1. Introduction
The evaluation of the mechanical behaviour of concrete is always considered essential, while the evaluation of the durability of this construction material is sometimes overlooked. Concrete with high porosity allows the entry of external agents, which can lead to durability problems; hence, it is essential to study in detail several properties that are related to the durability of concrete.
The permeability to aggressive agents in concrete can be measured through several tests, such as evaluating the transport mechanisms associated with chloride ions, oxygen and carbon dioxide.
The analysis of the permeability of concrete to gases concerns the entire porous structure of this material, i.e., both small and large pores, due to the reduced size of the oxygen molecules. Therefore, the oxygen permeability test has a higher sensitivity than the water absorption tests, since the water molecules are larger than the oxygen ones [
1].
The transport mechanism of chloride ions in concrete is somewhat complex, and may involve diffusion, impregnation and capillary water absorption processes. Transport mechanisms vary widely with the microenvironment in which the structural elements are inserted. The penetration of chlorides into ordinary concrete is usually carried out through the continuous pore structure of the cementitious paste, the interface between the aggregates, and the paste (ITZ) and micro-cracks [
2]. The penetration of chlorides is, together with carbonation, the main factor responsible for the depassivation and corrosion of steel reinforcement in reinforced concrete.
In turn, the carbonation process begins with the penetration, by diffusion, of carbon dioxide (CO
2) into concrete, which, in the presence of moisture, reacts with the hydrated cement minerals and gives rise to carbonation. In other words, CO
2 in the atmosphere reacts with the alkaline components of concrete, namely by transforming calcium hydroxide (CH) into calcium carbonate. These chemical reactions of dissolution of the crystalline phases of concrete cause a decrease in the pH of the concrete pores’ solution, enabling the corrosion of the steel reinforcement, since the oxide film that protects the steel reinforcement is only stable in very alkaline environments [
3]. The carbonation resistance of concrete is generally determined through accelerated tests, in which the concrete specimens are subjected to high concentrations of CO
2 (5%). Thus, one of the problems of this type of test is to understand whether the results obtained evaluate rigorously the behaviour of concrete exposed to atmospheric CO
2 concentrations. It is recalled that the concentration, in volume, of CO
2 in the atmosphere usually varies between 0.03%, in rural areas, and 0.10%, in places with high population density [
4].
According to data presented by the United Nations, in 2015 the world population was around 7.3 billion, and this number is expected to reach 9.7 billion in 2050 [
5]. That report also indicates that, in 2015, for the first time, the majority of the world’s population already lived in cities. The migration of populations to urbanized areas brings benefits for global development, but also implies a significant increase in built parks, with an increase in the production of construction and demolition waste (CDW), whose environmental impact is extremely negative. Although the use of CDW already occurs, as aggregates in concrete in some constructions, the truth is that its incorporation still corresponds to occasional cases and it is not a common reality.
The use of recycled aggregates (RA) in concrete raises some important questions in terms of durability. These aggregates have physical properties and compositions that are very different from those of natural aggregates. The main difference is related to the greater porosity and, consequently, greater water absorption of RA. However, this variation depends on the RA’s source and production process. On the other hand, the roughness and specific surface of RA are usually higher and RA typically have more elongated shapes. Due to these factors, the effective water/cement (w/c) ratio of mixes with RA must be increased to maintain a given workability, which then leads to more porous cementitious matrices and interfacial transition zones (ITZs). Given this, the use of RA in concrete can decrease its durability [
6].
On the other hand, in the 20th century, the annual emission of carbon dioxide (CO
2) into the atmosphere increased from 1.5 to 25 billion tonnes [
7]. This unsustainable rate of emissions comes from several activities, including construction, which alone contributes to more than 6% of the global value through the production of cement. Thus, it is important to investigate the possibility of providing the construction industry with an innovative way of creating structural concrete with a positive environmental impact throughout its life cycle. This can be achieved through the creation of synergies in two distinct areas: replacement of the Portland cement by an alternative binder with lower environmental impact using CDW; and incorporation of RA from CDW.
According to this literature review, there are not many studies on the behaviour in terms of durability of concrete with CDW simultaneously used as aggregates and binders. The use of CDW in concrete has been analysed mainly in two different ways: analysis of the use of RA from crushed concrete; and evaluation of the use of RA from mixed CDW. The latter RA have highly variable composition, which makes their analysis more difficult. On the other hand, RA from CDW generally have higher water absorption, which causes some limitations on their use in concrete.
