Monitoring Apparent pH Value in Geopolymer Concrete Using Glass Electrode

Geopolymer concrete (GPC) is estimated to reduce 80% carbon footprint in construction projects compared with ordinary Portland cement (OPC). Previous studies on GPC have been focusing on assessment of paste or mortar while the long-term durability of GPC has not been extensively investigated yet. This research aims to monitor carbonation process of GPC by measuring in-situ apparent pH value under accelerated carbonation test in laboratory environment. Glass pH electrode is typically applied for liquid medium and considered to be unsuitable for OPC due to high alkaline condition. We have endeavoured to test the glass electrode in GPC assuming the alkalinity in GPC would be much lower than OPC. Two pH probe sensors were embedded into GPC specimens with different depth and seal condition. After 18 hours heat cures and 7 days curing in ambient environment, one of GPC specimens was then relocated to a carbonation chamber with 1% carbon dioxide concentration. The glass electrodes were found to survive the GPC environment with real-time apparent pH value registered since the GPC specimens were poured. It is also observed that the apparent pH values were affected significantly by the level of humidity inside the concrete. Saturation experiments were then conducted to provide valid pH readings. As a result, we successfully monitored pH decreasing from 11.1 to 10.76 after three months carbonation experiment. The results are also validated by comparing against the data acquired by traditional concrete grinding test. The research features the first trial of its kind and has contributed to comprehensive understanding about corrosion mechanism of the new GPC material.