1. Introduction
Global production of aggregates was 21 B tonnes in 2007 and 40 B tonnes in 2014 [
1,
2]. The most recent data presents value of 50 B tonnes of sand and gravel yearly consumption [
3]. According to Big Market Research (BMR), Aggregates Market Development by 2026 report [
4] and Grand View Research (GVR), Aggregates Market Size, Share and Trends Analysis Report [
1] the global aggregates market was worth about 430 B USD in 2018, and is still raising. It is predicted to be worth about 600 B USD in 2026. The GVR’s report showed that about 60% of aggregates excavated worldwide was used for concrete production and about 20% for road substructures [
1]. It was estimated that only about 100 M tonnes of concrete were recycled into aggregate in 2004 [
5]. In 2017 only in Great Britain 72 M tonnes of recycled and secondary sources aggregates were used, which is quite a large amount in comparison to 176 M tonnes of primary aggregates [
6]. Nowadays, limiting the use of natural aggregates to concrete is doubly justified.
Firstly, natural aggregates are, as the name suggests, a natural resource. Their extraction is expensive and has a huge impact on the environment [
2,
7,
8]. Sand and gravel mines occupy very large areas and cause a change in the groundwater level in their vicinity [
8]. Additionally, sand mining in some countries is subject to prohibitions or very high taxes [
9,
10]. Black market and illegal sand extraction exist for example in Italy, India, Vietnam, Malaysia and others [
9,
11,
12]. In other desert countries (e.g., Egypt, Algeria, United Arab Emirates), although there is a lot of sand, it is not suitable for use in concrete, due to the high fineness and too smooth texture of the grains [
2,
9,
12,
13] or the pollution of sea sand chlorides [
11]. However, both fields are studied [
13,
14,
15,
16].
Secondly, metallurgical waste—sludge, slag and dust from blast-furnace, steel and sintering processes can be used as a substitute for natural aggregates. This is doubly beneficial—the waste is recycled and the use of natural aggregates is further reduced. This waste is deposited in landfills that take up large areas. There is also a danger of hazardous compounds leaking to the ground and groundwater [
17,
18,
19,
20].
Therefore, it is worth considering the recovery and use of other materials as an alternative to natural aggregates—various types of industrial waste and demolition rubble from buildings such as bricks or concrete [
21,
22].
Recycled concrete aggregate (RCA) is a shredded waste generated during the demolition of concrete and reinforced concrete structures. The resulting aggregates are the subject of research to reduce the need for natural aggregates for concrete applications.
The use of RCA increases the demand for water and decreases the consistency of the concrete mix with the same effective water-cement ratio. This applies to mixtures containing 50% and 100% RCA [
23]. Similar results are presented in [
24]. The studies described in [
25] have shown that mixes containing 50% and 100% RCA and modified with a superplasticizer have reached the same consistency as the reference mix-therefore it is possible to control the consistency of mixes containing even large amounts of RCA.
No effect of the substitution of 50% and 100% RCA on the air content of the concrete mix was found [
23]. The studies described in [
25] showed that mixes containing 50% and 100% RCA show a slightly increased air content.
The full replacement of the natural aggregate with RCA results in a lower concrete mix density [
23]. The density of High Performance Concrete (HPC) containing RCA is lower compared to the control mix [
26]. The tests described in [
25] showed that mixtures containing 50% and 100% RCA have lower density in comparison with the control mixture.
The use of RCA may result in deterioration of the mechanical properties of concrete—this depends on the quality of the concrete used. It is stated that the use of 30% RCA as a substitute for natural aggregate has not resulted in a decrease in compressive strength [
27]. Increasing the amount of RCA to 100% reduced strength by about 10%. The strength of concrete containing 100% RCA is lower by about 8% after 2 days but higher by about 5% after 7 and 28 days [
23]. After 28 days a decrease in compressive strength by about 10% was found [
24]. Similar relationships were found for mortars containing 25%–100% fine RCA [
28]. Reduction of compressive and flexural strength and elasticity modulus was also described for HPC containing RCA after 7, 14 and 28 days [
26]. Concretes containing 50% and 100% RCA have lower compressive strength compared to control concrete, however, the use of concrete from railway sleepers demolition caused an increase in strength [
25]. The compressive strength of concrete containing RCA is 30% less than the reference concrete, and the compressive strength is 10% less. Static and dynamic modulus of elasticity decreased by about 20% [
29]. Similar results were obtained by other researchers [
30].
The use of RCA as a replacement for natural aggregate results in increased concrete absorbability. The absorbability increases from about 5.5% for concrete without RCA to about 8% at 100% replacement level. Increase of absorbability and porosity by 15%–38% and decrease of density by 4%–7% for HPC is described in [
26]. Studies described in [
25] showed that concretes containing 50 and 100% RCA have a lower density compared to control concrete. The density of concrete containing 100% RCA is lower than the reference concrete by 5%–6% and its absorbability higher by about 70% [
29].
The capillary action and permeability of concrete containing RCA is increased by about 20% compared to the reference concrete [
24]. The permeability of concrete containing RCA can be reduced to a lower level than for concrete with natural aggregates using supplementary cementitious materials such as phosphorus slag, blast-furnace slag [
31].
The use of fine RCA causes a decrease of the durability properties of concrete—an increase of absorbability and permeability, increase of carbonation depth and chloride migration rate [
32]. However, it is possible to shape and improve these properties using chemical admixtures—mainly superplasticizers. No significant decrease in durability properties of concrete using up to 30% RCA was found [
33]. Above this amount a faster corrosion of reinforcing steel was observed. Chloride diffusion coefficient increased in concretes containing RCA [
30,
34].
