In order to accelerate the application of magnesium oxide (MgO) expanding admixture in roller compacted concrete (RCC), aiming at common three-grading RCC used in water conservancy and hydropower engineering, the autoclave expansion of mortar specimens and the compressive strength of the autoclaved mortar specimens were studied by means of autoclave test and compressive strength test. Results showed that the balanced MgO content in RCC, which cannot be determined by the MgO content corresponding to 0.5% autoclave expansion rate of mortar specimens or the inflection point of the curve of autoclave expansion changing with MgO content like normal concrete, should be determined by the MgO content corresponding to the inflection point of the curve that compressive strength of autoclaved mortar specimens changes with MgO content to ensure long-term safe operation of RCC dams.

The transformation law of autogenous volume deformation of MgO-admixed concrete with specimen size was revealed through continuous observation on standard-size (Φ200 mm × 500 mm), medium-size (Φ250 mm × 500 mm), and large-size specimens (Φ250 mm × 600 mm) over 6 years. Besides, the pore parameters of concrete core samples obtained from autogenous volume deformation specimens in the 1st, 3rd, and 6th years of age were investigated. The results show that the autogenous volume deformation of MgO-admixed concrete increases with the increase in the MgO content or age. The expansion rate of the concrete specimen decreases after the age of 360 days, and the autogenous volume expansion deformation of the specimen tends to be stable after about 2 years. When the size of the specimen changes, the autogenous volume deformation of MgO-admixed concrete decreases with the increase of specimen size. During the age of 2–6 years, the expansion of medium- and large-size specimens is reduced by 6–10 and 15–20%, respectively, compared with the standard-size specimens under the same MgO content. With the condition of an appropriate MgO content, regardless of the size of the specimen, the pore structure of the concrete becomes better and better with the growth of age, the concrete becomes denser and denser, and the expansion caused by MgO hydration will not cause damage to concrete structures.

Review and summarize Baishan hydropower station that unexpectedly discovered the beneficial effects of MgO, Dongfeng hydropower station that actively used MgO as an expansive agent for the first time in the main project, Changshaba hydropower station that used MgO expansive agent for the first time in the full arch dam, Huangjiazhai hydropower station that broke through 6% of MgO content for the first time in the normal state concrete, and Dahe reservoir that broke through 6% of MgO content for the first time in roller compacted concrete, finding that the production process, transportation equipment, pouring process, observation instrument burying, etc. of concrete mixed with MgO are exactly the same as concrete without MgO; the long-term autogenous volume deformation of MgO concrete presents a good delayed micro-expansion discipline, which matches the long temperature drop process of the dam concrete; the use of MgO expansive agent in the dam concrete improves the concrete's anti-cracking ability, simplifies temperature control measures of dam concrete, speeds up construction progress, and saves project investment. At the same time, it points out the appropriate quality standards for the MgO expansive agent used in dam concrete and its soundness content which shall not be exceeded.