Lj. Yuan et al., ELECTRICALLY-INDUCED SHAPE CHANGES IN HARDENED CEMENT PASTES AND POROUS SILICA-GELS - THE DYNAMIC NATURE OF GEL PORE STRUCTURES DURING WATER TRANSPORT, Journal of the American Ceramic Society, 78(12), 1995, pp. 3233-3243
Electrically induced strain studies of hardened cement pastes mere per
formed using an inductance technique. Investigations were done as a fu
nction of electrical history, measurement frequency, moisture content,
and initial mater to cement ratio (w/c) during hydration. Expansive s
trains as large as 4.5 x 10(-4) mere observed under held strengths of
similar to 0.5 kV/cm in specimens which had been saturated with mater.
Both switchable and nonswitchable strains mere observed under cycling
with an ac electrical field. Evidence mas observed that the switchabl
e strains are due to redistribution of water within the specimen, wher
eas the nonswitchable strains are related to long-range mater transpor
t toward the surface of the specimen. Investigations as a function of
measurement frequency revealed a strong relaxation of the induced stra
in in the frequency range of 6.67 x 10(-3) to 1 Hz. It is believed tha
t ions in the diffuse double layer move under an electrical field and
that water is transported by electro-osmosis. Electrically induced str
ains then develop because of local swelling effects associated with a
saturation gradient. Investigations as a function of w/c ratio reveale
d that the induced strains are related to the volume fraction of gel p
orosity. Electrically induced strain measurements of porous silica gel
s with average pore sizes of 25 and 50 Angstrom mere then performed. T
he characteristics of the electrically induced strains mere found to b
e nearly identical to those for portland cement. It is believed that t
he characteristics of the electrically induced strains are controlled
by the entry and removal of physically absorbed mater within the gel p
ore structures.