According to information published by the European Aggregate Association, the demand for European aggregates is 3 billion tonnes annually [1
]. About half of natural (virgin) material is consumed by the construction industry, which also generates a large amount of waste material [2
]. Undoubtedly, virgin material can partially be replaced by other materials, such as recycled industrial material, including material made from paper industry waste. This recycled waste can be substituted for virgin aggregates that are used in various applications in the building sector in huge quantities, especially for roads and earthworks [3
]. Of course, the mechanical and environment criteria for recycled materials according to the national legislation must be satisfied.
Globally, 420 million tonnes of paper and paperboard are produced annually [4
], and production is growing. The production processes result in significant waste generation; 11 million tonnes of solid waste are generated per year in Europe [5
]. Approximately 70% of this waste is from paper recycling, for example, deinking sludge [6
]. According to the Integrated Pollution Prevention and Control Directive 1996/61/CE [7
], the paper industry is required to minimize the amount of waste and develop more sustainable technologies for waste treatment. There are also EU waste management legislative measures and policies [8
] implementing a waste hierarchy, with landfilling being the least desirable option and recycling the most, supported by increased taxes for landfilling. Recycled paper residues are a potential material that could be substituted for virgin raw materials from a technical and economical point of view [9
Examples of pulp and paper industry residue implementation have been presented by other authors [11
], but in general, most paper industry waste is burned in power plant boilers or landfilled. The production process with different fillers, pigments, and coagulates influences the type of paper ash. Also, the technology and temperature in the boilers have an effect [15
If paper sludge ash were to be used only as a binder in the construction industry, some problems due to the presence of lime would be observed, but it could be very useful for the stabilization of road structures or as a backfill material [17
]. Different mixtures of paper ash and paper sludge have been tested on a laboratory scale. A mixture of sand, paper fly ash, paper sludge, and cement has been used in laboratory research [18
]. The mixture reached a compressive strength of 0.8 MPa, which is high enough for use as a backfill material for a foundation structure, a structural fill, or a hydraulically bound layer in a road structure. If the paper sludge ash is mixed with recycled concrete aggregate (RCA), the mechanical properties are improved, especially the resistance to acid and sulfate attacks [19
]. Highly plastic clay soil was stabilized with paper sludge ash [20
] and the compressive strength increased enough (0.7, 1 MPa) for the mixture to be used without any other additives for a pozzolanic reaction. For mining backfill material, a mixture of paper sludge ash and sewage sludge ash [23
] was prepared. Both materials were mixed and calcinated at high temperatures. In addition to paper sludge ash, paper sludge can be used. Paper sludge was used with marine-washed sand, aggregate, and Portland cement [24
]. With this mixture, a compressive strength of 8 MPa was achieved. Remediation of contaminated soil by red mud was used with paper ash as a binder material [25
Most of the research is related to laboratory tests, but some field results have also been published. A road subgrade was stabilized in a length of 250 m with a mixture of paper sludge and cement in Portugal [26
]. The installed mixture achieved an unconfined compressive strength of 4.5 MPa. Paper fly ash was used for gravel road stabilization of a hydraulically bound layer [27
] in Spain. The hydraulically bound layer reached an unconfined compressive strength of 5 MPa.
Paper sludge ash is also used in the cement industry as supplementary cementitious material in mortar [28
], concrete manufacturing [30
], and the brick industry [32
When using ash as a building material, particular attention should be paid to the impact on the environment. Studies have demonstrated the wide applicability of ash, but it is necessary to carefully investigate the potential environmental impact and use technology that is appropriate for individual recycled materials [33
Investors and designers find it difficult to decide to use recycled material in construction due to a lack of knowledge about the material, technology of installation, high cost of production, and often a negative attitude towards all new materials [1
]. The objective of this research is to develop a new backfill material (composite) used behind retaining walls from the residue of paper industry production and promote the use of the recycled material with a pilot structure. Especially in mountainous regions, landslides represent a threat to roads and railways, which must be reduced by slope stabilization with different retaining walls. A new composite must have high enough unconfined compressive strength and shear properties, but at the same time has to allow elastic deformation before cracking. Until now, paper sludge ash and deinking sludge have been used in different mixtures, usually in mixtures with soil and other binders. At present, deinking sludge ash and deinking sludge are mixed together as a new composite in a precise ratio and compacted under strict conditions behind a retaining wall. This type of composite has not been tested in the laboratory so far, nor has its use been validated in field tests.
