Main characteristics of scientific production
Types of publications and evolution of scientific production
The studies included in figure 2 correspond to articles (69%), reviews (22%) and book chapters (9%). The first two articles, "New Building Materials from Industrial and Agricultural Wastes" and "Low Cost Building Materials Using Industrial and Agricultural Wastes", were published in 1978 [42, 43]. The last article was published in December 2021 "Development of White Brick Fuel Cell Using Rice Husk Ash Agricultural Waste for Sustainable Power Generation: A Novel Approach" . A significant increase in publications is evident from 2016 onwards. This trend is similar to that of another study on the application of agricultural waste ash in cement, which showed an increase in the number of publications since 2016 .
Of the studies analysed, 74 percent were published between 2017 and 2021. These results coincide with those of similar studies, which indicate that in the last five years, there has been an increase in the scientific study on the use of AWB in concrete [18, 22]. This leads to the conclusion that the policies and strategies on sustainable development, which have become more relevant since 2015 with the 2030 Agenda, have boosted scientific research on this subject. Furthermore, it supports previous research findings that highlight the important role of the SDGs and circular economy and bioeconomy strategies on the increase of publications on AWB valorisation in the last five years [9, 45].
Publications by country
India is the country with the highest number of publications (22%), followed by Malaysia (13%) and Egypt (10%). The countries shown in figure 3 published 84% of the publications in the sample. Other studies rank India as the country with the third-highest number of publications on agro-waste in concrete production [18, 22]. These results align with a previous study that analysed AWB valorisation alternatives and identified India as one of the countries with the highest scientific production on the subject . China and India are two of the world's leading producers and consumers of cement. India's cement production increased significantly in 2017 compared to 2016 . In addition, these two countries have focused their bioeconomy policies on industrial and high-tech innovation .
In Malaysia, the construction sector is of great importance and has seen significant growth in recent years [46, 47]. Similarly, the regulations governing the construction industry in this country promote sustainable and environmentally friendly construction [48, 49]. Furthermore, the government prioritises this type of construction in its “Green Public Procurement (GPP)” guidelines . For example, it encourages the use of organic fiber in the cladding materials of public buildings . This could be a reason for the particular interest of these countries in the use of AWB as a sustainable raw material for the construction sector.
The co-occurrence of the keywords network was made using all keywords as analysis unit (author keywords and index keywords). Terms that are part of the search equation were excluded from the VOSviewer keyword list. The keyword network (figure 4) groups 41 terms in 5 clusters. Cluster 1 (red) is the central cluster with 13 items. The most relevant terms in this cluster are “compressive strength”, “water absorption”, “mechanical properties”, “thermal conductivity”, and “thermal insulation”. These descriptors reflect the importance given by the studies to the analysis of the physical, mechanical and thermal properties of AWB and the bio-based products obtained from its use. This cluster also includes "rice husk ash" as the main type of by-product obtained from rice husks’ incineration and widely used to manufacture sustainable building materials [51–53].
India is one of the world's leading producers and consumers of rice [23, 54], which also explains the country's interest in valorising this type of AWB. Cluster 2 (green) groups 11 items, mainly related to biomaterials and bio-based products obtained from AWB. For example, "supplementary cementitious mat", "cements", "binders", and "concrete". Similarly, this cluster highlights the term "silica" as one of the main components of AWB ashes and is of particular interest in manufacturing bio-based products [22, 29, 55, 56].
Cluster 3 (blue) consists of 8 items, including "sustainable development", "sustainability", "recycling", "waste disposal" and "developing countries". These descriptors are associated with the main benefits that can be derived from the use of AWB in the construction sector. Cluster 4 (yellow), with eight items, integrates the word "straw" as another main type of AWB used in the construction sector, mainly in the form of "fibers" because of its high "cellulose" content, which makes it an ideal by-product for the “reinforcement” of building materials [57–59]. The keyword "bagasse" forms cluster 5 (purple). This term refers to the by-product of the extraction of sugar cane juice . The ash obtained from the burning of this sugar cane bagasse has important qualities that improve the properties of different building materials [52, 53, 56]. India is the second-largest producer of sugar cane after Brazil . This also suggests a correlation with the number of studies corresponding to this country.
