Scanning transmission electron microscopic studies of calcium silicate hydrates
Authors: Wenzel Olivia , Schwotzer Matthias , Scherer Torsten , Chakravadhanula Venkata S. K. , Gerdes Andreas 
Published: December 2016

Cement‐based materials play a vital role in our modern day concrete environment. During service life, they are often in contact with different aqueous solutions, which lead to structural and chemical changes and in turn, a possible deterioration of this material. Transport processes and intrinsic material properties are key components of such reactions. Evaluations on transport mechanisms, as well as shrinking and creep processes, require a detailed picture of the pore system. To this day, it has not been fully understood yet even though the nanometer small gel pores of the calcium silicate hydrate (CSH) phase have a significant impact on material performance. This calls for additional characterizations on a much smaller scale. However, the gel pore structure of the CSH is complex, heterogeneous and inaccessible making evaluations problematic. For this purpose, an analytical approach on multiple scales was applied. To determine the potential impact of gel pores on material properties and thus, performance, a structural characterization of a model cement paste was performed using scanning transmission electron microscopy (STEM) along with gas adsorption measurements (multi‐point BET) in addition to standard mineralogical techniques. The model cement paste consisted of synthesized and hydrated tricalcium silicate. For this purpose, the focused ion beam (FIB) preparation technique was used, which is a fairly novel method concerning cementitious building materials. The challenge was the preparation of the FIB‐lamella due to the water content, the unstable nature of the sample's constituents and the high porosity of 20‐30 %. Manual adjustments had to be performed throughout the entire milling process in order to prevent the disintegration of the lamella. Furthermore, it was necessary to build multiple robust frames by depositing high amounts of platinum. The lamella was analyzed with the FEI Titan 80‐300 TEM in low‐dose (80 keV) mode. STEM images reveal the complex structure of the CSH. It consists of 3‐5 nm thin CSH foils oriented to one another in a honeycomb structure. The pore size spans from only a few nanometers up to 50 nm with a maximum at 15 nm. These gel pores touch and intersect each other producing a sponge‐like conduit network. A similar pore width distribution was also observed with BET. Multiple selected area diffraction (SAED) patterns reveal the volatile nature of this material. The first pattern showed a broad diffuse ring between 2.5 and 3 Å, which can be attributed to CaO. In the following diffraction patterns, this ring becomes more pronounced with many small individual reflexes (Si‐Si 1.6 Å) and further rings appear. These observations point towards a crystallization induced by the incident beam. This study shows that such advanced techniques can also be applied to building materials. The information gained is not only valuable to basic research of cementitious materials but also to applied problems related to deterioration processes. Further research and practice will be devoted to improve the FIB handling and technique for cement‐based materials and to the analysis of other cement‐based compounds, in order to investigate the relationship between nanoscale features and deterioration reactions.