Following the reaction degree of amorphous phases in cements is very challenging. It becomes even harder when some additions (supplementary cementitious materials) are employed. Nowadays, calcined clays are attracting a lot of attention as it is possible to reduce the clinker factor by 50%, which leads to a 40% CO2 cement footprint reduction. Thus, the aim of our overall project is to follow the reaction degree of amorphous anhydrous cement phases (for instance metakaolin) to yield different amorphous phase(s) (f.i. C-A-S-H gel) by combined X-ray micro-computed tomography (μCT) and powder diffraction (PD) analyses at different ages of hydration. Furthermore, some microstructural features, like porosity, can also be mapped out which are important for durability.