Keywords

1 Introduction

The building sector plays a vital role in national and international sustainability goals. According to a 2019 report by the International Energy Agency (IEA), it accounts for 36% of global final energy use and 39% of CO2 emissions, making it one of the most significant areas of energy use and emissions [1]. In this context, the role of insulation materials has grown significantly, as they are a deciding factor in reducing the environmental impacts of buildings over their lifetime. Almost a third of all buildings in the EU are over 50 years old, and over 75% are not designed to be energy efficient. Updating or repurposing these structures can cut annual CO2 emissions and energy use by 5–6% [2, 3].

The environmental benefits of natural fibers made from agricultural waste by-products include their biodegradability, renewability, recyclability, composability, and potential to reduce greenhouse gas emissions. Using these fibers, it is possible to reduce emissions produced by insulation materials that are in the market now, reduce agricultural waste, and make it part of a circular economy [4]. Together with a renewable construction material resource such as timber, it is possible to incorporate lightweight bio-based aggregate materials in timber frame structures effectively.

Traditional mineral binders such as Portland cement and lime are used to ensure the effective use of natural fibers in buildings [5]. However, they are associated with a high carbon footprint during their production, and therefore, alternative binders are often considered. Starch, gypsum, and geopolymer could be used as potential binders in bio-composite production [6]. Gypsum is a globally available substance primarily employed in the construction industry (plasters for renderings, plasterboards, prefabricated blocks, etc.). It is adaptable, a thermal insulator, a humidity regulator, non-combustible, and an acoustic absorber. Due to their chemical stability, gypsum materials can be fully recycled. As a result of this, researchers are interested in finding ways to use gypsum in construction sector materials to reduce existing large amounts of construction waste [7]. Starch is a biopolymer found in the photosynthetic tissues of plants that is both abundant and renewable. Starch is a cost-effective and biodegradable material with universal properties, composed primarily of amylopectin (75%) and amylose (25%). In Europe, starch is mainly extracted from potatoes, wheat, and maize. In investigations to produce thermal insulation composites, natural fibers such as hemp were combined with starch as a binder, resulting in improved mechanical performance [8]. Geopolymers have been gaining attention as an alternative to traditional cement-based materials in construction and various industrial applications. Geopolymers are inorganic polymers that can be formed from the reaction of aluminosilicate materials with an activating solution. They offer several advantages, including reduced carbon dioxide emissions, improved durability, and potentially lower production costs compared to Portland cement [9]. Many of these alternative materials have the potential to reduce the environmental footprint of construction materials, while there is a limited number of articles dedicated to ecological impact calculations.

The life cycle analysis (LCA) approach is a valuable tool for comparing the environmental performance of different insulation materials. It is consistent with the life-cycle thinking idea that should be used to ascertain the environmental consequences of insulating materials. LCA studies typically aim to either support decisions for more environmentally friendly materials or identify environmental hotspots in manufacturing building materials. Regarding materials, the goal of LCA studies is typically to support decisions for more environmentally friendly materials [3, 10].

2 Methodology

2.1 Life Cycle Assessment Application

This study aims to evaluate the environmental impact of novel bio-based insulation materials. This study performed a cradle-to-grave comparative LCA on bio-composite materials using three different binders: starch, geopolymer, and gypsum. The LCA was conducted by ISO standards 14040/44 and utilized the SimaPro 9.4 software. Most of the processes were modeled using the Ecoinvent 3.0 database, and the ReCiPe 2016 Midpoint (H) V1.07 method was used to calculate the environmental impacts.

2.2 Goal and Scope Definition

This study aims to compare the environmental impacts of the bio-based insulation materials developed in this research with traditionally used insulation materials for timber structures. The LCA results will provide insights into the overall environmental performance of the bio-based insulation materials, including potential environmental hotspots that could be targeted for further improvement.

The functional unit (FU) is the standard measurement used to compare products. Density, thickness, and thermal conductivity are three primary features and characteristics that can be used to classify insulation materials [10]. Thus, functional units were chosen to provide one square meter (m2) of insulation board with a U-value of 0.105 W/(m2·K). This U-value ensures normal operation in cold climates, such as Latvia. Functional units are identical for all three bio-composites thus it is possible to compare these materials.

