Methods for cementing a subterranean well and maintaining zonal isolation involve preparing a cement slurry that contains water, an inorganic cement and an expanding agent. The slurry is placed in the annular region between casing and the formation or between two casing strings. After the cement sets, the expanding agent reacts and causes the set cement to be in a state of compression within the annular region. The casing dimensions may fluctuate in response to a temperature change, a pressure change, a mechanical disturbance resulting from a well intervention, or mud contamination or a combination thereof. The expanding agent may further react and maintain a state of compression within the annular region. The state of compression in the annular region may be monitored by acoustic impedance measurements.