In situ pH monitoring in accelerated cement pastes

The fast pH changes in ordinary Portland cement pastes (with and without C₃A), caused by the addition of an Al₂(SO₄)₃ based setting accelerator were in situ monitored by optical pH sensors. The results obtained clearly demonstrated the ability of this novel method to determine the pH with a high temporal resolution. Additionally, the distribution of the accelerator within the cement paste was assessed by fluorescence imaging with the use of a fluorescein tracer. Instantly after the addition of the accelerator and the subsequent formation of ettringite, the pH dropped from ~13 to ~10. The pH drop and recovery rate was controlled by the accelerator dosage and availability of Ca²⁺ and OH⁻. The methods presented would help to predict and assess the effects caused by chemical admixtures (e.g. setting accelerators) on the cement hydration progress.