The input data for calculations of bio-composites based on three types of alternative binders are summarized in Table 1. The production process and technological properties of the binders and bio-composites are described in the previous articles [12,13,14]. Cradle-to-gate system boundaries were used as the use stage is similar for all compared materials.

Table 1. Data of bio-based materials for FU [12,13,14]
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2.3 Life Cycle Inventory

Data shown in tables are normalized according to FU before putting in SimaPro. Production of hemp shives was used from a study developed before [5]. Inputs in SimaPro for bio-composites are shown in Table 2.

Table 2. Quantities of material and energy inputs as referred to in the FU of the study (1 m2 of composite with U-value 0.105 W/(m2·K)), in each of the three different bio-composites.
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For comparison with traditional materials, a wooden frame (see Table 3) was added for all three bio-composites. Traditional materials for comparison were chosen from studies developed before [16]. Additionally, all materials were also compared to standard wooden structures. The composition of the standard wooden structure is shown in Table 3.

Table 3. Quantities of material and energy inputs as referred to in the FU of the study (1 m2 of composite with U-value 0.105 W/(m2·K)), for standard timber structure
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3 Result and Discussion

The results are examined individually for each type of bio-composite, analyzing separate impact categories of materials to assess which one has the greatest influence. Subsequently, all bio-composites and the whole assembly for one functional unit (FU) are compared with traditional timber frame constructions and insulated block materials.

The global warming impact category result for the bio-composite with starch is 79.23 kg CO2 eq. (refer to Fig. 1). The materials causing the most significant impact in the bio-composite are potato starch (56.12 kg CO2 eq.), glycerol (48.76 kg CO2 eq.), and electricity (43.42 kg CO2 eq.). In the production of bio-composite in a laboratory setting, electricity stands out as one of the significant contributors. However, in industrial production, the result would likely be considerably lower.

Hemp shives capture CO2 during growth, resulting in a negative impact in the global warming potential category, thereby improving the overall result of the bio-composite. Among the components, potato starch contributes most significantly to the impact categories. To diminish the impact of potato starch, considering the use of starch from alternative sources could be beneficial. Starch is obtainable from the food industry as food waste, agricultural by-products, and other sources. Utilizing starch recovered from food waste could notably decrease the impact of the bio-based composite across various LCA categories, contributing to its integration into the circular economy and promoting sustainability [15].

The composition of the starch bio-composite is not optimized to reduce its environmental impact but rather crafted to achieve maximum strength parameters. Considering the obtained results, it is possible to optimize the quantity of added additives (such as sodium silicate and glycerol) since reducing them could lower the material's overall environmental impact significantly without adversely affecting its functional properties.

Fig. 1.
figure 1

LCA results in global warming impact category for bio-composite with starch.

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Gypsum is a mineral binder with a relatively small carbon footprint; it generates only 0.098 kg/CO2 eq. Compared to 0.857 kg/CO2 eq. For Portland cement per 1 kg of binder [Ecoinvent database]. Hence, the overall environmental impact of the bio-composite is negative because the binder has a minimal environmental impact of –17.59 kg CO2 eq. (refer to Fig. 2). This stands as one of the significant benefits of using gypsum binders.

As the filler used, hemp shives absorb CO2 during their growth period, which results in a negative overall carbon footprint for the bio-composite. It absorbs more CO2 than is emitted during its manufacturing process. This is also related to the fact that there are not many additional substances in its composition; other emissions come from electricity usage during material production and drying processes.

Nevertheless, improvements could also be made in the composition, especially concerning the binders, because gypsum is relatively easily recyclable and available as a waste product. For instance, they are using recycled construction gypsum or phosphogypsum, which arises during the production of fertilizers. If directly used from the fertilizer production line, emissions related to gypsum could be entirely eliminated as there would be no need for additional incineration processes [16].

Fig. 2.
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LCA results in global warming impact category for bio-composite with gypsum.

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The highest impact among the examined experimental materials is the bio-composite material with geopolymer, which amounts to 64.49 kg CO2 eq. Per functional unit (FU) (refer to Fig. 3). The primary contributor in this case is metakaolin, as anticipated based on the literature review [17]. Metakaolin alone generates 99.63 kg CO2 eq., surpassing the material's impact itself; the hemp shives and their captured CO2 mitigate the overall impact of the material. However, the impact of the alkaline solution and electricity is also notably high.

