The chapter delves into the innovative use of polyurethane waste from the automotive industry to produce eco-cement cobblestones. It highlights the environmental benefits of this approach, such as reducing CO2 emissions and landfill waste. The study includes a detailed experimental design, manufacturing process, and extensive testing of the cobblestones, including mechanical strength, abrasion resistance, and fire performance. Notably, the research demonstrates that these eco-cement cobblestones meet or exceed standard requirements, while also improving environmental performance. The Life Cycle Assessment shows that incorporating waste into cobblestones reduces global warming potential and other environmental impacts, making a strong case for the adoption of these sustainable construction materials.
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Abstract
With the aim of implementing a circular economy in all manufacturing processes, by reducing the use of raw materials, while minimizing the use of natural resources and valuing several industrial wastes, efforts are focused on the development of new techniques that allow the use of wastes in the construction sector, in order to turn them into raw materials for the development of new materials, thus achieving all the processes towards a sustainable environment.
With this purpose, different precast cement cobble has been manufactured using recovered polyurethane waste from complete vehicle roofs generated in the automotive industry, turning them into a raw material. Depending on the amount of waste used, with progressive substitutions of sand of 20%, 40% and 60% of polymer waste, the final properties are achieved according to the application requirements, both in the fresh and hardened state, reaching the values required by current standards.
As well as water properties and microstructure, accelerated aging tests in freeze-thaw cycles and crystallization of salts have been tested, stablishing the compressive strength before and after, guaranteeing the properties in outdoor environments in which these materials can be placed.
The characterization has been completed with tests on the generation of volatile organic compounds (VOCs), fire resistance and Life Cycle Assessment (LCA).
In this way, the development of innovating solutions is achieved, valorizing waste that is generated in significant quantities, being able to be used in prefabricated products to be used in the building sector.
1 Introduction
The construction sector is one of the most polluting sectors today, as it generates CO2 emissions of about 10 GtCO2, increasing by 5% since 2020 and 2% more than the maximum obtained in 2019 [1]. In this sector, which uses a large amount of non-renewable raw materials, one of the main materials used for more than a century has been cement, for the manufacture of concrete and mortar [2]. The impact that the manufacture of these materials has on the environment is very high, and that is why the European Union has established the objective of trying to reduce these impacts and get closer to a green economy through several action plans [3]. The impact of the manufacture of these mortars and concretes could be reduced by using other raw materials, replacing either part of the cement or the aggregates used. Several studies have analyzed the use of some industrial sub products such as complementary cementitious materials (SCM) [4], volcanic dust and recycled concrete aggregates [5], recycled concrete aggregates and fly ash [6], polyurethane foam wastes [7] or lime sludge, evaluating the impacts that these substitutions have with respect to the conventional products used [8]. For all these reasons, and with the aim of trying to obtain new construction products in order to reduce the dependence on non-renewable raw materials, this research tries to use waste from the construction sector in mortars to manufacture prefabricated products. Thus, new ecological products are obtained using industrial waste, avoiding landfill and its consequent costs, and the amount of aggregate used in mortars, which is a non-renewable natural material, is reduced, trying to get closer to a more sustainable model in a sector as polluting as the construction sector.
2 Experimental Design
The research that has been carried out is the obtaining of lightened prefabricated products in the shape of cobblestones, in which industrial waste from the automotive sector is incorporated. The waste comes from the inside of complete vehicle roofs made of polyurethane and other materials such as adhesives or cardboard. They are incorporated crushed into prefabricated cobblestones, replacing part of the aggregate used in the manufacture of these cement mortars, with the aim of reducing waste treatment costs and preventing them from being deposited in landfills. They are designed to be used in both building and civil engineering.
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2.1 Raw Materials
The prefabricated cobblestones are made with cement CEM I 52.5 R, washed river sand of rounded and slightly angular shapes, typical of natural granular aggregates and a non-ionic additive with a high hydrophobic/hydrophilic composition, that improves their properties by reducing the amount of water needed for mixing. The cement/conglomerate dosage is 1/3, the conglomerate being the addition of the aggregate and the waste in the cases in which this is part of the prefabricated cobblestones. A first reference dosage is manufactured and subsequently the amount of aggregate is substituted by residue, in volume, of 20%, 40% and 60%, incorporating the additive that provides a better compaction of the matrix due to the lower water demand of the mortar, thus improving its mechanical properties.
