PROPAGATION OF ULTRASOUND THROUGH HYDRATING CEMENT PASTES AT EARLY TIMES

Measurements of the velocity and amplitude of ultrasonic compressional waves propagating through neat and accelerated American Petroleum Institute class G cement pastes undergoing hydration are compared with the predictions of the theory of elastic wave propagation through fluid-saturated porous media. The compressional wave velocity is observed to decrease slightly during the first few hours after mixing, followed by a rapid rise as the solid phase becomes interconnected. It is shown that this change in behavior results from a change in character of the observed wave as hydration proceeds. At early times the observed wave involves essentially motion of the fluid phase while at longer times it involves essentially motion of the solid frame. Ultrasonic waves are therefore sensitive to the point at which the solid phase becomes interconnected. This point is of practical significance since the connectivity of the solid phase is responsible for the load-bearing capacity of set cement. The early-time decrease in velocity results from an increasing tortuosity of the pore space due to the formation of hydration products while the increasing velocity at later times results from a stiffening of the porous frame. Wave propagation at early times involves motion of the fluid phase and is extremely sensitive to the presence of air bubbles. Cement pastes containing a sufficiently large number of air bubbles are found to act as high-pass filters over the frequency range employed. This results from the resonant scattering of ultrasound by the bubbles. At longer times the solid frame becomes interconnected and propagation of the low frequency components becomes possible. The amplitude and center frequency of these components are observed to increase with increasing connectivity of the solid phase.