111 / 2022-04-25 17:20:13

Full process of calcium silicate hydrate decalcification: Molecular structure, dynamics, and mechanical properties

C-S-H gel; decalcification; molecular dynamics; cement; lamellar mineral

C-S-H gel is the origin of cohesion of cement and deteriorates with decalcification. However, the molecular process underlying its complex chemical reactions and structure/properties evolution is mostly unexplored. Herein, the influence of decalcification on calcium silicate hydrate (C-S-H) is studied by molecular dynamics and a series of experiments.



The deterioration of C-S-H gel can be categorized into chemical and physical damages (e.g., mechanical damage). Decalcification may be the only way of chemical damage for C-S-H gel at room temperature. There are two stages during C-S-H decalcification: interlayer Ca first dissociates and then is followed by intralayer Ca. Complete dissociation of interlayer Ca occurs at the Ca/Si is around 1.1 for OPC, whose initial averaged Ca/Si is 1.7.



Dynamically, the dissociation of interlayer Ca leads to the shrinkage of volume and interlayer spacing of C-S-H molecules, and yet has little impact on the lamellar configuration. In contrast, intralayer Ca dissociation enables the disintegration of Ca-Si layers and it results in that the dynamic characteristics of Ca and Si species change from lattice vibration at fixed sites to diffusion, which makes possible the Si species dissolving. The lamellar structure is gradually destroyed and the semi crystal is transformed into amorphous structure. The volume and interlayer spacing expand, and the mobilities of Ca and Si species can be up to that of adsorbed water.



Chemically, since the lack of Ca that electro neutralizes negatively charged silicate chains, the O in silicate chains would capture the electrons of surrounding species, such as water molecules’ H and Si. Attracting electrons of Si enables the formation of Si-O-Si bonds, that is, polymerization reactions. Attracting electrons of the H in water enhances the dipole moment of adsorbed water and improves structural hydrophilicity, and macroscopically improves the anti-evaporation ability at high temperature. On the other hand, the OH bonds of some water molecules may be broken under the electrostatic attraction of silicate chains, thus hydrolytic reactions occur to produce Si-OH and Ca-OH, which enables the transformation from adsorbed water to chemically bound water. Each Ca dissociation is companied by the decomposition of 0.6 water molecules.



Mechanically, for both molecular and macro/micro scale, compressive properties gradually decrease with decalcification, while tensile properties first decrease and then increase. Tensile property is closely associated to interlayer bonding strength that is weakest at Ca/Si of 1.1~0.8 and enhances to some extent with further decalcification, whereas the interlayer bonding has relatively small impact on compressive behavior.