The European Union has adopted two regulatory packages moving towards a circular economy. The first of these packages was introduced in 2015 with the announcement of the Circular Economy Action Plan and addressed five priority sectors where changes would accelerate the transition to a circular economy [1
]. The changes and new regulations addressed plastics, food waste, critical raw materials, construction and demolition waste, biomass, and intermediates. In December 2019, the European Commission announced the European Green Deal, a development strategy to accelerate the transition to a circular economy [2
]. In general, a circular economy is defined as an economy “where the value of products, materials and resources is maintained in the economy for as long as possible and the generation of waste is minimized” [1
], which manifests itself as the efficient use of natural resources and the minimization or complete elimination of waste. To efficiently use materials, the key is to design a system in which a closed-loop process allows the minimization of material consumption while simultaneously allowing the economy to reuse or recycle said resources [3
Construction is one of the industries that generates a significant amount of waste, accounting for 25–30% by weight of total industrial waste [4
]. In countries of the European Union, the construction sector produces approximately 800 million tonnes of construction and demolition waste (CDW) annually [5
]. This is one third of the total amount of waste produced each year [6
]. Until now, sustainable disposal in the manufacturing process of structural and nonstructural concrete has been considered the best solution [7
]. In addition, the increased demand for construction materials [8
], the increased degree of industrialization [9
], and the depletion of natural resources [4
] force the further development of materials based on alternative raw materials. The use of waste materials to produce construction materials is a desirable development direction [10
], but further research is necessary to improve their properties.
A group of materials known for waste utilization is geopolymers. They are inorganic polymeric materials based on aluminosilicate precursors (such as fly ash, metakaolin, blast furnace slag, etc.) [11
] activated by alkaline activators (mainly sodium, potassium) [12
]. The final properties of the materials obtained depend on aluminosilicate precursors as well as the activator used, including their molar ratio [14
]. Given their strength properties, which are comparable to those of conventional concrete, and much smaller carbon footprint, they are drawing considerable attention among researchers [16
]. One of the directions of research on geopolymers is the search for alternative sources of aluminosilicate precursors. To advance the circular economy goals, materials with considerable potential are brick and concrete waste.
Şahin et al. in 2021 [17
] compiled most of the research performed on geopolymers based on construction and demolition waste (CDW); the conclusion was the possibility of using the materials in the production of geopolymers, although more research is needed to develop knowledge about this group of materials [17
Aldemir et al. [19
] conducted studies of geopolymer composites based on demolition construction waste. The geopolymer composites included clay brick, tile, hollow brick, concrete waste (rubble), and glass. As an alkaline activator for the geopolymerization of these materials, 8 M sodium hydroxide dissolved in water was used, to which other activators—calcium hydroxide and sodium silicate—were added after 6 h. Two types of geopolymer concrete were produced, including: (1) NGC—tile, red clay brick, hollow brick, glass, concrete waste, slag, fly ash, and natural aggregate and (2) NGC-R—tile, red clay brick, hollow brick, glass, concrete waste, slag, fly ash, and recycled aggregate. All CDW materials were crushed and milled. The compressive strength tests of the samples showed values of 37.5 MPa and 36.6 MPa, respectively. In turn, the tensile strength values were NGC—2.56 MPa and NGC-R—2.37 MPa [19
Komnitsas et al. [20
] investigated the compressive strength of concrete-based geopolymers, bricks, and demolition tiles according to the grain size of the raw materials. Compressive strength was tested 7 days after samples were produced. The demolition materials were activated with 8 and 10 M sodium hydroxide solution with sodium silicate and water. The hardening temperature was 80–100 °C. The best results were obtained for geopolymers based on ceramic tiles at 57.8 MPa and bricks at 49.5 MPa. In turn, geopolymerized concrete achieved 13 MPa after 7 days of aging. The study also showed that a smaller fraction of raw material in geopolymerization allows for higher compressive strength [20
Ilcan et al. [21
] investigated the rheological properties of geopolymers based on construction and demolition waste as they applied to 3D printing technology. Geopolymer composites were made on the basis of 80% clay precursors, which are a mixture of hollow brick, red brick, and roof tiles; 10% concrete waste; and 10% glass. The mixtures were activated with various combinations of sodium hydroxide, solution, calcium hydroxide, and sodium silicate. The best compressive strength, tested after 28 days, was obtained for samples activated with a solution consisting of three activators (NaOH, Ca(OH)2
)—36 MPa. The worst compressive strength was obtained for an alkaline activator based on a NaOH solution dissolved in tap water—11 MPa [21
D’Angelo et al. [22
] investigated the feasibility and potential of crushed brick waste (CWB) in the production of geopolymers to build precast components. Samples were obtained by curing a geopolymeric mixture at 60 °C for 3 days and aging for 28 days aging at room temperature. The results of the flexural and compression tests reached maximum values of 2.85 ± 0.73 MPa and 5.34 ± 0.66 MPa, respectively. The samples were deduced to have mechanical performance similar to that of gypsum produced from waste glass and ceramic waste. The conclusion was that CBW can be used successfully as a raw material in the construction of precast components [22
Youssef et al. [23
] examined the potential for reuse of waste brick (WB) by alkaline activation in a new geopolymer brick. Brick manufacturing was achieved by mixing WBs, ground granulate blast furnace slag (GGBFS), and sand with a solution of hydroxide and sodium silicate. The impact on properties based on variables was investigated, with the variables being the addition of GGBFS in different amounts, the molarity of sodium hydroxide (NaOH), and the silicate to sodium hydroxide ratio (Na2
/NaOH). A maximum compressive strength of 89.91 MPa was obtained for a GGBFS/WB ratio of 80/20, an 8 M NaOH molarity, and a silicate/hydroxide ratio of 2/1. Comparably, for a GGBFS/WB ratio of 0/100 and analogous conditions, the compressive and flexural strength reached 38.96 MPa and 7.30 MPa, respectively [23
In the literature, studies related to the development of geopolymers based on construction waste can be found, but to a much lesser extent than the commonly used aluminosilicates. This article uses research methods based on the analysis of the literature and the production of experimental geopolymer samples. Examples from world literature were analyzed in which the impact of replacing raw materials commonly used in the production of geopolymers with waste materials from building demolitions, replacing them completely in the mixtures produced, was analyzed. The authors of this article focused on the partial replacement of metakaolin and fly ash in geopolymers.