Main approaches to scientific production - Systematic review
Type of agricultural waste biomass used and main form of by-products
Figure 5 shows the main types of AWBs assessed in the investigations. A total of 32 AWB types were identified. Forty-eight percent of the studies used rice husk, and thirty-four percent used sugar cane bagasse. These are the two main AWBs generated in the highest volumes worldwide [23, 52]. In the investigations, straw from cereal crops was the third most analysed type of AWB (28%). The main type of cereal crop from which the straw is derived is wheat (65%), followed by rice (50%). To a lesser extent, straw from sugar cane, barley, sorghum, rye and oats was studied. Another study analysing approaches and alternatives for AWB utilisation identified cereal straw as the most relevant, mainly from wheat and maize .
Coconut husk is also one of the main types of AWB used in the research (24%). To a lesser extent, maize cob (20%) and oil palm (18%) and maize shells (6%) were identified as relevant. Other types of AWB that were considered as raw material in the investigations were: cork, banana and pineapple leaves and/or peels, stalks of maize, soybean and cotton crops, olive pomace and/or olive oil mill residues, coffee husks, nuts and cassava, and grape sprouts and seeds. As can be seen, the vast majority of crop parts (leaves, stalks, fruits, seeds, sprouts) are used to manufacture new bio-based products. Contrary to , it is evident that more than half of the studies in the sample used more than one type of AWB [14, 26, 60–63]. This was possibly to improve the properties of the final products .
Rice husks and straw, wheat straw, maize stalks and cob, and coconut husks were the main types of AWB used as raw materials in the first investigations in 1978 [42, 43]. Regarding the main form in which this type of AWB is used, it was found that more than half of the studies in the sample (55%) used it in form of ash. Twenty-nine percent used this biomass in the form of fiber. Other forms identified were granules and/or small particulate shredded material (20%). Most of the studies analysed the physical, chemical, thermal and other properties of the by-products (ash, fibers and AWB particles). This was mainly to characterise them, evaluate their potential, determine the best alternatives for their use and/or define optimum substitution percentages [29, 57, 58, 65].
The main parameters evaluated were; specific gravity, surface area, bulk density, particle size and fineness, water absorption, porosity, microstructure, and thickness [6, 18, 66–68, 22, 24, 28, 34, 36–38, 63]. Concerning chemical properties, they analysed the cellulose, hemicellulose, and lignin content. Similarly, percentages of Loss on Ignition (LOI) and chemical components such as; SiO2, Al₂O₃, CaO, Fe₂O₃, Na₂O, P2O5, MgO, MnO, K2O [7, 18, 37, 38, 52, 54, 55, 57, 58, 61, 63, 69, 24, 70–79, 26, 80–82, 27–30, 35, 36]. Other main properties evaluated were thermal conductivity, microstructure and sound absorption [6, 18, 83, 22, 28, 31, 32, 37, 61, 66, 75].
Figure 6 shows the five categories into which the biomaterials obtained from AWB were grouped according to their use. Fifty-one percent of the research focuses on obtaining bindings, aggregates and/or additives in soil, cement and/or concrete. Supplementary Cementitious Materials (SCMs) are the most studied in this first category. Seventy percent of the research evaluated the potential use of AWBs as an alternative material for the total and/or partial replacement of cement in concrete [7, 15, 35, 36, 42, 52, 54, 56, 60, 63, 65, 69, 18, 72–76, 84–88, 22, 89, 23, 24, 27–29, 34]. The relevance of this type of biomaterials is also reflected in their evolution. From 1978 to the present day, they have been studied as an alternative for the valorisation of AWB in the construction sector. One of the first biomaterials obtained from agricultural waste by the Central Building Research Institute in India in 1978 were pozzolanic reaction additives for cement production. Contrary to  and in line with the findings of , there is a large and long-standing field of research in AWB-based SCMs for use in concrete production.
In this same category, 24% of the studies in the period 2004 to 2021 used AWBs as a total and/or partial substitute material for conventional fine and/or coarse aggregates in concrete [25, 31, 93, 37, 38, 67, 68, 84, 90–92]. A smaller percentage (3%), between 2019 and 2021, analysed the potential of AWBs as additives and/or aggregates for soil stabilisation and/or improvement of geotechnical properties [70, 71, 94]. Other studies used a similar classification for the categories of use of agricultural residues in concrete [15, 67]. The second category (figure 6) integrates the studies (22%) that evaluated the use of AWBs as biomaterials for brick production. According to the number of studies and the publication period (1978-2021), this is the second most common type of AWB used in the construction sector.
