1. Introduction
Waste generation during the execution of construction activities (construction and demolition waste—CDW) has prompted researchers from several countries to develop projects aimed at making construction waste reuse viable [
1,
2,
3,
4,
5]. A material that can form part of this waste is Portland cement concrete (PCC), which, when crushed, is transformed into a recycled concrete aggregate (RCA). Re-incorporating an RCA into the productive cycle reduces its final disposal volume and the exploitation of quarry to obtain stone materials. Waste minimization complies with the principles of efficiency and Sustainable Development Goals 7, 12, and 13, promulgated by the United Nations—the UN. However, RCAs differ in some physical, chemical, and mechanical properties in relation to natural aggregates (NA), mostly due to the presence of bonded mortar [
6]. The presence of mortar, which originates at a weaker interfacial zone between the binder and RCA, increases the porosity and water absorption of RCAs, and also reduces their strength and mechanical performance when used in the production of asphalt mixtures [
7]. In
Figure 1, the different conformations of RCA are shown, taking into account the fractionation due to the crushing process. These conformations indicate that the structure of the RCA is heterogeneous since it can be made up of natural aggregate, mortar, or a combination of both.
Standard physical and mechanical laboratory tests for checking the quality of NA are used to evaluate RCAs and their potential performances in asphalt mixtures [
8,
9]. However, the study of the chemical and mineralogical properties of RCAs provides further information about the phenomena occurring in mixtures of RCAs and other materials [
10]. Besides, the chemical laboratory tests evaluate the presence of undesirable substances or elements in these aggregates, such as chlorides, sulfates, carbonates, and the contaminants absorbed from their original project sources [
6,
8,
9].
The chemical and mineralogical compositions of RCAs are varied and do not follow a general pattern in terms of its elements, compounds, and concentrations due to the various RCA sources and the different dosages of their original concrete components [
6,
8,
11]. Further, the chemical composition of RCAs has not been researched extensively [
6,
8], which makes it challenging to achieve a standardized procedure for their use [
12]. On the other hand, studies highlight the importance of applying image analysis to determine aspects such as residual mortar after the use of RCAs in concrete mixtures, estimation of porosity distribution, and degradation characteristics within concrete [
13,
14,
15].
Cement is a constituent of PCC. Cement has a significant effect on the chemical composition of an RCA because cement is present in the attached mortar, which is considered the weakest part of PCC. Cement is produced from mineral materials such as limestone and gypsum, an alumina base, and silica naturally found as clay or shale [
16]. Limestone, which is the cement base, is composed of 60% calcium carbonate (CaCO
3) and the rest (40%) include clay, silica, and dolomite. However, RCAs can contain salts formed from potassium (K) and cobalt (Co), and potassium (K) and iron (Fe), which can cause aging in asphalt pavements, as they can oxidize asphalt cement [
17]. Based on the chemical and mineralogical composition, the following concepts from previous investigations explain the effects of RCAs on asphalt mixtures: (a) The presence of salts in the aggregate indicates that there is a higher electrical conductivity. Therefore, a lower electrical resistivity makes the aggregate susceptible to the penetration of chloride ions when embedded in concrete [
16,
17]. (b) When there are manganese salts in the aggregates, and the aggregates enter into contact with air, an oxidizing effect occurs in the asphalt cement, generating premature aging [
17]. (c) The presence of iron oxides can cause aging and deterioration of the asphalt pavement when catalytic reactions are generated in the asphalt cement [
18,
19]. (d) The presence of magnesium in a mineral form called periclase can cause a volume increase in contact with water, owing to its hygroscopic property, causing stresses on the internal structure [
20]. (e) The carbonates of calcium and magnesium (dolomites) are prone to generate an alkali-carbonate expansive reaction through the dedolomitization process. This process forms brucite Mg(OH)
2 and regenerates the alkaline hydroxide in the concrete. Generally, Mg(OH)
2 formation weakens the cement paste junction and the porous zone in the periphery of the aggregate, which generates hygroscopic characteristics and affects its physical properties. (f) The regeneration of the alkali ion (OH)
− in a solution makes dedolomitization a continuous process that can affect the recycled aggregates, since it produces an increase in volume and possible generation of fissures [
21], hence the need to incorporate aggregates with low alkaline reactivity [
22]. (g) The carbonation process occurs when the concrete is exposed to atmospheric contaminants, which favors the appearance of microfractures. It reduces the material strength due to the cycles of crystallization and carbonation expansion related to the alternate wetting and drying of the material [
23]. (h) The different conditions and environmental factors to which the RCA concrete is exposed to during its useful life are other factors that can cause weakening and increase porosity. These exposures tend to favor chloride and sulfate attacks [
16]. (i) A property of the RCA that affects the adherence behavior with asphalt cement is pH [
24,
25]. Adhesion is favored at a higher pH (alkaline), as is the case for limestone aggregates. Adhesion decreases when the aggregates are acidic or neutral, such as aggregates containing aluminates and silica (i.e., basalts or granites).
