A test procedure for monitoring changes in the amplitude of horizontally polarized ultrasonic shear waves from the surface of hydrating cement paste is presented. A theoretical framework based on a poro-elastic idealization of the hydrating cement paste is developed for interpreting the ultrasonic reflection data. The poro-elastic representation of hydrating cement paste is shown to provide simultaneous, realistic estimates of porosity and shear modulus for hydrating cement paste through setting and early strength gain. The porosity predicted by the poro-elastic representation is identical to the capillary porosity predicted by Powers' model. The shear modulus of the poro-elastic skeleton was found to agree favorably with the value of shear modulus obtained from a vibration-based measurement. Data from cement paste with different water-to-cement ratios indicates a very non-linear relationship between capillary porosity and shear modulus. While there is a steady decrease in the porosity in the first 24 h, there is initially an exponential increase in the shear modulus up to the end of the acceleration period, which is followed by a period with a slower rate of increase. © 2011 Published by Elsevier Ltd.