The third category (20%) integrates studies that evaluated materials and/or bio-composites for specific use in structural and/or reinforcement applications. The composite materials mentioned in most of the studies in this category are made from a mixture of plastic polymers and natural fibers . Ten percent of the research in the period 2013 to 2021 analysed AWBs as specific materials for thermal and/or acoustic insulation of buildings (category 4). A smaller percentage (4%), category 5, grouped more recent studies (2016-2021). These studies evaluated the potential use of AWBs as a full and/or partial substitute for traditional asphalt binders and/or aggregates used in road construction. In 1978, agricultural residues were also used as substitutes for hydraulic components in bricks and other products such as boards and panels without synthetic binders. Some of these biomaterials were incorporated into walls and shade roofs for livestock [42, 43].
Main Bio-based products
Table 1 summarises the main bio-based products evaluated by the sample studies. These bio-based products were manufactured from mixtures of the above biomaterials with other types of waste and/or materials. For example; bamboo fiber and leaves [22, 36, 61, 76], bauxite process wastes , sheep wool , fluid catalytic cracking residue, ceramic sanitary ware, waste from beer filtration , construction demolition waste (C&DW) [53, 70], granulated tires [14, 95], glass powder/fiber [63, 96], sawdust [15, 29, 38, 42, 55], wood [27, 29, 36, 86, 87], sunflower stalks and seed [91, 97], egg shell powder , cow dung , reclaimed asphalt pavement, reclaimed asphalt shingles , water treatment plant sludge [80, 98], and recycled plastics .
Materials traditionally used in the construction sector, such as lime, sand, and Portland Cement, were also used to prepare the mixtures from which the bio-based products in Table 1 were obtained [7, 52, 70–72, 75, 90, 91]. This extensive listing of each category highlights the wide variety of bio-based products in which AWBs can be used. Furthermore, the diversity of industrial and/or other wastes with which they can be combined further enhances the overall waste valorisation.
Most of these bio-based products have traditionally been used in the construction sector, especially in categories 1 and 2. However, it is noticeable how the terminology has evolved in recent years. The most recent research refers to agro-concrete/cement, agro/bricks, sustainable cement/concrete, sustainable bricks, green concrete, ecological concrete, eco-friendly bricks, thermally efficient bricks, zero cement concrete, sustainable bio-modified asphalt, sustainable green highways [18, 28, 52, 56, 64, 68, 78, 91, 101]. These new concepts align with recent policy approaches on sustainable development and, more specifically, with the circular economy and bioeconomy [5, 12].
In addition, novel uses and futuristic bio-based products for the construction sector are evident. For example, bricks made from rice husk ash can be used as an alternative sustainable energy source . Category 3 integrates an extensive and varied list of bio-based products, demonstrating the potential of AWB for use as a biopolymer in various applications in the construction sector . Furthermore, 78 percent of the studies in this category have been published in the last five years, confirming the important evolution and relevance of bio-composites for structural and/or reinforcement applications. Bio-based products for buildings' thermal and/or acoustic insulation have mainly been analysed in the last four years.
Bio-based products in category 5 are entirely novel. A few recent, related pieces of research show that it is an emerging approach. This is also an indicator of important new alternatives that may emerge for the valorisation of AWBs as high value-added biomaterials. In this category are also terms from new models of sustainable development, such as "Sustainable bio-modified asphalt" . Furthermore, they refer to using these bio-based products to construct sustainable greenways . Other studies identified biomaterials and/or bio-based products similar to those in table 1, mainly from categories 1 to 4 [15, 31]. However, none of them includes those used for road construction.
The influence of AWBs on the properties of bio-based products depends on their characteristics, the processes they undergo and the proportions in which they are mixed with other materials, among other aspects [22, 92]. Therefore, most of the studies in the sample analysed considers physical, chemical, mechanical and other properties of the bio-based products. This was to identify the effects of AWBs and determine compliance with the standards of the regulations governing the quality of building materials . Table 2 summarises the main parameters evaluated for each of the properties.
