Relative humidity‐dependent expansion of magnesium oxide expansive agent in concrete

Abstract Magnesium oxide (MgO) expansive agent is widely used in hydraulic dams in recent years. It provides good expansion properties to compensate for concrete shrinkage in high relative humidity (RH) environments. However, it is uncertain whether it can still work well in civil building concrete with low RH and prone to water evaporation. This work evaluated the volume deformation of concrete with the same strength grade and MgO content under various RH environments, including internal RH (water–binder [w/b] ratios of 0.32 and 0.42) and external RH (sealed and dry curing). Then, a new calculation method to assess the RH‐dependent expansion of the self‐deformation ratio of MgO expansive agent was presented in this paper. Results show that all concrete with MgO exhibit expansive properties within 28 days. The expansion ratio of concrete with MgO remains relatively stable following 7 days of sealed curing but diminishes progressively in a dry environment. High w/b and sealed curing maintain the RH above 92%, thus enhancing the MgO hydration degree and MgO expansion. Sealed curing results in a 22%–42% increment compared to dry curing in the expansion ratio of MgO itself at 14 days, and in the 0.42 w/b group is 10%–27% higher than that in the 0.32 w/b group. The well‐hydrated MgO reduces the most probable pore diameter and cumulative volume. The findings of this work are of great significance for promoting the widespread application of MgO expansive agent.