Investigating the evolution of the cementitious pore solution during the first 15 Minutes of hydration via dynamic crossflow filtration

The analysis of cementitious pore solutions provides valuable insights into the chemical properties of the cement clinker phases during cement hydration. Accordingly, the pore solution chemistry of Portland cement has been extensively studied. However, the first pore solution samples are usually obtained after a few minutes of hydration time. Therefore, pore solution data often lacks data points in the first seconds and minutes of hydration. During this initial stage, ettringite forms as the first nanostructured hydrate phase, creating new surfaces and decreasing the water-to-solid ratio, thereby influencing the rheology of fresh cementitious materials. A method to sample the pore solution during this stage with a large sampling frequency has been missing. This study demonstrates the application of dynamic crossflow filtration for obtaining early cementitious pore solutions at a high time resolution and to investigate their chemical composition under varying conditions. We systematically vary the water-to-cement ratio, temperature, and cement chemistry, and analyze the temporal evolution of ion concentrations using ICP-OES. The earliest pore solution sampling is feasible at 15 seconds after the start of hydration for high water-to-cement ratios. The results show that increasing the water-to-cement ratio reduces ion concentrations, particularly affecting calcium sulfate nucleation, while lower temperatures slow down cement dissolution and hydration product formation. Additionally, our findings confirm that the filter pore size (200 nm vs. 400 nm) does not impact determined ion concentrations, validating the exclusion of nanoparticles from the filtrate. The new method of ultrafast pore solution sampling at very early times of hydration is expected to be particularly helpful for hydration studies of the aluminate phase of Portland cement.