In line with the findings of other studies, it was identified that the main tests bio-based products were subjected to are compressive strength, density and water absorption [6, 35, 101]. Compliance with these parameters is essential to guarantee the obtained bio-based products' quality, longevity, and durability [18, 38, 101–103].
Main types and properties of AWBs used by category
Rice husk and sugarcane bagasse are two of the main types of AWB used as biomaterials for the production of cement, concrete and bricks. The main form in which this biomass is used is ash, followed by a smaller percentage of fiber. These findings are in line with other researchers that highlighted rice husk ash and sugar cane bagasse as the most studied alternative materials used in building materials, mainly in the concrete industry [18, 22, 24, 28, 31, 38, 87]. Table 3 summarises the three main types of AWBs that are used for each category. It also highlights the most relevant aspects that impact the improvement of the properties of the bio-based products obtained.
A study analysing the use of industrial and agricultural wastes in cement identified general properties and applications of AWBs similar to those of category 1 . Of the three types of AWB prioritised for each category according to the number of studies that have used them, it is evident that rice husk ash is the most versatile type of waste that can be used in all applications in each category. One of the first researches from 1978 highlights rice husk as a highly reactive pozzolanic material, which makes it a good alternative for producing new cementitious materials .
Several studies indicate that the main component of agricultural residue ash is silica [22, 29, 55]. However, some of them point out that silica in rice husk ash and bagasse ash is significantly higher (between 60-95%) [18, 34, 51, 56, 87, 92]. In turn, the pozzolanic nature of these ashes  makes them efficient biomaterials for improving the physical, mechanical and thermal properties of bio-based products. Among them: strength, durability, workability, porosity, thermal conductivity, and other properties that are highlighted for each type of biomaterials in table 3.
Cereal straw fibres have significant use in the production of bricks and biomaterials used as fillers in polymeric matrices for structural reinforcement (increased strength and stiffness) and thermal and/or acoustic insulation [57–59, 83, 96, 100, 108, 110–112]. Fiber length and width affect bio-based products' physical, mechanical, and thermal properties [58, 110, 112]. In the manufacture of bio-composites, these fibres replace all or part of the wood [58, 111]. Coconut husks are another by-product with great potential due to their high natural fiber content . A study identified coconut and rice husks as one of the main AWBs used to produce biopolymers for construction applications . In line with the findings of other studies, it is evident that fibrous AWBs such as rice husks, cereal straw and bagasse are suitable for thermal and acoustic insulation applications [31, 39, 66].
Some of the research in the sample does not specifically highlight additional positive effects on bio-based products resulting from AWBs. However, most of them confirm that the use of AWBs to manufacture bio-based products is technically feasible from a technical point of view. Mainly because these bio-based products meet the properties and performance requirements for building materials. In addition, they are similar to commercial products and conform to regulatory specifications [6, 18, 28, 37, 67–69, 76, 80]. On the other hand, concerning the properties of bio-based products, most studies emphasise that a generalised substitution of AWBs is not possible. Therefore, it is essential to achieve mixture compositions with optimal AWB substitution percentages [22, 51, 56, 65, 87, 106].
To improve the properties and/or guarantee the performance of most of the bio-based products in table 1, studies suggest substitution percentages between 5 and 15% by weight of AWBs [7, 22, 67, 70, 78, 90, 101, 104, 116, 26, 28, 29, 31, 35, 52, 53, 55]. These findings align with a study that identified similar percentages, between 5 and 10% AWB for fired clay bricks . Several studies in the sample identified that higher substitution levels could generate counter (negative) effects on bio-based products' properties and/or performance [29, 35, 55, 68]. However, higher percentages of AWB (30-50% of oil palm shell) were suggested for the construction of roads with medium and/or low traffic . Also, in other applications used to improve the thermal performance of buildings (21-63%) [14, 33]. On the other hand, lower AWB percentages (2-4%) were considered for soil stabilisation [71, 94].
Main limitations and/or disadvantages of the use of AWB in bio-based products
As indicated in table 4, several of the studies analysed point out some limitations and/or disadvantages derived from the use of AWBs as biomaterials for the production of bio-based products. Increased water absorption, reduced workability, and increased loss on ignition are some of the main disadvantages identified [15, 55, 92, 114]. Some of the limitations and/or disadvantages in table 4 coincide with those identified by other studies that analysed them for specific applications, such as thermal and acoustic insulation [21, 32].
