The role of hydrotalcite in chloride and sulphate resistant properties of ground granulated blast furnace slag concrete

Chloride induced corrosion is the dominant cause of the deterioration of reinforced concrete structures. The penetrating aggressive chloride ions destroy the passive film around the reinforcement and cause localised corrosion which may subsequently result in complete destruction of the structure. Furthermore, concrete suffering from delayed ettringite formation resulting from a sulphate attack experiences non-uniform localised expansion and undergoes loss of its mechanical properties becoming, in extreme cases, crumbly and soft. The inclusion of ground granulated blast furnace slag (GGBFS) in concrete greatly restricts the mobility of chloride and sulphate ions. To date, the improved corrosion resistance of GGBFS concrete has been attributed to the formation of Friedelâs salt while many theories have been proposed to explain why it is effective in resisting sulphate attack.The author believes that the formation of Friedelâs salt is not sufficient to account for the remarkable improvement in chloride binding in GGBFS concrete. The author draws attention to the fact that high magnesia content in GGBFS leads to the formation of hydrotalcite (also known as anionic clay) during hydration. Hydrotalcite is used to remove chlorides or any other anions from wastewater due to its distinctive anion exchange property. Furthermore, sulphate (SO42-) ions possess the second highest affinity towards the inter-layer of hydrotalcite due to its anionic electrostatic potential energy. Therefore, this research focused on the hypothesis that hydrotalcite formation is the major reason for the enhanced chloride binding and improved sulphate resistant property of GGBFS concrete.In order to examine this hypothesis, the chloride binding ability of synthetic hydrotalcite was measured. X-ray diffraction (XRD) was conducted to identify and quantify the hydrotalcite in GGBFS and binary blend pastes containing GGBFS and ordinary Portland cement (OPC) in different proportions, and also to distinguish hydrotalcite from Friedelâs salt. Chlorides, in the form of NaCl, were added during mixing to assess the chloride binding abilities of these pastes by measuring their amounts of free and total chlorides. Furthermore, XRD was performed to investigate the effect of the NaOH activator on the stability of ettringite in GGBFS and binary blend pastes. In addition, the degrees of deterioration in activated GGBFS and binary blend cube specimens exposed to a sodium sulphate solution were observed.The results demonstrated the effectiveness of synthetic hydrotalcite in removing chlorides from saline water with XRD revealing that the formation of hydrotalcite occurred in GGBFS pastes and was accelerated in the presence of NaOH. Quantification showed that hydrotalcite was the most abundant of the crystalline phases in the activated GGBFS pastes and its proportion was correlated with the GGBFS contents in the binary blends. Quantification further confirmed that the crystalline proportion of hydrotalcite was more than five times that of Friedelâs salt in the activated GGBFS paste. Chloride measurements showed a significant reduction in the free chloride contents in GGBFS pastes. Free chloride gradually decreased with increases in the amounts of GGBFS in the blends. Furthermore, XRD demonstrated that ettringite was completely decomposed into its constituents in the presence of NaOH while quantification ascertained the formation of considerable amounts of hydrotalcite in the activated binary blends with higher GGBFS contents. Also, alkali activated GGBFS and binary blends specimens with higher GGBFS content offered enhanced resistance against aggressive sulphate ions.The results obtained strongly establish the prominent role of hydrotalcite in offering enhanced resistance against chloride and sulphate ions in GGBFS concrete