Expansive cements for use in cementing subterranean wells contain an encapsulated gas-generating substance. The gas-generating substance comprises one or more materials that release hydrogen, nitrogen or both. The gas-generating substance is encapsulated by a coating comprising a polymer. The coating prevents premature gas release at the surface during slurry mixing, and promotes gas release at a desired location in the subterranean well. The released gas may also control the internal pore pressure of the cement slurry, thereby inhibiting the invasion of formation fluids into the borehole.