On the other hand, most of the studies in table 4 also identified mechanisms to avoid and/or reduce the negative effects of AWBs on bio-based products. Applying appropriate processing methods including both pre-and/or additional treatment processes is crucial. These include immersion of fibers and/or ashes in chemicals, cooking, drying, filtering and/or screening [18, 22, 91, 107, 110, 112, 114]. Some research has also suggested the incorporation of other types of materials and/or micro-organisms [29, 35, 73]. In line with the approaches of , the findings of this study confirm that the indicated pretreatments and/or additional processes are necessary under certain circumstances and/or for certain types of biomaterials or bio-based products. A wide variety of biomaterials and bio-based products (table 1) can be obtained through direct utilisation or minimal transformation processes.
These identified corrective actions and/or improvement alternatives (table 4) reinforce the approach on the feasibility of using AWB as a biomaterial for the production of bio-based products. However, it is important to consider that some of the studies in the sample suggest that further research is needed to improve the utilisation of AWB, understand its influence on bio-based products and ensure its application in large-scale structures [18, 22, 34, 35, 38, 68, 74, 96].
Feasibility of the use of bio-based products
Table 5 shows the main aspects highlighted by the sample studies for each dimension of sustainability. The environmental dimension is the most relevant. More than sixty percent of the analysed studies highlight, in a general and/or specific way, contributions towards the improvement of the environment derived from the use of AWBs in the construction sector. Positive impacts on air, soil and water resources are comprehensively listed in table 5. Among the main benefits mentioned are reduction of carbon dioxide emissions and global warming [34, 35, 54, 56, 69, 84, 87, 91, 105]. Similarly, aspects related to the efficient management of AWB such as volume reduction, recycling, recovery, reduction of landfills and open burning [6, 22, 64, 78, 86, 102]. Reducing energy consumption and increasing energy efficiency is another environmental highlight [6, 14, 57, 78, 105]. On the other hand, some of the studies refer to the green economy and the circular economy as basic strategies, which are also supported and/or promoted by the valorisation of AWB in the building sector [14, 36, 76, 87].
Secondly, economic aspects were also analysed in the research. Some studies carried out profitability analyses and/or technical/economic feasibility studies [28, 74, 82, 107]. These analyses indicate that the manufacture of bio-based products is cost-effective and they can be suitable products to compete in the market. Furthermore, their use reduces the cost of construction. A key aspect they highlight to make costs feasible is using local AWB to reduce pre-treatment and/or transport costs [28, 74]. This aspect contributes to the socio-economic viability of bio-based products . Although they do not perform this analysis, other studies highlight that bio-based products made from AWB help the obtainment of economic benefits derived from reducing the cost of biomaterials, structures and/or construction in general [15, 64, 78, 86, 101, 102].
The findings for this economic dimension confirm the theorisation of a study that analysed the use of AWB in concrete production . This concerns the low number of studies that have conducted economic analyses to estimate the costs of producing bio-based products from AWBs and determine their feasibility. However, this study adds new evidence that this type of analysis has been conducted for various AWBs. For example, in addition to rice husk ash, as indicated by , costs have been evaluated for the use of coconut husk , sugarcane bagasse  and pineapple leaf fibers . However, given the large variety of AWB types identified (32 types); it is evident that a low percentage has been evaluated from an economic point of view. This could be one of the key factors necessary to boost bio-based products' application and commercialisation. So far, as derived from the findings of this study have mainly been developed at an experimental level. One study points out that some bio-based products used for insulation are in their early stages of development and will still have a long way to go before reaching the market .
A smaller percentage of the publications (11%) highlight some aspects related to the social dimension. Mainly, the reduction of housing and infrastructure costs in rural areas and/or developing countries [87, 93, 100, 115], the creation of new jobs [32, 105], among other aspects associated with the health and well-being of the population. One of the publications from 1978 already pointed out that the use of AWB in the manufacture of building materials contributed to solving waste disposal problems and reducing the costs of transporting materials. It also generated savings in production costs and energy consumption . This broad analysis of contributions for all dimensions reinforces the theorisation of this first publication. It indicates the relevance that the integration of environmental and social aspects as pillars of sustainable development has gained in research. However, it is essential that such studies include broader feasibility analyses, integrating all dimensions (economic, social and environmental) from local contexts.