2.2 Manufacturing Process
For its preparation, the materials are mixed, on the one hand, the complete roof waste with the cement and aggregate, and on the other hand, the water and the additive. They are kneaded in the concrete mixer and then placed into the molds where they are vibrated and compacted. The manufactured cobblestones have dimensions of 200 × 100 × 60 mm3, complying market requirements.
The prefabricated cobblestones have been tested at 28 days, complying with the indications established by European standards. Samples have been manufactured in accordance with the regulations for each of the tests carried out, so that the required conformity criteria are always satisfied in each one of them.
After a visual examination of the cobblestones, which must not present defects such as cracks, exfoliations or efflorescence, the behavior of the cobblestones has been studied with tests carried out to determine their possible application in building and civil engineering.
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3 Results
The results obtained when testing the prefabricated cobblestones with the different dosages replacing aggregate with polyurethane waste show how. in general, they satisfy the current standard on cobblestones EN 1338:2004 “Concrete paving blocks. Specifications and test methods” (Table 1).
Table 1.
Properties in fresh and hardened state.
Dosages. Substitution sand-waste
Bulk density (kg/m3)
Hardness Shore (C)
Mechanical strength (MPa)
Abrasive wear resistance (mm)
Slip resistance (USRV)
Water absorption (%)
Reaction to fire
Reference
2317
93.1
4.8
14
62
4.4
A1
20
2179
90.3
5.1
14
56
5.1
A1
40
2152
85.7
4.3
16
59
5.9
A1
60
2053
84.6
3.8
17
60
7.6
A1
3.1 Density and Hardness Shore C
The tests were carried out on all the dosages described in accordance with the standard for cobblestones. The results obtained show how the density decreases progressively as the amount of aggregate replaced by residue increases, due to the lower density of the waste compared to the aggregate it replaces (Fig. 1). These enhanced results also allows a better workability, the handling, transport and the installation on site. Shore C hardness also decreases as the amount of aggregate replaced by waste increases, which allows a lower resistance to external penetration, although the values are still high, so this decrease is not very significant (Fig. 2).
Fig. 1.
Bulk density
Fig. 2.
Hardness Shore C
×
×
3.2 Mechanical Strength
All the values obtained of the mechanical strength are above the minimum value established in the standard of 3.6 MPa, including those of greater substitution, such as those of 60% replacement of aggregate by waste, whose value of 28-day fracture is 3.8 MPa (Fig. 3).
In the case of the 20% substitution of aggregate for waste, the compressive strength result is even higher than the reference value, improved thanks to the effect of the additive, which means that less water is required for mixing, improving the compaction of the matrix, with the consequent improvement of its mechanical properties.
Fig. 3.
Mechanical strength
×
3.3 Abrasion Resistance and Slip/skid Resistance
The results obtained in the abrasion resistance test show in Fig. 4. All the dosages achieve the requirements of the standard, as they are less than 20 mm, so they can be classified as Class 4, Marking I. In all cases, the results are very close to the reference values without residue, being very satisfactory results. The results obtained in the abrasion resistance test are important because their use in floor pavements exposes them continuously to external friction, which can lead to accelerated aging.
Regarding the results obtained in the Slip/Skid Resistance (USRV) test, the results obtained show values close to the value obtained in the reference dosages, and always higher than 45 USRV, so that adequate values are always obtained.
Fig. 4.
Abrasion resistance (mm) and Slip/skip resistance (mm).
×
3.4 Water Absorption
The values obtained after carrying out the water absorption tests show how the values increase as a greater amount of waste is incorporated into the cobblestones. (Fig. 5) Almost all the values accepted by the standard except those of replacing 60% of the aggregate with residue. In this sample, an absorption of 7.6% is obtained while maximum value accepted by the standard is 6%. This result is due to the great amount and characteristics of the waste, which has polyurethane and other components such as cardboard, a compound capable to captivate more water. Therefore, all dosages except this one can be classified as Class 2, B marking.
Fig. 5.
Water absorption (%).
×
3.5 Fire Performance
Prefabricated cobblestones, according to the EN 1338:2004 standard "Concrete paving blocks. Specifications and test methods" belong to class A1 of the reaction to fire test, without the need for testing, since they meet the requirements for external fire performance.
3.6 Accelerated Aging Tests in Freeze-Thaw and Salt Crystallization Cycles
In order to evaluate the behaviour of the cobblestones when placed in outdoor environments, accelerated aging tests have been carried out in freeze-thaw cycles and salt crystallization, establishing the compressive strength before and after, and consequently it can be evaluated the pertinence to use them in these conditions. (Table 2).