To reduce the environmental impact of geopolymers, it could be possible to use waste products such as fly ash, which have a significantly lower environmental impact compared to metakaolin [17]. Yet, this substitution would significantly affect the rest of the composition and the desired properties, making such a replacement more complex. However, in further research, an in-depth analysis should be conducted regarding the increased utilization of waste products in these geopolymer-based bio-composites.

The traditional building materials considered in this study encompass aerated concrete blocks, expanded clay cement, and clay ceramic blocks insulated with either stone wool or polystyrene foam. These brick constructions are more prevalent than new material systems, primarily employed in private house construction [16]. Six functional units were constructed using these materials, with each brick material insulated using either of the two insulation materials. All functional units maintain an identical U-value of U = 0.105 W/(m2·K) as used in this study. The finishing layers across all types remain consistent without facade cladding and internal render.

Fig. 3.
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LCA results in global warming impact category for bio-composite with geopolymer.

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Among all materials, bio-composites with gypsum exhibit the most favorable results since they are the only materials that capture more CO2 than they produce, amounting to −13.94 kg CO2 eq. Walls constructed from various brick materials generate significantly more CO2 (up to 136 kg CO2 eq.) compared to traditional timber construction per square meter (32.18 kg CO2 eq.). This discrepancy primarily arises due to the substantial quantity of material used in brick block production, which also contributes to their greater mass, ensuring heightened thermal inertia. However, this advantage diminishes compared to the developed materials, as they possess relatively high thermal inertia. This characteristic stems from both the fillers and the binders used.

The traditional timber frame construction shows higher results than the gypsum bio-composite with a timber frame, and these results are generally lower than traditional masonry constructions. However, it should also be noted that the composition of the gypsum bio-composite could be improved to reduce its impact on the environment further, for example, by replacing gypsum made from fresh raw materials with phosphogypsum.

If these same exterior walls of the building were made from the developed gypsum bio-composite, which sequesters −13.94 kg CO2 eq. per square meter. Thus, by choosing the gypsum bio-composites developed in the project and replacing the exterior walls of an average private house with these materials, it is possible to save 149.38 kg CO2 eq. per square meter. Exterior walls typically account for 15 to 20% of total embodied carbon emissions, so it can be concluded that by using one of the materials developed in the project, the building's total embodied carbon can be reduced by approximately 20%. It should be noted that these materials can also be used for interior wall construction, and by changing the composition, they can also be used for underfloor and roof insulation. This would allow for even greater CO2 savings (Fig. 4).

Fig. 4.
figure 4

LCA results for comparison of insulation materials in wooden frame with traditional building materials.

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4 Conclusion

Three types of bio-composites made from different binders, gypsum, geopolymer, and starch, were examined. The materials were compared to each other and to traditionally used insulation materials with timber frame construction, assuming one square meter of wall with a specific U-value as the comparative unit. Similarly, all materials were compared to different traditional construction types with brick blocks and various insulation materials.

Bio-composites that use starch as a binder have a relatively high environmental impact, 82.89 kg CO2 eq. per m2 of wall. The most significant impact comes from the use of potato starch as a binder, as well as from the addition of glycerin and electricity required for the curing of bio-composites. To improve the environmental impact of this material, it would be possible to use a composition with reduced starch content, reduce the use of additives, or use starch from other raw materials.

Geopolymer bio-composites have slightly better results than starch bio-composites but are still relatively high, producing 68.15 kg CO2 eq. per square meter of wall. The main impact in the case of geopolymers comes from the use of metakaolin and the electricity required for curing. However, like with starch bio-composites, it is possible to improve the material's impact on the environment by reducing the use of additives, choosing alternative raw materials, and optimizing the curing process.

Gypsum bio-composites show the best results in terms of environmental impact, as they are capable of storing 13.94 kg CO2 eq. per square meter. This is mainly due to the low environmental impact of gypsum as a binder and the low drying temperature required. However, the environmental impact of gypsum can be further reduced by using phosphogypsum or recycled gypsum.

Results from this study show that all three developed bio-composites show a more negligible impact on the environment in the global warming impact category than traditional building materials. However, compared to standard timber structures, the only bio-composite with a lower impact on the environment is with gypsum binder. Geopolymer and starch binder bio-composites show a higher impact than standard timber structures, although it is worth noting that these bio-composites are produced on a small scale, which outputs a higher impact from energy consumption.