Based on the information presented above,
Table 1 summarizes the results of physical property tests, and
Table 2 summarizes the results of XRF tests reported by several researchers. The results of these tests depend on the RCA sources.
To understand more about the chemical and physical behavior of RCAs in asphalt, it was necessary to carry out some laboratory tests to identify their relationships.
RCA has been investigated in various proportions, sizes, and fractions as a replacement for NA in asphalt mixtures. However, the performance results obtained for the mixtures do not show a tendency to affirm acceptable behavior when RCA is included. The design characteristics of the mixture containing recycled concrete, and the mineralogy of its aggregates, mark the properties of RCAs such as density, absorption, and wear [
35]. The replacement ratio affects the behavior of the resulting concrete. In general, the aggregates are subjected to physical and mechanical tests to evaluate the quality of the aggregates used in asphalt mixtures. However, as mentioned earlier, there are chemical and mineralogical characteristics that affect these mixtures.
Reference [
9], reported the mineralogy of RCA in different areas of Portugal, with predominant results of quartz, calcite, K feldspar, and sodium feldspar; they also identified a high concentration of polluting species such as chlorides and sulfates that were not suitable for RCAs to be reused [
9].
Reference [
33] provided important data to be considered in the development of correct recycling strategies, based on the chemical and mineralogical compositions of different granulometric fractions of RCAs [
8]. Reference [
6] identified that replacements up to 30% of coarse RCAs in concrete mixtures do not influence the chemical composition in terms of the main components (SiO
2, Al
2O
3, and CaO), revealing a direct correlation between the chemical composition of solids and the leaching of ions through ICP-AE analysis of eluates. That highlighted the importance of a correct characterization of the leaching behavior of these new materials [
33]. Given the importance of the chemical characterization of RCAs, the present study aimed to analyze the chemical properties of RCAs from two demolition sources in an area of the Colombian Caribbean and a source of NA (recycled concrete aggregate of a building—RCAB; recycled concrete aggregate from a pavement—RCAP; natural aggregate—NA) to evaluate their potential influences on the behavior of asphalt mixtures. This should make a contribution to the knowledge about the characterization of these concrete residues according to the area to which they are exposed within their useful lives. Chemical stability was analyzed from the concentrations of the reactants that could be involved in potential asphalt–mineral reactions. The study presents an alternative approach that involves the chemical properties of RCAs as a criterion for defining the behavior of asphalt mixtures. The samples were evaluated by different techniques to identify the properties of the NA and RCAs (RCAP and RCAB). The characterization tests carried out were: X-ray fluorescence spectrometry (XRF), diffraction spectrometry, X-ray diffraction (XRD), soluble ion analysis by UV–Vis spectrophotometry, atomic absorption analysis, loss on ignition (LOI), determination of mass, percentage of organic impurities, and pH.
Figure 2 shows the flow chart used to characterize, analyze, and evaluate the properties of the NA, RCAB, and RCAP aggregates for use in asphalt mixtures.