Carbonation, decalcification and recalcification of Calcium-Silicate-Hydrate

Background The hydration of cement is a process involving multiple compounds and steps. From the moment water is added to the cement mix, cement grains start dissolving to form an ionic solution. Oversaturation of ions leads to the precipitation of hydration products, in which Calcium-Silicate-Hydrate (C-S-H) is the main constituent phase, bearing most of the calcium in hydrated cement. The physical and chemical properties of C-S-H is of great interest when it comes to designing cementitious materials. Characterization of C-S-H in cement paste is, however, complicated due to its complex structure, lack of long-range order, variation in Ca/Si ratio across the parent specimen, and potential admixtures of Ca(OH)2 (portlandite), CaCO3 (calcite, vaterite or aragonite) or unreacted C3S. Preparation of C-S-H In an effort to understand C-S-H structure, physical and chemical properties, studies have extensively used homogeneous, synthetic single-phase C-S-H prepared from the direct reaction of CaO and SiO2, or the double decomposition of sodium silicate and calcium nitrate. The main advantage of synthetic route is the possibility to easily modify Ca/Si ratio of C-S-H, since Ca/Si is arguable the major factor influencing C-S-H structure. On the opposite, C-S-H obtained from hydrated C3S or OPC paste ensures simlarities to the natural process of hydration. Degradation of C-S-H One of the major concerns when it comes to testing on synthetic C-S-H stems from the popular employment of its powder form. Processes such as carbonation and decalcification are heavily governed by mode of transport within the samples of interest. Likewise, decalcification is a combination of dissolution and diffusion, with the diffusion usually being the rate-limiting process. Therefore, the mode of transport in the specimens will be the governing factor to decalcification. The transfer of carbonation and decalcification knowledge from C-S-H powder to C-S-H embedded in cement matrices requires synthetic C-S-H to be prepared in a 3D shape. In this project, C-S-H from synthetic route will be studied in parallel with C-S-H from natural hydration. Both powder and bulk forms of C-S-H are subjected to similar environmental conditions such as CO2 concentration, relative humidity, NH4NO3 concentration. Such comparison provides an insight into how degradation behaviours differ between different types of C-S-H. Recalcification The process of recalcification was reported twice in the literature. Recalcification promises a reparation to degraded structures, restoring Ca/Si ratio and homogeneity across the samples. Effects of recalcification on different types of C-S-H will be investigated in this project. Research questions: 1) What are the differences in the crystal structure of synthetic C-S-H compared to C-S-H at the same Ca/Si but prepared from hydrated C3S paste, and from hydrated Portland cement? 2) How do different types of C-S-H respond to chemical degradation (carbonation and decalcification), and to recalcification? 3) Are decalcification and recalcification reversible processes? Can recalcification be used to repair Ca/Si of degraded structures? If yes, will C-S-H crystalline structure be the same as the intact reference?