery applications. Furthermore, cork serves as an electrical insulator and a dielectric material, and it is highly resistant to decay [4]. It exhibits exceptional stability and boasts good resistance to fire. These unique attributes are a result of its closed-cell structure as a cellular material (see Fig. 1) [5].

Fig. 1
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A Visual appearance of natural cork; B cellular structure of cork observed at SEM (Cortiça)

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Sustainability of cork over conventional materials

Cork products have ecological characteristics of re-generation makes it one of the most sustainable and energy efficient for construction.

Cork’s sustainability advantage is multi-fold. It is renewable, as the cork oak tree regenerates its bark every 9–12 years without being cut down over their long lifespan, which can exceed up to 200 years. This helps in sequestering carbon dioxide in the process. Furthermore, its extraction process is energy-efficient and generates minimal waste, as almost all parts of the cork are used. Compared to conventional materials, such as concrete or synthetic insulation, cork has a significantly lower carbon footprint [6].

This process doesn’t involve the use of harmful chemicals or extensive machinery. Additionally, cork forests serve as an important carbon sink, helping to mitigate climate change. Cork oak forests support diverse ecosystems and wildlife. Sustainable cork harvesting practices help in protecting these ecosystems. Cork is a natural material that biodegrades over time, without leaving harmful residues in the environment. When discarded, cork products break down naturally, contributing to reduce landfill waste.

Cork is a durable material that can last for many years. Products made from cork, such as flooring, wall coverings, and furniture, are known for their longevity, reducing the need for frequent replacements and conserving resources [4].

A possible way to think of the cork raw material transformation process is the creation of expanded cork agglomerate. The cork industry globally utilizes over 280,000 tons of cork annually. Nevertheless, a substantial portion of the raw cork, typically around 20–30%, is discarded upon arrival at the processing facility, primarily in the form of cork dust.

Due to the high energy level of this powder, it is frequently burned to create steam and/or power that is used in the manufacturing. All other industrial cork wastes are also valorised or repurposed in different ways. Thus, no cork is wasted.

Discovering practical applications for this discarded cork could carry significant economic and environmental benefits. One potential avenue is incorporating cork as an aggregate in concrete admixtures, where it could serve as a partial substitute for both sand and coarse aggregates.

Ecological aspect

This cork product is made entirely through the use of superheated steam, utilizing generators powered by discarded cork material, with no additional substances introduced and no cork resin-based agglomeration. This results in a completely ecological and natural product, offering a significant advantage that is challenging for competing materials to match.

The utilization of cork products in this manner also carries great environmental significance, as cork is a renewable and long-lasting resource that aids in carbon dioxide (CO2) absorption. Moreover, when cork oak trees are periodically harvested, they can yield between 250 and 400% more cork than if they were not harvested, which contributes to greater CO2 absorption. [6] Consequently, the use of cork products supports their sustainable production and the growth of additional cork forests, leading to increased CO2 sequestration. [2]

In order to approximate the amount of CO2 absorbed by cork building goods, we can perform the following calculations:

  1. a)

    Coverings (walls, floors, and ceilings) = 10 million m2/year, based on a 4 mm average thickness and a 450 kg/m3 average density, or 18.000 tons annually.

  2. b)

    Insulation = 150.000 m3 annually, based on a density of 120 kg/m3, or 18,000 tons annually. Corresponds to 36,000 tons each year on a worldwide scale.

With cork’s average carbon content of 57.37%, 36.000 tons of cork correspond to 20,653 tons of carbon annually, which means that 75.673 tons of CO2 are absorbed annually (CO2/C = 3664). [7]

Although these are approximations and only relate to Portuguese production calculations, they nonetheless serve as a measure of the ecological significance of cork goods and do not account for other cork products such expansion joints or degranulated cork.

In conclusion, hardly many items can match cork’s ecological harmony [7].

Nearly all cork products can be recycled, which offers the significant benefit of maintaining the carbon sequestered by the cork tree throughout the products’ long lifespans, thus delaying its release back into the atmosphere. Additionally, if there are no further uses after their useful life, cork products can be utilized for energy production. They have a high calorific value, and when incinerated, the CO2 emitted is equivalent to the amount originally fixed in the material, a process often described as “carbon neutral” [8].

