Multi-scale quantitative study on dielectric properties of C–S–H synthesized by different molar ratio of Ca/Si

Abstract

The main hydration product of cement is C–S–H. C–S–H has a great influence on the electromagnetic properties, but there are few studies on the independent study of the electromagnetic properties of C–S–H. Therefore, nanocrystalline C–S–H was synthesized by CaO and nano-SiO2, multi-scale analysis method was adopted, and the electromagnetic properties of its hydration products were analyzed from the mechanism. C–S–H powders with Ca to Si molar ratio (C/S) of 0.8, 1.2, 1.6, 2.0 and 2.5 were synthesized and tested by the coaxial method. Four types of materials were defined in this paper: HPB (composed of nanocrystalline C–S–H, Ca(OH)2, CaCO3 and pore phase); CSH (composed of nanocrystalline C–S–H and pore phase with diameter less than 5 nm); P-CSH (composed of pore phase and CSH phase); C-CSH (composed of Ca(OH)2, CaCO3 and CSH phase). The complex permittivity and complex permeability of HPB with different C/S at 2–18 GHz were tested. The influence of molecular structure, interlayer physically bound water, phase types and content of HPB on the electromagnetic properties were investigated by XRD, MIP, N2 adsorption, TGA and 29Si NMR spectroscopy. The results showed that with the increase of C/S, the permittivity of CSH, P-CSH, C-CSH and HPB were decreased. Both Ca(OH)2 phase and pore phase can reduce the permittivity and loss tangent of HPB. It was worth noting that the reduction degree of pores phase on the permittivity of the HPB were about 4 times than that of Ca(OH)2 phase (or CaCO3 phase). Meanwhile, the results confirmed that the carbonization of Ca(OH)2 phases had no effect on the electromagnetic properties of HPB.