Assessing the combined water of cement pastes: comparing solvent exchange and silica gel as hydration stoppage methods

The assessment of the combined water is a reliable method to monitor the hydration of cement pastes. This method is also being used to screen different SCMs and cement formulations. However, to obtain reliable data it is necessary to stop the hydration of specimens, removing the free water. In this sense, this work evaluates the effect of different hydration stoppage methods on assessing the combined water of cement pastes. Three procedures were investigated, two using solvent exchange (RILEM and CEMtec), including the one recommended by RILEM TC 238, and the other using silica gel (Silica4h). Firstly, two cement formulations (OPC and LC50) were evaluated using thermogravimetric analysis and X-ray diffraction. The results of combined water at 7 and 28 days were similar independently of the hydration stoppage technique. A repeatability test was also conducted and demonstrated that all three methods presented good repeatability with a low coefficient of variation. Then, six Brazilian commercial cements had their hydration stopped at 7 and 28 days using the studied procedures. The results of combined water were comparable regardless of the used method. A qualitative analysis was also carried out indicating that for the assessment of combined water the Silica4h is the most practical and cost-effective method to stop the hydration of cement pastes and RILEM is the fastest procedure. Finally, both procedures with solvent exchange require attention due to their health and safety issues, solvent disposal, and acquisition.