Other uses of the RCA include: roads substructures, earth dam construction [
35,
36,
37], paving blocks and structural blocks and bricks manufacture [
37,
38], bituminous-aggregate mixes [
39,
40,
41,
42], road pavements [
43], self-compacting concrete [
44,
45,
46], polymer concrete [
47], shotcrete [
48,
49], concrete exposed to elevated temperatures [
50], soil stabilization [
51], pervious concrete [
52], reactive powder concrete [
53], artificial reef formation [
37].
Recycled aggregate concrete (RAC) is a concrete in which all or part of the natural aggregate is replaced by waste materials [
54]. Many researchers have drawn attention to the problem of limiting the use of natural aggregates and have tried to use different materials as a substitute.
A lot of waste from the metallurgical industry has been involved in concrete technology. The main example is the use of ground granulated blast furnace slag and fly ash as a component of cement and concrete [
55]. Blast furnace slag, electric arc furnace slag, fayalite slag, basic oxygen furnace slag, and copper slag are widely used as an aggregate for concrete [
56,
57,
58,
59,
60,
61,
62,
63,
64,
65,
66,
67]. Other metallurgical wastes such as steel scales, chips and iron ore wastes are used as well [
68,
69,
70].
Another steel making process by-product is sludge. The use of sludge as an aggregate or aggregate component is not widely recognized. It is related to the fineness of the material causing a huge increase in water demand and the content of heavy metals, which may cause an extension of the initial setting time of the cement [
71].
Thermally treated steelmaking sludge was used to manufacture the lightweight aggregate [
72]. Sludge was sintered with clay to create pellets—results are promising but pellets manufacture is a high energy consuming process.
The research presented in this article focuses on the technological conditions of using metallurgical waste in its original form and as a component of RCA. Such an approach to the possibilities of metallurgical waste management has not been widely studied before.
Metallurgical sludge waste was added to concrete in three forms:
Component of rounded RCA as a partial replacement of fine aggregate (see
Section 2.2)
Component of crushed RCA as a partial replacement of fine aggregate (see
Section 2.3)
The former method is generally suitable for manufacturing of non-structural concrete elements, weak concrete layers or curb fixing, etc. Both latter methods may be used in situation while the concrete mix is left on the building site or the concrete do not meet requirements and need to be recycled. Then the metallurgical sludge waste together with admixture is added to concrete mix to form RCA. Additionally, the second method may be used to recycle elements manufactured with concrete containing metallurgical sludge waste and reuse it as RCA after crushing.
4. Conclusions
To summarize the research, the authors draw attention to the technological possibilities and environmental benefits of using both metallurgical sludge waste in a least processed form and aggregates made with its content.
The first possibility is the disposal of metallurgical sludge waste itself in concrete. Metallurgical sludge waste is originally stored in landfills that take up space that can be used in another, more favorable way. In addition, it can cause soil or groundwater contamination. Therefore, using it as a partial substitute for sand is environmentally beneficial. In addition, the consumption of sand, which is a natural resource, and its reserves are limited and its excavation is regulated in many countries by law. One could go further in this consideration and think that once the lifespan of concrete structures or elements containing metallurgical sludge waste has expired, such concrete could be used as RCA, after crushing.
For the production of ready-mixed concrete and its use on the construction site, there are two possibilities if there is an excess of mix or the mix does not meet the requirements. The first is to add waste and superplasticizer to the mix to improve consistency and then pour it onto a foil-covered surface and after hardening, crush the resulting composite and use it again as crushed RCA. A second option is to use a special admixture to recover the excess concrete mixture and process it into round RCA.
The authors present below the technological aspects of using metallurgical sludge waste and recycled concrete aggregate:
The use of metallurgical sludge waste or crushed or round RCA to produce concrete deteriorates the consistency. This is associated, in the case of metallurgical sludge waste, with its high water demand, and fineness and, in the case of RCA, with its high absorbability.
The use of the superplasticizer will result in additional costs, but without its use it will not be possible to use metallurgical sludge waste over 60% or RCA over 50% due to the deteriorated concrete consistency. Additionally, concrete mix recovery agents can increase costs. However, it is certain that the use of waste will have a positive environmental effect.
The use of metallurgical sludge waste as a replacement for sand in concrete does not affect the air content, density of the concrete mix or the density of the hardened concrete.
Using RCA as a replacement for coarse aggregate increases the air content in the concrete mix. This is due to the high porosity of RCA. RCA does not have a significant effect on the concrete mix density, but it does affect the density of the hardened concrete. This is due to the fact that RCA has a lower density compared to natural aggregate and, when present in the mix, contains water which evaporates during curing.
The use of metallurgical sludge waste as a replacement for sand in concrete increases its absorbability and slightly increases the depth of water penetration under pressure. It is connected with its higher porosity. If round RCA is used, the absorbability of the concrete is also increased. The permeability of the concrete increases as well. This may jeopardize the durability of components made of such concrete and requires further testing.
The use of metallurgical sludge waste as a replacement for sand in concrete improves the mechanical properties after 7 and 28 days of curing up to 60% of metallurgical sludge waste content. However, even replacing 90% of the sand with metallurgical sludge waste does not adversely affect the compressive strength. RCA’s use as a replacement for natural aggregates has a positive effect on the compressive strength of the concrete at all testing dates. The improvement in mechanical properties is associated with a reduction in the effective water-cement ratio. This is due to the high water demand of both metallurgical sludge waste and RCA. Water absorption by the aggregate reduces the amount of water in the cement paste and thus the porosity and strength of the resulting cement matrix.
According to the advantages and disadvantages the best performance of concrete containing RCA are while crushed RCA is used. However, it has to me mentioned that different methods may be used accordingly to situation.