Microstructural analysis of young cement pastes using impedance spectroscopy during pore solution exchange

Impedance spectra were collected in situ during the exchange of the aqueous phase with an organic solvent in young (1-28 days old) ordinary portland cement (OPC) pastes at two water-to-cement ratios (w/c = 0.35 and 0.70). The high initial values of dielectric constant (10³-10⁵) and the rapid decline upon exchange support a model of dielectric amplification where layers of calcium silicate hydrate (C-S-H) gel separate capillary pores. When the liquid in pastes of w/c = 0.35 was exchanged with isopropanol, the residual dielectric constants were typically between 10²-10³, indicating that C-S-H has a dielectrically amplified microstructure. After methanol exchange, however, the residual dielectric constant was much smaller, 10-100. This suggests there are two kinds of water within C-S-H gel in pastes of w/c = 0.35 - that which is exchangeable by both solvents (capillary pores) and that which is exchangeable by methanol but not isopropanol (gel pores). For pastes of w/c = 0.70, differences due to solvent types were negligible. These observations suggest that the nature of C-S-H varies with w/c ratio.