Torgal et al. [
8] analysed the oxygen permeability of concrete with RA and recycled cement from four types of ceramics (ceramic bricks, double-fired white stoneware, sanitary ware and single-fired white stoneware). The authors found that the replacement of 20% Portland cement with recycled cement from ceramics leads to maintaining the oxygen permeability of concrete. For two of the wastes (double-fired and single-fired white stoneware), there was even a slight improvement in this property (lower than 10%). In turn, the authors observed an improvement of 20% and 30% in oxygen permeability with the total replacement of coarse and fine aggregates, respectively. The authors explain this result with the highest degree of hydration in the cementitious paste of concrete with RA.
Thomas et al. [
9] evaluated some properties of precast concrete elements in which coarse RA and recycled cement were used. In this experimental campaign, six concrete mixes were characterised: a reference mix; two mixes with 25% and 50% (by weight) RA from mixed CDW; three mixes similar to the previous ones but with recycled cement with low clinker content (cement produced with 25% of ceramic waste). As in other studies [
6], it was found that the oxygen permeability coefficient increases with the replacement of natural aggregates with RA. The authors [
9] also concluded that this increase is higher in concrete with recycled cement and with the use of 50% of RA (higher than 300%) than in concrete with Portland cement and with the use of 50% of RA (higher than 40%).
Bravo et al. [
6] studied the replacement of natural aggregates (fine and coarse) by RA from mixed CDW from four recycling plants. To this end, the authors analysed the oxygen permeability of concrete produced with 0%, 10%, 50% and 100% of fine or coarse RA. Total replacement of the fine and coarse aggregates caused increases in this property of more than 43% and 91%, respectively. Thomas et al. [
9] also obtained significant increases in this property with the incorporation of coarse RA from CDW. The authors obtained increases of 50% in the oxygen permeability test performed at 28 days.
Torgal et al. [
8] also evaluated the resistance to the penetration of chloride ions in concrete with RA (ceramic) and recycled cement (from four types of ceramic waste). The authors concluded that the use of RA and recycled cement significantly improved this property, obtaining decreases in the diffusion of chloride ions between 12% and 70%, and between 23% and 29%, respectively.
Qin and Gao [
10] analysed the influence of the use of recycled cement produced from concrete waste (0%, 10%, 20%, 30% and 50% of the total weight of the binders) on the resistance to the penetration of chloride ions of cementitious composites. The authors concluded that the use of 50% recycled cement increases the permeability to chloride ions by more than 300%. However, the authors found that this increase is reduced to around 200% when the concrete is subjected to accelerated carbonation curing.
Some studies have also analysed the carbonation resistance of concrete with recycled cement. Kim [
11] evaluated the behaviour of self-consolidating concrete with recycled cement from concrete (0%, 15%, 30% and 45% of the total mass of the binders). The author found that the use of this recycled cement causes a significant increase in the carbonation depth of the concrete (between 2.3 and 6.9 times).
Sáez del Bosque et al. [
12] also evaluated the carbonation resistance of concrete with coarse RA from CDW (25% and 50% of the total coarse aggregates) and recycled cement (with 25% of ceramic wastes). Regardless of the type of cement, the average carbonation depth was slightly higher in materials with 25% or 50% recycled aggregate than in the reference concrete. The use of this partially recycled cement produced from ceramic waste led to an increase in the carbonation depth at 28 days from 4.2 mm to 5.0 mm (increase of 19%).
The present investigation follows previous ones that intended to analyse the mechanical behaviour, water absorption, shrinkage and thermal performance of concrete with RA from CDW and recycled cement [
13,
14,
15]. In these previous investigations, the authors observed that the use of GRC enhanced concrete environmental performance. At 10% replacement, it lowered the CO
2 emitted in concrete manufacture by 7.5%, and at 25% GRC by 18.7%, relative to concrete made with ordinary Portland cement (OPC).
There are already some studies that address the permeability of concrete with recycled cement (mainly from ceramic wastes) or with RA from CDW. However, no previous study has evaluated the permeability to aggressive agents (chloride ions, carbon dioxide and oxygen) of concrete with simultaneous replacement of the two elements evaluated in this investigation. To fill this scientific and technical knowledge gap, this study analyses the effect of replacing Portland cement with ground recycled cement (GRC), at 10% (R10) or 25% (R25). This assessment was carried out on concrete with 100% NA, 50% NA and 50% mixed recycled aggregates (MRA). This experimental campaign involved the evaluation of the permeability to the aggressive agents of these mixes through the performance of oxygen permeability tests, resistance to the penetration of chloride ions and resistance to carbonation. Finally, the evolution of the carbonation front in these mixes was studied with the increase in the exposure time for mixes in different exposure classes, according to the forecasting model proposed by EHE-08 [
16]. This assessment in this type of concrete has not yet been carried out in previous investigations.