None of the studies to date have dealt with changing the strength characteristics of the material during mixing and installation. Here, the time of transport of the material from the place of mixing to installation is crucial. In the study, we found that, over time, the strength properties decrease significantly, which may be crucial for the stability of the retaining wall. The study notes that the materials in the laboratory must also be tested in terms of installation time in order to provide the designer with relevant data regarding the geomechanical characteristics of the composite.
The pilot retaining wall structure promotes the concept of a circular economy from idea to laboratory tests, installing the structure, and monitoring it over a long period of time. The new composite and the technology were tested at a construction site and then monitored over a longer period of time. This gives us information about the details of construction and proves that the structure is stable, usable, and meets all the technical and environmental standards available to investors, designers, and contractors. Pilot structures could help suppliers, investors, designers, and contractors identify the factors hampering the use of recycled materials in the construction sector as well as provide strategies that can be adopted to form an economical and sustainable product.
A new composite from paper residues was developed as a backfill material for retaining wall structures. Preliminary tests were performed in an accredited geomechanical laboratory, and the results were later verified with field and laboratory measurements on a pilot structure.
A composite has to have geomechanical properties that are high enough to be used as a backfill material behind a retaining wall structure in an unstable area near a railway line threatened by landslides. Several mixtures with different contents of DS and DSA were initially tested to choose the optimal composite with the proper geomechanical properties. Two of them were studied in detail within the present research to estimate the effect of their composition upon particular basic characteristics.
No studies to date have dealt with the changing strength characteristics of material during mixing and installation. The time of transport is an important parameter for using this mixture as a backfill material. Results from the laboratory showed that the mixture has to be moistened above the wopt before being transported and used at the construction site within 4 h.
Based on the laboratory results, it was decided to use composite D70/30 as the backfill material for the retaining wall. The composite is a new mixture that has not yet been tested in a laboratory or installed in the retaining wall structure at the construction site. The composite with 30% DS had a high enough qu and shear strength but allowed small deformations before failure. The high shear characteristics of the composite allowed for a slimmer retaining structure to support the unstable slope behind it. At the same time, the DS in the composite enabled ductile behavior of the structure and prevented it from brittle failure.
In 2018, the composite D70/30 was used as a backfill material of the retaining wall structure built by gabions in the south part of Slovenia, near the railway line, for landslide stabilization. All laboratory and field tests confirmed the physical characteristics measured in the research phase and confirmed that the environmental requirements are being reached.
At the construction site, the material was installed in 30 cm layers. Each layer was compacted and controlled to reach the optimal moisture and maximum density. Before, during, and after the construction, landslide stability was assessed, and environmental monitoring was performed. With the test results from the pilot structure presented in this paper, the technology of mixing and compacting was improved. Mixing is usually not a problem in the laboratory, but at a construction site, a large quantity of material has to be mixed with the proper technology. The monitoring system confirmed that the retaining wall with the composite stabilizes the landslide near the railway. The composite was an impermeable material, and precipitation did not influence its stability. As the area covered with such an impermeable layer was not very wide, the impact on the groundwater recharge was limited. However, one should consider such a structure as a groundwater barrier that disturbs shallow groundwater flows. Thus, the use of this kind of structure should be combined with a properly designed drainage system, which minimizes the effect of groundwater disturbance. In the case of the retaining structure presented within this paper, vertical and horizontal drainage were employed to enable effective drainage of water behind the retaining structure.
On the other hand, the low density of the composite from paper residues also has several advantages. There is great potential for its use in construction on soft ground. In such a case, the utilization of very light materials, like the new composite, could prevent large settlement.
The pilot retaining wall structure represents a practical case for future investors, designers, and construction companies to encourage them to use recycled materials from the paper industry instead of virgin materials. The new composite and its installation technology were successfully tested. The presented circular case between the paper industry and the construction sector shows the advantages for both sides. Instead of disposing of the waste in a landfill for a very high price (in Slovenia, 80–150 €/tonne), the paper manufacturer processes the waste into a composite for a retaining wall structure. Contractors, meanwhile, get a composite that is cheap and has better deformation properties than the virgin material.