۱٫ Introduction

۲٫ Significance of the research

۳٫ Materials and experimental procedure

۴٫ Results and discussion

۵٫ Conclusions

Declaration of competing interest

Acknowledgements

References

Abstract

Alkali-activated concretes are being considered as an alternative environmentally friendly construction materials compared to the ordinary Portland cement. However, although these materials have reported mechanical properties comparable to those of OPC, the corrosion of reinforcing bars is a major durability issue that needs to be studied. This article studies the corrosion performance of an alkali-activated binary reinforced concrete (AABC) based on natural volcanic pozzolan (NP) and ground blast furnace slag (GBFS) exposed to chlorides ions. OPC concrete was used as reference material. To carry out the study, accelerated chloride ingress methods (impressed voltage, wet-dry cycles, and saltwater immersion (3.5% NaCl)) were used. Monitoring of the corrosive process was carried out using the techniques of half-cell potential, linear polarization resistance, polarization curves and curves of current intensity versus time. As complementary techniques, surface electrical resistivity and resistance to chloride ion penetration were evaluated. AABC showed higher resistance to chloride ion penetration compared to OPC concrete, a reduction of the charge passed up to 60% at 360 days of exposition. In general, the various accelerated chloride ingress techniques employed in the present research revealed that reinforcing steel embedded in AABCs have a higher resistance to corrosion compared to steel bars embedded in Portland cement (OPC)-based concrete. It should be noted that the B constant values obtained for AABC differ from the values commonly used in (OPC)-based concretes. In addition, the results indicate that the ranges of corrosion probability, such as half-cell potentials or corrosion currents, specified for OPC concretes should be checked for its application in alkali-activated concretes.

Introduction

Reinforced concrete based on ordinary Portland cement (OPC) is one of the most widely used materials in the civil industry due to its versatility, properties and low cost. One of the most important factors for the construction industry is the durability, which is determined by the useful life of the material under real service conditions, including its exposure to aggressive environments [1]. Durability is not an intrinsic property of a material, since it depends on the conditions of service, so that in concrete it is known to vary according to the type of environment to which it is exposed. One of the biggest threats that reinforced concrete faces is the corrosion of reinforcing steel due to cracking, delamination, and in severe cases, the partial or total collapse of a structure. The primary cause is the exposition to aggressive environments such as CO2 and/or the entry of chloride (Cl−) ions [[2], [3], [4], [5]]. These agents have been responsible for most structural damage and, therefore, a significant increase in investment in structure maintenance and repair. Chloride ions can come from a range of sources – marine environments, the use of de-icing salts (≈NaCl), chemical substances coming into contact with the concrete, or they may be present in the concrete mix, as happens with the inclusion of some types of additives (CaCl2) during production. Chlorides diffuse through the concrete and, on reaching a critical concentration near the steel, cause destruction of the passive layer and initiate the corrosive process, generating pitting corrosion in the steel [6,7]. In addition to concrete durability, a major concern is the environmental impact associated with production, especially relating to the use of Portland cement (OPC). Indeed, the OPC industry generates annually 2 billion tons of CO2, corresponding to 8–۱۰% of greenhouse gas emissions into the atmosphere [[8], [9], [10]]. For this reason, research applied to the development of alternative low carbon footprint cementitious materials is a priority for the construction sector [11]. Among these alternatives, alkali-activated materials (AAMs) have caught attention. These are obtained from chemical interaction between an alumina-silicate material (precursor) and a strongly alkaline solution (activator) [[12], [13], [14]]. AAMs are synthesized at temperatures between 25 and 100 °C, while OPC production temperatures rise to 1460 °C to obtain clinker. AAMs can thus be considered as potential low carbon cements [15]. These types of materials also have structural mechanical strengths and physical characteristics such as reduced permeability and thermal stability [16,17].