Abstract
The development of alkali-activated materials (AAMs) as an alternative to Portland cement (PC) has seen significant progress in the past decades. However, there still remains significant uncertainty regarding their long term performance when used in steel-reinforced structures. The durability of AAMs in such applications depends strongly on the corrosion behaviour of the embedded steel reinforcement, and the experimental data in the literature are limited and in some cases inconsistent. This letter elucidates the role of the chemistry of AAMs on the mechanisms governing passivation and chloride-induced corrosion of the steel reinforcement, to bring a better understanding of the durability of AAM structures exposed to chloride. The corrosion of the steel reinforcement in AAMs differs significantly from observations in PC; the onset of pitting (or the chloride ‘threshold’ value) depends strongly on the alkalinity, and the redox environment, of these binders. Classifications or standards used to assess the severity of steel corrosion in PC appear not to be directly applicable to AAMs due to important differences in pore solution chemistry and phase assemblage.
Highlights
Alkali‐activated materials (AAMs) are the products of the reaction between an aluminosilicate source and an aqueous ‘activator’, which supplies alkaline constituents, usually alkali‐metal hydroxide, silicate, carbonate or sulfate [1,2]
Classifications or standards used to assess the severity of steel corrosion in Portland cement (PC) appear not to be directly applicable to AAMs due to important differences in pore solution chemistry and phase assemblage
AAMs can be broadly categorised into systems with high Ca content, such as alkali‐activated slags, where the phase assemblage is dominated by a calcium‐aluminosilicate hydrate (C‐A‐S‐H) type gel; and low Ca systems such as alkali‐activated fly ashes, where the main reaction product is a three‐dimensional alkali‐aluminosilicate hydrate (N‐A‐S‐H) type gel [3,4,5]
Summary
Alkali‐activated materials (AAMs) are the products of the reaction between an aluminosilicate source (usually industrial by‐products such as slags from the iron and steel industry, coal fly ashes from thermoelectric plants, among others) and an aqueous ‘activator’, which supplies alkaline constituents, usually alkali‐metal hydroxide, silicate, carbonate or sulfate [1,2]. Mechanisms of steel reinforcement corrosion in AAMs, either due to lowering of the pH due to carbonation, or due to chloride ingress, can differ from those typically identified in PC‐based materials [9,10,11]. Given the differences in pore solution chemistry and phase assemblage between AAMs and PC, both the initiation and propagation timescales in a service life model are expected to be different. To work toward resolving this question, this letter provides new insight into the mechanisms governing passivation and the onset of chloride‐induced corrosion of the steel reinforcement in AAMs, and proposes a new approach for classifying AAMs according to their internal redox chemistry, when considering the likely durability of steel rebars embedded in such materials in the presence of chloride
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