Abstract
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.
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