1. Introduction
The linear production model has accentuated the problem of resource scarcity, greenhouse gases emission and waste generation [
1]. Hence, the need emerged for a circular economy model, where waste is reintroduced as a new alternative resource in production processes [
1,
2].
The construction related sectors are also responsible for the social and economic development of society; however, this growth has been at the expense of high environmental impacts, due to the use of large amounts of natural resources, namely energy, water, soil, raw materials and other materials [
3,
4,
5]. To achieve a higher level of sustainability and to increase the sector’s contribution to a circular economy, these sectors have sought, among other solutions, new raw materials and efficient ways to process them to combat the depletion of natural resources. A possible strategy is to incorporate wastes or by-products from the sector itself or from other industries but keeping the same technical performance as the traditionally used natural raw materials [
6,
7,
8].
Concrete based products are widely used in the world as construction materials. This particular sector needs large amounts of resources (aggregates, binder, water and additives), which have to be sustainably obtained [
8,
9]. One must prevent soil erosion and ecosystem destruction resulting from large-scale extraction of natural raw materials such as the case of its major component (aggregates). In addition, the production of the traditionally used binder (cement) has been associated with high environmental impacts, due to the need for large amounts of energy to be spent in the manufacturing of clinker [
8,
10].
In addition to the construction sectors, there are other industrial sectors such as the pulp and paper sector that also consume large amounts of raw materials, including wood and various chemical products, or other resources such as water and energy. A substantial volume of solid wastes is also generated in the pulp and paper industry, including lime mud or lime ash, lime slaker grits, green liquor dregs, boiler ash, fly ash, primary and secondary sludges, which are mostly disposed in landfills [
11,
12,
13,
14,
15,
16]. The European pulp and paper industry produces around 38 Mton of air-dried pulp, representing approximately a quarter of the value of world pulp production [
13]. Quina et al. [
13] indicates an average production value of this lime waste of about 15 kg of lime ash per ton of air-dried pulp. Considering these production values, the pulp and paper industry thus generates a significant annual amount of this waste.
Lime ash result from the causticizing process and consist mainly of calcium carbonate (CaCO
3) in its chemical composition [
17], sharing similar features with the limestone filler used in construction materials [
18]. Thus, several researchers have studied solutions for reusing the lime-based wastes in construction materials. For example, Modolo et al. [
19] assessed this waste incorporation in cement mortars while others [
18,
20] have studied the production of clinker with fly ash, lime mud and biological sludge. Madrid et al. [
21] have also tried using lime mud and sawdust in the production of concrete blocks with improved thermal properties. The incorporation of lime mud and fly ash in the anorthite production for the ceramic industry has also been successfully tested [
17]. Mymrin et al. [
22] studied the production of composites using the wastes from the pulp and paper industry, including dregs, grits and lime mud. Jia et al. [
23] have studied the sealing of mine tailing landfills with wastes from the pulp and paper industry (dregs, fly ash and lime mud).
After validating the technical performance, the environmental performance was also assessed using the life cycle assessment method ((e-)LCA) [
8], in order to identify the benefits and trade-offs of the best solution, following ISO standards (from ISO 14040 to ISO14044) [
24]. Some authors have also analysed social and economic impacts using the social (S-LCA) and economic (LCC) life cycle assessment method, respectively [
12,
25,
26,
27].
Regarding the concrete products sector, several researchers [
4,
8,
24] have carried out life cycle assessments to compare different products. For example, Colangelo et al. [
4] analysed four concrete mixes containing different wastes, including construction and demolition waste (CDW), incinerator ashes, marble sludge and blast furnace wastes. They found out that the concrete with wastes was more sustainable than the plain concrete. Turk et al. [
8] evaluated three mixtures of concrete with foundry sand steel slag and fly ash and concluded that the waste-added concrete had a beneficial scenario for the concrete industry. Domagoj et al. [
24] evaluated the use of sewage sludge ash (SSA) in the production of concrete as a partial substitute of cement and found that incorporation of up to 10% did not affect technical performance and, at the same time, reduced the impact of the global warming potential (GWP).