Applications of cork in the building industry

Cork has several applications in the construction industry due to its unique properties, which include insulation, durability, and sustainability [9]. Here are some applications of cork in construction:

Cork polymer composites (CPC)

When cork is incorporated into a polymer matrix using melt-based technologies, such as combining it with high-density polyethylene (PE) and polypropylene (PP), it produces new possibilities for cork composite material. These composite materials were produced using twin-screw extrusion, injection moulding, compression moulding, pultrusion, and compression moulding techniques. These materials are referred to as cork-based composites (CPC). To improve the mechanical characteristics and the link between the cork and polymer, different reinforcement techniques were applied [10].

These findings indicate that CPC materials possess the following attributes:

  1. i.

    Stability in dimensions with decreased absorption of water.

  2. ii.

    Even dispersion and distribution of cork particles within the polymer matrix.

  3. iii.

    Improved fire resistance of the matrix in addition to having superior acoustic and thermal insulation qualities.

  4. iv.

    An impressive array of mechanical characteristics.

In the second stage of the study, researchers produced samples of Composite Cork Panels (CPC) through compression moulding, and they conducted a comprehensive assessment of the material’s properties, as depicted in Fig. 2. The results of this assessment were particularly noteworthy when compared to commercially available products like high-density fibreboard (HDF) and medium-density fibreboard (MDF). The findings indicated that CPC exhibited several favourable properties, making it a promising alternative in various applications. These properties included minimal water absorption, excellent acoustic insulation, hardness that was comparable to established materials, and noteworthy fire resistance [11].

Fig. 2
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Cork polymer composite board

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What sets cork apart even further is its advantage in terms of cost and environmental impact when compared to conventional inorganic fillers and reinforcements. Cork-based materials benefit greatly from the incorporation of polymeric matrices, which not only enhance the overall performance of the material but also open exciting possibilities for novel applications across industries. These applications extend to diverse sectors such as furniture manufacturing, transportation, aerospace, and building and construction.

Moreover, research into materials like Wood Plastic Composites (WPC) demonstrates a parallel trend. WPCs combine wood fibres and cork, creating natural fibre composites blended with polypropylene, resulting in materials with enhanced technical characteristics. These WPCs find use in a wide range of applications, including outdoor furniture, decking, naval construction, and other relevant areas. This diversification in applications illustrates the adaptability and versatility of cork-based materials in catering to the evolving needs of various industries, emphasizing sustainability, cost-effectiveness, and superior technical attributes [2].

In essence, the ongoing research and development in the field of cork-based materials and natural fibre composites are not only expanding the possibilities for sustainable construction and manufacturing but also offering innovative solutions that can contribute to more environmentally responsible and economically viable industries. These developments signify a promising and dynamic future for cork in a variety of applications, underscoring its value in contemporary material science and technology.

Cork based sandwich panels

The research centres around the creation and evaluation of hybrid sandwich panels featuring a novel core material composed of cork-based plastic composites. These panels have been rigorously compared with a range of alternative construction materials, showcasing their remarkable competitiveness. Notably, the cork-based core material stands out for its versatility, combining the lightweight and insulating properties of cork with the durability and structural integrity of plastic. This innovation opens up a wealth of possibilities for sustainable construction materials. Moreover, the research places a strong emphasis on environmental responsibility, utilizing by-products from various industrial processes, such as gypsum from power plant flue gas desulfurization and recycled textile fibres from old tires. These components, when integrated with degranulated expanded cork, result in a composite material ideal for use in partitions and walls, serving both new construction and building rehabilitation projects.

The environmental benefits of cork, combined with the repurposing of industrial waste products, make this composite material not only eco-friendly but also highly adaptable to the dynamic needs of the construction industry. Additionally, its potential for enhancing energy efficiency and insulation is of particular significance in a world increasingly focused on sustainable building practices. As a result, this research paves the way for a promising future in construction materials, balancing performance, and sustainability in a rapidly evolving industry.

