**1. Introduction**

There is extensive scientific information on the potential of alkali-activated concrete as a sustainable material to replace ordinary Portland cement due to its low energy cost, high compressive strength, rapid setting and hardening, and its resistance to fire, acid, and saline solutions compared to ordinary Portland cement [1]. However, the electrochemical characteristics of the concrete–steel system have become the subject of recent studies due to the valuable life guaranteed—for at least 50 years—for structures built with reinforced concrete [2]. These are the challenges facing developers and marketing agencies engaged in large-scale construction [3].

Alternative types of cement are obtained by mixing different minerals, residues, and industrial by-products [4]. These materials, on some occasions, do not meet the specifications to be classified as materials suitable for incorporation into cement and concrete [5]. However, using alkaline activation technology, industrial waste and byproducts can be transformed into new materials with low energy consumption, high durability, and excellent mechanical performance [6,7]. A specific type of said cement is obtained from blast furnace steel slag, which uses a relatively simple preparation of materials [8]. The metallurgical industry generates considerable amounts of this industrial "waste", but it is not correctly used [9]. Therefore, studying blast furnace slag as a raw

**Citation:** Aperador, W.; Bautista-Ruiz, J.; Sánchez-Molina, J. Effect of Immersion Time in Chloride Solution on the Properties of Structural Rebar Embedded in Alkali-Activated Slag Concrete. *Metals* **2022**, *12*, 1952. https://doi.org/10.3390/met12111952

Academic Editor: RenatoAltobelli Antunes

Received: 11 October 2022 Accepted: 11 November 2022 Published: 15 November 2022

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material for manufacturing new cement is of grea<sup>t</sup> scientific, environmental, and economic interest [10]. The research results aimed at characterizing these materials can benefit both slag-producing companies and those that manufacture construction materials [11]. The former companies can improve the disposal of their waste with less environmental impact. From the low-cost slag, the latter companies will obtain cement with excellent mechanical, durability, and preparation properties [12].

Reinforced concrete is one of the most versatile construction materials, due both to its properties regarding its service and wide range of applications, as well its low cost [12]. However, the reinforcing steel in this structure is susceptible to corrosion, considerably reducing the useful life of structures built with this type of material [13,14].

In this study, the degradation of a type of non-Portland cement obtained from blast furnace steel slag and activated alkali was evaluated. This type of cement is of grea<sup>t</sup> interest because it avoids the emission of CO2 during its manufacture. Estimating the porosity determined the degree of deterioration suffered by the steel embedded in the concrete as a function of the evaluation time. The hydrated samples are also characterized by 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR) to determine the structure of the formed calcium silicate hydrate (C-S-H) gel. This mixture formed a C-S-H gel, constituted mainly of silicon in the middle groups, in chains in the disilicates. The effect of the slag was remarkable in improving the non-Portland cement, i.e., in the porous matrix, the concrete was found to considerably reduce the current passing through as a function of time, showing a reduction in porosity and an increase in impedance because of the generated pozzolanic reaction.

#### **2. Materials and Methods**

#### *2.1. Preparation of Concrete Specimen*

The study mixtures were obtained by cementing granulated blast furnace slag activated with sodium silicate (Na2SiO3) at a 5% Na2O, expressed as a percentage by slag weight to be incorporated. The SiO2/Na2O ratio used was 2.4. The manufacture of the concrete mixtures in both cases contained a cementitious material dosage of 300 kg/m3. It was assumed that the water + activator/slag solution ratios were equivalent; the ratio was 0.5 to obtain an adequate slump of 80 mm.

The type, composition, and size of the fine and coarse aggregates corresponded to gravel with a maximum size of 19 mm, a specific gravity of 2.94 g/cm3, a compact unit mass of 1860 g/cm3, a loose unit mass of 1700 g/cm3, and an absorption of 1.3%. The sand employed in the mixture had a specific surface area of 2.47 g/cm3, a compact unit mass of 1670 g/cm3, a loose unit mass of 1580 g/cm3, and an absorption of 2.9%. Curing was carried out with a relative humidity of approximately 85% and a constant temperature of 25 ◦C.
