According to the World Bank [1], waste generation on Earth continues to increase due to strong population growth, reaching more than 2 billion tons in 2016 and is estimated to reach about 3.4 billion by 2050. In Brazil, for example, according to the sustainability report of Brazil Steel Institute [2], steel production generates more than 600 kg/ton of byproducts and waste, including slag, which alone accounts for an average of 100 to 150 kg of this value. In 2017, about 59% of this amount was reused and recycled, leaving over 12 million tons of steel slag in storage.
Considering the need to reduce the consumption of natural resources used as raw materials in construction, some studies have been carried out to investigate the technical aspects related to the use of EAFS as an alternative material for the partial replacement of cement [3], as an aggregate in mortars [3], in the composition of bricks [4], concrete [5], self-compacting concrete [6], roller compacted concrete [7], embankment fills [8], and asphalt mixtures [9, 10], and in stabilizing soils as a substitute for cement and lime commonly used for this purpose [11–20]. In the latter case, it is noteworthy that the stabilization mechanisms are similar to those of the most common stabilizing agents, cement and lime, producing cementitious compounds as the final product [21].
Calcium oxide (CaO), the predominant element in several primary electric steel slags, promotes several actions that change the properties of a soil. First, the addition of CaO to soil decreases the moisture content of the soil due to its hydration reaction [CaO + H2O = Ca(OH)2 + heat]. This reaction increases the pH and the content of Ca2+ ions in the interstitial water of the soil due to the dissociation of calcium hydroxide [Ca(OH)2 = Ca2+ + 2OH-]. As a result, cation exchange takes place between the adsorbed ions on the surface of the clay particles and the calcium cations of the calcium oxide, resulting in a thinner double layer with lower repulsive forces, causing flocculation of the clay portion of the soil [22].
At the same time, high pH values lead to slow dissolution of minerals such as silica, alumina, and feldspar, which combine with calcium and water to form cementitious compounds such as calcium silicate hydrate (C-S-H), calcium alumino-silicate hydrate (C-A-S-H ), and calcium aluminate hydrate (C-A-H) [22–25].
Carbonation is a phenomenon that occurs naturally in any cementitious or pozzolanic compound when it comes into contact with air. The carbonation process occurs in two stages. First, carbon dioxide dissolves in water to form carbonic acid (CO2 + H2O = H2CO3), and second, this acid reacts with calcium hydroxide to form calcium carbonate [H2CO3 + Ca(OH)2 = CaCO3] [22].
Conventionally, the carbonation reaction that occurs in reinforced concrete is considered unfavorable because it reduces the durability of the material, but the opposite occurs in soil stabilization [26]. When soils treated with lime-based materials are subjected to the action of carbon dioxide, physical and chemical transformations occur that affect the behavior of the resulting mixture in the long term and increase its strength and stiffness [25].
The accelerated carbonation technique simulates carbonation in the field with an intensity higher than the material's service level. This technique aims to determine the behavior of materials subjected to carbonation in a short time [27, 28].
CO2 capture and storage and mineral carbonation methods play an important role in reducing the amount of CO2 in the atmosphere. Mineral carbonation uses industrial, natural, and waste minerals that have calcium or magnesium in their composition so that they can capture CO2 through the carbonation process [29]. According to Mohammed, et al. [29], mineral carbonation improves soil strength because the new carbonation products expand and fill voids in the soil, which increases strength. Carbonation in stabilized soils is limited by pressure and CO2 concentration.
The durability of soil-lime mixtures is the maintenance of engineering properties above the minimum design values during the service life of the construction [25]. Various causes of reduced durability of soils chemically treated with lime-based materials include seasonal variations in moisture (wetting and drying cycles), acid rain, freeze-thaw cycles, calcium leaching, and carbonation of calcium hydroxide and cementitious compounds [30–33]. According to Xu, et al. [32], carbonation is one of the key properties for evaluating the durability of lime- and cement-based materials.
The diffusivity of carbon dioxide in a solid material depends on the opening of the pore structure and thus on the presence of water in the pores. In water, this diffusivity is about 10,000 times lower than in air, and carbonation is delayed when the moisture content of the material increases [22, 32]. In addition, carbonation is an efficient method to reduce the leaching of heavy metals in steel slag, preventing the emission of these metals into the environment [34].
Vitale, Deneele and Russo [25] studied the effect of carbonation in samples subjected to the air-curing process on the mechanical behavior [by the CIU (Consolidated Isotropic Undrained) triaxial compression test] and chemical behavior of soil samples treated with lime. It was found that carbonation produced lower porosity due to the growth of calcium carbonate crystals. Sealed curing in a humid chamber followed by unsealed curing in air was observed to produce hydrated compounds in the first phase, but when cured in air, these compounds were carbonated and formed calcium carbonate, which is less resistant than hydrated compounds and promotes weakening of the soil-lime structure. The carbonation reactions of the chemical compounds calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H) are shown in Equations (1 and (2.
$$C-S-H+{CO}_{2}\to {CaCO}_{3}+{H}_{2}O+{SiO}_{2}$$
1
$$C-A-H+{CO}_{2}\to {CaCO}_{3}+{H}_{2}O+{Al}_{2}{O}_{3}$$
2
Haas and Ritter [35] evaluated the structural behavior of a lime-stabilized soil embankment after 34 years. The authors found that the carbonation process was not detrimental, but increased the strength of the material, and that lime was still present to continue the carbonation process.
There are few studies in the literature on the occurrence of carbonation and its effects on the durability of soils treated with lime-based materials [25]. More recently, studies have been developed that address this aspect in more detail [22, 25, 32, 36–47], but there is still no study in the literature that presents the influence of carbonation processes in tropical soils stabilized with EAFS, especially considering the mechanical tests that apply to structural materials of asphalt pavements (e.g., resilient modulus and CBR index).
Given the ability of EAFS to capture carbon dioxide and its beneficial effects on the environment, the need to reuse EAFS must be thoroughly investigated given its high reactivity with CO2 and its low cost. The objective of this study was to evaluate the influence of 2 different carbonation processes on the mechanical behavior of mixtures of tropical soils and Electric Arc Furnace Slag (EAFS) compacted at optimum moisture content and normal Proctor energy.