Cementitious slurries capable of controlling and/or preventing strength retrogression during the cementing of a formation contain a bismuth salt or derivative thereof. Bismuth trioxide is preferred. The cementitious slurry may further contain a density modifying agent and/or strength enhancer. The cementitious slurries of the invention are suitable for use over a broad temperature range of from about 45.degree. F. to about 500.degree. F.