A review on the durability performance of alkali-activated binders subjected to chloride-bearing environment

Globally, there has been a lack of sustained efforts by different industries to decarbonize the environment. Research on building materials has focused on the synthesis of alternative cement-free binders utilizing industrial by-products and natural materials to reduce greenhouse gases generated from cement production. Alkali-activated binder (AAB) is one such binder. Numerous studies have thoroughly investigated the characterization and mechanical properties of AABs, which proved that they are superior or equivalent to those of conventional concrete. Although AAB's resistance to acid and sulfate attacks is excellent, the exposure of these binders to the chloride environment has received limited attention to investigate its performance. Unlike C-S-H produced during OPC hydration in a layered structure, either C-A-S-H, N-A-S-H, or both are formed, depending on the constituents of the source material and the composition of alkaline activators. Similar to the binder structure, the variation in precursor material constituents and a host of other parameters influence the performance of these binders when subjected to aggressive environmental conditions. A critical state-of-the-art review of AAB resistance to corrosion caused by chloride is presented in this paper. The study revealed a shortage of data on AABs' long-term performance, especially in a chloride-bearing environment, under aggressive exposure conditions. Moreover, the studies conducted do not consistently prove that these binders are resistant to chloride penetration. Some difficulties have been noted in the literature regarding electrochemical measurements to determine the corrosion of reinforcing steel in AABs. In the case of AABs, the standard procedure adopted for OPC concrete may not be applicable. In light of the current literature, the review suggests that prospective studies in this field should focus on the creation of standards and specifications for AABs. Considerable attention is required to evaluate the performance of these binders when exposed to a chloride-laden environment, particularly the chloride transport mechanism, electrochemical reactions at the steel–concrete interface, and chloride binding capacity in view of the multi-phase ionic built-up in the pore solution of these binders.