Evolution of C-S-H in lime mortars with nanoparticles: Nanostructural analysis of afwillite growth mechanisms by HRTEM

This work studies the formation of calcium-silicate-hydrates (C-S-H) in lime mortars prepared with additions of nano-Ca(OH)(2) and nano-SiO2 at 23.3 degrees C. Mineral identification was carried out by X-ray diffraction after 10, 30, 90, and 120 days of curing. The nanoscale study starts from the generation of amorphous phases until the development of crystalline phases. Observations of binder mortar made by transmission electron microscopy (low magnification and high resolution TEM) after 30 and 110 days of curing showed the formation of two types of C-S-H with different degrees of crystallinity depending on the curing time. The development of short-range order C-S-H globular phases was visible after 30 days. C-S-H evolved into lamellar crystalline phases visible after 110 days of curing. The crystalline phase corresponds to the C-S-H known as afwillite (Ca-3(SiO3OH)(2)center dot 2H(2)O), first reported to affect cement and concrete's mechanical and hydration properties. It appears as isolated fibers, growing epitaxially along the edges of the calcite (product of the carbonation of Ca(OH)(2)), and advancing inward aided by atomic defects (grain boundaries, stacking faults). In addition, high-resolution transmission electron microscopy tools and electron diffraction simulation confirmed a monoclinic symmetry for afwillite crystals. These results contribute to analyzing the presence of crystalline/amorphous C-S-H in lime mortars, providing information on the structure of afwillite and its possible effects on the binder materials.