Despite previous research, still there is still a need for further research on the effective processing of CDW waste. It is worth mentioning that this type of waste requires recycling methods dedicated to particular geographical regions because of differences between used building materials, including raw material availability, building technology, and climate. The aim of this research is to use waste materials from building demolition to replace metakaolin and fly ash in geopolymer mixtures and to determine their mechanical and physical properties. To achieve the stated goal, ground clinker brick, concrete waste, metakaolin, fly ash, and a technical solution of sodium hydroxide with aqueous sodium silicate with a molarity of 10 M were used. The CDW from brick and concrete were mixed in GP to evaluate processing possibilities when the separation of waste is not possible. All the waste used came from south Poland. In the case of CDW, origin is quite important because of differences between the characteristics of the building materials used in different regions. The novelty aspect of the article is connected to the specific type of waste. This particular waste was not investigated before as a material for the geopolimerization process.
The results of prior research and the research carried out by the authors of this article prove the possibility of producing geopolymer materials using materials derived from the demolition waste of buildings and structures. A summary of the results of the exemplary literature can be found in Table 6
The results show the possibility of using construction and demolition waste (CDW) as materials in the production of geopolymers. The mechanical properties are in the range of geopolymer materials received also from other authors (Table 6
). In light of the research conducted, it is feasible to manufacture geopolymer composites using materials such as clay brick, concrete waste, roof tiles, or other elements of construction demolitions. The works provided demonstrate the potential of useful materials composed on industrial byproducts (fly ash) and CDW that provide environmental benefits [38
]. This approach is important from the point of view of implementation of circular economy, which emphasizes the essence of using recycling materials, saving natural resources [40
], and reducing the ecological impact of building materials [43
]. Furthermore, the research highlights the possibility of using mixed CDW for the manufacturing of geopolymer concrete. The potential advantages of this approach are stressed in the literature because the separation of CDW is an easy process [45
]. The literature indicates the prospect of manufacturing this kind of material with reduced environmental impact, but there are still practical uses of it for the construction industry [48
To reduce the impact of construction on the environment, it is necessary to search for new solutions and technologies that favor the development of a circular economy. The sustainable development of building materials is the main driving force behind research and application work in search of environmentally friendly products for use in construction. The production of geopolymers as materials to replace Portland cement leads to several environmental, economic, and social benefits: it reduces the amount of CO2
released into the atmosphere [17
]; allows the use of secondary raw materials, which reduces the use of natural resources; and lowers costs due to cheaper waste materials [50
]. Due to the growing public awareness of activities that aim to reduce the impact of all types of materials on the environment, the area of geopolymers is still being developed [51
]. An additional advantage of geopolymerization is the possibility of immobilizing various hazardous substances, thus securing waste landfills [52
It is worth stressing that the topic undertaken in the article is important for the development of a circular economy, especially in the area of closing the loops in material recycling. The results obtained show that the elimination of CDW and the continual safe use of natural resources (raw materials), including the reduction of the impact on the environment and resource deficiency, is possible by the use of geopolymerization technology. In particular, the results of the research could find practical applications in the building industry. The use of waste materials such as CDW allows one to limit environmental impact, improve material circularity, and thereby bring benefits to society.
Two types of geopolymer composites were created on the basis of both a natural raw material—metakaolin—as well as a secondary raw material—fly ash—and waste materials—clay brick, concrete waste. Research on concrete cements included the determination of the strength properties in terms of partial replacement of the most commonly used materials (metakaolin, fly ash, volcanic tuff, slag) for the production of geopolymers. The results obtained show the possibility of manufacturing useful construction materials based on industrial byproducts (fly ash) and CDW. Based on the analysis of the particular research results, the following conclusions can be drawn.
The densities of the materials were 1.76 g/cm3 and 1.81 g/cm3. These values are typical for solid geopolymer materials.
The mechanical properties of the composites obtained are reasonable and allow them to be applied in the construction industry. The compressive strength and flexural strength were 20.1 MPa and 5.3 MPa, respectively, for samples with metakaolin additive. Geopolymers containing fly ash achieved 19.7 MPa of compressive strength and 3.0 MPa of flexural strength. The values obtained for flexural strength are typical for geopolymers. The compressive strength is below the preliminary expectation but comparable to the data obtained from the literature.
SEM analysis provides useful insights into the mineralogy and microstructure of the produced geopolymers. This research shows the coherent and solid structure of the material obtained. The results of the EDS analysis are typical for geopolymer concrete.
To improve the properties of geopolymer concrete synthesized from byproducts and mixed CDW and next apply this type of composite to civil engineering, further research connected with other materials, such as fire resistance and other research, is necessary. In addition, optimization of material properties by applying desirable additives will be desired.