Plugging expanding cements ensure a better quality of insulation of productive oil and gas-containing layers from aquifers. We examined the effect of additives in the oxide form of expansion on the physical-mechanical properties of cements and plugging mortars. It was established that the introduction of calcium oxide and magnesium oxide additives in the amount of 20 % by weight leads to an increase, respectively, by 20 ‒ 40 % and 12 ‒ 36 % in cement water consumption, and reduces by 20 ‒ 32 % and 52 ‒ 64 % strength of the cements with additives in comparison with additive-free cement. It should be noted that the introduction of a magnesium oxide additive results in a more significant reduction of cement strength (by 40–47 %) and plugging mortars strength (by 29–44 %) in comparison with the samples with a calcium oxide additive. Cements and plugging mortars with a calcium oxide additive yield a much greater increase (by almost three times) in the dimensions of samples than a magnesium oxide additive. The negative impact of magnesium oxide on the strength of cements and plugging mortars can be explained by the features of hydration products of this additive. Magnesium hydroxide is formed in the form of fibers and flakes and it does not consequently participate in the formation of crystalline structure of cement stone. At the same time, calcium hydroxide can, in the process of hydration, form calcium hydrosilicates and portlandite, which somewhat reduces the negative effect of this additive on the strength of cements and plugging mortars. In addition, with an increase in temperature, based on data from electron microscopic analysis, there occurs the formation of a larger mass of magnesium hydroxide due to the formation of smaller fibers instead of the large lamellar ones. The total weight of new formations increases ‒ the expansion of the hardening mass of the plugging mortar also grows. However, these new formations of magnesium hydroxide do not possess strength and, in general, cement stone essentially loses strength compared to the cement stone with a calcium oxide additive. This confirms inappropriateness of using magnesium oxide as the additive in the production of expanding plugging cements.