Monitoring Early Cement Hydration with Coda Wave Interferometry

The inorganic chemical reaction of hydraulic binders, such as cement with water, is a complex mechanism and a vital part of every building process involving concrete. Dur ing this hydration reaction, the system develops strength due to the formation of various solid hydration phases over time. In the case of ordinary Portland cements (OPC), the main hydration phase is called C-S-H phase, and is responsible for the early- and long-term strength development of cement. Therefore, the formation of this phase determines the time for continuing construction work, e.g. removing formwork or concreting of the following structures. Under laboratory conditions and for relatively small samples, methods such as quantitative X-ray diffraction analysis are able to monitor the progress of hydration over time. However, it is more challenging to monitor the hydration of larger structures under non-laboratory conditions. In this study, we propose a sensitive and robust monitoring method to detect and quantify the formation of various hydration phases during the early hydration of OPC using Coda Wave Interferometry, based on the evaluation of ultrasonic waves with embedded piezoelectric sensors. We investigated the signal correlation and velocity perturbation of ultrasonic signals during the first 24 hours of OPC hydration. We identified periods during which these parameters varied significantly and periods during which the signals stabilized, in our case approximately 18 hours after initiation of the reaction. Simultaneously, we monitored the hydration of OPC using similar but smaller samples by means of heat flow calorimetry and a Vicat penetration test. We predicted the evolution of the main hydration phases’ mass by thermodynamic modeling and validated our results as well as the assignment of characteristic periods to the formation of specific hydration phases in the time domain. Our proposed method adds to the spectrum of ultrasound-based methods for hydration monitoring. Our approach aims at a more sophisticated prediction of the early-age strength development of cement and concrete, even under site conditions and for large structures.