Humidity Driven Transition from Insulator to Ionic Conductor in Portland Cement

Area of Science:

• Materials Science
• Electrochemistry
• Physical Chemistry

Background:

• Ionic conduction is crucial for cementitious materials.
• Understanding cement's electrical properties is vital for durability and performance.
• Anhydrous and hydrated cement pastes exhibit complex conduction mechanisms.

Purpose of the Study:

• To investigate ionic conduction in anhydrous cement and hydrated cement pastes.
• To determine the influence of aging, temperature, and humidity on conductivity.
• To identify charge carriers and assess the impact of environmental factors like CO2.

Main Methods:

• Electrochemical impedance spectroscopy was used to measure bulk and interfacial conductivities.
• Cement samples were aged for 5-25 days.
• Samples were subjected to controlled humidity (100% RH) and temperature (50-100 °C) conditions.
• Exposure to CO2 and subsequent acid treatment were performed.

Main Results:

• Bulk conductivity ranged from 10^-3 to 10^-2 S cm^-1, independent of hydration level.
• Interfacial conductivity varied from 10^-7 to 10^-3 S cm^-1, dependent on structural defects.
• Conductivity increased significantly at 100 °C, especially after moistening.
• Hydroxide ions were identified as the primary charge carriers.
• CO2 exposure decreased conductivity, but acid treatment restored bulk conductivity.

Conclusions:

• Ionic conduction in cement is primarily governed by hydroxide ions.
• Temperature and humidity significantly impact cement conductivity.
• Structural defects play a role in interfacial conduction.
• Carbonation negatively affects conductivity, but it can be mitigated by acid treatment.