Assessment of Chloride Resistance and Binding Mechanism of Portland-Limestone Cement (PLC) Concrete

Abstract

Concrete production is responsible for approximately 8% of global CO₂ emissions, primarily due to the high clinker content and energy demands associated with Portland cement (PC) manufacturing. In response to growing environmental concerns, Portland-limestone cement (PLC), which partially replaces clinker with interground limestone, and the use of supplementary cementitious materials (SCMs) such as fly ash and slag have been widely promoted as low-carbon alternatives. However, the durability performance of high-limestone PLC systems remains a topic of concern, particularly in terms of chloride ingress resistance and associated binding behavior. This study investigates the chloride resistance and chloride binding capacity of fifteen PLC concrete mixtures, incorporating varying limestone replacement levels and SCM types, under a water-to-powder ratio (W/P) of 0.4. A combination of bulk diffusion testing and acceleration tests was employed. Chloride binding was assessed using both acid-soluble and water-soluble extractions to quantify total and free chloride contents. The results show that SCMs significantly enhance chloride resistance. A strong correlation was observed between electrical resistivity and the apparent diffusion coefficient (Da), emphasizing the role of pore structure in chloride resistance under PLC systems. Moreover, the Langmuir isotherms provided the best fit for chloride binding behavior across all pure PLC mixtures, and analysis revealed that the Al₂O₃/SO₃ ratio, rather than the Al₂O₃ content, is a more reliable indicator of chloride binding capacity. While fly ash contributed to enhanced chloride binding, the effect of slag was less dependent on the limestone content due to its different chemical binding mechanisms. These findings underscore the need for a high quality control of cement manufacturing along with optimized concrete mix proportioning to ensure the reliable performance of high-limestone, low-carbon PLC mixtures in chloride-exposed environments.