At the level of production of aggregates for the concrete sector, Estanqueiro et al. [
3] carried out a comparative life cycle assessment between natural and recycled aggregates and demonstrated that the scenarios of recycled aggregates produce less impacts than the traditional solution (natural aggregates). Hossain et al. [
5] carried out a comparative study on the production of aggregates (natural, CDW and waste glass) for lower grade concrete products and found that the impacts of recycled aggregates were lower than those of virgin aggregates.
The pulp and paper sector has also been the subject of several life cycle assessment studies. For example, Lopes et al. [
28] applied the methodology to printing and writing paper to compare the environmental impacts resulting from the use of heavy fuel oil and natural gas. They found that the natural gas promotes lower impacts than heavy fuel. Santos et al. [
12] carried out life cycle assessment to quantify the social impacts of the value chain on different stakeholders. Waste from the pulp and paper industry has been studied in different recovery scenarios to identify the solution with fewer environmental impacts. For instance, Costa et al. [
29] evaluated the environmental impacts of various recovery solutions for building materials with waste and compared them with the landfill solution, finding that all recovery solutions produce fewer environmental impacts than the current solution. Teixeira et al. [
30] evaluated the technical and environmental performance of the recovery of fly ash resulting from the burning of biomass (CVB) and coal (CVC) in the production of mortar and verified that the emissions of formulations with fly ash and of biomass are lower than those of the reference mortar without residues. Deviatkin et al. [
31] evaluated the environmental performance of five de-inking sludge valorisation processes and reached the best valorisation scenario in the use of that waste in the cement industry. Mohammadi et al. [
32] evaluated the environmental performance of biological sludge used in the production of bioenergy through incineration, or in the production of biochar through pyrolysis or hydrothermal carbonization (HTC). These authors reported that all three systems produce lower environmental impacts than the traditional solution (landfill disposal).
The aim of this study was to assess sustainability of the precast concrete produced with lime ash (the waste from the cellulose industry) as an alternative raw material, based on a circular model approach. The case study was built and tested in a real scale industrial dimension. The economic, environmental and social impacts of the linear solution i.e., production of precast concrete along with the deposition of lime ash in landfill, and the circular solution i.e., precast concrete with incorporation of lime ash as an alternative raw material, were evaluated and compared according to the industrial scale scenarios.
2. Experimental Approach
In the case of precast concrete, the concrete is produced in a producer’s manufacturing facilities, moulded into elements of different shapes and sizes, which is then be assembled in the work field intended for the planned use. The various types of precast concrete elements today are standardized products and a target of CE marking, due to the existence of standards that define its requirements, characteristics, and test methods for this type of products. For example, columns are linear elements, regulated by the product standard EN 13225 which defines, among other parameters, a minimum class (C20/25) for concrete to be used in reinforced elements, such as columns [
33].
From an experimental point of view, this work involved the design, implementation and monitoring of four frames in an industrial scale with elements in precast concrete. One frame, used as reference, was manufactured with the standard formulation of precast concrete, and three other frames were manufactured using the same precast concrete developed with a waste (lime ash) fully replacing all the natural limestone filler used in the standard precast concrete formulation (4.4 wt.% of filler). The aim was thus to promote, in a circular economy perspective, the industrial symbiosis between two industrial companies located less than 25 km from each other, making the waste of one of them (from the pulp and paper sector) into a raw material alternative in the construction sector company (concrete products manufacturer).
This represents a change in the paradigm of the current linear economy model of concrete production, which uses natural aggregates in its production combined with the fact that the pulp and paper sector still sends lime ash and other wastes to landfill disposal [
13]. Hence, this was tested as a replacing raw material for natural filler in the production of precast concrete, to create an alternative to the traditional raw material, but without changing the processing conditions and the final characteristics of the precast concrete. Concrete elements were produced on an industrial scale following a traditional linear economy model (without wastes) or following a circular model (with wastes). Note that the amount of waste (lime ash) used in this work to replace the natural filler was small (around 4 wt.%) but intentionally so, it was enough to produce an impact while intending to locally exhaust the waste produced in the cellulose plant, without resorting to long-distance transports (>25 km).
