Abstract
The pore solutions of alkali-activated slag cements and Portland-based cements are very different in terms of their chemical and redox characteristics, particularly due to the high alkalinity and high sulfide content of alkali-activated slag cement. Therefore, differences in corrosion mechanisms of steel elements embedded in these cements could be expected, with important implications for the durability of reinforced concrete elements. This study assesses the corrosion behaviour of steel embedded in alkali-activated blast furnace slag (BFS) mortars exposed to alkaline solution, alkaline chloride-rich solution, water, and standard laboratory conditions, using electrochemical techniques. White Portland cement (WPC) mortars and blended cement mortars (white Portland cement and blast furnace slag) were also tested for comparative purposes. The steel elements embedded in immersed alkali-activated slag mortars presented very negative redox potentials and high apparent corrosion current values; the presence of sulfide reduced the redox potential, and the oxidation of the reduced sulfur-containing species within the cement itself gave an electrochemical signal that classical electrochemical tests for reinforced concrete durability would interpret as being due to steel corrosion processes. However, the actual observed resistance to chloride-induced corrosion was very high, as measured by extraction and characterisation of the steel at the end of a 9-month exposure period, whereas the steel embedded in white Portland cement mortars was significantly damaged under the same conditions.
Highlights
Steel reinforcing elements embedded in Portland cement (PC)-based concretes are protected from corrosion by a thin oxide film that is formed and maintained on rebar surfaces due to the high pH of the surrounding concrete (Böhni, 2005)
The core aim of this study is to evaluate the corrosion behavior of steel embedded in sodium silicate-activated blast furnace slag (BFS) mortars exposed to four different environments: an alkaline solution representing pore solution chemistry to prevent leaching, an alkaline chloride-rich solution to induce chloride transport without alkali leaching, plain water, and standard laboratory conditions
The current density values measured for the alkali-activated slag mortars and the blended White Portland cement (WPC)-slag mortars immersed in alkaline solution and in water were high, which would usually be taken to indicate high or very high risk of corrosion, but extracted rebars did not show evident pits or corrosion product layers
Summary
Steel reinforcing elements (rebars) embedded in Portland cement (PC)-based concretes are protected from corrosion by a thin oxide film that is formed and maintained on rebar surfaces due to the high pH of the surrounding concrete (Böhni, 2005). Various studies have demonstrated that a mixture of blast furnace slag (BFS) (even up to 70%) blended with Portland cement affected the corrosion process of steel rebar; the presence of BFS improved the corrosion resistance by slowing down chloride ingress, and decreased the corrosion rate and delayed the time at which corrosion occurs (Huang et al, 1996; Gu et al, 2000; Cheng et al, 2005; Yeau and Kim, 2005; Song and Saraswathy, 2006; Topçu and Boga, 2010; Garcia et al, 2014) This advantageous formation of a dense, impermeable microstructure through generation of additional calcium silicate hydrate (C-SH)-type reaction products is evident at higher BFS content and after a longer period of curing (Yeau and Kim, 2005; Topçu and Boga, 2010). Slag-blended Portland cements, and alkali-activated slag cements, generate hydrotalcitegroup phases as a hydration product, and this provides chemical binding effects (Ke et al, 2017a,b) that can further restrict the ingress of chloride
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