Cork-based agglomerates are being investigated as a possible sandwich component core material; these agglomerates have specific properties and are intended to be lightweight, high-performing, and low-maintenance structures [3]. Some contend that the current core materials have low structural flexibility and can have significant negative environmental effects. Thus, it has been proposed that hybrid sandwich panels with non-traditional materials—such as cork-based materials—may provide better results in terms of performance, financial gain, and environmental benefits. Cork’s characteristics make it especially well-suited for damage tolerance when subjected to impact loads; low-speed impact tests and four-point bending tests are used to characterize the material’s residual strength. These tests showed that this type of core material has distinct advantages over others [2].

In contrast to glass fibre-reinforced plastic, a cork-based plastic composite material has been proposed and studied in terms of its mechanical characteristics, financial benefits, and environmental impact. Investigations into the use of this material in lightweight, rigid panels also showed that cork composites outperform other core materials, such as wood-based and other plastic core materials.

Furthermore, since sandwich composites made of carbon fibre and synthetic foam core usually have subpar acoustics, there is an increasing need for noise reduction in these materials. In order to address this, research was done on the use of natural cork and carbon fibre composites in sandwich structures. By combining these two elements, a sandwich composite with superior noise attenuation properties was produced that maintained weight and mechanical performance. Its achievement of a notable 250% enhancement in damping performance and increased operational durability is noteworthy. Cork replaces synthetic foam, which is in line with eco-friendly materials and tries to lessen carbon emissions. Improved vibration and acoustic performance in applications like wind turbine blades and airplane cabins are among the anticipated improvements [12,13,14].

In a different research study [15], two types of plywood composites with layers of cork were made and put through mechanical testing. In one composite, there was a plywood board with a cork core, and in the other, there were layers of cork on the plywood core. These composites measured mechanical qualities were contrasted with those of conventional plywood and particleboard. The results showed that even though the cork-layered plywood had a significantly lower density than particleboard, it still had better mechanical qualities. Moreover, density and production costs were found to be lower than with conventional plywood.

Additionally, a patented cork core was developed for use between two surface skins in a sandwich panel [16]. This core consists of at least two overlapping layers of cork agglomerates, creating regular microcavities.

Furthermore, another core material for sandwich panels [17] was developed, comprising a composite made by combining a thermoplastic resin with cork powder. The production of this product involved injection moulding.

Block of concrete with cork

The infusion of cork into concrete for the purpose of designing carefully optimized shapes represents a groundbreaking development in the construction industry. This innovative approach has the potential to revolutionize building practices by offering lightweight structures that excel in terms of thermal and acoustic performance, all without compromising the essential physical and mechanical characteristics of concrete. The resulting cork-infused concrete blocks stand out as versatile construction elements, akin to traditional bricks, suitable for a wide range of applications.

Furthermore, the creation of a prefabricated vertical partition system that incorporates cork is an ingenious solution that promises remarkable thermal and acoustic properties. This material not only reduces the overall cost of construction but also significantly decreases labour requirements and waste production during the building process. Such advancements contribute to an eco-friendlier construction industry by reducing the ecological footprint. These innovations are poised to play a pivotal role in shaping the future of construction, offering a harmonious blend of sustainability, cost-efficiency, and high-performance building materials [2].

Computer aided designing

There are intentions to incorporate structural building components constructed from cork and the utilization of “three-dimensional” components characterized by irregular shapes and relief patterns. The objective is to enhance architectural significance, primarily by leveraging CAD/CAM technologies, especially in the latter stages of the production process. These technologies provide substantial geometric flexibility and offer opportunities for extensive customization [2].

Cork based damping materials

The widely accessible cork agglomerates and cork rubber composites, with their diverse compositions, have the potential to function as a material solution for damping capacity, while also providing low mass density and thermal and acoustic insulation properties.

Additionally, composition cork shows itself to be a very attractive option for sandwich panels that are light and sound-absorbing. Particularly for applications involving vibration damping, extensive comparative research was carried out on carefully selected composition cork materials [18]. The results show that an air spring/viscous-based mechanism controls the low-frequency behaviour of these materials.

Densified insulation corkboard

Expanded corkboard, sometimes referred to as current insulation corkboard (ICB), is the source of the material (shown in Fig. 3). This ICB is made entirely of natural ingredients and doesn’t include any extra binding agents. A broad range of operating conditions can be used to produce the denser material, yielding products with densities ranging from the maximum usually found in ICB, which is typically between 250 and 300 kg/m3, to 750 kg/m3 or even higher.