Table 2.
Mechanical resistance after accelerated aging tests.
Dosages. Substitution sand-waste
Mechanical strength after frost/thaw cycles (MPa)
Mechanical strength after crystallization of salts (MPa)
Reference
6.5
5.3
20
6.1
5.6
40
6.2
7.7
60
4.4
5.1
The freeze-thaw test was carried out and subsequently, the cobblestones subjected to these cycles were tested in compressive strength. The results after the test give higher values than the initial ones, improved in all the dosages, and always comply with standard since it is never lower than 3.6 MPa (Fig. 6).
Fig. 6.
Mechanical resistance after accelerated aging tests (MPa).
×
The salt crystallization test has also been carried out, and in the compressive strength results obtained after this test it is possible to see how, as in the previous case, the mechanical strengths are higher than those initially obtained. The result obtained in the dosage of 40% replacement, increases up to 7.7 MPa. This result is probably due to the effect of the additive, which, when combined with the mortar with waste, compacts the matrix in such a way that its resistance value improves notably, with very satisfactory results obtained.
3.7 Life Cycle Assessment (LCA)
It has been evaluated the environmental impact of the manufacture of these cobblestones, to quantify the impact of replacing part of the natural aggregate used in cement mortars by a waste product from the automotive industry, comparing it with the reference ones. For this purpose, a Life Cycle Analysis (LCA) of the different dosages of the prefabricated cobblestones has been carried out. The functional unit used is 1 m2 of cobblestone. It was calculated using the SIMAPRO program, with the CML-IA baseline V3.06/EU25 method.
Fig. 7.
Environmental impacts of the studied precast cobblestones.
×
As can be seen in Fig. 7, the results show certain variations in the studied impacts of the cobblestones with waste with respect to the reference ones. In the case of Global Warming (kg CO2 eq), the 20% substitution increases slightly with respect to the reference value, possibly due to the additive, which means that, although less aggregate is used, it has to be considered in the overall estimate of impacts. However, for the others substitutions, the results obtained are lower than the reference values, improving this parameter by including the waste in our cobblestones. Therefore, we consider these results really positive, since the use of an industrial waste replacing a natural resource, such as aggregates, even when using the additive, contributes less to global warming than the reference cobblestones currently used in the market.
Regarding eutrophication, an important parameter in our environment, the values obtained in the dosages with waste are very similar to the values of the reference cobblestones, so it is a parameter that is not significantly influenced by the use of waste or additives.
The ozone layer depletion has also been studied, which is reduced when we use waste replacing the reference aggregate.
Finally, soil and water adification has also been studied, a parameter that benefits when waste is used. As the amount of aggregate replaced by waste increases, this value is slightly reduced, so we obtain positive results by reducing this parameter.
It can be concluded that, by incorporating waste to replace part of the aggregate in the cement mortars, the environmental impacts generated are reduced. For all these reasons, we consider that the environmental results are slightly better than those of the cobblestones currently on the market.
4 Conclusions
The research carried out involves the study of prefabricated cobblestones manufactured with cement mortar made up of cement, aggregate, water, an additive and a waste of industrial origin with which the aggregate of the reference mortar is substituted in different percentages, in volume. With these substitutions, products lighter than the reference mortar are obtained, with the consequent savings in the base structure and the improved workability that this provides. Several tests have been carried out to study its behaviour, in terms of mechanical resistance, resistance to abrasion resistance and slip/skip resistance, water absorption, fire resistance, as well as accelerated aging tests in freeze-thaw cycles and crystallization of salts. In all of them, except in some specific cases with a 60% substitution of waste for aggregate in the absorption test, the cobblestones comply with the requirements of the standard. Finally, the environmental impact of the inclusion of these wastes in the precast products has been evaluated by carrying out a Life Cycle Analysis (LCA) of all the dosages tested. The parameters analysed show how the incorporation of these wastes into the precast products improves their environmental performance, so that the use of these ecological cobblestones could be considered, making the construction sector more circular and greener, by using wastes as raw materials, avoiding their deposit in landfills, reducing the use of natural materials, and thus reducing both energy and environmental costs.
Acknowledgements
Authors gratefully acknowledge the Regional Government of Castilla y León (Junta de Castilla y León) and by the Ministry of Science and Innovation MICIN and the European Union NextGenerationEU / PRTR.
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