New Insights into the Role of Portlandite in the Cement System: Elastic Anisotropy, Thermal Stability, and Structural Compatibility with C-S-H

Portlandite, Ca(OH)2, is a primary product in cement hydration, on which some biases or misconceptions persist, such as portlandite has inferior mechanical properties because of its layered microstructure and crystal brittleness. In this work, the mechanical properties of portlandite are systematically investigated by using first-principles methods and atomic force microscopy. No dramatic difference among average elastic constants is observed between portlandite crystal and C-S-H gel, but only a slight drop in elastic modulus, at ∼12%. In fact, portlandite even has better performance under uniaxial tension loading in the a and b directions in comparison to that of C-S-H. However, portlandite is quite anisotropic with its Young\u27s modulus anisotropy reaching 3.50, because of its stacking layers in the c axis. We have found that because of its small yield stress in the c direction (1.5 GPa) and softening behavior, portlandite can be classified as a very soft material. By using atomic force microscopy, we find that the indentation modulus of a perfect portlandite is about 15.32 \ub1 4.25 GPa. Moreover, we have also studied the size effects as well as defects in the portlandite crystal by using both molecular dynamics simulations and electron microscopy. On the basis of the results, we proposed a new C-S-H gel growth model by postulating an "epitaxial growth on monolayer portlandite template", which explains and elucidates the reactive hydration process, in a dynamic sequence of "protonating, attracting, and stacking", during C-S-H nucleation and growth