Fig. 3
figure 3

Samples of different densified insulation corkboard [ICB] materials

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In order to densify ICB and achieve irreversible densification, the boards are heated and then hot pressed under precise pressure, temperature, and time conditions. A smoother surface and better qualities appropriate for numerous new applications are produced by this process. Crucially, production and application diversification are made possible by the ease with which the manufacturing process can be incorporated into current production techniques.

Economic studies have demonstrated that this material can compete effectively with some products available in the market, such as wood-based materials and other cork-based materials. Potential uses for this densified material encompass floor coverings, wall, and ceiling coverings, false (suspended) ceilings, screen and door panels, skirting boards, sandwich panels, and furniture, among others [17].

Cork microparticles as reinforcement and filler agent

The majority of structural adhesives are very stiff and strong, but they are not tough or ductile. Different techniques have been used to increase their resilience. Natural cork microparticles were added to a brittle epoxy adhesive’s mechanical property in a research study [19]. To stop cracks from spreading, these 125–250 μm cork particles were mixed with a particular epoxy adhesive. Tensile and impact tests demonstrated that, as long as the particles were dispersed uniformly throughout the mixture, the mechanical properties were affected by the ratio of cork particles to resin.

The cork industry generates a fine waste material called cork powder, which is light and granular. Efforts were made in research [20] to explore the potential of utilizing this waste as a filler in paper applications. Initially, the brown cork granulate needs to undergo refining to achieve the correct particle size distribution. Cork granulate can be added up to 15% of the total weight without having an adverse effect on the inter fibre connections. Because of its porosity, cork’s main benefit in this situation is its capacity to regulate ink when using ink-jet printing inks. The fibres from pine and eucalyptus can be used in this manner. One drawback is that the resulting sheets’ colour makes them inappropriate for high-brightness materials like writing paper. However, it can be used for a variety of purposes, including packaging paper.

Cork-gypsum composites

Cork and plaster can be mixed effectively, allowing for the creation of various new building materials with different ratios of these components. In terms of acoustical insulation, this composite material doesn’t absorb sound but instead reflects it. However, it offers good thermal insulation properties.

It’s recommended for use in building applications like partitions. Different types of cork granules can be incorporated, typically accounting for 10–20% of the total weight. These composites have a lower density (ranging from 0.0 to 1.0 g/cm3) compared to similar plasterboard products, which typically have a density of over 1.2 g/cm3. Nonetheless, to enhance their mechanical properties, additional reinforcing agents are required. [21]

Lightweight polymer mortar with cork granules

Two sets of mortar formulations were investigated, featuring varying ratios of resin to sand by weight (binder to fine aggregate). Within each set, cork was present in different amounts, ranging from 0 to 45% of the total aggregate volume. Flexural and compressive tests were conducted to assess the mechanical performance of these cork-modified polymer mortars. The study examined the impact of both the cork volume fraction and the resin-to-sand weight ratio on the material’s behaviour.

The results revealed a linear decrease in properties as the cork volume content increased. The use of cork in the modified mortars led to a more gradual reduction in specific properties due to the lower density of the material. Overall, this research indicated that incorporating cork into polymer-based concrete led to lighter materials with enhanced compressive ductility [22].

Case studies

Ecork hotel in Portugal (Fig. 4).

Fig. 4
figure 4

Façade of the Ecork hotel, Portugal

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Location: Évora, Portugal.

Area: 6300 m2.

Nestled amidst olive and cork trees in Portugal’s Alentejo region, Hotel Ecork, designed by Jose Carlos Cruz, is purportedly the world’s first cork-clad hotel. Located in the city of Evora, this hotel is made up of a restaurant and entertainment area covered in cork, and it has about 56 guest rooms spread across several nearby bungalows [23, 24].

Cork is produced primarily in Portugal, which makes it an excellent choice for cladding because it is recyclable and offers superior climate insulation.

The adaptability and effectiveness of cork as a building material are demonstrated by this hotel. Cork, which acts as an acoustic and thermal insulator, covers the entire building’s façade (Fig. 5).

Fig. 5
figure 5

Façade of the Ecork hotel, Portugal

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Very small openings are present on the outer walls of the building, creating large uninterrupted surfaces of the material.

