Cement is the most widely used construction material and its demand in recent decades has increased exponentially. Cement is usually mixed with aggregates and water to form concrete. It is widely used in construction because it can be easily applied by being poured into molds (high fluidity) and can harden itself into a strong material over time (high strength). However, a material is unlikely to have both high fluidity before setting and high strength after setting. Therefore, optimizing the rheological and elastic properties of the cement mix is important for achieving high performance.
The hardening process of cement is divided into five stages based on heat evolution rate: pre-induction (I), induction (II), hydration rate increasing or setting (III), hydration rate decreasing (IV), and low hydration rate (V) stages. The induction stage is a crucial stage for the workability of concrete because it flows easily during this stage. The induction stage lasts from a few minutes to several hours, after which the setting stage starts. The setting stage is also important in that the concrete begins to develop its strength at this stage.
Despite the importance of these two stages, factors and mechanisms remain unclear that influence and determine the chemo-mechanical changes in concrete and the resulting length of the early stages. So far studies on the cement hydration process have been conducted using methods such as electron microscopy, X-ray diffraction, IR spectroscopy, and thermal analysis. While these characterization tools have provided information on the hydration process and the microstructure of each cement hydration phase, these tools have been less successful in tracking changes in chemical composition. These methods either require sample preparation such as polishing and grinding, or involve processes such as heating or dehydrating, which can eventually lead to sample damage or undesired changes in chemical composition.
Therefore, in-situ and operando identification and characterization methods such as Raman spectroscopy were implemented. In-situ characterization method requires non-destructiveness and high speed. Current state-of-the-art confocal Raman spectroscopy systems have high speed (<200 ms/spectra), high spatial resolution (<360 nm), and high depth resolution (less than 5 µm). Raman microscopy is stain-free and requires minimal or no sample preparation. In addition, water does not cause interference in Raman microscopy. Not only does Raman spectroscopy meet both requirements, but it also evinces strengths in accurate in-situ characterization in various other ways.
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