An Experimental Study on a Paper-Based Sensor for Chloride Ion Measurement in Fresh Concrete

The chloride ion (Cl–) content in concrete strongly affects reinforcement corrosion and durability, and recent increases in the use of alternative fuels in cement manufacturing have raised concerns regarding unintended chloride introduction. This study proposes a simple paper-based sensor for rapid on-site chloride quantification in fresh concrete. The sensor was fabricated by sequentially impregnating filter paper with AgNO3 and K2Cr2O7 solutions to form a silver dichromate (Ag2Cr2O7) reactive layer. When exposed to chloride-containing solutions, AgCl precipitates form along the strip through capillary-driven transport, and the precipitation length is used as a quantitative indicator. Sensors prepared with AgNO3 and K2&Cr2O7 concentrations ranging from 0.005 to 0.5 N were evaluated using chloride solutions of 30-1000 ppm. Lower reagent concentrations (≤ 0.05 N) resulted in significantly enhanced sensitivity, enabling clear visual detection even at 30 ppm Cl~. Time-dependent measurements showed that more than 84% of the final precipitation length formed immediately after immersion, with reaction equilibrium reached within approximately 20 minutes. These results demonstrate that the proposed paper-based sensor enables rapid, equipment-free, and visually interpretable chloride measurement, showing strong potential for practical on-site application in fresh concrete quality control.