Climate of the place is rainy and cold in winters and dry and hot in summers, thus wind plays a good role in temperature decrement. Thus, the building has large courtyards, specifically designed to take maximum benefit of the crosswinds and air circulation, which will eventually reduce the power consumption [23].

Challenges and solutions

The primary challenge associated with using cork in construction is its initial higher cost compared to traditional building materials. However, it’s important to emphasize that this upfront expense is often outweighed by the numerous advantages that cork brings over the long term.

Cork’s exceptional thermal and acoustic insulation properties are key contributors to its cost-effectiveness. When incorporated into building structures, cork effectively minimizes heat transfer and sound transmission. This translates to substantial energy savings, particularly in terms of reduced heating and cooling bills. As a result, buildings constructed with cork materials tend to be more energy-efficient and cost-effective to maintain. Over time, these energy savings contribute to a favourable return on investment, compensating for the initial cost differential.

Another critical aspect of cork’s cost-effectiveness is its durability and resilience. Cork is naturally resistant to pests and decay, reducing the need for maintenance and repair. This not only extends the material’s lifespan but also lowers ongoing maintenance costs, further justifying the initial investment.

Furthermore, cork’s status as a sustainable resource is vital. It is harvested without causing harm to cork oak trees, aligning perfectly with eco-friendly building practices. Its renewability is complemented by the fact that cork is recyclable, making it an environmentally responsible choice in construction. But on contrary, the decay rates for cork products in use are highly uncertain, as they are not grounded in empirical data but rather assumed based on typical cork product usage. Similarly, the decay rates of cork products in landfills are also subject to significant uncertainty [25].

As the demand for sustainable building materials continues to grow and processing techniques for cork improve, there is a strong expectation that cork’s cost will decrease, making it a more competitive and cost-effective choice for construction projects. Additionally, ongoing experimentation with cork and its potential as a composite material is likely to lead to the development of innovative cork-composite materials that offer enhanced properties while remaining cost-competitive with conventional building materials.

In regions that provide incentives or certifications for sustainable construction practices, such as LEED (Leadership in Energy and Environmental Design) certification, the use of cork can further offset the initial expenses. These incentives can make cork an even more attractive and economically viable option for environmentally conscious building projects.

In summary, although cork may require a higher initial investment, its exceptional energy efficiency, longevity, and sustainability contribute to substantial long-term cost savings and a reduced environmental footprint. As the construction industry increasingly focuses on sustainable and eco-friendly practices, cork stands as a promising and cost-effective choice for the future of construction materials.

Conclusion

Cork’s significance as an exceptional material for sustainable construction is rooted in its remarkable combination of renewable attributes and exceptional insulation capabilities. One of its standout features is its renewability, as it can be harvested from cork oak trees without causing any harm to the trees themselves. This sustainable harvesting process means that cork is an environmentally responsible choice, and it can be harvested repeatedly from the same trees, making it a long-term, sustainable resource.

Cork’s natural thermal and acoustic insulation properties are equally noteworthy. When used in construction, cork effectively reduces heat transfer and noise transmission, resulting in more comfortable indoor environments. This translates to lower energy consumption for heating and cooling, as well as a quieter and more peaceful living or working space. These qualities have a substantial impact on reducing energy bills and enhancing the overall quality of buildings.

While it’s true that the initial costs of cork-based materials may be somewhat higher than traditional alternatives, the long-term advantages far outweigh these concerns. Over time, the energy savings and the reduced environmental footprint of buildings constructed with cork materials become evident. Lower energy bills, decreased reliance on heating and cooling systems, and a reduced carbon footprint all contribute to the financial and environmental benefits of cork.

In an era where sustainability is gaining increasing importance in the construction industry, cork’s unique blend of eco-friendliness and energy efficiency positions it as a promising alternative to conventional building materials. It aligns with the growing emphasis on constructing environmentally responsible and energy-efficient structures, meeting the demands of both today’s eco-conscious consumers and stringent environmental regulations. Cork not only contributes to energy-efficient buildings but also offers a sustainable approach that benefits the environment and future generations. As the construction industry continues to prioritize sustainability, cork is poised to play a pivotal role in the construction materials of the future.