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Dyfuzja jonów chlorkowych w zaczynie cementowym poddanym działaniu nasyconego roztworu NH4Cl Chloride ions diffusion in hydrated cement paste immersed in saturated NH4Cl solution
Abstract
The goal of the research presented in this paper was the study of
chloride ions diffusion in cement pastes made from two cements
with various tricalcium aluminate content. For the measurements
of free Cl– ions concentrations the potentiometry of models water
solutions obtained from powdered cement paste, was used.
Simultaneously, the changes of paste phases composition were
analysed.
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Solubility relationships between Friedel’s salt and hydroxy AFm suggest an essentially complete solid solution with only minor discontinuities. Solid–aqueous partitions were determined by equilibration of 11 compositions at 10 mol % intervals at ∼20 °C in experiments of 6–17 months’ duration. Aqueous chloride concentrations >2mM, approximately, are required to incorporate significant Cl in hydroxy AFm; the process is complete, giving essentially pure Friedel’s salt, at a chloride content of ∼14 mM. The self-generated pH of the solid solutions are high: pH >12 for all compositions. The implications for diffusion of chloride ions through cement paste are discussed.
Pore solution study has been carried out on 2.43 and 14% C3A hardened cement pastes. Data have been analyzed in conjunction with the data developed in two pore solution studies made by Page and Vennesland and Diamond using 7.37 and 9.1% C3A mature cement pastes. The results show that C3A and alkali contents of a cement have significant effect on its chloride-binding capacity. For similar alkali content, the levels of free chlorides in the pore solutions of 2.43 and 9.1% C3A cement pastes are respectively 4.7 and 2.8 times more than in a 14% C3A cement. The alkali content of a cement appears to have an inhibiting effect on its chloride-binding capacity. However, this effect is overshadowed by a conjoint strong elevation of the OH− ion concentration in the pore solution due to cement alkalies, causing a net lowering of the Cl−/OH− ratio which roughly ascertains corrosion risk. Threshold chloride values have been evaluated for different C3A cements. The threshold chloride content for a typical Type I portland cement with C3A upto 8% and Na2O equivalent upto 0.60%, may be taken as 0.4% chlorides by weight of cement. However, for a similar alkali cement with a high C3A content of about 14%, the chloride threshold value is 2.5 times higher and may be taken as 1.0% by weight of cement.
It is well known that solid calcium hydroxide may act to inhibit chloride induced corrosion of steel in concrete. This hypothesis has been recently extended to include the inhibitive and aggressive nature of other solids that exhibit pH dependent dissolution behaviour. Concrete constituents that resist a local fall in pH may inhibit corrosion, while bound chloride released by such a fall may participate in corrosion initiation. In this work the pH dependent solubility of chloride in concrete is demonstrated. It is shown that most of the bound chloride is released as the result of the rapid dissolution of at least two hydrated phases in chloride contaminated OPC concrete. This occurs at pH values that are high (above 11.5) compared to that considered necessary to sustain local passive film breakdown at the site of a nucleating pit. Thus, in theory, the corrosion risk presented by bound chloride at the steel–concrete interface may be very similar to that presented by free chloride.
The relative tendency of different portland cements to remove chloride ions from the mix water by forming chloro-complexes is an important factor in the study of the corrosion of the embedded steel. The tri-calcium aluminate (C3A) phase of the ordinary portland cement (OPC) plays a dominant role in binding the chloride ions, but little quantitative information is available in the literature concerning the relative influence of the tetra-calcium aluminoferrite (C4AF) phase of the cement. The present investigation is an attempt to elucidate the role of C4AF in the sulphate resistant portland cement with respect to chloride binding. It is concluded that the chlorides in SRPC are bound in the form of the chloro-complex C3F.CaCl2.10H2O which is derived from the C4AF phase of the cement.