Novel Acid-Resistant Hybrid Mortar with Calcium Sulfoaluminate and Zeolite for Impressed Current Cathodic Protection of Bridge Infrastructure

Impressed current cathodic protection (ICCP) systems can experience acidification, which deteriorates the interface between the anode and the anode backfill mortar. This deterioration may necessitate premature intervention to remove and reinstate the backfill and, in some cases, replace the anode. If left unaddressed, acidification ultimately leads to debonding between the anode and the backfill mortar, resulting in the failure of the ICCP system. This paper presents the development of a specialised acid-resistant hybrid mortar designed for ICCP systems used to protect reinforced concrete bridges in marine environments. It also investigates the effects of acidification on the physical and mechanical properties of the proposed anode backfill mortars. Additionally, the study characterises acidification products from both field-extracted ICCP systems and laboratory-based accelerated testing, providing deeper insights into the acidification mechanisms. Novel mortar samples were subjected to varying concentrations of hydrochloric acid (HCl) under accelerated testing conditions. The incorporation of supplementary cementitious materials (SCMs), calcium sulfoaluminate (CSA) cement and zeolite significantly enhanced the strength and durability of the backfill mortars in acidic environments, while maintaining compliance with the electrical resistivity requirements (20–100 kΩ·cm) for ICCP systems. The lowest compressive strength loss observed in the developed hybrid mortar was 54% after 28 days of immersion in 5% HCl and 83% in 15% HCl. Microstructural analyses revealed that gypsum formation and chloride–sulphate competitive binding interactions are key mechanisms contributing to the improved acid resistance, particularly in CSA cement-containing formulations.