Free and bound chloride relationships affecting reinforcement cover in alkali activated concrete

Abstract

This paper investigates the free chloride profiles, diffusion parameters and chloride binding capacity of an alkali activated concrete (AACM) together with a control Portland cement (PC) concrete. Ggbs based AACM concrete specimens with different molarity of activator were exposed to a 5% NaCl solution for 540days to determine their free chloride diffusion properties. The relationships between the free and bound chloride concentration were determined by applying Freundlich and Langmuir isotherms. The required cover to steel reinforcement for corrosion prevention is derived to satisfy the limiting thresholds of free and bound chloride concentrations.The results show that Fick's second law of diffusion applies to the free chloride profiles of AACM concrete and provides higher values of diffusion coefficients than a similar grade of PC concrete. The relationship between the free and bound chlorides is defined by the Langmuir isotherm. PC concrete has higher chloride binding capacity than AACM concrete for both water and acid soluble chlorides. Less concrete cover to steel reinforcement is required in AACM than PC concrete when calculated by using the bound chloride concentration threshold limit. The values of cover based on the corresponding free chloride limit in AACM concrete are higher than its bound chloride values. Ca 6 Fe 2 O 6 .CaCl 2 .10H O kuzel's salt (KS) ISE ion selective electrode D ref diffusion coefficient at reference time t t ref reference age (days) m age factor C ref surface chloride concentration corresponding to the time k constant for surface chloride concentration C-S-H calcium silicate hydrate CH portlandite AFm monosulfoaluminate AFt ettringite C-S-H calcium silicate hydrate N-A-S-H sodium aluminosilicate hydrate SCMs supplementary cementitious materials

Chloride-initiated corrosion in alkali activated reinforced concrete

“…The corresponding values for AACM 2 are 6.67%, 8.02% and 10.70% while for AACM 3 they are 7.71%, 9.05% and 11.93% [16]. Hence, fewer chloride ions diffuse through the pore matrix of AACM concrete relative to PC concrete [35], which will influence chloride induced corrosion of embedded steel.…”

“…Therefore, at 690 days of exposure the acid soluble chloride concentrations would have considerably exceeded the permissible level for corrosion initiation of 0.40% by weight of cement which is stipulated in standards for PC concrete [22]. However, corrosion in AACM concrete is controlled by its free chloride in the pore fluid and an acid soluble chloride threshold is therefore not applicable [35].…”

“…High chloride ingress occurred in concrete up to 860 days due to both the diffusion and capillary absorption of chloride ions occurring simultaneously when concrete was exposed to cyclic wetting and drying [2,52]. There is insignificant depletion of the free chloride concentration in both AACM and PC concrete between 860 and 1750 days due to the release of physically bound chloride into their pore solution [23,35].…”

“…Values of 3 and above have been given by tests using more representitive environments (45,54). Many recent researchers [5,8,14,39] have similarly investigated corrosion of steel bars in simulated pore solutions of alkali-activated concrete while neglecting the influence of physical chloride binding [23,35] and the redox environment [5] around the steel interface in AACM concrete.…”

Chloride binding in cement paste with calcined Mg-Al-CO3 LDH (CLDH) under different conditions

“…Firstly, there is a competition between OHand Clduring their adsorption in the interlayers of AFm hydrates of compositon, [Ca2Al(OH -)6•nH2O] + , and the former enters the layered structure of AFm prior to the latter. In other words, low pH results in more adsorption sites for free chlorides to form Friedel's salt [34]. Secondly, an increased pH of the chloride solution increases the dissolution of Friedel's salt.…”