Degradation of C–S–H(I) at different decalcification degrees.

The deterioration of cement-basted materials serviced in leaching environments is strongly dependent on the decalcification behavior of calcium silicate hydrates after complete dissolution of calcium hydroxide. This paper examines the deteriorating condition of calcium silicate hydrates (hereinafter referred to C–S–H(I)) samples with the decalcification degree which is controlled by immersing samples into leaching solutions with different NH4Cl concentrations. The composition, chemical structure, micromorphology and pore structure of un-leached and leached C–S–H(I) samples were determined using XRD, TG, FTIR, 29Si NMR, SEM, TEM and N2 adsorption–desorption measurements. Results reveal that the decalcification degree of C–S–H(I) samples increases with the increasing NH4Cl concentration with the Ca/Si atomic ratio decreasing from 0.91 to 0.02. The area of flocculent micromorphology of C–S–H(I) samples increases with the decalcification degree due to the continuous generation of cross-linking C–S–H(I) and amorphous silica gel. In addition, the decalcification behavior damages the crystallinity, coarsens the pore structure and decreases the chemically-bound water content of C–S–H(I) samples. Molecular dynamics simulations prove that increasing NH4Cl content results in weaker connectivity stability and more breakage of Ca–O bond in C–S–H(I) molecules. [ABSTRACT FROM AUTHOR]