Design of a Multi-Ion Detection System Based on IoT Technology and Its Application in Cement-Based Materials

Simultaneous multi-ion detection is important for interpreting leaching, corrosion, hydration, and solidification processes in cement-based materials, because these processes are controlled by coupled ion migration, binding, and precipitation-dissolution reactions. Conventional methods such as pore-solution extraction, ion chromatography, inductively coupled plasma optical emission spectroscopy, and single-ion potentiometric measurements provide useful chemical information, but they generally rely on discrete sampling or isolated ion channels and therefore have limited ability to capture time-aligned multi-ion evolution. In this study, an IoT-based in situ multi-ion detection system was developed by integrating ion-selective electrodes for Cl^-, Ca²⁺, F^-, and H⁺ with an ADS1115 analog-to-digital converter, an ESP32 microcontroller, and a voltage amplification module. The system achieved minimum resolvable concentrations of 10^-5 M for Cl^- and F^- and 10^-4 M for Ca²⁺, while maintaining pH measurement over the range of 2-12. Ten consecutive measurements at 0.01 M showed relative standard deviations below 0.12%, indicating good short-term repeatability under laboratory calibration conditions. Interference and temperature tests showed that Br^- and NO₃^- affected the chloride channel at high concentrations, Ca²⁺ reduced free F^- activity through Ca-F precipitation equilibrium, and the temperature drift of Cl^- and F^- electrodes changed direction with concentration, whereas the Ca²⁺ response decreased monotonically with increasing temperature. When applied to phosphogypsum-cement hardened pastes, the system captured rapid Ca²⁺ release, low-level F^- fluctuation controlled by Ca-F interaction, non-monotonic Cl^- release, and alkaline pH evolution on the same time axis. Compared with existing single-ion or offline methods, the proposed system provides synchronized in situ evidence for interpreting coupled ion leaching in cement-based solid-waste systems.