Microscopic features of non-hydraulic calcium silicate cement paste and mortar

Non-hydraulic calcium silicate-based cement (CSC) is a newly developed binder that hardens upon carbonation reaction. The primary components of CSC are low-lime calcium silicates, including rankinite, wollastonite and pseudo-wollastonite. During the carbonation, CSC binder forms Ca-modified silica gel and CaCO3. Microscopic evaluation of the carbonated CSC paste using the 29Si {1H} CP-MAS NMR revealed that the Ca-modified silica gel phase consists primarily of Q3 and Q4 species, indicating a substantially higher degree of polymerization in comparison to that of the C-S-H. The elastic modulus and hardness of this gel phase were found to be close to those of the high density C-S-H. A composite phase, formed by the intermixing of Ca-modified silica gel and CaCO3, was also identified using SEM X-ray microanalysis and nanoindentations. The distribution of the microscopic phases in ITZ was similar to that observed in the bulk paste region for this system.