Furthermore, these findings coincide with the findings of other studies regarding the potential of AWBs for the production of bio-based products and their contribution to sustainability [31, 54, 105]. Although this suggests positive aspects in advancing the application of bio-based products on an industrial scale, it is important to bear in mind some considerations. For example, identifying and quantifying locally available agricultural residues is key. This is to ensure that there are no supply constraints [18, 32, 76].
European Union projects
Table 6 summarises the projects related to the valorisation of AWB in the construction sector. Of these projects, 67% were financed by the first EU framework programme for research and innovation - Horizon 2020. The oldest project, dating back to 1994, was carried out with the "Research and Technological Development in the Field of Industrial and Materials Technologies FP3-CRAFT (1990-1994)" programme. This project used straw and husks to obtain mineral binders . The second older project carried out between 2004 and 2007 was part of the EU's "Focusing and Integrating Community Research" programme (2002-2006). Rice straw was also one of the main inputs for this project, which produced composites for structural components as a bio-based product . The SYNPOL project, which produced biopolymers from rice straw, was part of the FP7-KBBE programme "Cooperation: Food, Agriculture and Biotechnology" .
In general, straw is the most commonly used type of AWB for producing bio-based products such as cement, wooden boards, thermoplastic adhesive, biopolymers and composites. However, in line with the findings of the research analysed, it is evident that these projects have used different types of AWB in recent years, which confirms their potential for obtaining high added-value products. All the applications of AWBs prioritised by the projects are similar to those identified in the research analysed. The focus on the production of biopolymers and bio-composites confirms that this type of AWB application has become more relevant in the construction sector in recent years. This makes sense considering that it has been one of the fastest evolving areas of the bioeconomy .
Other projects identified in the CORDIS database obtained eco-sustainable concretes from other types of waste such as plastic, electrical and electronic equipment, municipal solid waste and pneumatic components [120, 121]. They also made ceramics from the sludge from wastewater treatment . Most of the projects in table 6 address all three pillars of sustainability. They emphasise that from an environmental point of view, there are benefits associated with improved energy consumption, waste reduction and reduction of CO2 emissions [117, 123, 124]. Similarly, the creation of new jobs in the bio-based products sector and the improvement of industrial competitiveness . Economic benefits are derived from the reduced costs of new bio-based products . This is in line with the approaches of the European bioeconomy strategy .
Besides the manufacturing of bio-based products, some of the projects in table 6 include market potential analysis, marketing improvement strategies, and other activities aimed at improving the information on technical aspects of bio-based products to boost and/or enhance their market share [123–125]. The opening of bio-based markets through educational tools and campaigns aimed at improving knowledge and increasing public acceptance of bio-based products was also one of the objectives of the projects [126, 127]. This is a crucial aspect in advancing the application of bio-based products on an industrial scale.
Furthermore, the EU encourages synergies between European and Indian research programmes dedicated to biowaste conversion and biomass production through such projects . Sixty-seven percent of the projects were implemented in 2016-2021, indicating that the Horizon 2020 programme (2014-2020) prioritised the deployment of projects focused on bio-based alternatives to improve the sustainability of building products. This is to some extent because this research and innovation framework programme is the primary source of funding for the bioeconomy in Europe . Some of the projects are specifically framed within the action line "Societal challenges" of the Horizon 2020 programme, which prioritises sustainable agriculture and the bioeconomy. In general, these projects highlight the contributions of the bio-based construction sector to the consolidation of the European circular economy and bioeconomy.
These two models have become essential axes for sustainable development in Europe. For 2021-2024, the EU has included the bioeconomy among the vital strategic orientations for research and innovation (Horizon Europe) . This represents an opportunity to further strengthen the bio-based construction sector, especially in the production of bio-based products from AWB and improve the market's functioning for these products . The circular approach of the bioeconomy has demanded new lines of research on biomass valorisation alternatives in the construction sector. This has increased the number of studies and projects on this subject in the last five years. The resources that have been allocated through projects have been and will continue to be vital to the application and consolidation of the circular economy and bioeconomy as the primary strategy for sustainable development.