Combined effects of carbonation and salt solution on chloride desorption, pH, and composition of fly ash blended pastes

Abstract

The most consumed human-made material, concrete, is permeable and chloride-bearing deicers penetrate the concrete, causing deterioration. Chloride ions exist in concrete in two forms: free and bound chlorides. Free chlorides are the primary cause of chloride-induced corrosion because they can move in concrete pores, reaching the rebar surface to initiate corrosion. Bound chlorides are those chloride ions that are adsorbed by the concrete matrix and cannot actively participate in reinforcement corrosion. Although chloride binding is a beneficial process, chloride binding is reversible. In addition, most concrete infrastructures are subject to environmental attacks such as carbonation, causing pH reduction in the concrete pore solution and subsequently resulting in the release of bound chlorides (chloride desorption). The combined effects of carbonation and chloride attack on fly ash containing pastes are not well-studied, particularly concerning their impact on chloride desorption, pH changes, and compositional changes. This investigation aims to address this gap by exposing fly ash-blended cement paste to NaCl salt solution and carbonation. This study quantifies chloride desorption, monitors pH change, and analyze the compositional changes in pastes, including the measurement of Friedel's salt, portlandite, and calcite contents. Samples were first exposed to NaCl solution for 35 days and then 4 weeks of accelerated carbonation, during which their pH and composition at different depths were monitored. XRD and TGA tests were utilized to identify and quantify the main phases. The results show that carbonation reduced the pH from 13 to nearly 11 in OPC pastes and 10 in fly ash pastes. The inclusion of 15 % fly ash led to weakened carbonation resistance compared to OPC. After combined exposure to the salt solution and carbonation, the calcite content, representing carbonation, in fly ash pastes significantly increased by 190.9 % after one week carbonation and 282.1 % after two weeks of accelerated carbonation. Friedel's salt content, indicating bound chloride, dropped by roughly 78 % in OPC paste and 87 % in fly ash paste after one week of carbonation and completely dissolved after two weeks of carbonation. The results emphasize the importance of considering chloride desorption, similar to chloride binding, which has been incorporated into service life models. This negligence can lead to an overestimation of the service life of concrete infrastructures.