Distinguishing between hydrated, partially hydrated or unhydrated clinker in hardened concrete using microscopy

Hydration of clinker particles is since long a topic of interest in both designing and optimizing cement composition and its quantity used in concrete. The interest for carefully observing and also quantifying the type or stage of clinker hydration in hardened cement paste is twofold. Firstly, the characteristics of the hydration features can give useful information on the actual hydration mechanisms (e.g., diffusion versus dissolution). Secondly, considering the different hydration stages more quantitatively can help to study for example the amount of hydration or hydration rate in different types of concrete. In this paper, it is shown that polarization and fluorescent light microscopy (PFM) and backscattered electron microscopy (BSE) can be used to quantify different clinker hydration types in hardened cement paste. Different stages or types of hydration could be distinguished in the clinker grains: (A) unhydrated clinker core without a visible hydration rim, (B) hydrated clinker ‘imprint’, (C) hydrated clinker core with an unhydrated clinker rim and (D) unhydrated clinker core with a clear hydration rim. These different types of clinker hydration can be quantified using point counting. In fluorescent light, the hydrated clinker zones contain more fluorescent dye compared to the unhydrated clinker parts and also to the rest of the cement paste. This suggests that the clinker may have partly or fully dissolved, creating porosity and that the hydration products have precipitated further away from the clinker particle, resulting in a less porous hardened cement paste. In other words, dissolution and precipitation processes might play a role in the hydration mechanism. Quantifying these different clinker types could also be useful for estimating for example the amount of hydration that took place, or for quantifying the ‘overcapacity’ of unhydrated clinker. The latter gives interesting information to possibly reduce the amount of clinker and as such lower the CO2 footprint of concrete