Prior to implementation, the waste and the traditional raw material (natural limestone filler) were also characterised in terms of particle size analysis, density and mineralogical composition, to assess any eventual differences. During full-scale implementation, concrete specimens with and without waste were also collected to assess fundamental characteristics, such as mechanical compressive strength, hardened concrete density, water absorption and the capillarity coefficient of the concrete. All the four frames together with the characteristics of the precast concrete were monitored for 1 year. After this technical validation of the concrete produced in both contexts (circular and linear model), the sustainability of the developed circular solution was evaluated by comparing it with the linear solution. This evaluation was based on a life cycle (LC) assessment of environmental, economic and social impacts that this simple industrial symbiosis process causes.
3. Case Study—Description and Technical Assessment Results
As mentioned, the implementation of the circular model allowed not only to use lime ash as an alternative filler in the precast concrete industry but, at the same time, to avoid its landfilling in the pulp industry. The precast concrete industry produces a variety of products, such as the linear concrete elements (trusses and pillars) used in this study.
In this industrial-scale trial, implementation of the circular model involved the following 12 steps (
Figure 1): 1. Lime ash storage at waste producer; 2. Lime ash transportation to the precast concrete factory; 3. Concrete mixture ready for application; 4. Preparation of the reinforcement bars and moulds for beams and columns execution; 5. Internal sensors installation for future in situ monitorisation; 6. Concrete discharge into the form and execution of concrete samples for monitoring tests; 7. Curing and drying process; 8. Columns and beams for the frames; 9. Land preparation and direct foundations execution; 10. Columns assembly; 11. Beams and girders assembly; 12. Final frames structure.
To produce the precast concrete columns, the standard formulation shown in
Table 1 was used. Each column consumes 2.1 m
3 of concrete and has a section of 0.5 × 0.4 m
2 and 10.25 m in height, being its strength class the C30/37 one. In the case of concrete, the aggregate is based on the EN 12620 product standard, which identifies the minimum requirements that the aggregates must meet (particle size analysis) and the set of properties to be characterised (density and quality of fines). Thus,
Table 2 shows the properties of lime ash and natural limestone filler (ref. Betocarb-OU), which were characterised in the laboratory. The results with lime ash are like those of the natural filler it replaced, representing, according to the formulation in
Table 1, a filler percentage of 4.4 wt.% (104 kg per m
3 of concrete).
Figure 2 shows lime ash and natural filler particle size distribution (EN 933-10). A filler usually presents 70% of particles below 63 microns and, as it is possible to observe, lime ash particle size is within the defined minimum limit for a filler, making it possible lime ash to be used as such.
In step 6 of the implementation, 12 samples were collected from the circular (concrete with lime ash) and linear model (reference concrete) cases to evaluate the compressive strength, the concrete density in the hardened state at 7, 28, 180 and 365 days of curing (
Figure 3), as well as three samples for water absorption and capillary coefficient (
Figure 4) measurements with curing time (for 28, 180 and 365 days). Internal sensors placed for temperature and shrinkage control showed similar results during this time.
The harmonised standard for precast concrete establishes a minimum compressive strength class for columns and, at the beginning of the pilot design, that was defined to be C30/37. The results in
Figure 3b are according to the required class from the project design. Moreover, the standard deviations in the results shown in
Figure 3 and
Figure 4 were so negligible that their respective statistical deviation bars would not appear in these figures scales.
Regarding water absorption and capillarity coefficient, the behaviour of concrete containing lime ash is also like that of the reference concrete. For monitoring purposes, concrete pH and chlorides content control were also carried out at the beginning of test and after 6 months of immersion in distilled water. No substantial differences between the two compositions were found in these results. In conclusion, this technical assessment showed that the introduction of this waste did not hinder the quality and performance of the standard precast concrete. In
Figure 4 one should consider that as the cement hydration proceeds over time, the capillary pores of the concrete decrease and, therefore, water absorption from the concrete decrease with the concrete curing. Furthermore, the reduction of pores in the concrete will also contribute to reducing the capillary water absorption over curing time.