Chloride transport and the resulting corrosion of steel bars in alkali activated slag concretes

Abstract

As the relative performance of alkali activated slag (AAS) concretes in comparison to portland cement (PC) counterparts for chloride transport and resulting corrosion of steel bars is not clear, an investigation was carried out and the results are reported in this paper. The effect of alkali concentration and modulus of sodium silicate solution used in AAS was studied. Chloride transport and corrosion properties were assessed with the help of electrical resistivity, non-steady state chloride diffusivity, onset of corrosion, rate of corrosion and pore solution chemistry. It was found that: (i) although chloride content at surface was higher for the AAS concretes, they had lower chloride diffusivity than PC concrete; (ii) pore structure, ionic exchange and interaction effect of hydrates strongly influenced the chloride transport in the AAS concretes; (iii) steel corrosion resistance of the AAS concretes was comparable to that of PC concrete under intermittent chloride ponding regime, with the exception of 6 % Na 2 O and Ms of 1.5; (iv) the corrosion behaviour of the AAS concretes was significantly influenced by ionic exchange, carbonation and sulphide concentration; (v) the increase of alkali concentration of the activator generally increased the resistance of AAS concretes to chloride transport and reduced its resulting corrosion, and a value of 1.5 was found to be an optimum modulus for the activator for improving the chloride transport and the corrosion resistance.

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

“…The specimens were totally immersed in the alkaline medium, and no oxygen entered into the specimen, and this led to strongly reducing conditions where sulfide was the dominant redox-active species. Moreover, any available oxygen can be consumed locally by the sulfide species becoming oxidized, either in part or in full (Holloway and Sykes, 2005;Ma et al, 2016). In those aqueous systems, where aqueous sulfide is in excess over Fe 3+ , iron is partially reduced and the Fe 2+ generated can react with S 2− to precipitate as FeS (Wan et al, 2017).…”

“…However, fewer studies have been published regarding the actual corrosion process of steel embedded in alkali activated slag mortars in the presence of chloride (Holloway and Sykes, 2005;Torres Gomez et al, 2010;Park et al, 2015;Ma et al, 2016). AAS mortars activated with Ca(OH) 2 presented a higher corrosion resistance than AAS mortars activated with KOH and NaOH activators, which was attributed to the formation of Ca(OCl) 2 to remove free chloride from the pore solution, and thus to mitigate the corrosion risk (Park et al, 2015).…”

“…AAS mortars activated with Ca(OH) 2 presented a higher corrosion resistance than AAS mortars activated with KOH and NaOH activators, which was attributed to the formation of Ca(OCl) 2 to remove free chloride from the pore solution, and thus to mitigate the corrosion risk (Park et al, 2015). Ma et al (2016) found that an increased alkali concentration in the activator, and the use of a sodium silicate activator with a modulus (molar ratio SiO 2 /Na 2 O) of 1.5, gave a desirable microstructure which improved the resistance to chloride transport and corrosion. Moreover, the corrosion rate of the steel bars was significantly influenced by the free sulfide concentration in these alkaline concretes, consistent with a recent classification proposed by Mundra et al (2017b).…”

Steel corrosion in reinforced alkali-activated materials

“…Therefore, elemental sulfur could play a role in inhibiting pitting corrosion only if sufficient oxygen is available at the SCI. Higher corrosion rates [48], higher i corr and lower R p values [42,49] have been reported for steel-reinforced AAS exposed to chloride solutions in comparison to those observed in PC. However, visual examination of the steel rebars in each of these studies found that the extent of corrosion for steel embedded in AAS was much lower than those in PC, for instance, no evidence of corrosion was observed by Criado et al [42], Fig.5, which contrasts strongly with the electrochemical observations.…”

Influence of slag composition on the stability of steel in alkali-activated cementitious materials

“…The drop in the redox potential is a result of the release of reductants from the slag during its reaction (whether induced by alkali activation or in PC blends), and the inability of the cathode to generate a current that overcomes the HSoxidation peak. Ma et al [31] reported that the corrosion rate generally decreased with an increase in the concentration of sulfide due to the generation of a strongly reducing environment.…”