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 Al-2(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.