Alkali Activated Slag Mortars Provide High Resistance to Chloride-Induced Corrosion of Steel

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 behavior 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 (WPC and BFS) 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 characterization of the steel at the end of a 9-month exposure period, whereas the steel embedded in WPC mortars was significantly damaged under the same conditions.

Chloride-induced corrosion of steel embedded in alkali-activated materials: state of the art

“…), found by Glasser et al [21], and these concentrations are similar in AAS cements [12]. At the beginning the sulfides are oxidised to sulfate by the oxygen available in the system, thus depleting the oxygen concentration at the steel, creating a reducing environment and leading to a more negative value of corrosion potential [18,35], as seen in Figure 2 a). The reducing environment would favor the formation of Fe(II) rather than Fe(III), which forms the α-Fe 2 O 3 responsible for passivation [20].…”

Recent progress in low-carbon binders

“…Ma et al [222] linked chloride diffusivity, electrical resistivity, and corrosion testing of reinforced alkali-activated concretes, and highlighted the importance of sulfide (provided by blast furnace slag when used as a precursor in AAMs) in defining the corrosion rate post-initiation. The role of sulfide has also been identified in studies of steel corrosion in simulated alkali-activated slag pore solutions [223][224][225] [226], and in various types of mortar specimens [227][228][229]. The very high pore solution pH of some AAM binders has also been shown to generate unconventional threshold-like relationships in chloride initiation, and also to give chemical protection of steel reinforcement even at high chloride concentrations [230][231][232].…”

Chloride‐induced steel corrosion in alkali‐activated fly ash mortar: Increased propensity for corrosion initiation at defects

“…[ 15 ] In addition, recent work has highlighted the importance of the redox conditions in the pore solution of alkali‐activated binders, which is mainly controlled by sulfides from slag in the binder, for chloride‐induced corrosion of embedded steel. [ 9,16 ]…”