Chloride in cement

Abstract

Portland cement–steel composites frequently take up chloride from their service environment. The degradation processes and test methods are described through a critical review of the literature. Plain cement paste is not much affected by chloride except for increased solubilisation of cement solids, but chloride is detrimental to the passivation of embedded steel. Emphasis is placed on establishing the underlying physicochemical concepts and mechanisms of corrosion, and integrating these into a holistic picture. Simplistic calculations and test conditions probably do not reproduce well the complex reactions with cement components occurring at the steel–cement interface and the concept of a ‘threshold' – defined in terms of a critical chloride content or chloride:hydroxide ratio – is just an approximation at best: other cement components are involved, as is the oxygen activity at the interface. Chloride binding can retard the migration of chloride in concrete and specimen calculations are presented showing that the main binding mechanism arises from the AFm, [Ca2(Al,Fe)(OH)6].X.nH2O, content of the paste. Suggestions are made to enhance the value of experimental approaches to corrosion.

Anion capture and exchange by functional coatings: New routes to mitigate steel corrosion in concrete infrastructure

“…The binding of Cl − by cementitious phases can significantly suppress Cl − /OH − in the pore solution and thereby increase the service life of infrastructure [23–25]. For example, Cl − binding can occur by ion exchange into alumino-ferrite monosubstituent (AFm) compounds, or by sorption onto C-S-H or other compounds [26].…”

“…ACE by AFm’s is significant as (i) it operates in a Cl − concentration range relevant to typical ingress conditions (≈ 14 mmol/L [29]), (ii) it has a much larger binding capacity per unit mass than Cl − sorption by C-S-H [25], and (iii) Cl − species taken up by AFm’s are more strongly bound into their structure (i.e., reflecting structural incorporation), than binding by the C-S-H which represents weaker physisorption. Therefore, the Cl − binding capacity of cementitious formulations is linked to the mass fraction of AFm phases present [30].…”

“…Therefore, the Cl − binding capacity of cementitious formulations is linked to the mass fraction of AFm phases present [30]. Even though the specific binding capacity of AFm phases is high, the total binding capacity of OPC-based formulations is limited, since AFm’s comprise only 5 % to 15 % by mass of the solids in a hydrated OPC binder, offering only limited retardation of steel corrosion [19,23–25]. The Cl − binding capacity of OPC-based binders can be increased by the addition of supplementary cementing materials (SCMs: e.g., ground-granulated blast-furnace slag [31,32], fly ash [33]) which increases the Al 2 O 3 /SO 3 ratio of the mixture, leading to the enhanced formation of AFm’s at the expense of AFt [19,34,35].…”

“…The Cl − binding capacity of OPC-based binders can be increased by the addition of supplementary cementing materials (SCMs: e.g., ground-granulated blast-furnace slag [31,32], fly ash [33]) which increases the Al 2 O 3 /SO 3 ratio of the mixture, leading to the enhanced formation of AFm’s at the expense of AFt [19,34,35]. However, these effects are confounded with changes in the transport properties (e.g., permeability, and diffusivity) and the limited reaction of the SCMs that become relevant when OPC is replaced by SCMs at high levels [25,34,36]. As a result, while SCMs do indeed contribute reactive alumina that is required for AFm formation, they do so at a level that only slightly increases the mass of AFm’s formed.…”

Editorial

“…The first paper by Galan and Glasser (2015) is concerned with the state of chloride in cement, its reaction with concrete and its reaction with the embedded steel bar and diffusion process. It compactly and precisely summarises and reviews up-to-date knowledge of chloride.…”