Effects of HVFA with the addition of bottom ash, NaOH, and CaCO₃ on self-compacting concrete (SCC) in tidal environments

The study evaluates the impact of exposure to tidal zones over 240 days on the mechanical, microstructural, and durability properties of High Volume Fly Ash - Self Compacting Concrete (HVFA-SCC) for coastal infrastructure applications. The concrete mixture was designed with 60 % and 70 % fly ash (FA) as a replacement for cement, 20 % bottom ash (BA) as a substitute for natural fine aggregate (NFA), 0.05 molar sodium hydroxide (NaOH) as an activator, and 5 % calcium carbonate (CaCO₃). A comprehensive analysis of workability, flexural strength, displacement, compressive strength, microstructure, chloride ion concentration, and sulfate ion concentration was conducted. The FA content increased, and BA's addition significantly reduced the flow diameter. The combination of NaOH and CaCO₃ successfully modified the rheological properties of the mixture, meeting the SCC criteria. All HVFA-SCC variations demonstrated improved flexural and compressive strengths up to 240 days, while the control mixtures experienced reductions in the flexural capacity of 15.98 % and 14.44 %, respectively. Using FA contributed to a denser microstructure, enhancing durability and flexural toughness. Microstructural analysis shows that HVFA-SCC-70 +BA is more resistant to sulfate attack than the control concrete, characterized by dense C-A-S-H and uniform distribution of elements, despite visible micro-cracks on the surface. The combination of HVFA-SCC-70 +BA with NaOH and CaCO₃ significantly enhances the durability of the concrete in tidal zones, keeping ion concentrations below the threshold level, although it is not as effective as HVFA-SCC-70.